List of standard NEMA motors

[roydm]

I have a few questions regarding motor selection for pump applications.
1/ For a North American application sshould I be specifying in HP or kW?
2/ Could someone point me to a list of standard NEMA motor HP/(kW) ratings in the range 1 - 400 HP
3/ I have one application for a motor where I want to run a centrifual pump on a VFD at a reduced speed. Do I have to de-rate the motor (ie install a larger horsepower motor) because it needs to deliver the calculated horsepower at a slower speed and a given horsepower at lower speed requires a larger torque.
Note: The mechanical engineer calculated the HP required at the lower speed.
Thanks in advance
Roy

 

[electricpete]

1 - NEMA motor ratings are specified in horsepower.

2 - Standard ratings are given in NEMA MG-1. You can get a free condensed version at the NEMA website. You can also see the standard ratings on page14 of 25 here:

http://www.joliet-equipment.com/pdf/techinfo/EASAHandbook.pdf

3 - I think generally you would derate by the factor below rated speed since voltage is reduced by the same factor to keep constant volts/hz. Very low speed would require additional derating due to cooling reduction which occurs at low speed. Maybe others can comment.




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[roydm]

Electripete,
Thanks for the quick response and the link.
The pump will be running 24/7 at a fixed speed somewhere around 3,000 RPM. Am I correct in thinking that if we require 200 HP at 3,000 then I need a motor 200 * 3600/3000
Roy

 

[TurbineGen]

Roydm, I would recommend a 250 or 300 hp motor for your application. If you think you might run the motor above 3200 rpm, I'd recommend the 300 hp. If I remember correctly, the power is porportional to the square of the rpm in a pumping apllication.

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If it is broken, fix it. If it isn't broken, I'll soon fix that.

 

[jraef]

If your ME has truly calculated the HP at 3000 RPM, then your formula Is correct. In A VFD driven motor, HP will decrease with speed, so if you NEED 200HP at 3000RPM and you are going to start with a 3450 RPM motor (remember, HP is at RATED slip speed, not synchronous speed), then 3450/3000 = 1.15 x 200HP = 230HP, so the next NEMA size will be 250HP. The power varying by the cube of the speed is something your ME would have had to do in order to properly determine the HP requirement at 3000 RPM. Assuming he knew that, then all YOU need to do s respond to his stated need.

Technically, you might be able to get away with a 200HP motor that has a 1.15 Service Factor, but only if you don't care about the long term life of the motor.


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[EEBubba]

My rule of thumb when dealing with Variable Speed applications is to not talk about horsepower. Rather I always try to keep it in terms of torque.

The old standard Equation of 5252.1*HP / RPM = Torque as a base then HP = Torque * RPM /5252.1

5252.1*200 / 3000 = 350 ft-lbs of torque.

Many motor suppliers sell a typical Premium efficiency design motor with Constant Torque Ranges on the nameplate. Typical ranges are 10-60 Hz, 6-60 Hz, 3-60Hz etc. As you are looking to run this motor at a 50 Hz equivalent any of the above would be proper. As the motor is capable of handling the load as a constant torque, you simply size the motor around the same torque number.

The HP motor you want to request would be in the 350*3600/5252.1 = 240HP Range, with a NEMA std break point of 250HP. Consult with the motor suppliers applications engineering and they will confirm the number or perhaps recommend one horsepower higher.

Regards,

 

[LuckyHobo]

Roydm

This question stems from your April 12, 2008 inquiry about electrical immersion heaters having a NEMA 4 terminal box that the manufacturer claims to be suitable for a Class 1 Div. 2.

I am faced with the same question. What did you end up doing?

 

[roydm]

LuckyHobo,
The NEMA 4 box was accepted without a purge.
Roy

 

[LuckyHobo]

Roy,

Who was the manufacturer of the 120KW electrical immersion heater? Perhaps they can help me with my delima.