A Beginner’s Guide to Troubleshooting a VFD/ VSD

VSD and VFD
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In the world of electric motors, no matter whether servo or gearedAC or DC there is a certain component that can make them run better. Better control often means better performance and this is exactly what variable frequency/speed drives do. These drives are able to control the speed or frequency at which electric motors operate. This way, a speed drive helps control the number of spins the winding makes in a certain time period. But how does a VFD drive control the speed of an electric motor?

For this, the drive needs a controller and an operator interface which help change the frequency of the voltage and thus, the speed at which the motor runs. If the frequency of the supplied voltage is less than 50Hz, then the motor will run slower, but if the frequency is higher than 50Hz, then the motor will run faster. But like every electrical component, VFDs are also prone to issues and knowing how to solve them can save you a lot of money.

How to Repair VFD Drive

repair VFD drive
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Output Phase Loss

When output phase loss occurs, the first thing you should do is to verify the balanced phase-to-phase output with a meter. The readings, even on a reliable VSD, may not be the ones you expect. If that’s the case, look for a bypass and verify the motor lead connections. Look for a line of sight disconnect. Your meter may ring when on the waveform that drives the phase loss.

Overload

If your VSD is experiencing overload, then you must verify its parameter sets upon initial start-up and then measure the DC bus. Check if there is a solid bus or if it has changed. Check for phase-to-phase balance as well and for balanced output currents. Make sure to look at incoming waveforms, especially when the drive is tripping at a higher speed.

Cooling

Oftentimes, a VFD speed drive may experience cooling issues which are often a result of cooling fans not working. The reason for this may be clogged fan filters or the ambient temperature being above the rating of your VFD drive. Use low-pressure air to blow out some heat sinks and set the cooling system free. You must keep in mind that some VFDs have their cooling fans configured so that they only run when the VFD is running. Others have their cooling fans set so that they start working at a certain temperature or after a specific period of time.

Instantaneous Overcurrent

When instantaneous overcurrent occurs check if there is a start-up problem or if your VFD drive has been running for a long period of time.  Make sure to check the input current in each leg so you verify the rectifier as well as the output current to verify the meter. You should also look to inspect the taping in the conduit box and if the leads are rubbed together. Sometimes, the location of the motor can also be causing instantaneous overcurrent – moisture and temperature can have a huge effect on efficiency.

Types of VFD/VSD Drives

VFD drive
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Traction

This type of VFD has an adjustable diameter of the contact path between two mating metal rollers. This gives traction drives a multi ratio which allows it to generate a variable output speed.

Hydraulic Hydrostatic

A hydraulic hydrostatic drive has a positive displacement hydraulic pump as well as a motor. In these drives, the volumetric fluid output of the pump is varied through valves or just by varying the displacement. 

Hydraulic Hydrodynamic

Hydraulic Hydrodynamic
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Also known as a fluid coupling drive, a hydraulic hydrodynamic speed drive has two impellers coupled with hydraulic fluid. The drive works by changing the volume of the fluid which then affects the degree of coupling from primary to output. This way, the variable speed can be obtained from all these processes.

Viscous

With a hydro viscous drive, you get a device with multiple discs on the input shaft. These discs are also pressed against another set of discs on the output shaft. A film of oil sits between the discs, and by changing pressure the discs squeeze. This helps vary the torque transfer and thus, the speed at which the motor operates.

Variable Pitch

This simple drive uses a pulley and belt. The pitch diameter of either one or both pulleys (depending on the drive) is adjustable which gives the drive a multi ratio and thus, a variable speed output.

Conclusion

VFD drive
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No matter what you use an electric motor for, a variable speed drive can make it run a lot better and more efficient. VFDs are not simple but neither are electric motors, so it pays off to add a little trinket of complexity to an already complex system than to spend a lot of time and money making it run better after it starts failing.