Inverter Duty Motors

This day being Labor Day, I am at home without access to my library. What I can remember, however, is that inverter duty motors need to have high insulation ratings.

That said, I've seen standard motors used with inverters without any problems...except for one.

One engineer installed inverter driven motors up at the 6th floor of a building with the inverter sitting inside the MCC at the ground floor. They were wondering why the motors were burning out so quickly. I showed them the voltage at the motor terminals which was over 600VAC from a 440VAC inverter. They relocated the inverters to the 6th floor and haven't had a problem since.

This effect is due to standing waves and would require more explaining than I'd care to do right now. Just keep your cable runs as short as possible. A good inverter manufacturer would put the appropriate cable length in the inverter's manual.

Thanks, thats great practical stuff. So, unless the inverter is located 'far' away from the drive, the standing wave phenomena that doubles voltage is unlikely to occur?

Basically, yes. The thing is you need to determine what frequency your motors are expected to operate at most of the time. If you remember your physics and that thing about string length and frequencies, you'll know what I mean.

A short cable length means that it would take a very high frequency to produce a standing wave which your inverter will probably not be able to generate.

The largest difference between inverter duty motors and non-inverter duty motors are...

1) They are designed to run at hotter temperatures as most inverter driven motors run at less then line speed. This reduction in shaft speed slows down the cooling fan and the amount of cooling the motor sees. The temperature rating of the motor's insulation is higher to compensate for the increased heating in the windings.

2) The insulation system is designed to withstand larger pulse voltage that are or can be present on inverter systems.

3) The coil windings are braced better to withstand magnetic wire motion in the winding. Wire motion can cause "abrasion" damage to the insulation system.

All and all it has a lot to do with what you are going to do with the motor/inverter.

For a good document from a reputable manufacturer about non-inverter duty motors on inverters... go see

http://www.usmotors.com/Datasheets/PDF_PDS/pds811-215.pdf

Rick...

Adding to the list, they often have insulated bearings to prevent damage due to induced currents.

Almost all manufacturers are building motors for inverter duty. It is well worth the additional cost (if any). If it is not inverter duty and you use it on an inverter and damage occurs, your warranty may be void.

You are probably right. The problem is that the motor manufacturers have an extremely long lead time for inverter duty motors (besides a higher price) which suggests that their volumes are not all that large. At the same time there are more inverters sold than ever before. So motor manufacturers either are not responding to the possible volume increases or users are just taking one size higher HP or frame and getting on with their applications. Comments?

In Canada /USA almost all Reliance motors (now Baldor Dodge Reliance) are stocked inverter duty. I believe Siemens are also. If you have HP & RPM (and range of speed operation), and what country you are in I may be able to give you more precise info.

Apart from the factors already raised by others, a suitable dv/dt filter will overcome the standing wave problem [always endeavour to keep both motor and VVVF drive close together]. The motor will obviously require an independantly drive fan, as the shaft driven fan of a TEFC motor is unsatisfactory as low speeds.

Contact me for the dv/dt filter is you need more details.

Greg

Thanks, Greg. Is it a LC circuit to filter out the harmonics? Yes, am interested ..

Another technique is to use a 3 phase output reactor to filter out the high frequency component. This will also allow you to use "standard" motors in most cases, but don't count on warranty support. These reactors are made by people like Hammond, and sold by all the VFD suppliers.

A few points to mention.

Flame proof application in the Oil & Gas industry (heavy industrial) usually suggests a maxiumum temperature rating for a motor normally refered to as your T-rating. Regardless of wether you are looking for inverter duty or not your T-rating needs to be compared with characteristics of your hazard. One way to lower a T-rating is to increase the size of the motor frame.

Inverter rating is a separate issue, directly addresses the motor insulation rating, inverter duty motors have a higher insulation rating. Inverter rated motors are more rugged in construction because of their severe duty.

A common practice is a power line reactor to clean up the power signal. Normally a standard TEFC motor can be utilized if you take the appropriate steps to limit excessive voltage at the motor terminals. The T-rating of a standard TEFC motor is pretty hard to accurately calculate if you are going to drive it with an inverter.

At this point it is worth pointing out that if you specify an inverter duty motor instead of a power line reactor you need to specify power cable with higher voltage insulation as well. Would be best to install the power line reactor, standard 600V rated cable, and an inverter duty motor. Some will argue for VFD cable regardless.

As you can see the question you are asking is not straight forward. The temperature/ flame proof part of the question could make the engineering more complicated that it seems.

You mention Flame Proof. Do you have the local codes you need to meet? NFP? CSA? NEC? CSA? IEC? City / State codes?

Do you mean explosion proof?

The problem with this forum is it is global and many laws are local.

Other posts are correct in pointing out the higher heat dissipation for a motor running more slowly than normal, partly because of its internal characteristics, and partly because its cooling fan is spinning more slowly. It is important to ensure that the Temperature Class rating of the motor is not compromised as a result.

To make sure, select a certified hazardous area motor-inverter combination, rather than piecing together discrete items of equipment, appropriate for the country of use so as to be on the safe side.