Speed of multi-pole 3 phase motors

[Frank Gardener]

Can someone please clarify this point for me?

Let's say a (non-standard) induction motor has 9 coils: numbered 1-9
in a clockwise direction. They are fed by 3 phase 60Hz power as
follows:

1, 4, 7 - 0 degree
2, 5, 8 - 120 degree
3, 6, 9 - 240 degree

So it is three pole, three phase.

Assuming 60Hz power is applied, is the speed of rotation of the shaft
60Hz or 60Hz divided by 3, being 20Hz?

Thanks,

Frank Gardener

 

[Jamie]

Can someone please clarify this point for me?

Let's say a (non-standard) induction motor has 9 coils: numbered 1-9
in a clockwise direction. They are fed by 3 phase 60Hz power as
follows:

1, 4, 7 - 0 degree
2, 5, 8 - 120 degree
3, 6, 9 - 240 degree

So it is three pole, three phase.

Assuming 60Hz power is applied, is the speed of rotation of the shaft
60Hz or 60Hz divided by 3, being 20Hz?

Thanks,

Frank Gardener

I think you maybe a little confused with how you have your question
lined up.. At least you're confusing me..

Lets assume for the moment Synchronous speed to make the math simpler..

All Examples at 60Hz.
Hz*2/Poles*Seconds = RPM's
2 pole motor

60*2/2*60 = 3600 RPM's

4 pole motor

60*2/4*60 = 1800 RPM's

6 Pole motor
60*2/6*60 = 1200 RPM's

8 Pole Motor
60*2/8*60 = 900 RPM's
Ett..
You may have noticed that i've taken the Hz*2 ? This is because you get
a motion per half cycle change.

Now because these types of motors are not synchronous types, They have
slip, so actually speed is ~ SynchronizeSpeed*.965. This varies
depending on the rotor design and application.

So a 1800 RPM motor is ~ 1750 RPM's and when fully loaded drops back
a little more on top of that.

With your numbers that you have displayed here, it looks like you are
depicting a 9 wire motor? Which is design to allow you to operate the
motor in 2 different voltages. The pole count does not matter for this
fact. I have seen some motors where you could bundle the poles to
select 2 different speeds..

If you do have a motor with 3 sets of stators all evenly offset, you
need to know the number of poles per stator winding and sum them. Each
pole should be a even number.

In a standard 9 wire 3 phase motor, the coils are not offset, but in
the same place, together.. You need to parallel them or series them,
depending on the voltage you are applying.. In some applications, motors
have for example, an electric brake that releases when energized with 230
volts how ever, the motor is wired for 460 volts, in whic case you
simply connect the brake in the middle of the series tap, which will
give you 230 volts and if the motor was wired to operate at 230 volts,
then of course, the brake would be connected to the main source coming in.


Jamie

 

[Frank Gardener]

I think you maybe a little confused with how you have your question
lined up.. At least you're confusing me..

Sorry if the way I posed my question was confusing. The motor analogy
was actually intended to make it simpler

It is basically a physics question. Forgetting standard motor
configurations ... if I have a ring of nine coils and feed them with
a 60Hz sinewave in the following phase order, is the field rotation
60Hz or 60Hz / 3 = 20Hz?

1: 0
2:: 120
3: 240

4: 0
5: 120
6: 240

7: 0
8: 120
9: 240

Thanks,

Frank Gardener

 

[Frank Gardener]

Are the spatial distributions of the above coild still as stated in your
original post? If so, then the field is rotating at 60 Hz (3600 RPM). A
two pole motor, as described by Jamie.

I see three phase three pole. Where are you getting two poles from?

If the each pole is only progressing a third of the circumference 60
times a second, then why is the combined rotational speed of the three
poles 20 not times a second?

Frank