Ordering Spare Motor to be compatible with existing relay settings 3

[electricpete]

For purchase of a motor for a new installation the manufacturer would typically provide motor performance data and thermal damage curve and the user selects relay setpoint curve to fit between the motor starting curve (current vs time) and the motor damage curve.

I am in the process of buying a replacement (spare) for a group of 3500hp 324rpm vertical induction motors. We would prefer that there be no relay setpoint changes necessary in order to install the spare motor since we plan on swapping motors periodically to accomplish maintenance (stator cleaning) and we don't want to have to adjust relays every time.

I'm considering two approaches:
#1 - Simple but restrictive approach: Tell the manufacturer (bidder) to recreate or exceed the existing motor torque-speed curve and thermal damage curve, and require that FLA and LRA be less than or equal to existing:
#2 - Motre complicated but less restrictive approach - Give the manufacturer/bidder our existing relay settings and ask him to give us a motor whose starting curve will lie below the existing curve by specified margin and whose thermal damage curve will lie above our settings.

Has anyone ever done this? Any comments? Any convenient way to express the required margin for option #2?

 

[teco]

electricpete:- you dont state how old the original motor is however if you have the original thermal withstand curves , current speed and speed torque curves then option#1 would be the best answer.
we are sure if you supply these curves to a large motor manufacturer they could build a machine to these specs.
we would however suggest you talk to a manufacturer and outline your concerns and your requirements.
kind regds.

 

[gordonl]

I would be worried about the additional cost of the motor, it may be worth while to put in an electronic relay and download new settings than to pay extra for the motor. Some relays can even store two sets of settings which can be changed back and forth at the flip of a switch.

Just a thought,
Gord

 

[motorman]

Motors of this size are going to be “custom” designed anyway. The hard part is getting a clear definition of what motor performances must be duplicated. This is where things can get costly because some manufacture’s traditional design rules using their stocked materials and processes along with years of testing which has verified their calculations, will optimize on maybe efficiency at the expense of maybe power factor while other companies optimize on different performances. Expect a lot of questions during the quoting stage. For example the spare design has more inrush; is that a problem? Or, we can’t handle the 80 RVS, do you really need that during the entire accelerating period? Or worse, you get the spare motor in service and then something like the bearing temperature starts alarming where it never did before on the other machine. Then here comes those late night phone calls. This is why, no matter what, duplicates are expensive. They are so risky to design; require so much engineering and there is no payback for all the up front work because the manufacture will never be asked to build this precise machine again. Another issue is you might want the benefit of being able to cannibalize the spare motor for emergency parts which helps justify the cost of going only with the OEM.

However, in your case, the requirement being limited only to nameplate plus the desire to not change relay settings, then option 1 is what most designers are used to dealing with.

These are big slow speed machines. I am guessing they are vertical motors driving pumps. With closed valve starting, the load is probably less than the NEMA minimum for this rating thus meeting relay settings may not be a big deal cost wise. Meeting the existing mounting dimensions could now be the most cost sensitive part.