VARIABLE FREQUENCY MOTOR CHARACTERS.

There are a number of different designs for motors that are intended to be used with inverters. Every manufacturer has their own specific way to approach this, so there is no EXACT specification ot consider. But in general, the issue involved are as follows:

1. The magnet wire insulation used in the stator windings must be capable of withstanding a higher peak voltage than was previously used in "standard" motors. This is because of a phenomenon specific to being used in PWM power circuits called "standing waves" and if you Google that term you can read more extensively on the subject. I the old designs, the motor insulation needed only be capable of withstanding peak voltages of 1.5X the line voltage, but in the case of PWM applications, the higher the better. Most inverter duty motors now use insulation rated for at least 2.5X line voltage.
2. Motor cooling is an issue. Motors that rely upon convection cooling from integral shaft mounted fans are not typically suitable for inverter operation, because as the motor slows, so does the fan. In addition, motor losses are highest at the lower speeds, so the combination of higher temperatures and less cooling is usually fatal to the motor. Inverter duty motors then usually have fans that are powered separately from the motor power (which requires an additional circuit) or are TENV designs, which end up with larger masses to dissipate heat by conduction.
3. Bearings and bearing races are an issue as well. Bearings must be capable of operation at the maximum seed you might possibly run the motor with an inverter, because now you have the ability to run it above its base speed. In addition, there is another phenomenon peculiar to inverter use wherein a static charge builds up in the rotor which attempts to flow to earth through the bearings, and in doing so, create an effect similar to EDM that etches and eventually destroys the bearings and/or races. So the best inverter duty motors will come with haft grounding bushings.

One good way to observe the quality of an inverter duty motor is to look at the "speed ratio" rating. This is a ratio of the minimum speed with relation to the base speed. A standard motor (non-inverter rated) may be OK for use at a 2:1 ratio, meaning it's OK to run t down to 50% base speed. A minimum rating I have seen on inverter duty motors is 10:1, where a 1500RPM motor can be run down to 150RPM. Good ones are at 600:1 and the best I have seen has a rating of 1000:1, so a 1500RPM motor can be run as slow as 1.5RPM continuously without trouble. The cost increases with the ratio, so you must decide on your cost/benefit based on your expected use.

Good luck.

Dear Mr. MSAMAD,

THANK YOU FOR YOUR REPLY AND I WILL GO THROUGH THE LINK

PROVIDED BY YOU AND ANY DOUBT i WILL COME BACK TO YOU.

DHAYANANDHAN.S,

INDIA

Dear Mr.JRaef,

I thank you very much for your reply. You have referred, in your reply,

PWM, TNEV Design, EDM, I request you to give full expansion for the

same. It is quite true that when speed of the motor falls the cooling

of the motor by fan where the fan is mounted on the shaft suffers

considerably.

Kindly mark a copy of your reply to this forum as well as

to my mail Id. sd610@rediffmail.com.

Thanks,

DHAYANANDHAN.S,

India.

These are industry standard abbreviations, Google will find them for you.

PWM = Pulse Width Modulation

TENV = Totally Enclosed, Non Ventilated

EDM = Electric Discharge Machining (similar to plasma cutting/welding)

When the motor has to run at low speeds like 1:10 ratio the shaft mounted fan no longer is able to supply the required cooling air.So normally force cooling is resorted to where a additionl blower driven by a motor is used which produces the cooling air required at all speeds.Of course one has to consider the reduction in heating as current reduces.Some of the machines also have insulated bearings .

It was a practice earlier to derate a motor by 10-15% and use.With advent of high speed PWM inverters the standard motor will fail quickly.The voltage spikes produced by this can result in premature failure .The other problem is the corona inception voltage level often is exceeded and can weaken the wire with normal coating.So specially coated wire with addition of some compounds to reist breakdown is used.Inverter motors are specially manufactured.