BHP reduction due to increase in poles 3

[neps3]

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.

 

[RadarRay]

I would have said the opposite. The torque of a motor in english units is (HP*5250)/base speed. When you rewound the motor, the base speed was reduced from 1800 to 1200, assuming 60Hz operation. Therefore I would have said the torque was reduced and the HP or KW remained about the same.
Although I should say I am more comfortable saying that about a new motors than a rebuilt one.

 

[DanDel]

Typically, the size of a lower-speed motor is greater than the size of the same HP higher-speed motor. If you have a four-pole frame motor and rewound it to six poles, keeping it to the same size, the HP should decrease. I will assume that the windings per pole are smaller for the six-pole configuration than the four-pole configuration.
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.

 

[acmotorengineer]

There are three common multispeed ratings:

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.

 

[electricpete]

acmotorengineer. Can you explain a little more about your comment: "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."

I would like to understand that comment a little more.

 

[RadarRay]

Excuse me for the correction but I meant to say the torque should have increased, which is what you would have found if you used the equation. I should read what I wrote before I submitted the post.
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.

 

[acmotorengineer]

The simple version:

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.

 

[jbartos]

Suggestion: Visit

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.

 

[acmotorengineer]

PS to my earlier

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.

 

[neps3]

I have found some discussion on this.
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.

 

[acmotorengineer]

for kantor: can you supply the equation for magnetising current, without poles?

 

[neps3]

acmotorengineer

Im = Ideal SC current x Dispersion Co-efficient

Further information should refer a motor design manual.

 

[neps3]

acmotorengineer

The magnetising current can be obtained from the circle diagram. If you need more details please refer a design manual.

 

[acmotorengineer]

to kantor: does anybody still use a circle diag?

 

[neps3]

acmotorengineer

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.

 

[jbartos]

Suggestion: Please, notice that the motor efficiency will decrease with motor rewinding, in general. Visit

http://www.elongo.com/pdfs/MaintainEfficiency990514.pdf

for more info

 

[neps3]

jbartos

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.

 

[electricuwe]

One of the basic rules of motor design is that for a given maximum airgap flux density and current density torque is proportional to bore volume.

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.

 

[neps3]

electricuwe

Thanks. That is correct.