Converting a 4 Pole Motor to 8 Pole

It would be easier to either:

• fit a gearbox, or
• drive it from a variable speed drive.

It is the load that determines the torque and the motor power, not the other way round. So, without knowing the nature of the speed/torque characteristic curve of the load the motor is driving, no-one can say.

Buy a new motor.

contact a motor rewinder

You need to check with a qualified rewinder to see if the number of slots existing on the stator will allow an 8 poles configuration.

For a 3 phase, 4 poles, the minimum number of slots is 3x4x2=24

Depending on the motor kWand other considerations by the maker, it can have more poles to accomodate the necessary coils (gauge of wire and number of turns...).

For an 8 poles motor, the minimum could be 48 slots.

In any case, the kW rating of the motor will be reduced due to the resizing of the wire gauge and the number of turns for each coil. Do not forget that since the rpm is reduced, you need to increase the torque to achievd the same kW output, and to do this, the current capacity of the winding will need to increase, hence the wire cross section and the cooling capacity... none of each could be accomodated on the same Frame. If you check any manufacturer's table, you will see that the Frame size of the motor will change, for the same kW output, with the selected number of poles (or rpm).

I got rewinded my 4 pole motor converted to 6 pole with 980 rpm.

Motor winders suggested that the motor kw is reduced to 10 kw now from original 15 kw.

But my purpose is solved now. My 15 kw 1470 rpm motor was drawing 22 A on load and now this rewinded motor 780 rpm is drawing only 13 A.

Previously I was using the motor with inlet damper of fan in partially closed condition, now it is fully open.

Comments invited on advantage and disadvantage on it

You are saying that this motor is driving a Fan. This is what we wanted to know from start. If I assume that this is a CENTRIFUGAL FAN, then we can have the following:

In this case, reducing the rpm from a Nominal 1500 (4 poles) to a nominal 1000 rpm (6 poles) will reduce the required power from the fan: P1/P2 = (N1/N2)^3 for the same conditions of flow and back pressure. the Qty of air pushed through will be less (Same damper setting).

In your case, (1500/1000)^3 = 3.375, and 15 kW/3.375 = 4 kW will now be needed.

Therefore, for the same restrictions settings as previously met (Almost closed dampers), your newly modified motor-fan would draw ~ 4 kW only! But also, less air volume.

You were drawing 22A which comes to full load of the previous 4 poles motor because, at the dampers settings, it was drawing that much power and blowing what you needed (I presume!).

Now, with the reduced speed, if you maintained the same restrictions, you would draw less air in, but you opened the dampers to full, making the motor to draw more power than the 4 kW. Since the rewinder says you have a 10 kW motor now, and the current is 13A, you are drawing 415 x 13 x 1.73 x 0.8 = 7,467 W (~ 7.5 kW).

If Your Process requirement is now met by this setting of fully opened dampers, and running OK, then you are better off than before: Less power for less air. Which meant that the previous 4 poles motor was over sized for the FAN-Process at your site (You could not close the dampers any more ...).

The 980 rpm is equivqlent to the 1460 rpm of the 4 poles configuration, which is very good: Good rewinder Guy!

I measured the kw of rewinded 980 rpm motor with energy analyser - 415 V , 14 A, 4.6 kw, 0.46 PF.

I think my previous motor (1470 rpm) was drawing 11.5 kw with 0.7 PF and 22 A

Congratulation to all participants specially LAA Lucke, for achieving this much power saving and making our earth safe.