A very basic question about VFD output frequency

Question

I know that a VFD's inverter stage creates alternating voltages at a desired frequency for the terminals of a delta induction motor(stator voltage freq.).

Normally without a VFD the nominal freq. for the motor terminal voltages are 50/60 Hz rated.

In case of VFD use, will the induction motor terminals(stator terminals) voltage frequency be limited to 50/60Hz? On the other hand I see such data where the VFD electrical output freq. is given upto 590Hz as follows:

Induction motors are manufactured for 50/60Hz systems. How come 590Hz limit possible for safe operation? Am I misunderstanding something here?

Answer 1

When you're setting up a Variable Frequency Drive (VFD) you will configure things like the min/max frequency, rated voltage, rated full-load amperage, and accel/decel time. Depending on how fancy of a VFD you've got, there could be dozens of other settings (comms., safety, digital inputs/outputs, control methods, etc).

As mentioned in some comments, many industrial motors are manufactured to 50/60 Hz depending on your geography, but this is not true of ALL motors. I believe ships and planes use higher frequency. Wikipedia says that spacecraft, submarines, machinery, and military equipment may be run on 400 Hz. Motors run at 400 Hz can provide similar power for a smaller size.

To answer your question -- 590 Hz out of the VFD is possible, but it is not safe for the operation of a 50/60 Hz motor. In this scenario, you would configure the VFD to 50/60 Hz. If you were using a 400 Hz motor, you would configure the VFD to 400 Hz.

Answer 2

The example nameplate linked in a comment shows a motor rated for 460 volts at 60 Hz. A motor like that can be operated below the rated 60 Hz frequency with proportional reduction in voltage. It is probably safe to operate as low as 30 Hz continuously at the rated load torque. The service factor for operation below 60 Hz would be 1.0 rather than the 1.15 marked on the nameplate. It is probably safe to operate over a limited range above 60 Hz at 460 volts and reduced torque such that the delivered mechanical power does not exceed the rated power marked on the nameplate. There may be other limitations published by the motor manufacturer. Standard motors like that have been used successfully with VFDs since VFDs were first available in the mid 1960's.

Today it is preferable to use motors that have been designed for use with VFDs. Such motors would have additional information for VFD use on the nameplate and published by the manufacturer. Many motors are designed for either fixed frequency or VFD operation. They may be identified as suitable for VFD use my nameplate markings or in catalog or instruction publications.

Some motors are designed primarily for VFD use. Their nameplates may have voltage and frequency markings that are only attainable with a VFD.

Answer 3

In theory you CAN operate a motor that was originally designed for 50 or 60Hz at 590Hz. But theory and reality have a way of diverging. For the moment though let's ignore most of the details and cover the big one.

AC Induction Motor design is dependent upon a relationship between applied voltage and frequency, what's referred to as the "V/Hz ratio", to produce a given amount of torque at a given speed. So a motor designed for 460V at 60Hz has a V/Hz ratio of 460/60 = 7.67 (to one), motors designed for 380V 50Hz have a V/Hz ratio of 7.6 (the similarity is not a coincidence by the way). Maintaining this V/Hz ratio (+-10%) allows the motor to produce rated torque at any speed, up to the design speed. If you only reduced the voltage without maintaining the ratio, the torque drops at the square of the voltage change, so the motor stalls very easily. What a VFD does is to allow you to attain that rated torque from the motor at any speed below the designed speed without stalling.

If you operate the motor at a frequency above that which it was designed for, the first issue is that you can no longer maintain the same V/Hz ratio; you "run out of voltage" because the VFD output voltage is limited to the VFD input voltage (or less). For example if a motor is designed for 460V 60Hz, the drive will provide 460V at 60Hz, but if you increase the frequency, you can no longer increase the voltage with it. So above the base frequency of the motor design, the effect is the same as decreasing the voltage; you begin to LOSE torque. At some point your motor will produce so little torque that it can no longer even accelerate the mass of its own rotor and it stalls. Such would likely be the case for a 50 or 60Hz motor that is given 590hz.

Long before you get there however, those other previously ignored reality issues will bite you; the bearings are not rated for that speed, the mechanical balancing of the rotor (not to mention the load) will likely become so bad that it flies to pieces, and the cooling system for the motor will cease to function, burning up the windings.

VFDs OFFER those higher frequencies because it is POSSIBLE to buy motors that are custom designed to operate at higher frequencies. These are often referred to as "spindle motors" because they are used in high speed machining applications.

Side note: the limit to 590Hz is the result of a world-wide restriction implemented to avoid having the VFDs be used in uranium enrichment centrifuges, which need to operate at frequencies somewhere above 600Hz (we don't know exactly where). Not all countries adhere to this restriction, so there are VFDs that can go to 1,000Hz and more. They just can't be legally sold in countries that DO adhere to that restriction, mainly North America and EU countries.