Control.com - Nerds in Control

Dear All,

I have heard that if we need to use an inverter, then the motor should be inverter duty.

In my case, I need to use an inverter (315kw, 400V) only to reduce the starting current. Once it is running it will run only at full RPM so if I use a standard motor do I need to de-rate the motor or do I need to use next higher size?

Please advice.

It seems that what you really need is a soft starter, not an inverter. A soft starter will ramp up the voltage until the motor is at full line voltage. The soft start does not generate an AC wave, it only lowers the voltage. It should work fine with any motor. An inverter generates AC power with a lot of high frequency noise, that is why you need the inverter duty motor. This is why cable lengths become an issue with inverters too. A load reactor may allow a standard AC motor and/or longer cabling to work reliably with an inverter also.

Bill Sturm

Most modern motors are built for inverter duty. You can run one that's not but run the risk of it failing. At the least use a VFD with load reactor, this will help reduce any switching spikes.
Roy

Dear All,

Thanks for your advices.

Mr. William Sturm, I have to use inverter (Soft starter is not enough) to reduce starting current as close as possible to maximum running current of the motor. We need to run it on a generator and the size is limited.

The conclusion is even if the inverter is for starting only, I have to use inverter duty motor and if the cable is longer than recommended by inverter manufacturer I need to use load reactor.

Anybody have any technical document or a link which says the inverter duty motor is necessary for inverter application.

Thanks in advance.

Are you trying to utilize an existing motor? As mentioned, most new motors are inverter duty. One of my points also, is that a load reactor will possibly allow you to use a non inverter duty motor with an inverter. It is not a cut and dry decision, it is a matter of motor life and there are many variables. The job of the reactor is to filter out all of that high frequency noise from the AC signal. The high frequency spikes are damaging to the insulation on older motor windings. Inverter duty motors do use better insulation.

You could buy a good inverter and a load reactor and see how long your existing motor lasts. Chances are, it will run fine for many years. Worse case, you may have to have it rewound someday. It depends also how critical this motor is to your business. Like I said, there are many variables, you need some good background info and make your own best decision.

Bill Sturm

An inverter will have lots of high frequency noise on its output. This rapid voltage change (dv/dt) puts additional stress on the insulation which can eventually cause premature insulation failure. Inverter duty motors have improved or increased insulation to handle this.

As mentioned by someone else, most motors these days are designed for inverter duty. If you have a question about your motor, then you should ask your motor manufacturer about the application. There is a very good chance that the motor is designed for inverter duty even if the catalog doesn't mention it.