×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

BHP reduction due to increase in poles
3

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.

RE: BHP reduction due to increase in poles

  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.

RE: BHP reduction due to increase in poles

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.

RE: BHP reduction due to increase in poles

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.

RE: BHP reduction due to increase in poles

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.

RE: BHP reduction due to increase in poles

  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.

RE: BHP reduction due to increase in poles

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.

RE: BHP reduction due to increase in poles

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.

RE: BHP reduction due to increase in poles

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.

RE: BHP reduction due to increase in poles

(OP)
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.

RE: BHP reduction due to increase in poles

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

RE: BHP reduction due to increase in poles

(OP)
acmotorengineer

Im = Ideal SC current x Dispersion Co-efficient

Further information should refer a motor design manual.

RE: BHP reduction due to increase in poles

(OP)
acmotorengineer

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

to kantor: does anybody still use a circle diag?

RE: BHP reduction due to increase in poles

(OP)
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.

RE: BHP reduction due to increase in poles

(OP)
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.
  

RE: BHP reduction due to increase in poles

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.

RE: BHP reduction due to increase in poles

(OP)
electricuwe

Thanks. That is correct.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources