Ordinary Motor on VFD

Let me get this straight , you can't determine the cause of the VFD tripping, so you want to substitute another motor to see if it's the VFD, or the motor that's failing...Is this the case?

http://www.industrialknowledgezone.com/5-rules-for-motor-vfd-compatibility/

Solar Eagle link advice is good, and you want to be conservative with a motor that expensive.

Your troubleshooting efforts are incomplete. It makes much more sense to find the cause of the problem than to just give it up to superstition. The ACS800 is a mature technology, so either the drive is defective or the motor/load has a problem.

You first need to tell your remote audience just what fault the drive displayed when it tripped. If no fault displayed, then make your RUN command as a single pair of wires, so no external problem is likely. If the simple RUN command version fails, with no fault indication, it is likely the drive itself has a problem.

The character of the load is essential to correct VFD operation. The ABB technical support may not understand the nuances of your unidentified driven machine, which then could make the VFD fall short of replacing a DOL starter.

Technically we should not use ordinary motor with VFD combination for the following reasons.

1. Ordinary motor runs at constant speed with small to blow air to cool the motor. When the speed is reduced by VFD the fan speed also will come down and hence air flow will be reduced leading to rise in winding temperature and lead to premature failure.

2. The ordinary motor uses un-insulated bearings. The motor for VFD application should be fitted with insulated bearing on one side or provide bearing insulation to avoid the shaft current circulation.

3. Normally the ordinary motor speed will be (synchronus speed – slip) and slip may be 4 %, where as the VFD motor speed may require to run it above the synchronous speed, and vibration severity will increase to the square of the speed. If you consider your speed required is 1.2 times than synchronous speed, the vibratin amplitude wi,, be (1.20) ^2 = 1.44 times and the rotor and bearings may not with stand.

4. The earthing may not be dedicated one for ordinary motor an earthing flat will be alf of the current capacity where as for VFD application, it should have dedicated earthing, and earthing should be 2.5 times more in cross section if G.I flats are used.

This topic is discussed in this forum 4 or 5 times. Pl. search in this forum and you will find many members have shared their experience and knowledge.

DHAYANANDHAN.S

I agree with most of the comments. First and foremost the motor needs to be inverter duty rated which all newer motors are. Rated and insulated to run 90% of rated speed. The drive will have a fault or trip code stored unless it is an external issue with the start or stop command. If there are no codes for the stop of the motor try running it with the drive itself, local. This will tell you what and where the issue is.

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CR4 is not a permission-giving entity.

One must be aware that the thermal losses from a motor that is driven from a VFD, as opposed to DOL, are slightly higher. This affects the cooling performance, especially when the speed drops below 100%. Provided the motor is protected internally thermally, as many larger motors are, then this characteristic will protect the motor going into temperatures it is not designed for. However, if correctly protected it may trip on high temperature.

The original posting is incomplete: <...drive was tripping after...some hours...> - on what parameter does it trip, please? Numbers?

Thanks for the update!

So, the drive was right, then, and the protection was doing its job. Great stuff.

To both questions; VFDs are good “fault finders” in that because of the nature of how they work, by pumping DC voltage pulses at very high frequencies and possibly creating capacitive effects that increase those voltages, any small seemingly insignificant flaw or nascent failure in the motor windings will become evident in a much more dramatic way shortly after being run from a VFD. Often times if you use standard measuring tools, such as 500VDC meggers, they will not see those flaws yet. I always recommend using 1000VDC meggers on 400 V class motors that are going to be run from VFDs.

And if you just used a DMM to measure the resistance, that is all but useless. All it tells you is that the motor insulation is not open circuit, but is just sufficient to handle a 9VDC battery...

Since there was no issue found with either motor or cable or drive, we went with varnishing of motor after winding inspection.

The motor is running healthy since last 3-4 months! But it still remains a big misery as what really was the problem that we could not pin point & resolve.

Thank you all for spending your valuable time and being ready to help.

The old motor( not an inverter duty motor) can be run on variable frequency drive. The following measures are required to run an ordinary motor on VFD.

1. If the motor is used to deliver less torque than its rated torque, the ordinary motor can be used. With VFD, the losses in the motor increases and necessary extra cooling is required.

2. The distance between the VFD and motor should be as minimum as possible to avoid higher voltage at the first turns of the motor caused due to standing waves of PWM inverter.

3. For the constant torque application throughout the speed range, the cooling of the motor at lower speed is required.

4. The shaft needs to be grounded through a carbon brush to prevent circulation of bearing current else the bearing may get damaged.

Related Article:Why V/f ratio is kept constant in VFD?

The simple answer is yes you can run a non vfd rated motor on a vfd.

I would need to know some specifics before I could begin to diagnose your issue.

What is the fault?

what is the application?

Was the drive commissioned by someone profficient with drives?

I'm guessing that this is a 175 Hp and would need to know your application to determine whether or not a VFD or across the line starter would be most beneficial.

Unfortunately you haven't provided enough information to properly offer a suggestion on anything other than if you can use an ordinary non-vfd rated motor.