BHP reduction due to increase in poles
BHP reduction due to increase in poles
(OP)
When a four pole 0.85 power factor 355kW motor is rewound to a six pole motor, the power output kW reduces approximately by two thirds with a lower power factor while maintaining the same Torque.
Any comments please.
Any comments please.





RE: BHP reduction due to increase in poles
Although I should say I am more comfortable saying that about a new motors than a rebuilt one.
RE: BHP reduction due to increase in poles
I'm not sure why you have a two-thirds reduction in power; that seems excessive. Did you check the efficiency by looking at the input current? The lower PF may be typical for this type of rewind, or maybe you have a difference in air gap causing the greater magnetizing current.
RE: BHP reduction due to increase in poles
Constant Torque: where as you might expect, the torque is constant.
Variable Torque: where the torque is reduces by the square of the change of speed. i.e. 4/8 Pole, the torque of the 8 pole winding is 25% of the 4 Pole winding.
Constant HP: where the torque increases inversly with speed. i.e. 4/8 pole, the torque of the 8 pole winding is 200% of the 4 pole winding.
However, if you're just rewinding a motor, you are usually limited by the magnetic densities of the rotor in going to a lower speed, and the stator if going to a higher speed. For 2,4,6 pole motors you almost always have to derate torque - sometimes severely - otherwise the motor will overheat.
I'm surprised you were able to hold torque. Probably an older motor.
And as you mentioned the performance is not very good.
RE: BHP reduction due to increase in poles
I would like to understand that comment a little more.
RE: BHP reduction due to increase in poles
Also the comment about lower base speed motors being fatter is true, which is why I made the comment about not being sure how more poles in a rebuild would ultimately affect the torque.
RE: BHP reduction due to increase in poles
The density of the magnetic field in the core =
1.745 x D x Bg
_____________
P x hc
where: D = rotor dia
Bg = air gap density
hc = the radial depth of the core
P = poles
since D and hc are fixed by the design and Bg is fixed by the winding;
increasing or decreasing P has a large effect on the core density.
i.e. going from 4 to 6 poles increases the Core density by 6/4 = 150%. If this density saturates it must be compensated by reducing the Bg.
And since Bg is proportional to torque -- reducing torque.
On very old motors the design density is very low and you can do pretty much what what you want, however at the expense of performance.
RE: BHP reduction due to increase in poles
http://www.pacificliquid.com/motorintro.pdf
where is stated that the motor with more poles/coils will make motor starting much easier especially under load.
RE: BHP reduction due to increase in poles
In the design stage, the motor engineer changes the rotor diameter rather than the Bg - which is the reason for different rotor diameters on the same stator diameter.
i.e. for 18.5 inch stator dia
Pole Rotor Dia
2 9.5
4 11
6 12
8,10 12.5
12,14 13
In real life, there is usually insufficient volume to justify capital expense for different rotors for all low speed motors - then all low speed designs are put on 6 Pole rotors - which degrades performance - i.e. low power factor.
In very large motors the high overall cost can justify designing a unique rotor, however.
RE: BHP reduction due to increase in poles
1.Change in the number of poles does not affect the magnetising current.
2.The flus per pole is inversely proportional to the number of poles.
3.The short circuit current varies inversely as the number of poles.
4.Hence conclude that the circle diagram varies inversely as the number of poles. (two thirds)
5.Hense the maximum power input varies likewise.
6.Decrease in short circuit current increases the dispersion co-efficient. Hence decrease in power factor.
7.Power output is given by
Power = T * omega
T1 = 355 / (2*pi*1500/60)
T2 = 235 / (2*pi*1000/60)
T1 is approximately equal to T2.
RE: BHP reduction due to increase in poles
RE: BHP reduction due to increase in poles
Im = Ideal SC current x Dispersion Co-efficient
Further information should refer a motor design manual.
RE: BHP reduction due to increase in poles
The magnetising current can be obtained from the circle diagram. If you need more details please refer a design manual.
RE: BHP reduction due to increase in poles
RE: BHP reduction due to increase in poles
I am not sure about who uses it. Have you done any pole revisions and if so did you achieve the output you wanted. I have the answer anyway and I will close this subject please.
RE: BHP reduction due to increase in poles
http://www.elongo.com/pdfs/MaintainEfficiency990514.pdf
for more info
RE: BHP reduction due to increase in poles
Yes you are correct, definitly the efficiency and the power factor decreases. I am actually arranging a rewind of a 355kW-1480rpm motor to a 990 rpm motor. We will arrange for type tests such as locked rotor current/torque etc etc. May be once completed I will post the results of this task. Only issue is that we can't get anything more than 235kW. Thats what my calcs give me. Anyway I will post the results on this particular thread please.
RE: BHP reduction due to increase in poles
Bore volume will not be changed due to the rewind.
Since with the rewind to a six-pole motor you will reduce rotational speed to 2/3 of a 4-pole motor and so power go down with the same ratio.
In an new design a six-pole motor in the same frame could be designed with a larger bore diameter since less yoke area is necessary compared to a 4-pole motor and hence higher power could be provided compared to a rewound 4-pole-motor.
RE: BHP reduction due to increase in poles
Thanks. That is correct.