Variable Frequency Drives (VFDs) are the most common starter choice in modern industrial applications for motor control. VFDs can improve energy efficiency and reduce wear and tear on mechanical components. But they pose a potential safety risk if not correctly installed or maintained. This article will discuss more technical insights on making VFDs safe for electric motors.
Basic Motor Selection for VFD Motor Control
It is essential to select a motor designed to handle the variable frequency and voltage output of the VFD. Check the following:
- Choose a motor rated for use with VFDs (inverter-duty), i.e. can handle high-frequency voltage spikes from VFDs. Even “inverter duty” motors differ from manufacturers. So, ensure the motors are in the 1600 VAC spike sustainability range for at least 1 msec with superior bearing lubrication.
- Select the motors based on application needs: variable torque or constant torque application. The application type is essential for proper motor control.
- Match motor power rating to VFD rating. The rating should be such that we should not operate less than 75% of the desired load & can go to rated loads when desired for maximum efficiency.
- Check that voltage and frequency ratings of the motor match the VFD output.
Insulation Class & Service Factor of Motors
The insulation class effect on Motor Control
The Insulation class of the motor is an important consideration when using VFDs. VFDs can generate high-frequency voltage spikes that cause electrical stresses on the motor insulation. This leads to insulation breakdown and motor failure.
Motors with higher insulation classes, such as Class F (125 C rise, heavy duty) insulation, are more suitable for VFDs over Class B (80 C rise, general purpose) and Class F (105 C, industrial use).
In addition, we should do the periodic tests of the motor for insulation resistance to ensure it functions correctly.
Service Factor for Proper Motor Control
Service factor indicates the motor’s overload capability over the rated motor power, i.e. 1.15 service factor motor can go up to 15% over the rated power.
But the motor manufacturers might say this is not valid on VFD-operated motors. So, getting this verified from motor suppliers for VFD duty motors is essential, especially on constant torque applications.
Motor Turn-Down Ratio
Check the motor speed range and variable speed control needed for your load. The motor turn-down ratio of signifies how much speed reduction from the motor base speed the motor can handle properly. This ratio plays an important role in motor control by your AC drives.
- General purpose motors: 10:1 turn-down ratio without overheating.
- Inverter duty motors: 20:1 turn-down with V/Hz control.
- Many loads do not need extremely high turn-down. Eg. a 10,000 cfm fan does not need to go below 500 cfm.
- Special motors for high turn-down (1000:1-2000:1) with vector VFD control. For applications that need high low-speed torque and high turn-down. Use vector control VFDs for such applications.
Bearing Protection for VFD Controlled Motors
Proper bearing protection is essential when VFDs for motor control, as they can generate electrical currents that can damage motor bearings. This can lead to premature bearing failure and motor downtime.
Bearing protection with insulated bearings that are designed to prevent electrical currents from flowing through the bearings.
In addition, grounding brushes or rings can be installed to provide a low-impedance path for electrical currents to flow away from the bearings.
Cable Selection
Proper cable selection is critical for ensuring VFDs as well as motors’ safe and reliable operation.
- Cables selection is based on the maximum current drawn by the VFD and motor and rated for the VFD’s voltage and frequency output.
- Cable size can also impact the ground capacitance & reflected waves, so higher-size cables can help counter the longer motor cable lengths, wherein reflected waves can be a problem for VFD-operated motors.
In addition, cable routing should be away from sources of electromagnetic interference (EMI), which can cause interference with the VFD and motor. Shielded cables can be used to reduce the effects of EMI.
Grounding and Bonding
Proper grounding and bonding are critical for ensuring the safe operation of VFDs and motors. Ensure proper bonding of all metal components together to prevent the buildup of static electricity that can damage electronic components.
In addition, a proper grounding for the VFDs will also ensure protection against electrical shock and reduce the risk of electrical fires.
VFD Carrier Frequency
Carrier frequency can smoothen up the output waveform more sinusoidal, resulting in better torque control & dynamic response. However, this can increase the reflected wave effect, causing frequent high voltage surges to the motor and motor temperature rise.
Typically VFDs have a carrier frequency from 2 kHz to 16 kHz or more. According to various studies, operating a VFD at 2 kHz vs 16 kHz can reduce the motor life by 60-70%. However, it leads to better motor control. So you have to see what results you are after & can choose the carrier frequency accordingly.
Operating beyond rated motor speed
We can operate motors over their rated speed i.e. a 50 Hz rated motor in India or a 60 Hz rated motor in the USA can go up to 400 ~ 1000 Hz using VFDs.
But motor OEMs are reluctant to give this clearance specifically above certain ranges for certain applications.
So we need to consider the motor control for such requirements & weigh in the advantages over the downside of motor performance & life.
One important point is that torque falls over the rated speed as the voltage can’t be increased beyond the rated voltage. Wouldn’t that break some basic laws if we could do that (pun intended)?
So, is it safe to use VFDs for motor control?
VFDs are not the perfect solution for your motors & this article doesn’t cover their advantages. But all the downsides of the VFDs are manageable to a large extent & measured against the advantages.
So if you need a starter for your motors, the VFDs can be the ideal choice & the above points can serve you best in making your decision.
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