Servo Drive Trips Over Current Even With No Load? The Hard Part Isn't the Power Module — It's the Invisible Current Anomaly
From Kollmorgen to the Kodak CTP Drum Driver: how DTP uses the communication protocol for precise static and dynamic servo drive diagnostics
Problem Summary
This is the story of a servo drive that kept reporting Over Current even with no load.
The drive had been repaired, tested, reinstalled, and returned seven or eight times. On the bench it looked normal every time. Back on the Kodak CTP machine, it eventually alarmed again.
Replacing the power module did not end the problem, because the power module was not the source of it. The exact root cause was not isolated by hardware replacement alone. What the repeated failures pointed to was something harder to see: an abnormal current signal inside the drive's own measurement chain.
The hardest drives to repair are usually not the ones that have burned out. They are the ones that look normal while a circuit parameter is slowly drifting away from its design value.
That is one reason DTP studies drive communication protocols and develops dedicated diagnostic software.
Field Symptoms
- The drive reports Over Current even with no load connected.
- The power module shows no short circuit; the power supply shows no obvious abnormality.
- The motor and its wiring show no obvious fault.
- After repair and bench testing, the drive runs normally.
- After reinstallation and a period of operation, the customer reports Over Current again.
Remove, inspect, repair, reinstall. Repeat.
For the service engineer, this is the most time-consuming category of fault. For the equipment user, it means repeated downtime, repeated removal and reinstallation, and repeated repair cost.
Where is the problem actually located?
Error Meaning: What the Drive Is Really Measuring
A servo drive is not a simple motor power supply. It is a precision closed-loop control system.
Taking the three-phase brushless servo drives common in industrial equipment as an example, the main internal blocks are:
- Motion controller — receives speed, position, or torque commands and executes the control algorithm.
- PWM power drive stage — uses MOSFETs, IGBTs, and similar power devices to control the U, V, and W output to the three-phase motor.
- Current sensing system — measures motor current through Hall current sensors or sampling resistors.
- Analog signal processing — op-amps, precision resistors, filter capacitors, and reference voltage.
- ADC conversion — converts the analog current signal into digital data for the controller.
- Feedback and protection system — performs closed-loop regulation and fault protection based on encoder, Hall, temperature, current, and voltage parameters.
The basic control path is:
Control command → controller → PWM → power module → three-phase motor → current and position feedback → controller.
In this chain, current sensing is a critical link.
The drive must know not only how fast the motor is turning, but also whether the actual motor current matches the control demand.
If the current sensing system itself goes abnormal, the controller can make a wrong decision even when the motor is not truly over current.
Figure 1: Control and power output structure of a typical drive. The controller drives the power module through PWM and logic circuits while receiving current, speed, position, and protection feedback. The diagram shows the structure of a specific drive under study and does not mean that all drives use the same chips.
Why High-End Drives Are Harder to Repair
Professional motion control manufacturers such as Kollmorgen place strong emphasis on control accuracy and safety protection in their drive designs.
These drives typically include multiple protection mechanisms, such as over current, over voltage, under voltage, over temperature, feedback faults, and power output fault protection.
These designs reduce the risk of severe damage from some faults.
But however complete the protection circuitry is, it cannot stop electronic components from aging over the long term.
For drives that have been running for ten years or more, the following hidden faults deserve particular attention.
1. Hall Current Sensor Zero Drift
In some systems that use bidirectional current sensing, the sensor outputs a fixed offset voltage at zero current.
For example, on a sensor powered from 5V with 2.5V as the zero-current center, the zero-current output should normally be close to its specified offset value.
If the sensor ages, the output may gradually shift.
The shift may be only a few tens of millivolts, but after subsequent amplification and digital conversion it can affect the controller's judgment of current.
It must be emphasized that the specific zero-point voltage depends on the sensor model. 2.5V cannot be treated as the standard value for all Hall sensors.
2. Analog Amplifier Drift
The current sensor is usually followed by an analog amplifier stage.
After long-term operation, changes in op-amp input offset, power supply rejection, or the state of the surrounding circuit can all lead to an abnormal output.
This kind of fault sometimes appears as:
- Data looks normal after power-up, then starts drifting after a period of operation.
- Normal when cold, abnormal when hot.
- One current-sensing channel always shows a deviation.
- Occasional over-current alarms with no obvious abnormality in the power module.
3. Precision Resistor and Capacitor Parameter Changes
Resistors and capacitors in the current-sensing chain are not just ordinary supporting components.
They may determine gain, filter bandwidth, reference voltage, and signal response speed.
When a resistor value drifts, a filter capacitor develops higher leakage, or power supply decoupling degrades, the current reading may shift or become noisy.
4. ADC and Reference Voltage Anomalies
Even when the sensor and amplifier outputs are normal, an unstable ADC reference voltage can still cause deviations in the digital value read by the controller.
For that reason, an abnormal current reading shown by the drive does not necessarily mean the Hall sensor has failed.
The Hall sensor, amplifier, filter circuit, ADC, reference voltage, and controller data must be analyzed as one complete measurement chain.
Diagnostic Path
The Biggest Difficulty in Traditional Repair: Not Knowing Where to Measure
For hard faults such as a shorted power module, a blown fuse, or a burned component, engineers can usually make a judgment with resistance measurement, a diode test, or an oscilloscope.
Hidden faults are different.
When a drive reports an over-current alarm, the open questions are:
Is the motor actually over current?
Or has the Hall sensor output shifted?
Is the op-amp stage abnormal?
Or has the ADC reference voltage changed?
It could even be grounding noise, thermal drift, or a control logic anomaly.
Without internal operating data, engineers can only work through a large number of test points one by one.
Some faults appear only after the equipment has been running for a period of time. A short run on the test bench looks fine, but the alarm returns after the drive is installed back in the CTP machine.
This is why traditional drive repair so often requires repeated installation and verification on the machine.
The difficulty is not that engineers cannot measure. It is that they do not know which circuit should be measured first.
DTP's Solution: Reading Internal Drive Data Through the Communication Protocol
To address this, DTP has studied the communication mechanisms and developed test software for several drives used in Kodak CTP equipment.
Through the drive control interface, internal status and operating parameters can be read on supported models.
Compared with measuring external circuits alone, communication-based diagnostics offer two clear advantages.
First: Static Testing Without Repeated Machine Installation
On some drives, it is enough to supply the control power as specified and keep the power output disabled to read part of the controller's internal parameters.
For example:
- Raw current sampling values
- Zero-current offset
- Controller operating status
- Drive enable status
- Temperature sampling data
- Fault and protection status
This means a service engineer can carry out part of the static testing on the workbench, instead of installing the drive into the machine every time.
It is particularly useful when judging current sampling offset and analog circuit drift.
Static testing, however, cannot replace power output, loaded operation, and safety protection testing.
Second: Dynamic Monitoring to Record Anomalies During Operation
Where test conditions permit, the software can also read operating parameters continuously to analyze:
- Motor start-up process
- Speed changes
- PWM output
- Current sampling changes
- Drive operating mode
- Alarm and protection status
By recording continuously, data changes before and after a fault can be analyzed.
In the past, all we knew was that the drive reported over current.
Now it is possible to go further and determine:
Is the actual current abnormal, or is the current sensing system itself abnormal?
These two situations require completely different repair directions.
What Was Checked: Kodak 8Up Driver Real-Time Monitor
Below is the test interface of the Kodak 8Up Driver Real-Time Monitor software developed by DTP.
Figure 3: DTP real-time drive monitoring software, showing controller information, operating status, current sampling, PWM parameters, and CSV history curves.
1. Communication Link and Controller Identification
From the screenshot:
| Item | Value Read |
|---|---|
| Communication port | COM6 |
| Baud rate | 57600 |
| Controller | MC70000 |
| Control type | Brushless DC |
| Firmware | 2.2 |
| Axes | 1 |
| Chips | 1 |
The software was able to read the controller version information, which confirms that the communication link and the corresponding query functions worked in this test.
This provides the basis for reading operating status and current data afterwards.
2. Drive Status Detection
The software read the following status values:
| Parameter | Returned Value |
|---|---|
| EventStatus (0x31) | 0x0008 |
| ActivityStatus (0xA6) | 0x03C0 |
| SignalStatus (0xA4) | 0x8007 |
| MotorMode (0xDD) | 1 |
| LoopMode (0x70) | 0x0001 |
| ActualVelocity (0xAD) | 0 |
The software interface decodes these status values further.
Visible here are:
- Capture Received
- Overtemperature
- PWM Disable
- Motor Mode
- Position Capture
- Quad A, Quad B, Index
- Amplifier Disable
The value of this information is that engineers no longer have to judge status only from the drive's external alarm LEDs. They can observe the status word returned by the controller directly.
In this screenshot, for example, ActualVelocity is 0, and the status includes PWM Disable and Amplifier Disable.
This indicates that the screenshot does not correspond to normal loaded rotation, but to a state suitable for observing certain static parameters.
Note that the ActivityStatus decoding also shows an Overtemperature status bit. Whether this represents a currently active temperature fault must be confirmed against the specific controller manual, the status bit definitions, and the test conditions. It cannot be concluded from the screenshot alone that the drive is overheating.
3. Static Current Measurement: Current A / Current B
This is one of the most valuable measurement functions in the software.
The data in the screenshot is:
| Parameter | Value Read |
|---|---|
| Current A | -32 raw |
| Current B | -32 raw |
| Software-estimated Current A | -0.016 A |
| Software-estimated Current B | -0.016 A |
Both current sampling channels show the same raw value: -32.
The software converts this into an estimated current of about -0.016 A.
One important clarification: this does not mean the drive is actually outputting -0.016 A.
With the power output disabled, the reading may include the ADC zero point, sampling offset, quantization error, or conversion deviation in the software.
Whether it is normal must be determined by comparing against data from a known-good drive of the same model under the same conditions.
If the two channels on a good board stay stable over time, while one channel on a faulty board shows an obvious offset, fluctuation, or temperature-related drift, the corresponding current-sensing channel can then be examined further.
This is exactly where communication-based diagnostics prove their value in hidden-fault repair.
4. PWM Output and Control Commands
The software also reads:
| Parameter | Value |
|---|---|
| PWM Phase A | -128 |
| PWM Phase B | 128 |
| PWM Phase C | 0 |
| AnalogCommand | -132 |
| MotorCommand | 0 |
These values reflect the relevant internal commands and PWM parameters of the controller.
Note that while the Amplifier Disable state is active, the PWM values read by the software cannot be taken directly as the three-phase voltage the power module is applying to the motor.
They may be internal registers or command-related values.
During repair, they should be interpreted together with the drive enable state, the actual output, and the controller definitions.
5. CSV Curves: Leaving Evidence of Intermittent Faults
The lower part of the screenshot shows four curves:
- Blue: AnalogCommand
- Red: PWM Phase A
- Green: Current A
- Purple: Current B
The currently opened CSV file contains 35 records.
The recording period is:
2026-09-19 12:23:53 to 12:24:46.
Within this data set, some command and sampling values change briefly.
Whether these fluctuations represent a real current anomaly still has to be judged together with the sampling period, raw values, operating mode, and the way the data is read over the communication link.
A spike in the curve alone is not enough to conclude that an over-current fault exists.
The important value of this curve-recording function is that results from different times, different states, and different drives can be compared.
For example:
Good drive versus faulty drive.
Cold state versus hot state.
Before repair versus after repair.
Static state versus running state.
Anomalies that used to appear only occasionally, and were hard to capture, can now be turned into an analyzable record through continuous sampling.
Repair Action: How the Software Helps Locate the Faulty Circuit
Taking an abnormal current-sensing channel as an example, DTP can apply a layered diagnostic method.
Step 1: Check the Static Current Offset
Read Current A and Current B in the specified test state.
Compare them with a known-good drive of the same model to confirm whether an obvious deviation exists.
Step 2: Check the Hall Sensor Output
If one channel deviates abnormally, measure the supply and the zero-current output of the corresponding Hall sensor.
Determine whether the offset already exists at the sensor output.
Step 3: Check the Analog Amplifier Circuit
If the Hall sensor output is normal but the controller reading is abnormal, continue by checking:
- Op-amp input and output
- Reference voltage
- Feedback resistors
- Filter capacitors
- ADC input signal
Step 4: Check Temperature Drift
For intermittent faults, a controlled temperature test can be carried out under safe conditions.
Observe whether the current sampling data changes abnormally with temperature.
Step 5: Re-test After Repair
After replacing a confirmed faulty component, read the same parameters again and compare them with the pre-repair data.
Finally, carry out the necessary dynamic and loaded verification.
The point of this method is not to have the software tell the engineer which resistor has failed. It is to narrow the fault range using internal data, and then complete component-level localization through hardware measurement.
Final Result: Fast, Accurate, Precise, and Cost-Saving
DTP summarizes the value of software-assisted repair in four words.
Fast: Finding Anomalies Quickly
Reading internal data through the communication interface reduces the time spent searching blindly for test points.
Accurate: Judging the Fault Direction Correctly
Distinguishing current-sampling anomalies, control-status anomalies, power-stage problems, and external load problems.
Precise: Locating the Faulty Circuit in Detail
Through data comparison and step-by-step measurement, narrowing the fault range down to a specific circuit and suspected component.
Cost-Saving: Reducing Repeated Removal, Reinstallation, and Repair Cost
For some discontinued, high-value drives, this reduces unnecessary whole-unit replacement and repeated on-machine testing.
In the past, experience and repeated trial and error. Today, data-guided repair.
That is the purpose behind DTP's dedicated diagnostic software.
Engineering Lesson
The lesson from this class of fault is that a drive cannot be judged by its power stage alone.
The Hall sensor, analog amplifier, filter network, ADC, reference voltage, and controller data form one chain. A single drifting link in that chain can produce an alarm that looks exactly like a real over-current event.
A second lesson is the value of comparison over assumption. A reading is only meaningful when compared with a known-good drive, the same test conditions, and ideally a hot and cold state.
A third is that software diagnosis and hardware measurement are complementary. Communication-based readings narrow the range; a meter or oscilloscope confirms the individual component.
And static tests, however useful, never replace power output, loaded operation, and safety protection testing.
Related Services
DTP's drive research and repair service covers a range of Kodak CTP Drum Drivers and related upgrade systems.
| Drive / Part Number | Associated Equipment |
|---|---|
| ACE500 / 601-00193 | Trendsetter Series IV/V, Achieve, Flexcel NX |
| Kollmorgen / 500-03349A | Magnus 800 |
| 601-00032C | Trendsetter Series III, News |
| ACE1200 / IDS1100 | Early Trendsetter-related drive configurations |
| 512-00898A | Trendsetter Series III 8Up upgrade kit |
| 512-00899A | Trendsetter Series IV, 1600-related upgrade kit |
| 512-00901A | Trendsetter Series V, Achieve-related upgrade kit |
Of these, 512-00898A, 512-00899A, and 512-00901A are upgrade kit numbers, not standalone drive models.
Different models use different controllers, communication interfaces, and firmware, so the data and test functions actually available must be confirmed for each specific drive configuration.
Beyond Kodak CTP, similar servo control and current-sensing technology is also widely used in industrial printing equipment such as HP Indigo and in automation systems.
DTP has long worked on Kodak CTP laser systems, electronic control boards, servo drives, and other critical industrial components.
We believe the future core competence in industrial electronics repair is not only soldering and component replacement, but the ability to explain a fault, locate its cause, and verify the repair result through data analysis.
For older and discontinued drives, DTP continues to improve its communication protocol research, real-time monitoring software, static testing, and dynamic verification capabilities.
CTA: Global Technical Cooperation
We welcome cooperation with CTP equipment service providers, independent service engineers, industrial electronics repair companies, and equipment distributors.
DTP can share its dedicated servo drive diagnostic and test software and provide related technical exchange and support, so that we can improve fault localization and repair capability for industrial drives together. The same software can also be developed for other drive models.
Through shared software, technology, and repair experience, we hope to build long-term partnerships with partners worldwide.
Contact DTP TECH
Website: www.allctp.com
WeChat Work: scan the QR code below for technical cooperation, drive testing, and repair service inquiries.
When you contact us, please provide the equipment model, serial number, drive part number, fault code, and a photo of the nameplate.
DTP TECH — Kodak CTP & Industrial Servo Drive Diagnostic Solutions
Making invisible faults visible, and industrial electronics repair more precise.
Which drive are you troubleshooting, and what exactly does it report — over current at idle, over current only under load, or an alarm that comes and goes with temperature? Send us the part number and we will tell you which internal parameters can be read on that model.