Asynchronous Motor with Slip Ring Rotor

The first question is to ensure with the supplier of those motors if their quoted motor is suitable to the existing application and how it would get along with that.

Gurus here will explain you some useful tips. But in order to have that, the information provided by the OP should be very clear.

If you want to increase speed from 992___ to 995___. Please consider what you will achieve if you have 3__ increased by the existing one. If it does nothing to help, why we need to change?

As hien-nguyenquoc said we need more information.

992 as against 995 RPM, but at what loading for each speed?
The rotor controller would be my main concern, what type is it?

Don't bother! it will work fine! There wouldn't be proportionate gain vis-a-vis the investments you are going to make for increasing the speed by just 3 RPM. So, don't waste money.

Assuming I understood the kind of motor you are havinG:

- Asynchronous with a wound rotor.

- The rotor has slip rings connected to a set of 3 starting resistors (3 phase motor).

- You are using a suuply of 50 Hz and the motor has 3 pairs of poles (6 poles) meaning that its synchronous speed is 1000 rpm

- If the above is what you are talking about, then you have no problems from the running speed difference. This speed is indicated as the full load droop speed.

- Also, as long as the input voltages and frequency are the same, the the rotor values are to insure that the contactors used are adequate for the new motor.

NO PROBLEM AT ALL -Go ahead Buy,Install,Run it as you did with the old one.MM

Why do you want to go for the same old type of non standard motor? You can use a standard squirrel cage motor with a VFD to obtain similar features. Of course, the cost aspect must be looked into if it is that critical.