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Oversize motors.....


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Oversize motor replacement!!!!!

We presently have 4 – 75 HP motors turning the contact wheels of an ACME centerless grinder. These are old Westinghouse motors with very little on the nameplates.

The motors are 575vac, 70FLA ….TFFC, 4.5t frame….1200rpm…that’s all on plate except Serial #…….

Measurements taken are …..595vac…..13amp no load……23 amp to run head with belt….and ……29amp (maximum) to grind bar (this will change depending upon size of bar and how much they want to take off)…these are measurement taken with clamp on meter.

I believe these motors are affecting or pulling down our power factor. Most of the other power used in the plant is used on rectifiers for platting and induction hardening.

These motor are started with a star / delta system.

They have been running this way for at least 15 years and due to the changes in the way Ontario charges for hydro, it must be better to change these out.


However I cannot find anything to help me sort out the payback to justify getting ride of these.

Or could someone lead me to the right spot to sort this out!!


Thanks Ron

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Hello ron_97


If you have a poor powerfactor, that can be caused by inductive loads such as motors, but it can also be caused by harmonics such as caused by rectifiers etc. If, as you say, a lot of your load is made up of rectifiers, then I would expect a poor power factor due to the high harmonic content and there is little that you can do about that except some very expensive filtering systems.


In regards to the motors, at no load, they are drawing 13 amps. We can assume that this is close to the magnetising current of the motor and so there will be a reactive current per motor of about 13 amps. The magnetising current is essentially independant of load so it is there all the time. This equates to about 13.4KVAR per motor or 53.6 KVAR in total.

In terms of a payback, this is difficult to predict unless one knows the full details of the tarrif being applied and also the details of the rest of the load. Rather than replacing the motors, you coud ad power factor correction. In this case I would recommend a correction of 10KVAR per motor or 40KVAR in total. This would reduce the loaded current of the motor from 29 Amp to 25.7 amps so would reduce the maximum KVA demand for that motor from 29.89KVA to 26.5KVA


Replacing these motors with smaller motors would reduce the starting current (no effect on the power bill) and would change very little else. if they spent a significant amount of time loaded. If they are predominantly unloaded, then you may get a reduction due to the reduction in iron loss, but that would assume that you use new motors with a lower iron loss. (not necessarily assumed to follow the size!!) You would still need to power factor correct to get any real advantage in power factor improvement.


Best regards,

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