Need tips on motor upgrade

[McCormick93]

Hello,
I'm a mechanical engineer working on a chilled water pump upgrade. Is there an expert willing to offer advice on the following? I have some old (1980 vintage) pumps that have 460/230V 50 hp motors running at full load, around 62 A. I would like to swap them out for slightly larger pumps which would require 60 hp motors to get the required flow. (I would load them to about 56 hp). According to my research so far, if I use premium efficiency motors (94%), the current should only be around 63 A.

So can I just install the new equipment without changing out the starters? The wire size to the motors is AWG 4 with 80A fuses. I don't know the rating on the overloads, they are obviously rated for the old 50 hp motors.

I have read that starting current on high efficiency motors is much higher than on old motors. Will I need a new, larger instantaneous overcurrent device? Some of the older starters don't appear to have an instantaneous device.

Thanks for any replies.

 

[jraef]

It is not the "starting current" that is different, it is the "instataneous inrush current" that is greater. Most people don't realize there is a difference, and the term "inrush current" is often misused to describe starting current. Instantaneous inrush current is the magnetization current of the windings, and is only present for 1-2 cycles. In the case of many energy efficient motor designs, the Inrush Current can be as high as 2000% (varies from mfgr to mfgr). This is the problem with retrofitting, because under the old NEC rules a circuit breaker could only have up to a 1300% instantaneous trip setting. This will likely give you nuisance trips on an energy efficient motor. Latest NEC rules take this into account and now allow up to 2000% IF the motor requires it. Bottom line, you may have needed to change a breaker, but fuses depend upon the fuse curve. Assuming you are using a time-delay motor starting fuse, find out from the fuse curve if the delay is long enough to withstand the higher inrush without blowing.

As to your starter, only you can tell. Read the max. FLA on the starter label. THAT is the only determining factor. HP labels are essentially a guide.



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