VFD Overvoltage Fault Causes

 

A VFD overvoltage fault is a prevalent drive protection function that occurs in variable frequency drives. It is triggered when the drive’s power supply voltage exceeds the rated value. The fault can be caused during starting/stopping, running processes, sudden load variations, and more. In general, it is vital to determine the cause and the period when the fault occurs to find the appropriate solution. The reason for the malfunction might be related to the power supply, motor and load, braking, drive settings, or external factors.

What is VFD Overvoltage Issue?

To understand what a VFD overvoltage is, we must first understand what goes on inside the VFD. A VFD or Variable Frequency Drive is used to control the speed of an AC motor by controlling the frequency and voltage of the power supplied to the motor. In the most simplified form, the flow of power in a VFD is as follows: AC Supply → Rectifier → DC Bus → Inverter → Motor. The incoming AC power is rectified to DC power by the Rectifier and stored in the DC Bus. The inverter converts the DC power to three-phase AC power and supplies it to the motor. The VFD monitors the voltage across the DC Bus and trips open when it senses an overvoltage condition in the DC Bus.

To put the function of the DC Bus in perspective, imagine that it is analogous to a water tank. The AC supply coming into the VFD is the inflow to the tank, whereas the power consumed by the motor is equivalent to the outflow from the tank. At steady-state conditions, the inflow equals the outflow. However, there are conditions where more power flows into the DC Bus than is being supplied to the motor, thereby causing the voltage across the bus to rise. When the detected DC bus voltage rises beyond the rated voltage that the drive can handle, the drive stops and throws an overvoltage fault to protect the power electronics.

The important thing to note is that the extra power coming into the DC Bus is not solely coming from the supply. The power supplied to the motor can, in some cases, flow back into the DC bus. One common scenario is when the motor is decelerating. Imagine a large fan connected to a motor rotating at 50Hz. Since this is a large piece of equipment, there will be rotational inertia associated with the motor. Let’s say that the VFD receives a command to decelerate the motor from 50Hz to 20Hz. It will take some time for the rotational inertia of the fan and motor to slow down, and during that time, the motor can act as a generator and send power back to the drive. As a result, the direction of power flow has reversed. While normally power flows from the drive to the motor, in this case, however, some of the power flows from the motor back into the drive. This causes the DC Bus voltage to rise, and if the drive is not able to dissipate or consume this power, the voltage will keep rising until the drive throws an overvoltage fault.

Using this same principle, we can analyze one of the real-world situations where a VFD overvoltage fault is likely to occur. Let’s say that you have a pump motor running at 50Hz. Your PLC sends out a command to stop the pump when the system detects that the desired pressure has been achieved. The stop command is executed, and the drive begins to slow down the motor. Say that you have programmed the drive to decelerate the motor to zero speed in 2 seconds. The motor, however, cannot get rid of its rotational inertia instantaneously, and so it keeps rotating and feeding power back into the DC Bus of the VFD. If the rate at which the motor was feeding power to the bus exceeded the rate at which the VFD could dissipate the power, the DC Bus voltage would rise, and the drive would throw an overvoltage fault.

From this example, it can be seen that a VFD overvoltage fault is not necessarily due to a condition where the supply voltage is too high. It only means that the voltage in the DC Bus was too high. The reason behind the excess voltage could be a combination of different factors, e.g., high supply voltage, transients in the supply, and power fed back into the bus from the motor. Having an understanding of this basic operating principle enables one to troubleshoot the different VFD overvoltage faults that are commonly encountered in the industry.

Note: Does a motor always send energy back to the VFD? A motor does not always send energy back to the VFD. During normal motoring operation, energy is transferred from the VFD to the motor, where it is converted to mechanical power to drive the load Energy is fed back to the VFD only during regeneration, which occurs during deceleration or when the motor is driven by the load. The amount of energy regenerated depends on the motor speed, inertia of the connected load, rate of deceleration, and many other factors. Therefore, in certain applications, a VFD may require braking or regeneration capabilities to handle the returned energy.

1. High Incoming Supply Voltage:

One of the most straightforward reasons for VFD overvoltage faults is excessive incoming supply voltage. A VFD (variable frequency drive) typically charges incoming AC current to a DC voltage within its internal rectifier stage. The voltage is then stored on the VFD’s DC bus in order to be inverted back to AC for driving the motor. Thus, if the incoming supply voltage is higher than the rated voltage for the drive, the DC bus voltage will also be higher than normal. This can lead to an overvoltage situation if the voltage exceeds the VFD’s internal overvoltage limit.

An example would be a VFD rated to accept 400-480 V AC. If, for instance, the plant’s distribution voltage was higher than the VFD’s rated voltage, the DC bus voltage would be higher than normal. The drive could then falsely (in the case of no regenerative current) trip on overvoltage.

It is important to differentiate between this type of overvoltage fault and an overvoltage fault caused by regeneration. In the case of regeneration, the DC bus voltage rises due to current flowing back from the motor/load into the VFD. This is different from the supply side of the drive having too high of a voltage. When checking the problem, it is important to check the actual voltage at the VFD’s line input terminals, rather than assuming that the voltage in the plant is fine. It is also important to note whether the overvoltage only occurs at certain times, for example, if there are large motors being turned off, as this may cause a momentary voltage spike.

2. Short Deceleration Time:

Short deceleration time can lead to overvoltage in the drive. It occurs mainly with large motors or high-inertia loads. When the motor is running, it has stored kinetic energy in the rotating parts (rotor, pump wheel, etc.). When the VFD is told to decelerate, it must shed this energy. If the deceleration time is too short, the motor can generate more energy and send it back to the drive. For example, if the drive is running at 50 Hz with a 2-second deceleration time and a stop command is received, the VFD will attempt to rapidly decelerate the motor from 50 Hz to 0 Hz. However, the inertia of the rotor and pump impeller will cause them to continue rotating. During deceleration, the motor acts as a generator and sends the energy back to the drive in a process called regeneration. This energy comes in the form of DC voltage on the bus, which can raise it past the overvoltage threshold of the drive, causing it to trip.

3. High Inertia Load:

A high-inertia load may cause the drive to experience overvoltage since the load stores substantial rotational energy. Generally, inertia refers to the property that enables a rotating object to maintain its rotational motion. In this case, the heavier and faster the equipment to be driven by the motor, the higher its inertia. For example, a large centrifugal fan operating at 50 Hz will start decelerating when the VFD requests to stop and slow down the fan. Ideally, the motor should slow down as the VFD reduces the frequency. However, since the fan has substantial inertia, it will keep spinning until it reaches zero speed. During the deceleration period, the motor can behave like a generator and produce enough energy to feed back into the VFD and drive the bus voltage beyond the rated value. As a result, high-inertia loads can cause overvoltage faults due to high regeneration energy. The relationship between high inertia and overvoltage can be summarized as follows:

1. High inertia load: Large amount of rotational energy stored

2. Motor slows down: Rotational energy is regenerated back to the VFD

3. Bus voltage increases: Overvoltage fault is triggered

4. Braking Resistor or Braking Circuit Problem:

A faulty, wrong size, or open circuit braking resistor could also be the main reason for a VFD overvoltage error. In some cases, the braking resistor circuit is used to absorb power from the motor. It is applied when a motor needs to slow down. When the motor slows down, it produces more energy than required. This surplus electricity would be returned to the VFD. Ideally, the braking circuit should absorb this extra energy and dissipate it away as heat.

For example, if a large motor is decelerating, the power that was fed back can cause the DC bus voltage to rise. The braking unit will activate the chopper to utilize the braking resistor in the circuit. The resistor will then dissipate the surplus energy as heat. This process prevents the bus voltage from rising beyond the allowable levels. However, if the resistor is open, wiring is wrong or faulty, physically damaged, of an incorrect rating, or the chopper circuit is not operating correctly, the resistor will fail to utilize the extra power.

When reviewing this issue, make sure you check the resistor connection, rating, and its physical condition. Also, review the braking unit and chopper functions and the VFD settings. Ensure the resistor is appropriate for use with the given motor’s braking requirements. A resistor suitable for intermittent service might not work well with a heavy continuous-duty motor.

Practical note: A braking resistor is not designed to prevent regeneration. The resistor only helps clear surplus energy from the bus link. The motor is still allowed to regenerate, which results in the power being fed back to the VFD.

5. VFD Hardware or DC Bus Measurement Problem:

One of the ways in which the VFD presents an overvoltage fault is falsely and this occurs when the power supply voltage is normal, not overloading the motor, and the braking system is operating normally. If all these aspects are in order, the issue might be with the VFD. The VFD uses a circuit to continually monitor the DC bus voltage and if there is a fault in the voltage sensing or measurement circuit, it can make the drive display an overvoltage alarm. Another genuine hardware fault could also be the cause.

This could include any of the following: DC-bus capacitor, voltage sensing circuit, power electronics, or control circuit. Therefore, simply resetting the drive is not a good solution because the problem would be continuous and might suggest the need for a hardware change. A good strategy for detecting this reason is by comparing it with a good external measurement of the bus voltage. This can be done if one has the right tools and knowledge to handle the VFD bus voltage and avoid injury.

If there is no problem with the external voltage but the VFD still displays the high voltage, one should investigate the DC-bus voltage’s internal measurement or control circuit. It is pertinent to note that the issue should be addressed after ruling out the possible external reasons first including high source, regeneration, short deceleration, and fault braking. The VFD bus voltages have the potential to be hazardous even when the power has been disconnected from the inverter. As a result, internal inspection and measurements must not be undertaken alone with little or no experience at all.

I have covered the general theory on VFD overvoltage fault causes. I have also not attempted to cover all the topics related to it, as it can vary from case to case. Once you are familiar with this type of technology, you can easily troubleshoot any issues related to it.

Thank you for reading the post. I hope you liked it and will find a new way in this type of technology.

Written by Viral Nagda, Industrial Automation Engineer with 12+ years of experience…