Introduction

The Daikin SUT00 Super Unit is a servo-hydraulic power system designed primarily for injection molding machines, die-casting machines, and other industrial forming equipment. It is not a conventional fixed-displacement hydraulic power pack, nor is it simply a general-purpose inverter. Instead, it is an integrated closed-loop electro-hydraulic system consisting of an IPM permanent-magnet servo motor, gear pump, pressure sensor, rotary encoder, servo controller, power module, current detection circuit, and hydraulic control components.

The SUT00 system regulates pump output flow by controlling motor speed. At the same time, it uses pressure feedback to perform closed-loop pressure control. Compared with a traditional hydraulic system using a continuously running induction motor, fixed-displacement pump, proportional valve, and relief valve, the SUT00 can adjust motor output according to the actual pressure and flow demand. This significantly reduces energy consumption and oil temperature during standby, pressure-holding, and low-load operation.

During repair work, the controller may require replacement of the control board, power-supply board, current detection board, driver board, or IPM power module. After these components are replaced, technicians often encounter a panel indication reading “StP.”

Because “StP” is not a conventional numerical alarm code, it is often misinterpreted as a controller initialization failure, software mismatch, missing program, incompatible power module, or defective replacement board. In many cases, however, “StP” simply means that the controller is in the Stop state. The controller has completed its basic power-up process but has not received a valid pump start command.

Nevertheless, after board replacement, “StP” should not be cleared by blindly shorting terminals or forcing the pump to start. The technician must verify the start/stop digital input, parameter settings, board compatibility, pump-controller matching parameters, pressure and flow commands, Ready status, encoder feedback, pressure sensor feedback, and thermistor circuit before attempting operation.

This article explains the meaning of “StP,” the start logic of the Daikin SUT00, the parameters that must be checked after controller replacement, the correct troubleshooting sequence, and the precautions required during the first trial run.

1. Basic Construction and Operating Principle of the Daikin SUT00 System

The Daikin SUT00 series mainly consists of the following components:

1. Three-phase servo controller
2. IPM permanent-magnet synchronous motor
3. Gear-type hydraulic pump
4. Motor rotary encoder
5. Hydraulic pressure sensor
6. Motor thermistor
7. Motor cooling fan
8. Main control board
9. Driver and power-supply board
10. IPM intelligent power module
11. Current detection board
12. External pressure, flow, and start/stop command interfaces

The machine controller normally sends two analog signals to the SUT00:

• Pi: pressure command
• Qi: flow-rate command

The SUT00 also sends monitor signals back to the machine controller:

• Po: pressure monitor output
• Qo: flow-rate monitor output

During operation, the controller calculates the target motor speed according to the Qi flow command. The encoder detects the actual motor speed, and the controller regulates the U, V, and W phase currents through the IPM power module so that the motor reaches the commanded speed.

As the hydraulic pressure rises and approaches the Pi pressure command, the controller transitions from flow control to pressure-limiting or pressure closed-loop control. It then reduces motor speed automatically to prevent the actual pressure from exceeding the target pressure.

Therefore, the SUT00 does not merely output a fixed three-phase frequency. It simultaneously evaluates:

• Pressure command
• Flow command
• Pressure feedback
• Motor speed feedback
• Motor current
• Motor temperature
• Controller temperature
• Digital start/stop input
• External interlock conditions

The SUT00 controller uses a three-phase 380–440 VAC power supply. The motor cooling fan usually requires an additional single-phase 215–245 VAC power supply. For the SUT00S13018 model, the rated maximum flow is 130 L/min and the maximum operating pressure is 17.6 MPa.

2. What Does “StP” Actually Mean?

“StP” is generally an abbreviation of Stop. It indicates that the pump motor is currently in the stopped state.

This is fundamentally different from an alarm.

A real fault alarm normally appears as a specific alarm number or code. It may also cause the Ready output to turn OFF and prohibit motor operation. By contrast, “StP” mainly indicates that the controller is not currently executing a pump run command.

When the operation panel displays “StP” steadily, it normally suggests that:

• The control-board CPU has started;
• Communication between the panel and the main control board is functioning;
• At least part of the low-voltage control power supply is operating;
• No immediate severe hardware alarm has been detected at power-up;
• The motor has not yet been enabled to run.

Therefore, “StP” by itself is not direct evidence of a failed IPM module or defective control board.

However, “StP” also does not prove that the controller is fully functional. Since the controller has not yet driven the motor, some faults may only appear after a valid start command is received, including:

• Encoder fault
• Motor phase loss
• IPM driver fault
• Current detection abnormality
• Motor overcurrent
• DC-bus undervoltage
• IPM protection trip
• Speed deviation
• Abnormal pressure rise
• Motor thermistor fault

For this reason, “StP” should be interpreted as the controller is stopped and waiting, not as confirmation that the repair has been completed successfully.

3. The SUT00 Is Not Normally Started from the Panel

The operation-panel keys are mainly used for:

• Switching monitor modes
• Viewing operating data
• Entering parameter-setting mode
• Viewing alarm history
• Modifying permitted parameters

Unlike a general-purpose inverter, the panel does not normally provide a direct RUN key for starting the hydraulic pump. The pump motor is primarily started and stopped through digital input DI1.

Parameter P00, identified as the DI_A start/stop signal switching parameter, determines the DI1 logic:

When P00 is set to 0, DI1 ON means Run. When P00 is set to 1, the logic is reversed and DI1 OFF means Run.

This is one of the most common reasons for an StP indication after a control-board replacement.

For example, assume the original controller used:

P00 = 0

The machine PLC sends 24 V to DI1 whenever the hydraulic pump is required to run.

If the replacement control board contains:

P00 = 1

the same 24 V input is interpreted as a stop command instead of a run command.

From the PLC side, the pump-start output appears to be active. From the SUT00 controller side, however, the stop condition remains valid. The panel therefore continues to display “StP.”

For this reason, it is not sufficient to verify that 24 V exists at the DI1 terminal. The P00 input logic must also be checked.

4. Main Reasons for an “StP” Display After Board Replacement

4.1 No DI1 Start Signal Is Reaching the Controller

An injection molding machine normally does not start the hydraulic pump immediately after power-up. Several interlock conditions may have to be satisfied first, such as:

• Emergency stop reset
• Safety door closed
• Main PLC running normally
• Oil level switch normal
• Oil temperature normal
• Motor thermal protection normal
• Servo controller Ready signal present
• No hydraulic-system alarm
• Machine in an operating mode that permits pump start
• Pump-start pushbutton or command activated

If any required interlock remains unsatisfied, the PLC may not energize DI1.

Common field problems include:

• DI1 wire omitted during reassembly
• Connector not fully inserted
• Ribbon cable misaligned
• DICOM not connected
• External 24 VDC supply missing
• Wrong DI1 polarity
• Failed PLC output relay
• Different terminal definition between original and replacement board
• Loose terminal screw
• Bent connector pin

Therefore, when “StP” appears after board replacement, the first task is to determine whether the controller is actually receiving the DI1 signal.

4.2 P00 Does Not Match the Original Controller

A replacement control board may still contain factory-default parameters, or it may have been removed from another machine.

Even if the board number appears identical, its stored parameters may not be suitable for the current machine. If P00 differs from the original setting, the start logic will be reversed directly.

The technician should enter parameter-setting mode, read the current P00 value, and compare it with:

• The original controller parameter record
• The machine electrical schematic
• PLC output logic
• Actual DI1 voltage changes
• Machine commissioning records

If no original documentation is available, the DI1 monitor status and machine start behavior can be used for diagnosis. However, parameters should not be changed casually and followed immediately by high-pressure operation.

4.3 Original Parameters Have Not Been Restored

A Daikin SUT00 controller is not fully interchangeable merely because the replacement board has the same appearance or board number.

During factory commissioning, the controller is configured according to:

• Actual pump displacement
• Pressure sensor characteristics
• Analog command range
• Hydraulic response
• Machine motion requirements
• Motor and controller calibration

If the original parameters are not restored after controller replacement, the system may run but exhibit serious performance problems, including:

• Insufficient maximum flow
• Failure to build pressure at low flow
• Incorrect pressure indication
• Actual pressure higher than commanded
• Abnormal motor speed
• Unstable pressure holding
• Slow response
• Pressure shock
• Speed alarm
• Incorrect flow scaling
• Machine motion speed different from the original condition

The manual specifies that original controller parameters should be restored after controller replacement. In particular, H15, H30, and P07 are closely related to the pump, pressure sensor, and controller matching calibration.

4.4 The Ready Condition Has Not Been Established

After power-up, the controller must complete internal initialization, including:

• Control power stabilization
• CPU program execution
• Memory reading
• Initial sensor checks
• DC-bus status detection
• Protection-circuit self-check
• I/O initialization

Only when the controller reaches the permissible operating state does the Ready output turn ON.

If Ready is not established, the external PLC may refuse to send the pump-start command. This creates a sequence such as:

1. SUT00 controller is not Ready;
2. PLC does not output pump start;
3. Controller remains in StP;
4. Machine controller waits for the hydraulic-unit Ready signal.

In this case, the technician must not focus only on DI1. The DO1 Ready output, alarm history, and internal monitor values must also be checked.

4.5 Compatibility Problems Between the Control Board, Driver Board, and IPM Module

The power module shown in the repair photographs is a Mitsubishi Electric PM100RL1A120 series IPM module. Such modules typically integrate power semiconductor devices, gate-drive protection, and fault-feedback functions.

When replacing an IPM module, the following points must be confirmed:

• Exact complete model number
• Voltage rating
• Current rating
• Pin assignment
• Module revision compatibility
• Correct mounting screw torque
• Good thermal contact with the heat sink
• Uniform thermal compound thickness
• Correct insulation-sheet condition
• Driver connector installed in the correct orientation
• No shifted control pins
• Correct DC positive and negative bus connections
• Correct U, V, and W output connections

If the IPM module or driver board is incompatible, the controller may remain normal while stopped but trip immediately when a start command is applied.

Therefore, static checks must be completed before DI1 is enabled.

5. Parameters That Must Be Checked After Controller Replacement

5.1 P00: Start/Stop Input Logic

P00 determines whether DI1 ON or DI1 OFF represents Run.

This is the first-priority parameter when the panel remains in “StP.”

5.2 P05: VMAX Analog Command Full-Scale Voltage

The SUT00 normally uses 0–10 V pressure and flow commands, but the actual full-scale input voltage is parameterized.

An incorrect VMAX setting may cause:

• A 10 V command to produce less than full output
• A small voltage to produce excessive pressure or flow
• Pressure not proportional to the machine setting
• Severe flow-scaling error

5.3 P06: PMAX Maximum Pressure Scaling

P06 defines the maximum pressure corresponding to the full-scale pressure command.

Although the SUT00S13018 has a rated maximum operating pressure of 17.6 MPa, the actual machine may be configured for a lower maximum pressure due to:

• Hydraulic circuit limitations
• Mold-protection requirements
• Machine design
• Process requirements

The system should never be tested immediately at 17.6 MPa simply because that is the model’s maximum rating.

5.4 P07: QMAX Maximum Flow Scaling

P07 defines the maximum flow corresponding to the full-scale flow command.

The SUT00S13018 is rated for up to 130 L/min, but the pump and controller are factory-matched and calibrated. The P07 value stored in a spare controller may not be correct for the installed pump assembly.

The manual requires the original P07 value to be written into the replacement controller.

5.5 H15: Q_EV Flow Correction Coefficient

H15 compensates for actual pump-output characteristics, particularly during low-flow operation and pressure build-up.

An incorrect H15 value may cause:

• Difficulty building pressure at low flow
• Unstable pressure holding
• Motor rotation without sufficient pressure rise
• Low-flow pressure-rise alarm
• Difference between calculated and actual pump output

The manual indicates that H15 may require adjustment if abnormal pressure rise occurs at a low flow setting after pump replacement or commissioning.

5.6 H30: PS_G Pressure Sensor Gain

H30 calibrates the pressure sensor feedback.

If H30 is incorrect, there may be a difference among:

• Actual hydraulic pressure
• SUT00 panel pressure indication
• Machine HMI pressure indication
• Po pressure monitor output

If the pressure sensor gain is too low, the controller may believe pressure is insufficient and continue increasing motor torque, causing excessive actual pressure.

If the gain is too high, the controller may reduce speed too early and fail to reach the required pressure.

After replacing the pressure sensor or controller, H30 must be restored correctly or recalibrated. The manual recommends comparing the controller reading with an accurate pressure gauge in pressure-control operation.

5.7 H21 to H28 Must Not Be Copied Directly

The manual specifically states that H21 to H28 are controller-specific calibration values.

Therefore, after replacing the controller:

• Machine operating parameters may be restored;
• H15, H30, and P07 may be transferred as required for pump matching;
• H21 to H28 must not be copied from the old controller into the replacement controller.

Doing so may corrupt the replacement controller’s own current-detection, hardware-calibration, or drive-compensation data.

6. Standard Troubleshooting Procedure When “StP” Is Displayed

Step 1: Confirm the Unit Model and Replacement Component Information

Record the complete nameplate and board information, including:

• SUT00S13018-10YA-N0218
• Controller manufacturing number
• Pump and motor manufacturing number
• Control-board number
• Power-board number
• IPM module model
• Current detection board number
• Software revision
• Hardware revision

The Daikin manual states that the motor-pump assembly and controller are tested as a matched pair before shipment. In principle, the controller and pump assembly should have the same manufacturing number.

When using replacement boards, at minimum confirm that the product series, power rating, software version, motor type, and feedback method are compatible.

Step 2: Perform Static Checks with Power Disconnected

Disconnect the three-phase power supply and wait at least five minutes for the DC-bus capacitors to discharge.

Then check:

1. No short circuit between the DC-bus terminals P1 and P2;
2. U, V, and W phase resistance characteristics are approximately symmetrical;
3. Diode-test characteristics from U, V, and W to the positive and negative DC bus are symmetrical;
4. U, V, and W are insulated from ground;
5. Motor winding resistances are balanced;
6. Motor insulation to ground is acceptable;
7. IPM module mounting surface is flat;
8. Mounting screws are secure;
9. Current detection board is installed in the correct direction;
10. All connectors are fully inserted;
11. Ribbon-cable contacts are clean;
12. No bent pins, cracked solder joints, or burn marks are present.

A standard multimeter can only provide a preliminary assessment. It cannot prove that the IPM module is dynamically healthy under load.

Step 3: Power Up Without Immediately Starting the Pump

During the first power-up after repair, do not immediately allow the PLC to issue a run command.

Observe:

• Whether the panel powers up normally;
• Whether “StP” is displayed;
• Whether any other alarm appears;
• Whether the control board produces abnormal sound;
• Whether the power-supply board heats abnormally;
• Whether the IPM module heats rapidly;
• Whether the DC-bus voltage is normal;
• Whether the Ready output is established;
• Whether the cooling fans operate as designed.

If a serious alarm appears immediately, resolve that alarm before any attempt to start the motor.

Step 4: Check the Actual DI1 Status

Measure the voltage between DI1 and DICOM with a multimeter.

Record the voltage under both conditions:

• Machine stopped;
• Pump-start command activated.

If the voltage does not change, the problem is most likely in the external control circuit.

Check:

• 24 VDC power supply
• PLC output
• Interposing relay
• Terminal wiring
• Emergency-stop circuit
• Safety-door circuit
• Ready interlock
• DICOM connection

If the voltage changes but the panel still displays “StP,” enter monitor mode and verify whether the controller internally detects the DI1 change.

Step 5: Read P00

Confirm that P00 matches the external DI1 logic.

For example:

• If DI1 changes from 0 V to 24 V when the machine requests pump operation, P00 is commonly set to 0;
• If DI1 is energized during stop and released during Run, P00 may be set to 1.

The correct setting must be determined from the original circuit and actual input status, not from assumption alone.

Step 6: Check Pressure and Flow Commands

Even if DI1 is in the Run state, the system may remain in a low-output standby condition if both Pi and Qi are zero or near zero.

The manual describes a standby condition when the pressure and flow commands are low. Parameter P15 BIAS defines the standby pressure behavior.

The following monitor values should be checked:

• Pi pressure command voltage
• Qi flow command voltage
• Pressure feedback
• Flow feedback
• Actual motor speed
• Digital input status

If the machine has issued a motion command but Pi and Qi remain at 0 V, inspect the analog signal wiring and analog common connection.

Step 7: Restore the Original Controller Parameters

The best method is to read and record all parameters from the original controller before removal.

If the original controller is completely damaged, search for:

• Machine manufacturer parameter sheet
• Injection molding machine commissioning records
• Another identical operating machine
• Previous repair records
• Parameter photographs
• PLC or HMI process-setting records

The parameters should be divided into three categories.

Machine Parameters That May Be Restored

These include P00, P05, P06, P07, P15, and pressure- and flow-response parameters.

Pump-Matching Parameters That Must Be Restored

These include H15, H30, and P07.

Controller-Specific Parameters That Must Not Be Copied

These include H21 to H28.

7. Low-Risk Procedure for the First Trial Run

Only after the previous checks are completed should the unit be started.

7.1 Place the Hydraulic Circuit in an Unloaded Condition

Avoid immediately performing:

• High-pressure clamping
• Injection
• Ejector operation
• High-pressure mold locking
• Driving a cylinder against its mechanical stop
• Long pressure-holding operation

Where possible, select an unloaded return path or a low-load machine movement so that the pump will not immediately build high pressure.

7.2 Use Low Pressure and Flow Commands

For an initial trial, use low command values, such as:

• Pressure command: approximately 0.5–1.0 V
• Flow command: approximately 0.3–0.5 V

The exact values must be selected according to the machine’s actual command scaling.

Do not apply 10 V pressure and 10 V flow commands during the first test. If H30, P06, P07, or H15 is incorrect, the result may be uncontrolled pressure or motor speed.

7.3 Perform a Short Jog Test

The first run may be limited to approximately one or two seconds.

Observe:

• Whether the motor rotates smoothly;
• Whether there is severe vibration;
• Whether the pump produces a sharp cavitation sound;
• Whether motor direction is correct;
• Whether pressure rises abnormally quickly;
• Whether the three-phase currents are balanced;
• Whether an alarm appears immediately;
• Whether the IPM module heats abnormally.

Stop immediately if any abnormal condition is detected.

7.4 Confirm Pump Rotation Direction and Oil Suction

If the U, V, and W phase sequence or the encoder matching relationship is incorrect, the motor may not operate correctly in closed loop.

Reverse pump rotation may cause:

• Failure to draw oil
• No pressure rise
• Abnormal pump noise
• Pump damage
• Incorrect encoder speed direction
• Speed deviation or overcurrent alarm

Motor rotation alone does not confirm correct pump direction.

7.5 Verify Pressure Feedback Accuracy

Use a mechanical pressure gauge or a calibrated electronic pressure gauge to compare:

• Actual hydraulic pressure
• SUT00 panel pressure
• Machine HMI pressure
• Po pressure monitor output

If the difference is significant, do not continue increasing pressure. Check:

• Pressure sensor
• Pressure sensor power supply
• Signal wiring
• H30 setting
• Analog monitor scaling
• Pressure sensor installation point

8. Common Incorrect Repair Actions and Their Risks

8.1 Shorting DI1 Immediately After Seeing “StP”

If P00 is configured for reverse logic, shorting DI1 may not start the pump. More importantly, forcing DI1 can bypass machine safety logic and cause unexpected movement.

The correct method is to read the DI1 state and confirm P00 first.

8.2 Failing to Restore Parameters After Board Replacement

The controller may appear to run, but the hydraulic performance can differ significantly from the original machine.

8.3 Copying All H Parameters from the Old Controller

H21 to H28 are specific to the replacement controller and must not be overwritten with values from the old board.

8.4 Applying Maximum Pressure and Flow During the First Run

This amplifies any error in pressure feedback, flow scaling, or current detection and may damage the replacement IPM module again.

8.5 Ignoring the Encoder Circuit

The SUT00 uses encoder feedback for speed closed-loop control. A loose connector, missing encoder supply, abnormal A/B phase signals, or incorrect direction can cause serious speed-control faults.

8.6 Ignoring the Motor Thermistor and Cooling Fans

The motor thermistor, controller fan, and motor cooling fan are important protection components. A missing fan supply may not cause an immediate power-up fault but can result in overheating during operation.

8.7 Applying a Megohmmeter Directly to Electronic Circuits

A megohmmeter can damage:

• Encoder
• Pressure sensor
• Control board
• IPM control terminals
• Analog input circuits

Insulation testing should be performed only on the motor windings and power cables after electronic components have been properly disconnected.

9. Criteria for Confirming a Successful Repair

A repair should not be considered complete merely because the panel exits “StP.”

A satisfactory result should include all of the following:

1. No abnormal alarm at power-up;
2. Ready output operates correctly;
3. DI1 status corresponds to the external start command;
4. P00 logic is correct;
5. “StP” disappears when valid run conditions are satisfied;
6. Motor runs smoothly at low speed;
7. Encoder feedback is stable;
8. Three-phase current is reasonably balanced;
9. Pressure command agrees with actual pressure;
10. Flow command produces the expected speed response;
11. No abnormal noise or vibration;
12. IPM module temperature rise is normal;
13. Motor and controller cooling fans operate correctly;
14. Low-pressure, low-flow, and pressure-holding operation are stable;
15. High-pressure and high-flow tests are introduced gradually without alarms;
16. Machine motion speed returns to the original performance;
17. Final parameter values are recorded and backed up.

Only after these conditions are met can the repaired SUT00 unit be considered technically reliable.

10. Conclusion

When a Daikin SUT00 servo-hydraulic control unit displays “StP” after replacement of the control board, power-supply board, current detection board, or IPM module, the indication normally means that the controller is in the Stop state. It does not automatically indicate a hardware fault.

The most common causes are:

• No DI1 start command
• Incorrect external 24 V or DICOM wiring
• P00 start logic different from the original controller
• Ready condition not established
• PLC safety interlock not satisfied
• Original controller parameters not yet restored

The correct solution is not to short terminals blindly or continue replacing power modules. The technician should follow a structured sequence:

static inspection → power-up initialization → Ready verification → DI1 monitoring → P00 verification → parameter restoration → low-risk jog test → pressure and flow calibration

Particular attention must be paid to H15, H30, and P07. These parameters are associated with flow correction, pressure sensor gain, and maximum flow scaling. At the same time, H21 to H28 are controller-specific calibration values and should not be copied from the old controller into the replacement controller.

The Daikin SUT00 is a multi-loop servo-hydraulic system integrating pressure, flow, speed, and current control. It should not be diagnosed using only the logic of a standard contactor-driven motor or general-purpose inverter. Reliable recovery requires correct power hardware, valid sensor feedback, correct digital start logic, proper analog command scaling, and accurate control parameters. Only when all of these elements are verified can the original pressure accuracy, motion speed, operating stability, and long-term reliability of the machine be restored.

Introduction

With the rapid development of industrial automation, variable frequency drives (VFDs) have evolved from simple motor speed regulators into intelligent drive systems integrating motor control, speed feedback, torque management, communication functions, and advanced fault diagnosis.

In applications requiring high speed accuracy and dynamic response, such as machine tools, cranes, textile equipment, printing machinery, packaging lines, and automated production systems, closed-loop vector control with encoder feedback has become increasingly common.

The AnyHz (Foster Technology) FST-650 series is a high-performance vector control inverter that supports multiple control modes, including:

• V/F control;
• Sensorless vector control;
• Closed-loop vector control with encoder feedback.

When the FST-650 displays the fault code Err20, the inverter has detected an abnormality in the encoder feedback system.

The meaning of Err20 is:

Err20 = Encoder Fault / PG Card Fault

However, in practical maintenance work, Err20 does not always mean that the encoder itself is damaged. Many real-world cases are caused by:

• Incorrect control mode selection;
• Wrong encoder parameter settings;
• Encoder wiring problems;
• Missing encoder power supply;
• PG card failure;
• Incorrect replacement configuration.

This article provides a detailed analysis of the working principle behind Err20, common causes, diagnostic procedures, and repair methods to help engineers quickly troubleshoot AnyHz FST-650 encoder-related faults.

1. Working Principle of Err20 Fault in FST-650 Inverter

1.1 The Role of Encoder Feedback in Closed-Loop Vector Control

A conventional inverter operating in open-loop mode controls the motor based on the output frequency and voltage.

For example:

• Frequency command: 50Hz;
• Output voltage calculated according to motor model;
• Motor speed estimated by inverter algorithm.

This type of control does not require an encoder.

However, in closed-loop vector control, the inverter must continuously know:

• Actual motor speed;
• Rotor position;
• Rotation direction;
• Speed deviation.

Therefore, an encoder is installed on the motor shaft.

The control process is:

Speed Command
      ↓
Vector Control Algorithm
      ↓
IGBT Output Three-Phase Power
      ↓
Motor Rotation
      ↓
Encoder Detects Actual Speed
      ↓
PG Card Processes Feedback Signal
      ↓
CPU Receives Feedback Data
      ↓
Adjusts Output Frequency and Torque

The encoder acts like the “eyes” of the inverter.

If the encoder signal disappears, the inverter cannot accurately determine the motor operating condition, so it triggers Err20 protection.

2. Main Causes of AnyHz FST-650 Err20 Fault

According to the FST-650 technical documentation, Err20 is mainly related to encoder feedback abnormalities. The possible causes include:

1. Incorrect encoder type setting;
2. Incorrect encoder wiring;
3. Damaged encoder;
4. Faulty PG feedback card.

In practical applications, these causes can be divided into several categories.

2.1 Incorrect Encoder Type Setting

This is one of the most common reasons for Err20.

The FST-650 supports different feedback devices, such as:

• Incremental AB encoder;
• ABZ encoder;
• UVW encoder;
• Resolver.

Different encoder types output completely different signals.

For example:

Actual hardware:

Incremental encoder
A+
A-
B+
B-

But inverter parameter setting:

UVW encoder

The encoder itself may be working normally, but the inverter receives an incompatible signal format.

The result:

Encoder signal abnormal
        ↓
No valid speed feedback
        ↓
Err20 alarm

2.2 Encoder Wiring Failure

Encoder systems usually contain several signal lines:

Any problem in these connections may cause Err20.

Common wiring problems include:

• Broken encoder cable;
• Loose connector;
• Poor grounding;
• Incorrect phase connection;
• Damaged shielding layer.

For example:

If the encoder power supply line is disconnected:

Encoder has no power
        ↓
No pulse output
        ↓
PG card receives no signal
        ↓
Err20

2.3 Encoder Damage

Encoders are precision electronic components. Long-term operation may cause:

• Optical sensor aging;
• Internal IC failure;
• Mechanical shaft damage;
• Dust or moisture contamination;
• Vibration damage.

Typical symptom:

The inverter powers on normally.

However:

When the motor starts running:

Motor rotates
       ↓
Encoder should output pulses
       ↓
No feedback detected
       ↓
Err20 appears

At standstill, the problem may not be obvious because the encoder is not generating speed pulses.

2.4 PG Card Failure

The PG card is the interface between the encoder and inverter CPU.

Its function:

Encoder signal
       ↓
Signal conditioning
       ↓
Voltage conversion
       ↓
Filtering
       ↓
CPU feedback input

If the PG card fails, even a good encoder cannot provide feedback to the inverter.

Typical PG card failures:

• Input circuit damage;
• RS422 receiver failure;
• Optical isolation failure;
• Power supply abnormality;
• Poor connector contact.

3. First Maintenance Step: Confirm Whether an Encoder Is Actually Required

A very common situation in the field is:

The inverter reports Err20, but the motor does not have an encoder.

This usually happens after:

• Parameter reset;
• Replacement of inverter;
• Second-hand equipment installation;
• Incorrect commissioning.

Example:

Original system:

Standard motor
+
V/F control

After parameter modification:

Closed-loop vector control enabled

The inverter starts searching for encoder feedback:

No encoder signal
        ↓
Err20

Check Control Mode Parameter

Enter the inverter parameter menu and check the control mode.

If the inverter is set to:

Closed-loop vector control

then the system must have:

• Encoder;
• PG card;
• Correct encoder parameters.

If the machine does not use an encoder, change the control mode to:

V/F Control

or:

Sensorless Vector Control

Then save the parameters and restart the inverter.

4. Complete Err20 Troubleshooting Procedure

The following procedure is suitable for field maintenance.

Step 1: Determine When Err20 Appears

Situation A: Err20 appears immediately after power-on

Possible causes:

• Wrong parameters;
• PG card failure;
• Encoder configuration mismatch.

Focus on:

• Control mode;
• Encoder type;
• PG card installation.

Situation B: Err20 appears only after motor starts

Possible causes:

• Encoder signal loss;
• Encoder cable problem;
• Encoder damage.

Focus on:

• Encoder output waveform;
• Cable continuity;
• Mechanical installation.

Step 2: Check Whether a PG Card Exists

Open the inverter control section.

Confirm whether a PG expansion card is installed.

If:

• Control mode = closed-loop vector;
• No PG card installed;

then Err20 is expected.

The solution is:

Change the control mode.

Step 3: Check Encoder Power Supply

Use a multimeter.

Measure:

Encoder +5V – GND

Normal value:

Approximately:

5V DC

If voltage is:

• 0V;
• unstable;
• significantly lower;

check:

• PG card power supply;
• Cable short circuit;
• Encoder internal failure.

Step 4: Check Encoder Output Signal

For incremental encoders:

Rotate the motor shaft manually.

The A and B channels should generate pulse changes.

Normal signal:

A channel:

0V → 5V → 0V → 5V

B channel:

Phase shifted 90° from A channel

If there is no signal:

Possible causes:

• Encoder failure;
• Missing power supply;
• Broken cable.

Step 5: Verify Encoder Parameters

Important parameters include:

Encoder Type

The inverter setting must match the actual encoder.

Example:

Actual:

AB incremental encoder

Parameter:

AB encoder

Incorrect:

Resolver

or:

UVW encoder

Encoder Resolution

Example:

Encoder nameplate:

1024 P/R

Parameter must be:

1024

Incorrect pulse number settings may cause:

• Incorrect speed feedback;
• Speed deviation;
• Err20 alarm.

5. Typical Field Repair Case

Fault Description

A machine equipped with AnyHz FST-650 inverter shows:

Err20

The motor cannot start.

Inspection Results

1. Motor has no encoder;
2. No PG card installed;
3. Inverter configured for closed-loop vector control.

Fault Analysis

The inverter entered closed-loop vector mode.

The CPU expected encoder feedback.

However:

No encoder signal existed.

Therefore:

Missing feedback
        ↓
Encoder fault detection
        ↓
Err20

Solution

Change control mode:

Closed-loop vector control
              ↓
Sensorless vector control

Save parameters.

Power cycle inverter.

Result:

Machine returns to normal operation.

6. Precautions When Replacing an Encoder

Replacing an encoder is not simply a matter of installing a new component.

Several factors must be considered.

6.1 Mechanical Installation

The encoder shaft must be:

• Properly aligned;
• Concentric with the motor shaft;
• Mechanically fixed.

Poor installation may cause:

• Vibration;
• Pulse loss;
• Feedback instability.

6.2 Rotation Direction Verification

If A/B phase sequence is reversed:

Symptoms:

• Motor rotates opposite direction;
• Speed feedback abnormal.

Solutions:

• Exchange A and B signals;
• Modify encoder direction parameter.

6.3 Encoder Resolution Matching

The replacement encoder must have the same resolution.

Example:

Original:

2048 P/R

Replacement:

1024 P/R

may cause:

• Incorrect speed calculation;
• Speed deviation;
• Control instability.

7. Difference Between Err20 and Other FST-650 Faults

Correct fault identification prevents unnecessary replacement.

Err19

Motor auto-tuning fault.

Common causes:

• Incorrect motor parameters;
• Auto-tuning failure.

Err20

Encoder feedback fault.

Focus on:

• Encoder;
• PG card;
• Feedback wiring;
• Encoder parameters.

Err21

EEPROM read/write fault.

Usually related to:

• Control board memory;
• Parameter storage failure.

8. Recommended Repair Strategy

For AnyHz FST-650 Err20 faults, follow this sequence:

1. Confirm control mode

Does the application actually require an encoder?

2. Check PG card

Verify:

• Installed or not;
• Connector condition;
• Power supply.

3. Check encoder power

Confirm:

+5V supply is stable

4. Check encoder output

Verify:

• A/B pulse signals;
• Signal quality;
• Cable condition.

5. Verify parameters

Confirm:

• Encoder type;
• Pulse number;
• Motor parameters.

Conclusion

The AnyHz FST-650 Err20 fault is essentially a protection response caused by abnormal speed feedback in the closed-loop vector control system.

Although the display message indicates an “encoder fault”, the actual cause may exist in multiple areas:

• Incorrect control mode;
• Wrong encoder configuration;
• Wiring problems;
• Encoder power failure;
• PG card damage;
• Encoder hardware failure.

In practical maintenance work, engineers should avoid immediately replacing the encoder. A systematic troubleshooting approach is more effective:

Control Mode Verification → PG Card Inspection → Encoder Power Check → Signal Measurement → Parameter Verification → Hardware Replacement

Understanding the operating principle of closed-loop vector control allows technicians to diagnose FST-650 Err20 faults faster, reduce unnecessary component replacement, and improve industrial equipment maintenance efficiency.

1. Application Background

In dust extraction fan retrofit projects, the variable frequency drive often needs to accept remote PLC control while retaining a local operating option for commissioning, maintenance, and emergency adjustment.

A typical requirement is as follows:

Remote mode: the PLC sends a 4–20mA analog signal to control fan speed.

Local mode: the operator sets the frequency directly from the VFD keypad.

LI1 is used as the run/stop command input.

LI3 is used to switch between local and remote frequency reference sources.

AO1 provides actual output frequency feedback to the PLC.

AO2 provides actual output current feedback to the PLC.

This control architecture is commonly used for dust extraction fans, exhaust fans, induced draft fans, supply fans, ventilation fans, circulation fans, and other centrifugal fan applications.

Although the wiring appears straightforward, commissioning problems are often caused by inconsistent parameter planning rather than incorrect wiring. Typical issues include selecting the wrong analog input channel, setting the wrong current range, confusing run-command switching with frequency-reference switching, or assigning unsuitable analog output scaling.

The Controlway IE series VFD provides two frequency reference channels, configurable digital inputs, AI1 and AI2 analog inputs, as well as AO1 and AO2 analog outputs. These functions make it suitable for PLC-controlled fan systems requiring both local and remote operation.

2. Define the Control Architecture Before Editing Parameters

Before changing any VFD parameters, the complete control structure should be defined clearly.

For this application, the recommended arrangement is:

Run command source: external terminals.

Run/stop input: LI1.

Remote frequency reference: AI2, 4–20mA signal from PLC.

Local frequency reference: VFD keypad frequency setting.

Frequency reference selection input: LI3.

AO1 output: actual output frequency, 4–20mA.

AO2 output: actual output current, 4–20mA.

The operating logic should be as follows:

In remote mode, the PLC sends a 4–20mA signal to AI2. The VFD converts this analog signal into the target operating frequency.

In local mode, the VFD ignores the remote frequency reference and follows the frequency manually entered through the keypad.

LI1 remains responsible for start and stop control in both local and remote frequency modes.

A critical distinction must be made between run-command source switching and frequency-reference source switching.

Run-command source switching determines whether the VFD start/stop command comes from the keypad, external terminals, or communication.

Frequency-reference source switching determines whether the speed command comes from AI2, AI1, keypad setting, communication, multi-speed inputs, or another source.

In many dust extraction fan projects, only the frequency reference needs to change between local and remote modes. The run/stop command remains controlled through LI1 or the PLC. In that case, the VFD must remain in external terminal run-command mode. Only the frequency source should switch between AI2 and keypad reference.

If the VFD is switched completely to keypad local mode, LI1 start/stop control may no longer function as intended. This is a frequent cause of commissioning confusion.

3. Run Command Configuration

The Controlway IE series normally uses parameter f002 to select the run command channel.

Typical selections are:

f002 = 0: External terminal run command.

f002 = 1: Keypad run command.

f002 = 2: Serial communication run command.

For this application, the recommended setting is:

f002 = 0

This ensures that the fan is always started and stopped through external terminal logic.

LI1 should be configured as a forward run command.

When LI1 is active, the VFD runs the fan.

When LI1 is inactive, the VFD stops the fan.

For a standard one-direction dust extraction fan, LI1 is normally assigned to forward run only. Reverse operation is generally unnecessary and may create process problems, reverse airflow, belt stress, abnormal duct pressure, or unexpected dust movement.

A recommended arrangement is:

LI1: Forward run command.

LI2: Unused, fault reset, or reserved.

Reverse operation: Disabled.

The stop mode should also be selected according to the fan inertia and process requirements. For most dust extraction fans, deceleration stop is preferred because it provides a controlled stop. Free-run stop may cause a large fan to coast for a long period, affecting process interlocks and safety sequencing.

4. AI2 as the PLC 4–20mA Remote Frequency Reference

In remote mode, the PLC analog output should be wired to AI2.

The standard scaling principle is:

4mA = minimum frequency.

20mA = maximum frequency.

Intermediate current values correspond proportionally to intermediate frequencies.

For example, if the fan maximum frequency is 50Hz:

4mA = 0Hz or the minimum allowed operating frequency.

12mA = 25Hz.

20mA = 50Hz.

In many fan applications, the lower limit should not be set to 0Hz. Fans may suffer from poor cooling, unstable airflow, resonance, or insufficient dust extraction at very low speed.

For example:

4mA = 20Hz.

20mA = 50Hz.

Under this configuration, the fan runs at 20Hz when the PLC output is 4mA and reaches 50Hz at 20mA.

The minimum frequency should be determined according to fan curve, motor cooling, duct resistance, process airflow demand, and mechanical vibration conditions.

The Controlway IE series allows separate main and auxiliary frequency references. The recommended configuration is:

f003 = AI2.

f005 = Keypad frequency setting.

f006 = Frequency reference switching mode between f003 and f005.

f021 = Single-channel frequency reference structure.

With this arrangement:

Main frequency reference: AI2, used for remote PLC control.

Auxiliary frequency reference: keypad setting, used for local manual adjustment.

LI3 is then used to switch between the two frequency reference sources.

5. LI3 as the Local/Remote Frequency Reference Selector

LI3 should be assigned to the frequency-reference switching function.

It should not be assigned as a normal run command, stop command, multi-speed input, reset input, or other unrelated function.

A typical operating definition can be:

LI3 OFF: Remote mode, frequency reference from AI2.

LI3 ON: Local mode, frequency reference from keypad.

The opposite logic can also be used:

LI3 ON: Remote mode.

LI3 OFF: Local mode.

Either method is acceptable, but the electrical design, PLC program, switch label, operation manual, and VFD parameter logic must all match exactly.

A common site problem occurs when the selector switch is labeled “REMOTE,” but the actual LI3 state causes the VFD to use keypad frequency. Operators then assume that the PLC system has failed even though the VFD is simply using the wrong reference source.

The panel door should clearly identify the operating condition, for example:

REMOTE: PLC AI2 4–20mA frequency reference.

LOCAL: VFD keypad frequency reference.

RUN/STOP: Controlled by LI1.

The logic input type must also match the PLC output type.

The Controlway IE series supports source logic and sink logic.

For PNP transistor outputs, source logic is normally used.

For NPN transistor outputs, sink logic is normally used.

For relay dry-contact outputs, the actual control supply connection and COM/0V wiring must be checked carefully.

An incorrect source/sink logic setting can cause LI1 or LI3 to operate in reverse, remain permanently active, or fail to respond.

6. AI2 4–20mA Wiring and Signal Considerations

The PLC analog output should generally be connected as follows:

PLC AO+ to VFD AI2.

PLC AO− to VFD analog common or signal COM.

Use shielded twisted-pair cable for the analog signal.

Ground the shield at one end only, normally at the control cabinet side.

Do not route analog signal cables in parallel with motor output cables for long distances.

Keep analog signal cables separate from U, V, W motor cables.

Where crossing is unavoidable, cross at approximately 90 degrees.

A 4–20mA signal is generally more suitable than a 0–10V signal in industrial fan systems, especially where cable runs are long and electromagnetic interference is present.

Advantages include:

Better immunity to electrical noise.

Less influence from cable voltage drop.

More reliable transmission over longer distances.

Ability to detect some open-circuit or signal-failure conditions.

If the PLC output circuit opens or the signal cable is damaged, the analog value may drop below 4mA. Depending on process requirements, the VFD or PLC should include low-signal detection, minimum speed protection, fault alarms, or interlock logic to prevent the fan from operating at an unsuitable low speed.

It is also important to confirm that the actual VFD hardware and firmware version supports AI2 current input. Older manuals or earlier VFD versions may describe AI2 differently. Before commissioning, verify the VFD model, terminal board type, firmware version, and applicable manual revision.

7. AO1 Output Frequency Feedback Configuration

AO1 is used to provide actual VFD output frequency feedback to the PLC.

Recommended configuration:

AO1 signal type: 4–20mA.

AO1 monitored value: Actual output frequency.

4mA = 0Hz.

20mA = Maximum operating frequency.

For a 50Hz fan system:

0Hz = 4mA.

25Hz = 12mA.

50Hz = 20mA.

AO1 normally requires two separate settings:

First, the electrical output type must be set to current output.

Second, the internal monitored variable must be set to output frequency.

These two settings do not conflict. One defines the electrical format of the output signal, while the other defines the process value being transmitted.

For example:

AO1 output type = 4–20mA current output.

AO1 monitored value = output frequency.

The PLC should then scale the received signal correctly. If 4–20mA corresponds to 0–50Hz, the PLC engineering conversion should be:

Actual Frequency = (Measured Current − 4mA) / 16mA × 50Hz.

Incorrect PLC scaling can make the display value incorrect even when the VFD output is functioning properly.

8. AO2 Output Current Feedback Configuration

AO2 is used to provide the actual motor output current to the PLC.

This signal can be used for:

Motor load monitoring.

Fan blockage detection.

Belt slip indication.

Fan impeller fouling analysis.

Filter blockage trend monitoring.

Overload warning.

Maintenance planning.

Recommended configuration:

AO2 signal type: 4–20mA.

AO2 monitored value: Actual output current.

4mA = 0A.

20mA = VFD rated output current or selected monitoring full-scale current.

For example, if the VFD rated output current is 38A:

4mA = 0A.

20mA = 38A.

The PLC engineering conversion would be:

Actual Current = (Measured Current − 4mA) / 16mA × 38A.

The selected full-scale current must be consistent in three locations:

VFD AO2 scaling.

PLC analog input scaling.

HMI display and alarm thresholds.

If these values are inconsistent, the PLC may show incorrect motor current, false overload alarms, or an inaccurate loading trend.

9. Recommended Parameter Logic Summary

Run command channel:

f002 = External terminal control.

Main frequency reference:

f003 = AI2.

Auxiliary frequency reference:

f005 = Keypad frequency setting.

Frequency source switching:

f006 = Switch between f003 and f005.

Frequency reference structure:

f021 = Single-channel reference.

LI1:

Configure as Forward Run.

LI3:

Configure as Frequency Reference Source Switching.

Logic input type:

Configure according to PLC PNP/NPN output type and actual wiring.

AO1:

Set as current output.

Set monitored value as output frequency.

Set output range as 4–20mA.

AO2:

Set as current output.

Set monitored value as output current.

Set output range as 4–20mA.

Motor and fan protection settings:

Set maximum frequency according to fan design limits.

Set upper frequency limit according to process requirements.

Set lower frequency limit according to minimum stable fan operating speed.

Set acceleration time according to fan inertia.

Set deceleration time according to fan inertia and required stop behavior.

Disable reverse operation unless reverse rotation is specifically required.

Select deceleration stop unless free-run stop is required by the process.

10. Recommended Commissioning Sequence

Do not place the complete PLC control system into automatic operation immediately. Commission the system in stages.

Step 1: Disconnect the PLC analog output or force the PLC output to 4mA.

Step 2: Confirm motor nameplate data, maximum frequency, acceleration time, deceleration time, and rotation direction.

Step 3: Confirm that LI1 starts and stops the fan correctly.

Step 4: Switch LI3 to local mode and verify that keypad frequency setting controls the fan speed.

Step 5: Switch LI3 to remote mode and verify that AI2 receives the PLC 4–20mA signal correctly.

Step 6: Output 4mA, 8mA, 12mA, 16mA, and 20mA from the PLC and confirm that VFD frequency changes linearly.

Step 7: Verify that AO1 feedback matches the actual VFD output frequency.

Step 8: Verify that AO2 feedback matches the VFD current display and a clamp meter reading.

Step 9: Verify PLC display values, alarm thresholds, trend curves, remote/local status, and interlock logic.

Step 10: Run the dust extraction system under actual process load and monitor airflow, duct pressure, fan vibration, motor current, filter differential pressure, and operating stability.

11. Conclusion

The most important point in dust extraction fan VFD retrofit work is not a single parameter value. It is the consistency of the entire control architecture.

AI2 provides the PLC remote 4–20mA frequency reference.

The keypad provides the local frequency reference.

LI1 controls fan start and stop.

LI3 switches between local and remote frequency references.

AO1 sends actual frequency feedback to the PLC.

AO2 sends actual motor current feedback to the PLC.

When the frequency source, run command source, digital inputs, analog inputs, analog outputs, PLC scaling, and operator switch labeling are all coordinated correctly, the system becomes stable, maintainable, and easy to troubleshoot.