Motor Rpm Based on Frequency, Poles and What? 4

[daveshei]

I understand a 3 phase squirrel cage motor RPM is 120*(Frequency/Poles).
120 is derived from what? How do you determine single phase motor RPM?

 

[DanDel]

I believe the equation, Ns=120f/P, where:
Ns = rpm
f = frequency in Hz(cycles per sec)
P = number of poles
is applicable for single, two-phase, or three-phase(or any-phase) motors, since it is based on the number of poles. I believe that a 4-pole, 3-phase motor has 4 poles per phase.
The 120, I believe, is derived from the change from Hz(cycles per sec) to rpm (revolutions per min). Look at another version of the same formula: Ns = 60f/p, where:
Ns = rpm
f = frequency in Hz(cycles per sec)
p = number of pole pairs
In this case, a 4-pole machine would have 2 pole pairs, and the multiplier is reduced to 60, which is the number used to change seconds(Hz) to minutes (rpm). To complete one revolution, a 4-pole motor has to pass through 2 pole pairs.
I'll admit I can't find any reference to this right away, but I do believe this was the explanation given to me at school.

 

[UKpete]

To add to DanDel's reply, the equations give the synchronous speed i.e. the theoretical maximum speed of the motor. On load the speed falls due to slip (maybe only 1 or 2%).

The single phase induction motor speed is found in exactly the same way as the three phase motor, there is no difference. They can also have 2, 4, 6... poles.

 

[d23]

All:

One point:

The number 120 in the formula refers to the electrical time degrees that seperates the phases . That is why it is a consatant for 50 hertz, 60 hretz etc...

DanDel:
I would guess your young. You have an excellent memory! Enjoy it while it last

 

[daveshei]

Of Course! That makes sense, thanks.

 

[jbartos]

Suggestion: Essentially, 120 = 2 x 60
where
2 is linked to 2/p or 1/(p/2) to consider a pole-pair, where p is number of poles
60 is coming from the conversion of seconds to minutes between rads/sec and revolutions/minutes. Therefore, it holds true for motors of various number of phases.
See for example reference:
1. The Electrical Machine Problem Solver, Staff of Research and Education Association, New York, NY, 1983, (Dr. M. Fogiel, Director),
Problem 10-2, Page 557

 

[d23]

jbartos

I'm not sure I understand your post. The formula for sync speed of an induction motor is:

120 *(F/Poles)

Will your formula return the same result if you have 2, 4 6... pole motors?

If I use your formula would I need to consider the operating frequency? I think your constant number of 60 assumes 60 hertz. Can it be changed to whatever as needed?

The 120 "assumes" (bad word) that you have a 3 phase power supply and the 3 phases are 120 degrees apart. Is your reference assuming a system other than 3 phase supply?

I'm not doubting you or your reference. I just don't understand the way you are presenting in your post.

 

[jbartos]

Suggestion: I am sorry, this Forum is essentially providing eng-tips for those who have some education and experience in electrical areas. I provided a direct answer to the original questions.
Starting with the simple relationship for motor speed:
wm=(1-s) x (2/p) x ws = (1-s) x (2/p) x 2 x f x pi, in rad/sec

1 rad/sec = 60 /(2xpi) revolution/minute

wm=(1-s) x (2/p) x ws = (1-s) x (2/p) x 2 x f x pi x 60 / (2 x pi) , in revolutions/minute
or
wm=(1-s) x (2/p) x ws = (1-s) x (2/p) x f x 60 , in revolutions/minute
or
wm=(1-s) x (2/p) x ws = (1-s) x 120 f /p , in revolutions/minute
This is where you see 120, this thread is referring to.
Motor phases, are not involved.
The formula is valid for a number of poles p, where p is a positive integer different from zero.
f is sometimes indicated as fs, synchronous frequency.

 

[UKpete]

d23 - as jbartos says, it is simply the conversion of units of time from seconds to minutes. If we quoted motor speed in revs/sec, there would be no x120 (i.e. 2x60) factor, just x2 (to account for the fact that by p is number of poles and not pole pairs). jb has stated the full formula including the small effect of slip in an induction motor.

I have one very slight addition to jbartos' last post, p must be an even number of course.

 

[DanDel]

d23 - thank you, but most people would not consider my age (44) young, although I do have a good memory.
I don't know why you assume me to be young with a good memory if you don't agree with my analysis of the original question; perhaps you can explain further.

 

[DanDel]

Oh, and stars to UKpete and jbartos for following up on my original explanation.