In industrial automation systems, analog outputs from inverters are widely used to transmit operating frequency, output current, output voltage, torque, power, and other real-time operating data to PLCs, HMIs, recorders, or supervisory systems. Although this appears to be a simple analog wiring task, field commissioning often reveals problems such as:

“The parameter has already been set correctly, but the PLC display is still inaccurate.”

A typical example involves a JT330S2 inverter configured to output a 0–20mA analog signal through terminal AO2 so that a Siemens S7-200 SMART PLC can monitor the motor’s actual running current. According to the inverter manual, the technician sets parameter:

F6-08 = 2

which selects “Output Current” as the AO2 output function.

However, during operation, the inverter display or clamp meter shows an actual motor current of approximately 20A, while the PLC displays only 10A — exactly half of the real value.

This type of issue should not immediately be interpreted as:

“The inverter is faulty”
or
“The parameter was set incorrectly.”

From a troubleshooting perspective, the problem is usually related to scaling mismatch between:

1. What signal the inverter actually outputs
2. What signal the PLC actually receives
3. How the PLC program converts that signal into engineering units

To solve the problem correctly, all three stages must be analyzed independently.

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1. Basic Function of AO2 Analog Output

The JT330S2 inverter provides two analog outputs:

AO1
AO2

These outputs can be configured to represent different operating parameters, including:

Running frequency
Set frequency
Output current
Output torque
Output power
Output voltage
Input analog values
Motor speed
Communication settings

In this application, the user wants the PLC to monitor the motor output current, therefore AO2 should be configured as:

F6-08 = 2

meaning:

AO2 output function = Output Current

This setting direction is correct. However, F6-08 only determines:

“What AO2 outputs”

It does NOT automatically guarantee:

“20A motor current = 20mA analog output”

Analog output accuracy also depends on:

Output mode
Zero offset
Gain
Full-scale mapping
PLC input scaling
Engineering unit conversion

Therefore, F6-08 is only the first step. The technician must still verify:

Whether AO2 is configured as current or voltage output
Whether AO2 outputs 0–20mA or 4–20mA
How the PLC analog module is configured
How the PLC program converts the raw analog value

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2. Why the PLC Displays Only Half the Actual Current

The key field symptom is:

Actual motor current: 20A
PLC displayed current: 10A

An error that is exactly “half” is usually not random. It almost always indicates a scaling mismatch.

Suppose the inverter internally defines:

20mA = 40A

Then when the motor current is 20A, AO2 only needs to output half-scale current:

Approximately 10mA

Now suppose the PLC program assumes:

20mA = 20A

Then the PLC interprets 10mA as:

10A

This creates the observed condition:

Actual 20A
→ AO2 outputs approximately 10mA
→ PLC interprets 10mA as 10A

Therefore, the issue is most likely NOT that AO2 is malfunctioning. Instead, the scaling relationship between the inverter analog output and the PLC engineering conversion is inconsistent.

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3. Measure the Actual AO2 Output Current First

One of the biggest mistakes in field troubleshooting is immediately changing parameters without measurement.

The correct approach is to first measure the real AO2 output current using a multimeter.

Unlike voltage measurement, current measurement must be performed in series.

The correct wiring method is:

Inverter AO2 → Multimeter mA Input
Multimeter COM → PLC Analog Input AI+
Inverter GND → PLC Analog M/COM

In practical terms:

Disconnect the wire between AO2 and PLC AI+
Insert the multimeter in series

The multimeter must be set to:

DC mA measurement mode

The red probe must be inserted into the:

mA terminal

and the black probe into:

COM

If the meter is accidentally placed in voltage mode, AC current mode, or connected in parallel, the reading will be incorrect and the meter fuse may even blow.

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4. Using Measured Values to Determine the Root Cause

During testing, record three values simultaneously:

1. Inverter displayed output current
2. Multimeter measured AO2 current signal
3. PLC displayed current

Measurements should be taken at multiple load points:

Low load (e.g. 5A)
Medium load (e.g. 10A)
High load (e.g. 20A)

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Case 1: Actual 20A → AO2 ≈ 10mA → PLC Displays 10A

This means the PLC display follows the analog signal correctly, but the engineering scaling range is too small.

The PLC likely assumes:

20mA = 20A

while the inverter output behaves like:

20mA = 40A

Typical PLC conversion formula:

Current = Raw_AI × 20 / 27648

should instead become:

Current = Raw_AI × 40 / 27648

This adjustment causes:

10mA → 20A

which restores correct display.

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Case 2: Actual 20A → AO2 ≈ 20mA → PLC Displays 10A

In this case, the inverter analog output is already correct.

The fault is entirely on the PLC side.

Possible causes include:

Incorrect analog module range
Incorrect PLC scaling formula
Incorrect HMI engineering scaling
Additional divide-by-two logic in the program
Incorrect analog module configuration

If the PLC raw analog value already reaches near full scale but the displayed engineering value is still half, the inverter is NOT the problem.

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Case 3: Actual 20A → AO2 Has Almost No Current Output

This suggests a hardware or wiring issue.

The technician should verify:

AO2 jumper/switch position
AO2 wiring
PLC analog input type
GND common reference
F6-08 parameter
F6-12 offset
F6-13 gain

On the JT330S2 control board, AO1 and AO2 can be configured as either:

Voltage output (V)
Current output (I)

If AO2 is still configured for voltage output while the PLC expects current input, readings will be abnormal.

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5. Recommended Inverter Parameter Settings

For transmitting actual motor current through AO2:

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AO2 Hardware Mode

Set AO2 jumper/switch to:

I = Current Output

Do NOT leave it at:

V = Voltage Output

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AO2 Function Selection

F6-08 = 2

meaning:

AO2 outputs motor current

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AO2 Offset

F6-12 = 0.0%

Normally keep default.

Improper offset adjustment can create false low-current readings.

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AO2 Gain

F6-13 = 1.00

Normally keep default initially.

If measurements confirm the AO2 signal is exactly half the desired value, gain can be increased:

F6-13 = 2.00

However, increasing gain also risks premature saturation.

Example:

Originally:

40A = 20mA

After doubling gain:

20A = 20mA

Then any current above 20A can no longer be represented properly.

Therefore, adjusting PLC scaling is usually preferable.

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6. PLC Engineering Unit Scaling Is Often Overlooked

Siemens S7-200 SMART analog modules typically convert analog current into digital values.

A common full-scale raw value is:

27648

for:

0–20mA

Engineering conversion formula:

Engineering_Value = Raw_AI × FullScale / 27648

The key question is:

What actual current does 20mA represent?

The PLC does not know automatically.

The programmer must define it.

If the PLC program assumes:

20mA = 20A

then:

10mA = 10A

If the inverter behavior is actually:

20mA = 40A

then the PLC formula must be updated accordingly.

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7. 0–20mA and 4–20mA Must Not Be Confused

Industrial analog current signals are commonly:

0–20mA
4–20mA

0–20mA:

0mA = zero signal
20mA = full scale

4–20mA:

4mA = zero signal
20mA = full scale

4–20mA allows wire-break detection because signal loss drops below 4mA.

The following three elements must always match:

Inverter output type
PLC analog input type
PLC scaling formula

If one side uses 0–20mA and the other assumes 4–20mA, scaling errors will occur.

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8. Why Special Output Modes Should Not Be Used Randomly

Some JT330S2 firmware versions include special AO2 scaling options such as:

100% = 1000A

These modes are usually intended for:

Special scaling
Communication mapping
Large-current systems
Manufacturer calibration

They are generally unsuitable for standard motor current monitoring applications.

For normal PLC monitoring:

F6-08 = 2

is the preferred choice.

Scaling corrections should then be handled through:

F6-13 gain adjustment
or
PLC engineering scaling

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9. Recommended Field Commissioning Procedure

A practical troubleshooting workflow:

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Step 1 — Verify Wiring

AO2 → PLC AI+
GND → PLC M/COM

For testing:

AO2 → Multimeter mA input
Multimeter COM → PLC AI+
GND → PLC M/COM

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Step 2 — Verify AO2 Hardware Mode

AO2 jumper = I

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Step 3 — Verify Parameters

F6-08 = 2
F6-12 = 0.0%
F6-13 = 1.00

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Step 4 — Record Operating Data

Measure:

Actual motor current
AO2 mA signal
PLC raw analog value
PLC displayed engineering value

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Step 5 — Analyze Results

Correct AO2 but wrong PLC display
→ PLC scaling issue

AO2 signal too small
→ Gain or scaling mismatch

No AO2 signal
→ Wiring/jumper/input mode issue

Unstable signal
→ Grounding/shielding/noise problem

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10. Wiring and Safety Considerations

Even though AO2 is a low-level control signal, inverter commissioning still involves dangerous power circuits.

Important precautions:

Never modify power wiring while energized
Do not short 24V, 10V, or GND terminals
Current measurement must be series-connected
Use correct meter terminals
Avoid shorting AO2 to GND
Ensure proper common grounding
Use shielded analog cables
Separate analog cables from motor cables

Many analog signal issues are actually caused by:

Improper shielding
Noise interference
Incorrect grounding
Mixed routing with motor cables
Floating analog commons

rather than parameter settings.

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11. Explaining the Problem to Customers

A practical explanation for customers is:

“Setting F6-08=2 only tells the inverter to output motor current through AO2. The PLC display depends on how many milliamps AO2 actually outputs and how the PLC converts those milliamps into amps. If actual current is 20A but PLC shows 10A, the scaling ratio is incorrect. First measure the real AO2 output current with a multimeter. If 20A corresponds to 10mA, either adjust PLC scaling so that 20mA equals 40A, or increase AO2 gain. If 20A already corresponds to 20mA, then the issue is entirely inside the PLC program.”

This explanation helps avoid unnecessary inverter replacement.

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12. Conclusion

When using the JT330S2 inverter AO2 analog output to transmit motor current to a Siemens PLC, parameter:

F6-08 = 2

is correct, but it is only the function selection step.

The common field symptom:

Actual 20A
PLC displays 10A

is usually caused by mismatch between:

AO2 output scaling
PLC analog input scaling
Engineering conversion formulas

The proper diagnostic method is to:

Measure the real AO2 mA signal first

using a multimeter connected in series.

Then determine whether the issue lies in:

PLC scaling
AO2 gain
Wiring
Output mode
Grounding

In most practical applications, correcting PLC engineering scaling is preferable to increasing AO2 gain, because it avoids premature analog output saturation and preserves full measurement range.

The key principle in troubleshooting analog output systems is understanding the complete signal chain:

Motor Current
→ Inverter Internal Calculation
→ AO2 Analog Output
→ PLC Analog Acquisition
→ PLC Engineering Conversion
→ HMI Display

By testing and verifying each stage independently, technicians can rapidly locate the true cause of scaling errors and avoid unnecessary hardware replacement or repeated blind parameter adjustments.

In CNC equipment, automatic feeding systems, packaging machines, printing machines, dispensing machines, winding machines, and many other automation mechanisms, AC servo systems are responsible for accurate positioning, speed control, and torque output. Once a servo drive reports an alarm, the machine usually stops immediately. Compared with a standard inverter, a servo drive has stricter monitoring logic because it does not only check current, voltage, and temperature; it also continuously compares command position, command speed, actual encoder position, and actual feedback speed.

For this reason, when a servo system fails, the technician should not only ask whether the motor can rotate. It is more important to analyze the relationship between command, feedback, load, wiring, parameters, and the mechanical transmission structure.

In field applications, the Zhishan K5 series AC servo drive may show the following symptom: the drive powers on normally, the servo can be enabled, forward rotation may work, but reverse rotation fails. Once a reverse command is given, the motor may not move, may move slightly and stop, or may immediately trigger Er24.1. In some cases, Er16.0 also appears.

This fault may look like a defective servo drive, but the real cause may be mechanical jamming, excessive load, abnormal encoder feedback, incorrect motor phase wiring, wrong direction logic, active reverse limit signal, brake not released, or improper parameter settings. To solve this problem correctly, the alarm meaning must be combined with the real machine behavior.

1. The Meaning of Er24.1: Excessive Speed Deviation

For the K5 servo drive, Er24.1 generally means excessive speed deviation protection. Speed deviation is the difference between the speed commanded by the drive and the actual speed fed back by the motor encoder.

For example, the PLC or motion controller sends a reverse rotation command to the servo drive. The drive outputs three-phase current to make the motor rotate in reverse. However, the encoder feedback shows that the motor speed does not reach the expected value, or the motor almost does not move. In this case, the drive determines that the difference between command speed and actual speed is too large. To prevent overcurrent, overload, mechanical impact, or loss of control, the drive stops and reports Er24.1.

Therefore, Er24.1 does not necessarily mean that the drive is internally damaged. More accurately, it means:

The drive has issued a motion command, but the actual motor movement does not match the expected result.

Common causes include:

motor blocked by mechanical load;

reverse direction mechanically jammed;

mechanical hard limit reached;

motor brake not released;

load inertia too large;

acceleration or deceleration time too short;

motor U/V/W wiring wrong or loose;

encoder cable loose or abnormal;

motor and drive not matched;

reverse direction signal logic incorrect;

speed deviation detection threshold set too small;

servo gain not suitable for the machine.

If the actual symptom is “forward rotation works, but reverse rotation fails,” the key point is not only speed deviation. The real question is:

Why does the motor fail to follow the command only in reverse direction?

2. The Meaning of Er16.0: Overload Protection

Er16.0 is usually related to overload protection. Overload does not always mean an instant short circuit, and it does not always mean that the drive power module is damaged. In servo systems, overload usually means that the motor current has exceeded the allowed range for a certain period of time. The drive judges that the motor or drive is carrying excessive load.

If the machine is mechanically jammed in reverse direction, the drive will increase output current in an attempt to make the motor rotate. But because of mechanical resistance, brake locking, wiring error, or abnormal feedback, the motor cannot reach the target speed. This may first trigger Er24.1. If the high current continues, Er16.0 may also appear.

In other words, Er24.1 and Er16.0 are often connected:

A reverse command is given.

The motor cannot rotate correctly because of mechanical load, limit signal, brake, wiring, or feedback problem.

The drive increases output current.

The encoder feedback speed cannot follow the command.

The drive reports Er24.1.

If high current continues, the drive reports Er16.0.

Therefore, when Er24.1 appears together with Er16.0, do not treat them as two unrelated faults. The correct diagnostic logic is:

Find out why the motor cannot follow the command first, then determine whether overload is the cause or the result.

3. When Reverse Rotation Fails, Mechanical Problems Are Highly Suspect

In real repair work, if a servo system can rotate forward but cannot rotate backward, the mechanical side must be checked first. Many technicians immediately suspect the servo drive and replace it. However, after replacing the drive, the same alarm may still appear because the root cause is not in the drive.

Common mechanical causes include the following.

The first is one-direction mechanical jamming. Lead screws, guide rails, belts, chains, gearboxes, feeding wheels, and clamping mechanisms may move smoothly in one direction but become tight in the opposite direction. Lack of lubrication, damaged bearings, worn lead screw nuts, misaligned belts, damaged gearbox teeth, or foreign objects may all cause reverse movement failure.

The second is a mechanical hard limit. If the mechanism has reached the end position, and the limit switch or software limit does not stop the axis correctly, the servo may command movement against a dead stop. The motor receives torque command but cannot move. Current rises quickly, actual speed remains very low, and the drive reports an alarm.

The third is brake failure. If the servo motor has an electromagnetic brake, the brake must be released before motion. If the brake power supply is missing, the brake relay contact is damaged, the brake coil is faulty, or the brake mechanism is stuck, the motor may hum, vibrate, or fail to rotate. Sometimes the brake is not completely locked but only partially released. This is more difficult to detect because the motor may rotate at no load but fail under load.

The fourth is asymmetric load. Many machines do not have the same load in forward and reverse directions. Lifting mechanisms, feeding systems, pressing rollers, and clamping systems may have very different resistance depending on direction. If reverse direction happens to be the heavy-load direction, Er24.1 and Er16.0 may appear more easily.

For this reason, when a K5 servo drive reports Er24.1 or Er16.0 and the machine cannot reverse, do not start by changing parameters. First confirm whether the machine is mechanically able to move in reverse.

4. The Most Effective First Test: Disconnect the Load

The fastest way to separate mechanical problems from electrical problems is to disconnect the motor from the mechanical load. This means loosening the coupling, belt, gear connection, or other transmission connection so that the motor can run freely without load.

Do not test at high speed first. Use a low-speed jog command, such as 30 rpm, 50 rpm, or 100 rpm, and observe whether the motor rotates smoothly.

Check the following:

Can the motor rotate forward without load?

Can the motor rotate backward without load?

Does the motor vibrate during reverse rotation?

Does it make abnormal noise?

Does Er24.1 still appear?

Does Er16.0 still appear?

If the motor runs normally in both directions without load, the servo drive, motor, and encoder are probably not the main problem. The focus should move to mechanical load, brake, limit switch, or machine process.

If reverse rotation still triggers Er24.1 without load, the problem is more likely in motor wiring, encoder feedback, control signal, parameters, motor, or drive hardware.

This simple test is extremely valuable because it can quickly divide the fault into mechanical side or electrical side. Many servo faults take too long to repair because the technician keeps adjusting drive parameters without first separating the motor from the machine.

5. Check U/V/W Motor Wiring and Encoder Feedback

A servo motor is not the same as a normal three-phase induction motor. For a normal induction motor, changing two phases can reverse the direction. But for a servo motor, U/V/W phases cannot be changed randomly. The drive output phase sequence must correspond correctly to the encoder feedback angle. If the motor phase wiring is wrong, or if the encoder feedback direction does not match the drive output, the servo loop may become unstable.

Possible symptoms include vibration, no torque, excessive current, overcurrent, speed deviation alarm, or overload alarm.

The following points must be checked carefully:

U, V, and W are connected to the correct drive terminals;

motor cable has no broken wire;

terminal screws are tight;

connector pins are not bent or pushed back;

motor cable is not damaged;

encoder connector is fully inserted;

encoder cable shield is properly grounded;

encoder cable is not bundled together with power cables for a long distance;

motor and drive belong to the same axis;

motor cable and encoder cable are not crossed with another axis.

Multi-axis machines are especially prone to cable mix-up. For example, the motor power cable may belong to axis A, but the encoder feedback cable may be connected to axis B. Once the motor and encoder are not matched, the drive cannot close the loop correctly. The result may be immediate alarm, vibration, or dangerous motion.

If the fault appeared after repair, transportation, rewiring, motor replacement, or drive replacement, wiring error must be treated as a high-probability cause.

6. Check Reverse Limit and Inhibit Signals

“Cannot reverse” may also be caused by external control signals. A servo drive usually receives signals from a PLC, motion controller, or control board, including pulse command, direction signal, servo enable, positive limit, negative limit, emergency stop, alarm reset, and inhibit signals.

If the reverse direction limit signal is active, the drive may block movement in that direction. The field symptom may look like the motor cannot reverse, or it may stop immediately when reverse command is given.

Check these signals:

positive limit input;

negative limit input;

forward inhibit input;

reverse inhibit input;

emergency stop input;

servo enable input;

pulse input;

direction input;

PLC output logic;

control common wiring;

input terminal function assignment.

Some machines use normally closed limit logic, while others use normally open logic. If a limit switch, PLC output card, wiring, or parameter has been changed, the input logic may become inverted. The machine may not actually be at the negative limit, but the drive may think the negative limit is active, so reverse movement is blocked.

The correct method is not only to watch whether the limit switch moves mechanically. The technician must confirm the actual input status seen by the servo drive. This can be done through the drive monitor function or by measuring the terminal voltage with a multimeter.

7. Check Whether the Brake Is Released

If the servo motor has a holding brake, the brake circuit must be checked separately. Many technicians assume that the brake will automatically release when the servo is enabled, but in actual machines this is not always true.

The brake often requires an independent 24 VDC supply. It may be controlled by a relay, PLC output, or drive output. If the brake power supply is missing, the relay contact is burnt, the brake coil is damaged, or the brake mechanism is stuck, the motor will be forced to rotate against the brake.

Check the following:

brake rated voltage;

whether brake voltage appears after servo enable;

whether the brake makes a clear release sound;

whether the motor shaft can rotate freely after brake release;

whether brake power supply capacity is enough;

whether relay contacts are burnt;

whether brake coil is open or shorted;

whether brake gap is abnormal;

whether the brake is mechanically stuck.

If the brake is not released and the servo is forced to run, Er24.1 and Er16.0 can appear together. The drive outputs current, but the motor does not reach commanded speed. The result is speed deviation and overload.

8. Parameter Adjustment Should Not Be the First Solution

When technicians see speed deviation alarm, they may want to increase the speed deviation threshold. When they see overload alarm, they may want to increase acceleration time, reduce gain, or modify torque limit. Parameter adjustment can sometimes reduce false alarms, but it should not be used to hide a real mechanical or wiring problem.

If the mechanism is jammed, increasing the speed deviation threshold only delays the alarm. The motor may remain stalled for a longer time, causing motor overheating, drive damage, or mechanical deformation.

If U/V/W wiring or encoder feedback is wrong, parameter adjustment cannot solve the root problem. It may only make the fault more dangerous.

Parameter checking should be done after mechanical and wiring checks. Important parameter groups include:

speed deviation detection threshold;

speed deviation detection time;

acceleration time;

deceleration time;

speed loop gain;

position loop gain;

torque limit;

electronic gear ratio;

pulse input mode;

direction signal polarity;

motor capacity setting;

encoder-related settings;

positive and negative limit input assignment.

Do not restore factory parameters blindly. Machine builders may have set electronic gear ratio, limit logic, pulse mode, gain, and control mode according to the actual machine. A careless factory reset may make the machine unable to return to its original working condition.

9. How to Distinguish Drive Fault, Motor Fault, and External Fault

A practical diagnostic sequence should follow this order:

external mechanical system;

motor and encoder wiring;

control signals;

parameters;

drive and motor hardware.

If the motor runs normally without load but alarms under load, suspect mechanical load, brake, guide rail, lead screw, belt, gearbox, or machine jamming.

If forward works but reverse fails, suspect reverse mechanical resistance, negative limit, reverse inhibit, direction logic, or reverse acceleration impact.

If both forward and reverse vibrate or lack torque, suspect U/V/W wiring, encoder cable, motor matching, or servo gain.

If the alarm appears immediately after servo enable, suspect encoder fault, motor cable fault, drive power module, motor winding, brake locking, or serious parameter mismatch.

If the alarm appears randomly, suspect loose connectors, poor shield grounding, encoder interference, terminal contact problem, unstable power supply, or intermittent mechanical jamming.

If replacing the drive does not change the fault, the problem is probably not inside the drive. It is more likely in motor, encoder, wiring, mechanics, or control signal.

If replacing the motor solves the problem, the original motor may have encoder, winding, or brake failure.

If replacing the encoder cable solves the problem, the original cable or connector has a hidden fault.

If the same drive and motor work normally on a test bench but fail after installation, the machine mechanism or control logic must be checked.

10. Recommended Field Troubleshooting Procedure

For a Zhishan K5 servo drive showing Er24.1, sometimes with Er16.0, and failing to reverse, the following procedure is recommended.

First, record the alarm timing. Confirm whether the alarm appears at power-on, at servo enable, during forward rotation, or only during reverse rotation. The timing is more important than the alarm code alone.

Second, check whether the mechanism can move backward. With power off, manually rotate the transmission mechanism if possible. Check for tight points, hard stops, abnormal noise, brake locking, or one-direction resistance.

Third, disconnect the load and test the motor alone. Run forward and reverse at low speed. If the motor runs normally without load, focus on the mechanical side. If reverse still alarms without load, focus on wiring, feedback, control signal, or parameter.

Fourth, check the brake. If the motor has a brake, confirm that it is really released, not only that the control signal exists.

Fifth, check U/V/W and encoder wiring. Confirm motor phase wiring, encoder cable, shielding, connectors, and axis matching.

Sixth, check control input status. Focus on negative limit, reverse inhibit, emergency stop, servo enable, direction signal, and pulse command.

Seventh, check parameters. Confirm speed deviation threshold, acceleration and deceleration time, torque limit, direction polarity, pulse mode, electronic gear ratio, and limit input function assignment.

Eighth, test again at low speed. Start with no load, then light load, then normal load. Observe current, speed feedback, machine movement, and alarm behavior at each step.

Ninth, use substitution testing only when necessary. A same-model motor, encoder cable, or drive can be exchanged for comparison, but wiring and parameters must be confirmed before testing to avoid causing new damage.

11. Common Mistakes During Repair

Several mistakes are common when repairing this type of servo fault.

The first mistake is increasing the speed deviation threshold immediately after seeing Er24.1. This may hide mechanical jamming and cause more serious damage.

The second mistake is assuming the drive power module is bad after seeing Er16.0. Overload is often caused by load or motion conditions, not necessarily by drive hardware failure.

The third mistake is repeatedly testing the machine without disconnecting the load. If the mechanism is jammed, repeated testing sends high current into the motor and drive.

The fourth mistake is swapping U/V/W phases casually. A servo motor cannot be treated like a normal induction motor.

The fifth mistake is checking only the motor power cable and ignoring the encoder cable. Servo control is closed-loop. Encoder feedback problems can also cause speed deviation and overload.

The sixth mistake is looking only at the physical limit switch but not the drive input status. A switch may move correctly, but the drive terminal may receive the wrong signal.

The seventh mistake is restoring factory settings blindly. This may erase the original electronic gear ratio, limit logic, pulse mode, and gain settings.

The eighth mistake is ignoring the brake. A brake that does not fully release is a frequent cause of overload and speed deviation alarms.

12. Conclusion

When a Zhishan K5 series servo drive reports Er24.1, the core meaning is excessive speed deviation. When Er16.0 appears as well, the system also has an overload condition. For the symptom “reverse rotation failure,” the correct conclusion is not to immediately condemn the servo drive. The most likely causes are reverse-direction mechanical resistance, active reverse limit, brake not released, excessive load, abnormal encoder feedback, wrong wiring, or incorrect direction logic.

The correct repair method is to separate the mechanical side from the electrical side first. Disconnect the load and test forward and reverse rotation at low speed. If the motor works normally without load, check the mechanism, brake, and limit signals. If the alarm still appears without load, check U/V/W wiring, encoder cable, control terminals, direction command, and parameters.

Parameter adjustment should be used only after the real cause is identified. It should never be used to cover up mechanical jamming or wiring errors.

A servo alarm is the result of closed-loop monitoring. Er24.1 means the actual motor speed does not follow the command. Er16.0 means the motor or drive is overloaded. Only by analyzing command, feedback, current, mechanical load, and control signals together can the technician locate the fault quickly, avoid unnecessary part replacement, reduce downtime, and prevent further damage to the motor, drive, and machine mechanism.

In CNC machine tools, turret mechanisms, feeder axes, clamping axes, and automated special-purpose machines, servo systems are responsible for precise positioning, fast response, and closed-loop control. Compared with a standard inverter driving an induction motor, a servo system has much higher requirements for motor power wiring, encoder feedback wiring, parameter matching, mechanical load condition, and control sequence. Once any of these links becomes abnormal, the servo drive may not merely show unstable speed or positioning deviation. Instead, it may directly trigger protective alarms such as overcurrent, encoder fault, overload, or excessive position deviation.

In the TECO JSDAP series servo drive, one common field alarm is RL-04, AL-04, or simply 04 on the keypad display. In actual repair work, many users tend to interpret this alarm as “high motor current” or “heavy load.” However, from a maintenance perspective, alarm 04 should not be treated as a simple overload. It is closer to an instantaneous abnormal current protection in the main power circuit. Possible causes include incorrect U/V/W motor phase wiring, abnormal encoder feedback, servo motor winding defects, damaged drive power module, mismatch between parameter Cn030 and the actual motor, mechanical jamming, or a brake that has not been released.

When a machine-tool customer reports that “the servo motor should normally rotate two turns, but now it only moves briefly and then reports RL-04 overcurrent,” this symptom has strong diagnostic value. It usually means the system is not completely faulty at power-on. Instead, the alarm occurs when the servo is enabled, the drive begins outputting current, and the motor just starts to respond. The diagnostic focus should therefore be placed on why the current rises abnormally at the moment of operation, rather than only looking at the auxiliary alarm displayed on the CNC screen.

1. Basic Meaning of the TECO JSDAP RL-04 Alarm

On the TECO JSDAP series servo drive, alarm 04 generally corresponds to drive overcurrent. From the drive’s protection logic, overcurrent does not simply mean that the load is slightly higher than usual. It means that the main circuit has detected current exceeding the protection threshold. To protect the IGBT/IPM power module, motor windings, and mechanical system, the drive immediately cuts off output and issues an alarm.

For alarm 04, the diagnostic direction normally includes checking whether the motor-side U, V, W wiring and encoder wiring are normal, and confirming that the wiring follows the standard connection diagram. If the alarm still exists after the power has been turned off for a period and then reapplied, the fault may involve the internal power transistor module of the servo drive or severe electrical noise interference.

This means RL-04/04 is not caused by one single fixed fault. It is a protective result for a class of abnormal output-current conditions. Maintenance personnel must judge the cause together with the timing of the alarm: whether it appears immediately after power-on, after servo enable, after the motor moves briefly, after a period of running, under mechanical load, or even when the motor is disconnected. Different timing points correspond to completely different fault ranges.

2. Why “The Motor Moves Once and Then Reports 04” Is Important

In field service, customers often describe the problem as “the servo motor moves a little and then alarms,” “normally it should rotate two turns, but now it stops immediately,” or “after reset, the same thing happens again.” This simple description actually contains several important clues.

First, the drive is not completely unable to power up. If the drive reports 04 immediately after control power or main power is applied, the first suspects would be the drive power module, current detection circuit, main-circuit short circuit, or serious internal component failure. In this case, however, the alarm occurs after operation begins, which means the drive can at least complete part of its initialization. The fault is triggered during output.

Second, the motor has made a short movement. If the motor can move briefly, it means the main circuit has probably output three-phase current, and the servo enable signal and part of the control logic are present. But when the motor starts and the alarm immediately appears, it suggests that the drive detects an abnormal closed-loop control condition. The current response may be much higher than expected, or the motor feedback may not match the drive output, causing the servo amplifier to increase current sharply before tripping.

Third, if the motor should normally complete two revolutions but now cannot, the axis may be involved in homing, turret indexing, clamping-axis positioning, or a fixed travel sequence. If the CNC screen also displays a message such as “clamping axis not returned to home,” that CNC message may not be the root cause. It may simply be the interlock result after the servo axis fails to complete its action. The true primary fault should still be judged from the servo drive display.

Therefore, “one brief movement followed by RL-04” should not be directly judged as motor failure, nor should it immediately be judged as drive failure. The correct approach is to distinguish mechanical load problems, motor feedback problems, power wiring problems, parameter mismatch, and drive hardware failure.

3. Common Cause 1: Incorrect, Loose, or Poorly Insulated U/V/W Motor Wiring

The U, V, W three-phase power wires of a servo motor cannot be swapped casually like those of a standard induction motor. For an ordinary three-phase induction motor, swapping any two phases mainly changes the rotation direction. But an AC servo system is closed-loop controlled. The current vector output by the drive must strictly correspond to the rotor position feedback from the encoder. If the U/V/W phase sequence is wrong, or if one phase is loose or intermittently connected, the drive may find that the motor feedback direction, speed, or phase does not match the expected response. It may then rapidly increase current to correct the error, eventually causing overcurrent protection.

In real field cases, U/V/W problems commonly occur in the following situations:

The motor or drive has been removed for repair, and the wiring was restored without a reference photo.

The terminal screws are aged or not tightened, and vibration causes poor contact during operation.

The motor cable has been worn by the drag chain, sheet-metal edge, or oil-contaminated area, damaging the insulation.

The motor connector has oil or water ingress, causing leakage or short circuits between pins.

The motor cable was replaced, but the wire colors do not match the original factory definition.

The drive output cables are bundled together with other strong-current cables, causing interference or insulation damage.

For field maintenance, wire color alone should not be used as the final judgment. The motor nameplate, connector pinout, original wiring diagram, and actual terminal marks must all be compared. After power-off and full discharge, check whether U, V, and W correspond correctly from the drive to the motor. Then measure the resistance of U-V, V-W, and W-U with a multimeter. The three values should be basically balanced. Next, measure insulation from U/V/W to the motor frame or ground. A standard multimeter can only detect serious short circuits. If insulation degradation is suspected, a megohmmeter should be used. In servo motors, insulation problems may not appear as a complete static short circuit, but may become obvious only when the drive outputs PWM voltage.

4. Common Cause 2: Abnormal Encoder Feedback

The core of a servo system is closed-loop control. The drive not only outputs three-phase current to the motor, but also reads encoder feedback in real time to determine rotor position, speed, and direction. If encoder feedback is lost, reversed, distorted, intermittent, or affected by broken wires or poor connector contact, the drive’s judgment of motor status becomes unreliable.

Encoder faults do not always immediately appear as a dedicated encoder alarm. In some cases, the encoder signal seems normal while stationary, but once the motor starts, vibration causes an internally broken conductor to lose contact, or the feedback position jumps. The drive may then output abnormal current and finally show overcurrent protection. In machine-tool environments, oil mist, coolant, metal chips, long-term vibration, and repeated drag-chain bending can all cause hidden encoder cable damage.

Encoder troubleshooting should be performed systematically. First, power off and unplug the encoder connector. Check whether the pins are bent, retracted, oxidized, or contaminated by oil. Second, inspect the cable sheath for crushing, pulling, or excessive bending. Third, check whether the encoder power supply is normal. Many systems use 5 V encoder power, but the actual value should be confirmed according to the motor and manual. Fourth, if a same-model motor or cable is available, cross-substitution is the most effective method. Encoder cables are one of the most easily overlooked but most common fault points in field service.

If the motor is separated from the mechanical load and still shakes, rushes briefly, or reports 04 as soon as it is enabled, while U/V/W wiring shows no obvious short circuit, encoder feedback should be placed very high on the suspect list.

5. Common Cause 3: Mechanical Jamming, Brake Not Released, or Clamping Mechanism Not Open

In machine tools, a servo motor often does not drive a light free-running load. It may be connected through a coupling, timing belt, reducer, ballscrew, turret, clamping mechanism, homing mechanism, or other mechanical transmission. If the mechanical side is not fully released, the servo motor may face a near-stall load at startup. The current rises instantly, and the drive may report RL-04.

The word “no-load rotation” must be clarified. Customers may use it in two different ways. One means the motor is physically disconnected from the machine and the motor shaft is truly unloaded. The other simply means the machine is running an “empty cycle” or homing program, while the motor is still connected to the mechanical structure. These two meanings are completely different for diagnosis.

If the motor remains connected to the mechanism, the following problems may cause overcurrent:

The mechanical brake has not released.

The turret clamping mechanism has not opened.

Hydraulic or pneumatic pressure is insufficient, so unclamping is incomplete.

The reducer is internally damaged or jammed.

The ballscrew, bearing, or guideway resistance is too high.

The coupling is eccentric, over-tightened, or deformed during installation.

The home switch or limit switch state is incorrect, causing the axis to drive into a mechanical stop.

The machine has been idle for a long time, and oil sludge, chips, or dried coolant has blocked the mechanism.

For servo motors with mechanical brakes, the brake power supply must be checked carefully. A brake usually requires an external DC 24 V control supply to release. The brake wires must not be mistaken for ordinary signal wires. If the brake is not released, the motor is effectively starting against a locked rotor, and overcurrent is almost unavoidable.

The most effective way to identify a mechanical problem is to disconnect the coupling, timing belt, or reducer and let the motor truly run without load. If the motor runs normally after being disconnected and no longer reports 04, the drive and motor are probably not the main cause. The fault should be traced to the mechanical side. If the motor is completely disconnected and still reports 04 immediately after movement, the mechanical side can largely be excluded, and the electrical system should be checked first.

6. Common Cause 4: Incorrect Cn030 Motor Matching Parameter

The TECO JSDAP series drive cannot run any motor arbitrarily. The drive must know the connected motor’s power, rated current, rated speed, encoder type, inertia class, and related characteristics. The parameter Cn030 is used for the motor/drive series matching setting. The diagnostic item dn-08 can be used to check the currently configured drive and motor combination. If the displayed combination does not match the actual motor, Cn030 must be corrected.

This point is especially important for second-hand machine tools, old equipment repair, drive replacement, and parameter initialization. Many field failures are not caused by damaged hardware, but by a mismatch between the drive parameters and the actual motor. For example, the drive may be a JSDAP-15A, but the connected motor may have a different encoder type, rated current, or power class. If Cn030 is set for another motor combination, the drive’s understanding of motor electrical angle, rated current, and feedback resolution will be wrong. The result may be vibration, abnormal movement, or overcurrent immediately after operation.

Parameter mismatch is common in the following situations:

The drive has been replaced, but the original parameters were not imported.

The drive was repaired and reset to default parameters.

A used drive was installed as a substitute, with similar appearance but wrong parameter settings.

The motor was replaced, but the old drive parameters were retained.

Only part of the motion parameters was restored, while the motor-series parameter was ignored.

Different JSDAP capacity ranges or encoder specifications were mixed incorrectly.

Therefore, when troubleshooting RL-04, hardware measurement alone is not enough. The drive’s diagnostic item dn-08 should be checked and compared with the actual motor nameplate. If the motor model, power, speed, or encoder specification does not match the drive setting, Cn030 must be corrected before trial operation. Repeated testing under wrong parameter conditions not only fails to solve the problem, but may also expand the damage.

7. Common Cause 5: Servo Motor Failure

Servo motor failure can also trigger RL-04. Common motor faults include winding turn-to-turn short circuit, three-phase imbalance, insulation breakdown to ground, encoder internal failure, bearing seizure, rotor demagnetization, or brake mechanism failure.

Turn-to-turn short circuit is a relatively hidden fault. It may not show as a complete U/V/W short circuit. When measured with a standard multimeter, one phase resistance may only be slightly different, or the difference may not be obvious. However, once the drive outputs PWM current, the shorted turns generate abnormal current and heat, causing the drive to trip on overcurrent. Motors that have overheated, been contaminated by oil or water, or suffered insulation aging are more likely to develop this fault.

Insulation degradation to ground is also common. In machine-tool environments, coolant, oil mist, and metal powder can enter the motor connector or junction area, causing leakage to ground. Servo drives are sensitive to output-side leakage and current abnormalities. When leakage current becomes large, the drive may report 04 or another main-circuit alarm.

Motor bearing seizure should not be ignored either. If the motor shaft feels tight, has periodic sticking points, produces abnormal noise, or feels as though it is scraping internally, the bearing, brake, or internal mechanical structure may be damaged. A motor with a brake cannot be rotated normally unless the brake is released, so the brake-release condition must be confirmed before judgment.

For the motor itself, the most effective method is still cross-substitution. If a same-model normal motor is available, install the original motor on a known-good axis, or connect a known-good motor to the faulty drive. If the fault follows the motor, the motor or encoder is confirmed as the likely cause. If the fault remains with the original drive or original machine axis, continue checking the drive, cable, and mechanical load.

8. Common Cause 6: Drive Power Module or Current Detection Circuit Failure

If the motor, cable, encoder, parameters, and mechanical load have all been excluded, the drive itself must be considered. The power module inside the JSDAP servo drive converts the DC bus voltage into three-phase output current for the servo motor. If the IGBT/IPM module is aged, partially shorted, or if the gate drive circuit or current detection circuit is abnormal, alarm 04 may appear during operation.

Drive hardware failure often appears in the following ways:

The drive reports 04 even when the motor is disconnected.

Any connected motor causes the same 04 alarm.

The drive runs briefly when cold but frequently reports 04 after warming up.

The output three-phase current is obviously unbalanced.

The motor produces abnormal squealing or vibration before alarm.

There is a burnt smell, visible explosion mark, heavy oil contamination, or dust accumulation inside the drive.

The power module shows abnormal readings in diode-mode testing.

After power-off and full discharge, an experienced technician may perform a preliminary diode-mode check between U/V/W and the DC bus points. However, this type of inspection must be done by qualified personnel. A servo drive contains high-voltage capacitors, and dangerous voltage may remain after power is turned off. Touching internal circuits before the charge indicator is off is unsafe.

If the drive power module is confirmed to be damaged, parameter reset or external cable replacement will not solve the problem. The drive must be inspected internally, including the IPM/IGBT, gate drive optocouplers, current sensors, DC bus capacitors, snubber circuit, power supply board, and control board. For old machine tools, the external motor and cables should also be checked for the original cause. Otherwise, the repaired drive may fail again after installation.

9. Correct On-Site Troubleshooting Sequence

For alarms like RL-04, the diagnostic sequence is very important. If the drive is removed and repaired immediately, time may be wasted. If the mechanical system is jammed and the operator repeatedly resets and retries the machine, the drive power module may be damaged. A reasonable diagnostic procedure is as follows.

First, confirm the true alarm source. CNC screen messages such as “clamping axis not returned to home,” “servo abnormal,” or “axis not ready” are often system interlock messages, not necessarily the root cause. A clear photo of the servo drive display must be taken to confirm whether the alarm is 04, AL-04, or RL-04.

Second, observe the alarm timing. If the alarm appears immediately after power-on, suspect the drive body, main-circuit short circuit, or serious wiring error first. If it appears after servo enable, focus on motor wiring, encoder wiring, and parameters. If it appears only at a fixed mechanical position, mechanical jamming, limit status, clamping mechanism, and program sequence become more suspicious.

Third, disconnect the mechanical load. Separate the motor from the coupling, timing belt, or reducer so the motor can truly run unloaded. If it runs normally unloaded, the mechanical side is the priority. If it still alarms unloaded, the electrical side is the priority.

Fourth, check U/V/W and encoder wiring. Confirm phase order, terminal tightness, connector condition, shielding, grounding, insulation, and possible internal cable breakage. Pay special attention to drag-chain cables and oil-contaminated connectors.

Fifth, check Cn030 and dn-08. Confirm whether the drive model, motor model, power, speed, and encoder specification match. This step is essential for second-hand machines and machines whose drives have been replaced or repaired.

Sixth, test the motor. Check three-phase resistance balance, insulation to ground, brake release, shaft rotation resistance, and encoder feedback condition.

Seventh, perform cross-substitution. Use a known-good same-model motor, cable, or drive for comparison. Cross-substitution is more reliable than guessing.

Eighth, evaluate drive hardware. If the drive reports 04 without the motor connected, or if the same alarm remains after replacing the motor and cable, the drive should enter the internal repair process.

10. Analysis of the CNC “Clamping Axis Not Returned to Home” Message

On some CNC lathes or special-purpose machines, the servo axis may not be a standard X/Z feed axis. It may be used for a clamping axis, turret, indexing table, feeder mechanism, or auxiliary homing mechanism. When the CNC screen displays “clamping axis not returned to home,” this does not necessarily mean the home switch is faulty. It may mean the servo axis reported RL-04 immediately after starting during the homing process, so the PLC never received the home-complete signal.

In this situation, the fault chain should be divided into two layers.

The lower-level fault is the servo drive 04 overcurrent alarm.
The upper-level fault is the CNC/PLC message caused by the servo axis failing to complete the required motion.

If only the upper-level alarm is handled, such as replacing the home switch, changing the PLC input, or forcing home completion, the real problem may remain. The correct method is to make the servo axis run stably first, then handle the homing logic. For clamping, unclamping, turret positioning, and similar mechanisms, pneumatic or hydraulic signals are often required before servo movement. If the unclamp signal is not complete and the servo axis is forced to move, the motor is effectively starting against a mechanical lock, so overcurrent is a reasonable result.

For JSDAP servo alarms on clamping-axis or turret mechanisms, the following items should be checked at the same time:

Whether the unclamping solenoid valve operates.

Whether air pressure or hydraulic pressure meets the required level.

Whether the clamped and unclamped position switches provide correct feedback.

Whether the mechanical lock pin has fully retracted.

Whether the homing direction is correct.

Whether the servo enable sequence occurs after unclamping is completed.

Whether an abnormal PLC input causes the servo to start under the wrong condition.

11. Maintenance Precautions

When dealing with RL-04, frequent reset and forced operation are not recommended. Every overcurrent trip stresses the power module, motor winding, and DC bus capacitors. If the actual fault is U/V/W short circuit, unreleased brake, or mechanical jamming, repeated trial operation can turn a repairable minor fault into a damaged power module.

U/V/W should also not be swapped casually to “test direction.” In a servo system, direction should be corrected through parameters, command settings, or proper motor matching, not by randomly swapping output phases like an induction motor. Incorrect phase sequence may cause closed-loop runaway and instantaneous current shock.

Encoder wiring must not be modified randomly either. Encoder feedback is a low-voltage high-speed signal. It should use proper shielded cable and be routed away from power wiring. Shield grounding should follow the original design. Multiple random grounding points can create interference loops. If the encoder cable has been soaked in oil or repeatedly bent in the drag chain, replacing the cable is often more effective than merely cleaning the connector.

For old machine tools, the cabinet environment also matters. Oil mist, metal dust, poor heat dissipation, fan failure, poor grounding, supply voltage fluctuation, and strong electrical interference can all reduce servo system stability. The drive should be installed in an environment with proper ventilation, limited dust and oil mist, reliable grounding, and sufficient heat dissipation.

12. Conclusion

The TECO JSDAP servo drive RL-04 or 04 alarm is essentially a drive overcurrent protection. It may be caused by a damaged drive power module, but in machine-tool field service, more common causes include abnormal U/V/W motor power wiring, encoder feedback problems, mechanical jamming, unreleased brake, clamping mechanism not open, or mismatch between parameter Cn030 and the actual motor.

For the symptom “the servo motor only moves briefly and then reports RL-04, while it should normally rotate two turns,” the most important diagnostic method is to disconnect the motor from the mechanical load and perform a true no-load test. If the motor runs normally after being disconnected, focus on the mechanical lock, brake, reducer, turret, or clamping-axis mechanism. If it still reports 04 when disconnected, focus on U/V/W wiring, encoder cable, motor body, Cn030/dn-08 matching, and drive hardware.

When repairing this type of fault, do not rely only on the CNC screen message, and do not immediately assume the drive is damaged. The correct procedure is: confirm the alarm source, observe the alarm timing, disconnect the mechanical load, check power wiring, check encoder feedback, verify parameters, test the motor, perform cross-substitution, and then repair the drive if necessary. This sequence reduces misjudgment, lowers unnecessary replacement cost, and prevents repeated trial operation from expanding the fault.

In actual machine-tool maintenance, RL-04 is not an isolated alarm. It is the protective result of interaction among the servo drive, motor, cable, mechanical structure, and PLC sequence. Only by analyzing it as a complete system can the real cause be found.