Can a Servo Drive Overvoltage Alarm Be Caused by a Missing Braking Resistor?
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Practical notes for CNC router, automation and industrial motion systems.
Yes, the servo drive overvoltage alarm can definitely be triggered due to a missing, incorrectly selected, or faulty braking resistor. The regenerative energy generated when the servo motor decelerates or stops is fed back into the DC bus; if this energy cannot be properly dissipated, the DC bus voltage rises above critical levels, triggering the overvoltage alarm.
Can a Servo Drive Overvoltage Alarm Be Caused by a Missing Braking Resistor? What Is It?
Servo drives used in industrial automation systems play a critical role in production processes by providing precise control of motors. One of the common alarm conditions these drives encounter is the “Overvoltage” alarm. This alarm is triggered when the DC bus voltage of the drive exceeds a specified upper threshold and stops the motor to ensure system safety. So, could the absence or incorrect configuration of the braking resistor be one of the main causes of this alarm? The answer is definitely yes.
A servo motor converts electrical energy into mechanical energy while in motion. However, when the motor needs to decelerate or stop, the mechanical energy is converted back into electrical energy. This phenomenon is called regenerative braking or regenerative energy generation. The electrical energy produced is fed back into the driver’s DC bus. If this fed-back energy cannot be consumed or dissipated by the system, the DC bus voltage rises rapidly. Servo drives have a specific DC bus voltage upper threshold to protect their internal components and the connected motor. When this threshold is exceeded, the drive triggers an overvoltage alarm to protect the system.
This is precisely where the braking resistor comes into play. Its purpose is to safely dissipate the excess energy returning to the DC bus during regenerative braking by converting it into heat. This resistor is controlled by a chopper circuit within the drive or by an external braking unit. When the DC bus voltage rises above a certain level, the chopper engages and connects the braking resistor to the DC bus, allowing the excess energy to be dissipated across the resistor. Therefore, if a braking resistor is missing, selected with an incorrect value, connected incorrectly, or if the chopper circuit fails, the regenerative energy will uncontrollably increase the DC bus voltage and trigger the overvoltage alarm.
Operating Principle and Technical Data
In servo drive systems, the primary cause of an overvoltage alarm is the DC bus voltage exceeding the nominal operating range. The main mechanism behind this situation is regenerative braking and the management of the energy generated during this braking process. When a servo motor decelerates or stops the load it is connected to, it behaves like a generator. If the motor continues to rotate due to its inertia or external forces, a voltage is induced in the motor windings, and this voltage is fed back to the drive’s DC bus. This fed-back energy is stored in the DC bus capacitors, causing the voltage to rise.
Drives have a specific tolerance range for DC bus voltage. For example, the nominal DC bus voltage of a drive powered by a 400V utility supply is approximately 560–600 VDC. However, the overvoltage alarm threshold is typically set to a higher level, such as 750–800 VDC. When this threshold is exceeded, the drive triggers an alarm and stops operation to protect its internal components (particularly the power modules and capacitors).
This is where the braking resistor comes into play. An internal or external braking unit (chopper) connects the braking resistor to the DC bus when the DC bus voltage reaches a specific activation threshold (for example, 680–720 VDC). This resistor dissipates excess electrical energy by converting it into heat, ensuring that the DC bus voltage remains within safe limits. The resistor’s value (ohms) and power rating (watts) must be carefully selected based on the application’s regenerative energy requirements. An incorrectly selected resistor (a value that is too high cannot dissipate energy quickly enough, while a value that is too low may overload the driver with excessive current; while an insufficient Watt rating may cause the resistor to overheat and burn out) or the complete absence of a resistor will inevitably trigger the overvoltage alarm.
| Parameter | Value/Description |
|---|---|
| DC Bus Voltage (Nominal) | Depends on the line voltage: ~1.414 × Line RMS Voltage (e.g., ~565 VDC for 400 VAC) |
| DC Bus Voltage (Chopper Activation) | Varies by drive model; typically 120–130% of the nominal value (e.g., ~680–720 VDC for 400 VAC) |
| DC Bus Voltage (Overvoltage Threshold) | Varies by drive model; typically 135–145% of the nominal value (e.g., ~750–800 VDC for 400 VAC) |
| Braking Resistor Resistance Value (R) | In ohms; determined based on the drive and motor power. Typically recommended by the manufacturer. |
| Braking Resistor Power Rating (P) | In watts, determined based on the amount of regenerative energy and the braking time (continuous/peak power). |
| Maximum Braking Duration | The application’s stopping time directly affects the braking resistor’s capacity. |
| Braking Ratio (Duty Cycle) | The ratio of the braking time to the total cycle time (e.g., 10% duty cycle). |
Field Considerations
- Proper Braking Resistor Selection and Sizing:
This is the most critical step. The resistance’s ohm value must fall within the range recommended by the driver, and its power rating (watts) must be sufficient to safely dissipate the maximum regenerative energy produced by the application. Factors such as the motor’s inertia, load size, braking duration, and cycle speed play a role in determining the required braking resistor power. Manufacturer catalogs and software serve as the primary references for proper sizing. The resistance’s ohm value must be set so that it does not exceed the driver’s maximum output current; otherwise, it may damage the driver. The power rating, meanwhile, must be sufficient to handle both continuous and peak braking power.
- Braking Resistor Connection and Wiring Check:
The braking resistor must be connected to the driver’s specified “Braking Resistor” or “DB” terminals with the correct polarity and tightness. Loose connections, corrosion, or incorrect wiring can prevent the resistor from engaging or functioning properly. Ensuring that the wire gauge is appropriate for the current the resistor will carry and that shielding is properly installed is important for electromagnetic compatibility (EMC). Voltage drops and interference over long cable distances must also be taken into account.
- Driver Parameter Settings and Monitoring:
Most servo drives allow you to adjust parameters such as the activation threshold of the internal braking unit (chopper) and the braking resistor values. Ensure that these parameters are set correctly according to the braking resistor used and the application’s requirements. Additionally, monitoring the driver’s DC bus voltage in real time is useful for detecting potential overvoltage conditions in advance and providing critical data during the troubleshooting process. Some drivers also allow monitoring the status of the braking unit (active/passive) or the power applied to the resistor.
- Mechanical Load and Motion Profile Optimization:
The amount of regenerative energy is directly affected by the inertia of the load driven by the motor and the applied motion profile. Very rapid acceleration and sudden stops (short braking times) generate a high amount of regenerative energy. If possible, optimizing the motion profile (by using smoother acceleration/deceleration ramps) to reduce the regenerative energy load can alleviate the strain on the braking resistor and reduce the risk of overvoltage. In applications where the load moves vertically (e.g., elevators, lifting systems), gravity can also amplify the regenerative effect.
Common Problems and Solutions
Common issues related to overvoltage alarms in servo drive systems and their corresponding solutions are detailed below:
- Problem 1: Braking Resistor Missing or Incorrectly Connected
Symptom: The overvoltage alarm triggers as soon as the motor begins to decelerate or immediately afterward. This is particularly noticeable with high-inertia loads.
Solution: First, check whether a braking resistor is present in the system and whether it is correctly connected to the appropriate terminals (typically DB+, DB-). Ensure that the connections are secure and free of corrosion. If there is no resistor, one suitable for the driver and the application must be selected and integrated into the system. - Problem 2: Incorrect Braking Resistor Value (Very High Ohm Value)
Symptom: The overvoltage alarm persists even though the braking resistor is connected. The DC bus voltage gradually rises and exceeds the threshold because the resistor cannot dissipate the energy quickly enough.
Solution: Check the resistor range recommended by the driver manufacturer. If the ohm value of the resistor in use is higher than the driver’s minimum resistance value or cannot keep up with the energy dissipation rate required by the application, it must be replaced with a resistor that has a lower ohm value but is still within the safe range for the driver. If the resistance is too high, it slows down energy dissipation. - Problem 3: Incorrect Brake Resistor Power Rating (Too Low Wattage)
Symptom: While the system operates without issues during brief braking, it triggers an overvoltage alarm in applications requiring continuous or intense braking. The braking resistor overheats, emits smoke, or the resistor element burns out.
Solution: Recalculate the application’s average and peak regenerative power requirements. The resistor’s nominal power capacity must meet the average regenerative power; its peak power capacity must be able to handle high energy spikes during sudden braking. A resistor with insufficient power capacity will overheat and fail or be unable to dissipate sufficient energy. If necessary, a resistor with a higher wattage rating or the parallel/series connection of multiple resistors may be required. - Issue 4: Internal/External Chopper (Braking Unit) Failure
Symptom: An overvoltage alarm occurs even though the braking resistor is correctly selected and connected. No heat generation or current flow is observed in the resistor when the DC bus voltage rises.
Solution: Check whether the driver’s internal chopper circuit or the external braking unit is functioning. You can monitor the chopper’s status via the driver’s diagnostic menu or check the chopper trigger signal using an external power source. A faulty chopper cannot connect the braking resistor to the DC bus, so energy dissipation does not occur. In this case, repair or replacement of the drive or external unit may be necessary. - Issue 5: Excessive Regenerative Energy (Exceeding Resistor Capacity)
Symptom: Even though all components appear to be correctly selected and functioning, an overvoltage alarm is occasionally triggered, particularly in highly dynamic or heavy-load applications.
Solution: Review the application’s motion profile. Extending braking ramps or using smoother transitions can reduce regenerative energy peaks. If this is not sufficient, a higher-capacity braking resistor or the use of multiple resistors may be necessary. Alternatively, using regenerative power supply units that feed energy back into the grid may be a more efficient solution, particularly in high-power systems that require continuous braking.
Expert Advice
Although the overvoltage alarm encountered by servo drives in industrial automation systems may seem like a complex problem, it is generally rooted in the improper management of regenerative energy. As detailed in this article, the absence, incorrect selection, or faulty connection of a braking resistor, or a malfunction in the control circuit (chopper), are among the most common and direct causes of this alarm. The excess energy generated as the servo motor decelerates or stops causes the DC bus voltage to rise above critical levels, triggering the drive to enter protection mode and issue an alarm.
For field engineers and technicians, adopting a systematic approach in the event of such an alarm is of vital importance. First, check whether an appropriate braking resistor is physically present in the system, then verify that the resistor’s ohm and watt ratings meet the requirements of the drive and the application. The integrity of the connections, the correctness of the cable cross-sections, and the proper configuration of the drive parameters must also be verified. Finally, the functionality of the drive’s internal braking unit or external chopper circuit must be tested. It should be kept in mind that sometimes the problem stems not from a single component, but from a lack of optimization in the system’s overall design (e.g., motion profile, load inertia).
As a professional recommendation, always refer to the catalogs and application notes provided by the drive and motor manufacturers, as these offer the most reliable information for selecting the correct braking resistor and configuring the system. If in doubt, do not hesitate to consult an experienced automation engineer or the manufacturer’s technical support team. A properly sized, correctly installed, and properly parameterized braking system not only prevents overvoltage alarms but also significantly enhances the system’s safety, service life, and operational efficiency. This is a fundamental requirement for ensuring that industrial processes continue uninterrupted and efficiently.
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