Why is it better to use an inverter rated motor (instead of a non-inverter rated motor) on a Variable Frequency Drive?

Unfortunately, "inverter rated" is what the manufacturer wants it to be.

Find a motor that complies with NEMA standard MG1 part 31. It calls out higher insulation ratings as well as 1600 volt winding insulation. This will help handle the issues that occur as the distance between the motor and the VFD increases.

As a note, most NEMA Premium Efficient motors conform to MG1 part 31

It has to do with higher insulation rating to mitigate or eliminate potential damage from the voltage spikes that are a result of the PWM output on a VFD. Spikes can easily exceed 1000V on a 460/3/60 system and the problem is magnified as you increase the conductor run length between the VFD and the motor.

There is also a difference between inverter ready and inverter rated. If you plan turning down the drive down to say 1000:1, you should use inverter duty. This has to do more with cooling of the motor, but many will also have shaft grounding or insulated bearing to handle shaft current that will damage the bearings.

inverter rated motors also have fans to cool them at low speeds.

Inverter duty motors have heavyer style of winds to offset heat and load for continous use and smoother operation. The insulation for the windings is heavier for heat disapation.

The terms "inverter-duty" and "inverter-ready" are marketing terms, not engineering terms. The key to knowing which motor to use on adjustable frequency drives depends entirely on the individual application for which it will be used.

You need to ask questions such as: 1. What will the cable distance be between the drive and the motor? 2. What is the system voltage? 3. What is the rise time of the AFD's output IGBTs? 4. Is the load constant torque, variable torque or constant horsepower? 5. What speed range is required? 5. Will the motor always be powered by the AFD or is across-the-line bypass required? 6. What is the motor's nameplate voltage and frequency rating? 7. What is the horsepower or kW rating of the motor? 8. Will the conductors between the motor and the drive be shielded or non-shielded? 9. What type of raceway will be used for the conductors between the drive and the motor?

The key to picking the proper motor based on the answers to these questions is to understand the difference between NEMA MG1 Part 30 which is titled "Application Considerations for Constant Speed Motors Used on a Sinusoidal Bus with Harmonic Content and General Purpose Motors Used with Adjustable-Voltage or Adjustable-Frequency Controls or Both" and NEMA MG1 Part 31 which is titled "Definit-Purpose Inverter-Fed Polyphase Motors".

For systems that are rated for 230 volts generally do not have a problem with reflected waves because the motor insulation systems are generally rated for 600V systems anyway. Reflected waves on 230V systems would have peak voltages no greater than 650 to 1000 volts.

On 380V or higher systems, if the cable distance between the AFD and the motor is relatively short (typically 25 feet or less) then there need not be any concern about reflected waves damaging the motor insulation. On retrofit situations where you need to re-use an existing motor and you can't determine the peak voltage withstand rating of the motors's insulation system the you can add devices either at the drive output or at the motor which can prevent reflected wave voltage spike damage.

NEMA MG1 Part 30.2.2.8 designates that for motors with a nominal insulation rating of equal to or greater than 600V that the peak voltage rating of the insulation system has to be equal to or greater than 1kV with a rise time of greater than or equal to 2 microseconds.

NEMA MG1 Part 31.4.4.2 designates that for the same nominal voltage rating as above the peak voltage rating of the insulation system nees to be 3.1 times Vrated with a rise time of equal to or greater than 0.1 microseconds. There is some debate as to whether Vrated in a nominal 480V system is 480V or the motor nameplate line-to-line voltage rating of 460V. The difference is relatively small, 1426V vs. 1488V.

The operating speed range has more to do with determining the ability of the motor to keep itself operating below it's rated temperature rise when operated below it's nameplate speed than with anything else. This is more of a problem for constant torque applications with very wide speed ranges that for any other type of application. This is especially true if the application requires that the motor operate at rated torque at zero speed as in some web handling applications.

There are some "vector-duty" motors that have very wide C.T. speed range capabilities but are not suitable for applications that require across-the-line bypass operation. That many be because they are really a NEMA Design A motor which has similar locked rotor torque capabilities to a NEMA Design B motor but the locked-rotor current is not limited by NEMA as it is in a NEMA Design B motor. This could cause upstream short-circuit devices to trip or fuses to blow when the motor is started in the bypass mode. Another consideration is that there are some "vector-duty" motors that are designed with non-standard volts-per-hertz ratios such as 460V/90Hz.

Everything is pretty much covered in previous comments, but here's a (fairly) concise listing.

As it operates, the drive does three things: it changes output frequencies at a very high rate, it uses a non-sinusoidal waveform to approximate a true sinusoid, and it cannot "turn off" one gating pulse before "turning on" another.

With the non-sinusoidal distribution, there will be overshoots and undershoots to the voltage waveform as the drive corrects itself and fires the next part of the power electronics. This causes a peak voltage to appear on the winding; typically, this can anywhere from 2 to 4 times the RMS value of voltage. ( See D Koehl's post above: 460 V nominal can see upward of 1500 V applied.) Unfortunately, these spikes cannot be "tuned out" - which means they are present to damage the strand, turn, and coil insulation of the winding.

The high frequencies required for switching the devices means particular care has to be taken in the grounding of the machine. These signals - although typically of low amplitude - can easily create a reflected wave effect, amplifying the damage they can do tremendously. ( See B Jacobs' post above.) These high-frequency effects can also interfere with the smoothing of the incoming voltage and current waveforms through distortion.

Since each device (thyristor, IGBT, IGCT, SCR, etc) need to be fired in sequence, and their outputs have to somewhat overlap to achieve a reasonably sinusoidal output, there will be more than one device "turned on" at once. However, the two devices have no inductive component: this means there is a "short circuit" between phases for a very brief period of time, from the winding perspective. This is also the effect called "Common Mode Voltage" or "Common Mode Noise". Not only does this create current spiking in the windings, it also makes grounding more difficult.

INVERTER-READY means that some of the extras required to protect the machine from the damaging effects of the drive output waveform are included. Typically, this means beefed up insulation on the winding and (usually) a choice of a certain pitch to help damp out prevalent harmonics. It may or may not include skewing of the rotor winding vs the stator winding. It would also include insulation of the non-drive bearing - but possibly not both bearings. The intent of an inverter-ready design is to start / run at a single speed (drive output frequency either 50 or 60 Hz, so machine runs at/near normal synchronism) on the drive. More winding cross-section is used, to help deal with the added heat from those current harmonics which squeak through.

INVERTER-DUTY (also sometimes listed as "inverter ready") means even more extras are built in. All the inverter-ready items are there, plus: insulation of drive end bearing (if not already included) and the addition of a shaft grounding method to drain any voltage / current buildup and provide a safe working environment. These may well include a separately-powered fan, to aid in ventilation when operating at load with the speed significantly below synchronism. The inverter-duty machine can be operated continuously at any load / speed combination within its nameplate rating.

Just a supporting anecdote... I can't tell you how many times I've heard essentially the same story. Someone wants to upgrade his tomato packing plant so he instals VFDs on the motors that have been in service since 1963 when they were first installed. Run them on the new VFDs and they fail in 12 months, (almost) every time.

Thanks everyone for your answers. They have all been helpful.

You might want to use an isolation transformer between the drive and the motor if the VFD is any distance from the motor.

To Larry Rineharts comment. I liken this to taking apart your wife's broken hair dryer to "fix" something. It worked, kind of, but when you touched the windings all the insulation falls off. Same thing happens with an old motor. Three things are motors enemies; heat, moisture, vibration. Older motors are probably Class B insulation and 1000 or 1200 volt level insulation. In addition it was probably abused and tired at best and then introduced to VFD power. The higher voltage spikes, dv/dt will additionally stress what insulation was left and cause a failure in the first few windings of the stator causing either phase-phase or phase-ground fault on the drive. Peculiar is that if the motor is allowed to cool and then meggered it could appear good but put back into service it will "fail" again after coming up to operating temp. The windings will grow with temperature and clearances decrease again causing the same failure to occur.

As John Jones notes a 3 or 5% reactor on the output at the drive will reduce the damaging affects of reflected wave. Attention should be paid to the VFD manual for "long lead length" requirement. Some manufactures state as little as 25 meters to 100 meters. This would assume using a new motor with current MG1 insulation. If an old motor it is good practice to use every time, cheap insurance. This is just adding reactance to help reduce rise time, voltage levels, and clip or choke. Losses are minimal and shouldn't affect VFD performance.
Note if long leads over 100 meters are used it will be necessary to consult the manufacture of the VFD for recommendations. Line loss, capacitance coupling, etc come into play and special VFD cabling and possibly reactor/filters may be required.

It is assumed that based on flow, effort and motor output you know the capabilities of your inverter.

Basically an inverter has three stages electronic or subsystems are:
1. - Stage grinding
Two. - Stage Filtering
Three. - Investment Stage

The advantage of using a variable frequency drive on an engine is that you can vary the speed by adjusting the frequency of the drive. This is beneficiable in many processes and industries such as food and automotive industries among others.

To Jimmy Moran's comment I would urge caution in applying an isolation transformer between the output of a drive and a motor. Most standard transformers are not designed to have a PWM voltage applied to the primary. Also, the transformer impedence is going to be in the 5% to 7% range which could introduce enough voltage drop that the motor would not be able to achieve it's rated output HP.

To the original question, almost all motors produces today have "inverter rated" wire in their windings, as it has already been pointed out this varies from manufacturer to manufacturer. what this typically comes down to is speed range, which is defined as how slow you intend to run the motor and how much torque will be required at that speed. Even inexpensive motors will usually default to a 2:1 speed range at full torque, so you could run the motor at half of its base speed (900 rpms on an 1800 rpm motor) higher quality motors will have 8 or 10 to one speed range and then "inverter duty" or "vector duty" will have much high speed ranges than can go to 1000:1 or greater. I would recommend you determine the speed required for you application and select a motor that has the appropriate speed range. If the motor is existing, be certain to check of the specs on that motor for windings and speed range compatibility.