True or false?

On a normal motor starter, i use a pushbutton, magnetic contactor & an OLR..
If I will install a VFD to control the motor speed, can I not use the contactor & OLR since the VFD has this on internal?
Im referring to Mitsubishi VFDs..

Yes you can replace the start stop switches, overloads and the contactor with a VFD.

The VFD has start stop overcurrent (or overtorque) built in BUT you must set the parameters controlling them.
Dan Bentler

True or false..

I can increase the speed of my motor beyond the supplied frequency (i.e. 60 Hz) on our area.

theripley said:

True or false..

I can increase the speed of my motor beyond the supplied frequency (i.e. 60 Hz) on our area.

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This depends upon the motor. Some motors are rated "inverter duty", usually meaning that it has heavier windings and bearings that will withstand a certain amount of abuse. I always try to use the 10% rule, meaning that I do not like to go more than 66 hz. on a normal, non-inverter rated motor. Also remember that running a motor too slow can cause problems too, as the fan on the back doesn't turn fast enough to sufficiently cool the motor down.

shoelesscraig said:

Some motors are rated "inverter duty", usually meaning that it has heavier windings and bearings that will withstand a certain amount of abuse. I always try to use the 10% rule, meaning that I do not like to go more than 66 hz. on a normal, non-inverter rated motor.

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Am i correct because I think not all motor can be connected to a VFD. There are motors intended for this application like the "inverter duty" you mentioned above. In this case, I am referring to an "inverter duty" motor. I am curious how did you connect a non-inverter rated motor with a VFD.

Also, are you saying it is possible to speed up an "inverter duty" motor beyond the supplied frequency on our area? Is this possible?

theripley said:

Am i correct because I think not all motor can be connected to a VFD. There are motors intended for this application like the "inverter duty" you mentioned above. In this case, I am referring to an "inverter duty" motor. I am curious how did you connect a non-inverter rated motor with a VFD.

Also, are you saying it is possible to speed up an "inverter duty" motor beyond the supplied frequency on our area? Is this possible?

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Yes, most VFD's will allow you to run a motor MUCH more than the original frequency, but that doesn't mean its a good idea. Here in the US, our line frequency is 60 hertz. The VFDs I use at work will allow me to turn the frequency up to 200 hertz or more. Of course, this will almost certainly destroy the motor.

And there is nothing different about the wiring on an inverter rated 3-phase motor versus a regular 3-phase motor. You can definitely hook a VFD to a motor that is not rated "inverter duty". I have a plant full of hundreds of motors that are almost all connected to VFDs. Maybe a half-dozen of them are "inverter rated". Been running like that for years and years.

Inverter rated motors are designed to cope with the higher voltages involved.

You can run the motor up to its rated mechanical speed (normally much higher than base speed) but you must remember that the motor will produce less torque at speeds greater than base speed.

It was stated in an earlier post that you can do away with the contactor. This may not be the case depending on the safety requirements/regulations; in some cases you may need two contactors if stopping the motor is safety critical. If there are no safety requirments then you can do without a contactor.

Nick

shoelesscraig said:

And there is nothing different about the wiring on an inverter rated 3-phase motor versus a regular 3-phase motor. "inverter rated".

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I am currently looking at a motor manufacturer catalog right now. There were listed different types of motors depending on the application, & there were motors for inverters. I have asked one of their TS about connecting an ordinary three-phase motor to an inverter, the TS said frequency variation cannot be made to an ordinary motor. I think I need a theory in here.

VFD drives first rectify the AC supply to end up with nominally around a 600vdc. Transistors are used to switch this and produce a waveform, I found a usefull introduction here:

http://datadog.com/pwm_tutorial.pdf

The waveforms are made up out of "lumps" of DC so whilst the rms voltage may not exceed 400V the instantaneous voltage will be at the level of the DC bus (nominally 600V). The other aspect that motor/circuit design must take into considderation is the rise time of the voltage, from 0 to 600 in a very short space of time; this is where poor earthing/screening of cables an motors causes radiated electromagnetic noise and induced currents in other conductors close by.

I once conducted an experiment where I inserted an amp meter in series with screen connection of a motor cable - it recorded 14mA of current just with the drive enabled and not even turning. Disconnected from earth the voltage on the screen rose to several hundred volts.

Nick

DickDV tells it like it is, in this thread.
http://www.plctalk.net/qanda/showthread.php?t=2336&highlight=snake