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

 

[TenPenny]

I think you're mixing several different concepts.

Motor efficieny refers to the amount of electricity you need to supply in order to get the particular output from the motor. So, in your example, a 100 hp motor with 93% efficiency will draw 107.5 hp worth of electricity when running at full load (100 hp). This is reflected in the FLA (full load amp) rating of the motor nameplate.

On top of that, motors will have a service factor (SF) of 1.0, or 1.1, or 1.15 for example, which means that your motor, for example, could safely be run at 100, or 110, or 115 hp without overheating.

Finally, to get to your question: If your pump has an operating point of 107.5 hp, you could conceivably use a 100 hp motor that had a SF of 1.1 or 1.15, and still be within the acceptable operating range of the motor; in my experience, the plant's electrical department will be the ones to decide if this is accepatble to them or not. Most industrial process plants electrical depts that I deal with will not allow this.

Now the additional piece of info: many users of centrifugal pumps will specify that the motors must be non-overloading at runout; if your pump requires 107.5 hp at its operating point, it might require 121 hp at runout, in which case most of my customers would insist on a 125 hp motor.

 

[electricpete]

The manufacturer is right. According to NEMA standards, the motor nameplate rating in horsepower is based on the maximum motor OUTPUT power the motor can continuously deliver. The electrical input power (converted to horsepower) would be slightly higher (~5% for 95% efficient motor) than the nameplate horsepower when operating at "rated conditions". i.e. rated horsepower applies to motor OUTPUT.

SF is something else to consider, as mentioned.

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[electricpete]

It may be a little superfluous, but an easy way to remember this in the US is that fact that motors are rated in horsepower vs kilowatt. Ignoring for the moment the existence of conversion factors, we normally associate horsepower with mechanical power (electrical power would be watts). So the horsepower rating is the output. This logic goes out the window for IEC motors used overseas. They still rate the motors based on output, but they use SI units, so there is no horsepower mentioned in the rating, only kw.

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