Selection of Motor Frame Size

Why you would be doing this is best answered by you.

There may be cases in which a lower rpm motor of the same hp would eliminate a reduction drive, but result in a larger frame. Or special motor features might also require this.

From reading some of your previous posts I thought you were a part of a design team. If you don't know why you're doing what ever you're doing, how are we to know?

Dear sir,

i am from engineering department(not a design) in cement plant.i have only 3 years of experience.still i am in the stage of learning the basic selection criteria for individual equipments.Kindly clarify the above post

regards,

N.Ganesan

On the general arrangement drawings for the installation will be the initials, if not the name, of the individual that carried out that design. Find that individual and ask the question. One of the reponses might be a signpost as to where to go next.

It certainly cannot be seen from here.

Generally a one frame larger motor is a method of providing a higher service factor motor usually a 1.15 service factor which allows the motor to operate above its normal nameplate rating. A normal method for obtaining a 1.15 SF is to get a normal frame size with a higher rated insulation. A typical motor will have Class B insulation. Class F insulation will provide a 1.15 service factor. Class H will provide 1.3 SF

In heavy industrial work it is advisable to use 1.15 SF motors. It doubles the life of the motor and takes care of unsepected overloads.

Going to a higher frame size for the 1.15 SF or 1.3 SF can create problems if you have not prepared for it. The larger frame size may have increased torque that is too high for the application and of course all of the dimensions are different. Check with the mechanical engineer and the supplier before going this way. The Class F or H insulation approach is much better.

"Generally a one frame larger motor is a method of providing a higher service factor motor usually a 1.15 service factor which allows the motor to operate above its normal nameplate rating. A normal method for obtaining a 1.15 SF is to get a normal frame size with a higher rated insulation. A typical motor will have Class B insulation. Class F insulation will provide a 1.15 service factor. Class H will provide 1.3 SF"

DennisL,

He is in India, they don't have Service Factor and he may not know what that means.

But to ganeshakce-electrical, the point made here is still most likely the issue. When someone PURPOSELY selects a motor size larger than what is apparently needed, it is usually because they are anticipating a need that may not be immediately evident to those that come along later. So in this case, the original designer may have experienced one or more of those kinds of needs in an event and said "Never again! I will henceforth always use a larger motor to avoid this scenario at all costs". Second guessing a decision like that is potentially a disaster waiting to happen; again. As was said by PWSlack, see if you can connect with the original designer. If not, you may want to avoid assuming there was a mistake made.

Good luck.

Dear Mr.JRaef,

greetings of the day.

Thanks for your valuable information.

i have seen that if we go for one size higher frame size the Motor cost & size is equivalent to next standrad KW in most of the cases.in some cases speed of the same KW motor is reduecd.( Pole & cost of the motor is inversally proportional).

in the above consideration.better we can for next KW motor for 1.15SF(cost of current rating of the winding will be increased.but else other equivalant to frame size).this is my doubt.kindly clarify.

You are a user, the designer is likely to know, as mentioned in the previous posts. But my guess would be the requirement of the higher starting power (especially if it is in pulverisers you are talking about)

In case it is not emptied (most of the cases it is not) before starting, the initial torque and power requirements would be quite higher than the steady state requirements.