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urjaengrs (Electrical) (OP)
31 May 07 5:16
Can anybody guide me regarding rating of VFD? how it is co-related to motor kW rating? Is it nescessary to de-rate VFD i.e. if motor kW is 90 kW, then VFD rating should be 110 kW. is it so?
Also, what other factors to be considered for determination of VFD rating as well as motor kW rating.

Please guide.
DickDV (Electrical)
31 May 07 8:07
Using kw or hp to size drives is a high-risk proposition.  Much better to size the drive, first, by continuous running amps, and then, second, by short-term peak overload amps.

This requires that you understand the continuous and peak torque requirements of the load, then the corresponding continuous and peak currents the motor will require to produce those torques, and finally, provide a drive that can supply those currents.

Only in the simplest of fan and centrifugal pump applications will kw or hp be reliable as a sizing technique.
CJCPE (Electrical)
31 May 07 8:09
VFD kW ratings are only used for estimating purposes. The VFD output current rating must be selected to be equal or greater than the full load current rating of the motor. De-rating should not be necessary unless the VFD is installed in a location that exceeds the rated maximum altitude or ambient temperature. Of course, the VFD output voltage and frequency must also be appropriate for the intended motor operation.
sunrays (Electrical)
31 May 07 8:14
The following would be the selection criteria for a Drive:

I would not suggest you to look at the kW Rating of the drive and motor but select the drive based on the FLC (Full Load Current) of the motor. The Drive continuous output current should be greater than the FLC of the motor.

Drives are generally rated for a specific current at 40 or 45 Deg. Ambient (depends on manufacturer), if your ambient is higher you need to derate the drive accordingly.  The derating factor would vary between manufacturers.

Drives these days come in dual ratings ie Normal Duty & Heavy Duty. Normal Duty is 110% overload for 60 secs., good enough for Fans & Pumps ie Variable Torque applications and Heavy Duty is 150% overload for 60 secs. basically Constant Torque applications like Hoists, Extruders etc. You need to identify your load and select the drive Duty accordingly.

You could be in trouble if you select a Normal Duty drive when the application demands a Heavy Duty Drive.
rovineye (Electrical)
1 Jun 07 14:23
Don't ignore the motor characteristics either.  A TEFC motor running at lower speed means that the cooling fan is running slower as well. Depending on the load (constant vs variable torque), operating speeds, and environment (ambient temperature) it may not like that and you may need an inverter duty motor.

 If you use a PWM there may be cable issues due to length of cable to the motor and "standing wave condition" (high peak voltages). That can damage motor windings. That is applicable to lenghts of 250 feet or more from what I have read.

 If the drive is being added an existing motor, and that motor's insulation is marginal, expect it to fail.

 
controlsdude (Electrical)
6 Jun 07 11:14
I agree
rating on hp alone is misleading
continuous current of the motor is probably the best way.
Then the manufacturers are a bit deceiving rating the drive in HP but giving a continuous current that would be almost the fla of that horsepower motor.
example
I think AB does this a lot.  So look at the vfd manufacturers spec carefully.
Mitsubishi drives go off of KW but does give you a continous load current rating.  But if you notice that the HP rating is higher for that drive.  Compare the ab and mitsu side by side and you will see a difference.
DickDV (Electrical)
7 Jun 07 7:41
Sizing a drive by continuous current only will often result in inadequate short term overload current capacity.

After sizing by continuous current, you must then review the short term current issues to be sure they are covered.

That's why there are heavy duty and normal duty rated drives.  Same continuous current rating but the first has 50% overload and the second has only 10% overload.

And, as I mentioned before, it is the load characteristics that determine which rating is needed.

In extreme cases, I've sized drives three hp classes larger than the motor just to get all the overload capacity the motor has to offer (about 220% for a NEMA B motor).

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