Sewage Pump Motor Selection

[vodeni]

I have a simple question. If a BHP is at certain level say 100 HP; and motor efficiency is say 93%; I always assumed that the motor rating should take in account the motor efficiency: in this case 100/0.93 = 107.5 HP. If the motor is rated: say 105 HP (the numbers are just for an example) does that mean that the motor will be slightly overloaded? All these years I thought that it would be and you should have motor with the rating higher than its shaft power, but the pump manufacturer is trying to convince me that that is not the case. Am I wrong to insist that the motor be rated > 107.5 HP. We are talking about the design point, where pump would run most of the time. Thanks for your help

 

[SteveWag]

Motors are rated in shaft HP. Efficiency is a measure of how much electricity is used to produce the rated shaft horsepower. Many motors also have a Service Factor, and can be operated in their Service Factor, slightly above rated power. In your example, a 100 HP motor is what is needed – UNLESS – there is a possibility that your calculated operating point moves. A move to the right will require some more HP. Make sure conditions are such that this won’t happen. I often specify that the motor be sized so as to be “non-overloading”.
Steve

 

[bimr]

What you should do is look at the pump curve. Use the HP rating from the pump curve. You do not apply the pump efficiency to the HP shown on the pump curve. The pump curves are based on actual pumps operating with actual motors so that the motor efficiency is already accounted for.

A centrifugal pump always operates on the pump curve. As the pump moves to the right on the curve (higher capacity@ lower head), the required HP increases with pumped flow.

As the curve moves to the right, notice that the HP increases. That is what is meant by overloading the motor. You must install the HP on the right side of the pump curve if your pump is capable of operating there.

The HP needs to be such that you do not overloading the motor.

 

[vodeni]

Thank you all; I understand concept an so on. My question was whether the manufacturer motor rating reflects shaft or input power. Steve clearly answered that it is shaft power. So basically we can have situation that 100 HP motor running at 100% load draws 107.5 HP (equivalent) and that means that the motor is rated 100 HP not 107 HP. That is all I wanted to clarify.

 

[bimr]

By definition, a motor of a given rated horsepower is expected to deliver that quantity of power in a mechanical
form at the motor shaft.

Motor losses are the difference between the input and output power. Once the motor efficiency has been determined
and the input power is known, you can calculate output power.

This brochure should help you:

http://www.shoemakerindustrial.com/Determining Load.pdf

 

[vodeni]

Thank you all again.