motors suitable for VFD

Standard motors can be fed from VSD (variable Speed Drives). Just an internal thermal protection is needed for the motor (thermistors for example).

For a standard motors, the rated speed should be reached when applying the nominal voltage under full load operation. All are given on the name plate of such a motor.

Using a VSD/VFD you can drive the motor at a speed greater than nominal (even greater than synchronous speed for induction motors), and then you can't load the motor with its nominal mechanical hp (i.e. power de-rating is considered), such a case is called constant power mode of operation.

Double rated speed can be reached via VSD (I guess) with no problem.

Regards... SAMAK

With prsent day VFD's which have an output close to sine wave Standard motors can be used. Also standard motors are now made with Class F insulation and are vaccuam Dried.As regards speed variation pl note in constant torque mode ie below synchronous speed for induction motor it is in ratio of 1:10 that is if 1500 is the syn RPM theoretically you can go down to 150RPm but stability and other considerations may not allow this.Also u need to know how long motor will run in low speed as in fan cooled motors the cooling reduces drastically and external cooling may be required. Such special motors are available in market.For low speed such as 1/10 syn speed another alternative is to use a geared motor with VFD.As regards higher speed above syn .speed which is the constant power mode one can up to twice the sped .But mechanical aspects will be the limiting factor and needs to be checked with motor manufacturer particularly bearings.Normally motor will be derated by about 10to 15% for heating due to harmonics while selecting for use with VFD.

you can use your motor up to 10 to 20 % of motor rated rpm but may reduce your troque as speed down hence first insure your application and then reduce your rpm till you can make your exisiiting work.

it is just regular practice.

For Scalar Control types of VSDs (VFDs) you may face a reduction in the generated torque at very low speed. Such a problem had been compensated via advanced types of VSDs of which based on Vector Control (or Field Oriented Control) or based on Direct Torque Control strategies. Now a day, you can drive an induction motor at very low speed (typically 3 - 10 rpm) while the motor nominal torque is utilized, then a ventilation problem comes up, so you have to use an external cooling fan.

Regards... Samak

This is a pretty good article which addresses your questions.

http://www.mt-online.com/articles/0202_vfd.cfm

Simply applying a VPI to an existing motor does not make it an inverter rated motor. Specially insulated wire must be used to protect it from the voltage spikes caused by gate switching. These voltages can be 4 to 5 times the nameplate voltage. The motor must be rewound with inverter duty wire to make it inverter rated. That is to say that any motor can operate on the VFD just at reduced life expectancy. Devices like line filters can help soften the blow of these voltage spikes.

The added heat due to voltage spikes in addition to operations at reduced rpm cause TEFC motors to run hotter due to reduces air flow.

Another problem with running motors on VFD's is shaft currents induced by the high frequency shifting. This is especially prevalent in larger motors. (>100hp) Insulation of the opposite drive end bearing usually takes this loop circuit out of the equation. Some motors will require both bearings to be insulated and the shaft grounded with a brush inside the bearing housing.

Hope this helps.

Regards,
Drew

May I ask you more details concerning the shaft current, its causes.

Let's post a discussion thread for this topic.

Regards... Samak

Here are a few links of interesting reading.

http://www.motionsystemdesign.com/Issue/Article/47423/Diffusing_current_trouble.aspx

http://www.plantservices.com/articles/2007/022.html?page=1

Regards,

Drew

Drew,

Your two posts discussed a topic I previously hadn't considered. Before reading them I was prepared to discount your contribution. Thanks.

VFD applications I have worked on always included a load reactor between the drive and the motor. Mostly we were working with existing motors in pumping applications. Since the reactors are significantly reducing the "choppiness" of the synthesized sine wave from the VFD, I would expect them to also significantly reduce the problems with currents and voltages imposed onto the shaft. This is something Mr. Yung's two papers you referenced did not discuss.

I would also suggest the addition of line reactors (they're the same type of component as a load reactor, just a different location) for all VFD installations. The reason is to reduce the percentage of odd harmonics imposed on the incoming power line. (One time, we had access to a good Dranetz unit and measured over 50% harmonics without the reactor.)

These would form a good research project topic for an upper division or even graduate EE student--looking at harmonics, waveforms, voltage spikes, and shaft currents for VFD applications with or without line/load reactors while using different PWM carrier frequencies. Anyone out there interested?

--John M.