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3-phase induction motor speed control

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qtommer

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hi there,

from what i know there are 3 factors which determine the speed of a 3phase induction motor which are:
i) frequency
ii) number of poles
iii) resistances of the windings.


As for premise (ii), the lower the number of poles, the faster the speed of the motor. Why is that so? Shouldn't the speed increase as there are more magnetic fields in the stator thus producing more e.m.f to drive the rotor?


help is greatly appreciated ;)

thanks!
 
increasing the number of poles decreases the physical travel for a single electrical cycle. or in other words a 3 pole motor only moves 120 degrees instead of 360 for a single cycle. that is also why the speed is 3600 for a single pole... there are 60 seconds in a minute and the motor is rated for 60 Hz so you end up with 3600 RPM.

applying more current only effects the torque... there are no brushes... for a given motor speed is only determined by operating frequency.

Dan
 
Torque - the greater the load, the higher the slip meaning a lower speed.
 
increasing the number of poles decreases the physical travel for a single electrical cycle. or in other words a 3 pole motor only moves 120 degrees instead of 360 for a single cycle. that is also why the speed is 3600 for a single pole... there are 60 seconds in a minute and the motor is rated for 60 Hz so you end up with 3600 RPM.

There are no single pole motors. Two poles is as low as you can get. One north pole one south pole.
 
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There are no single pole motors. Two poles is as low as you can get. One north pole one south pole.

"Three-phase motors have three salient poles per pole number, so a four-pole motor would have twelve salient poles. This allows the motor to produce a rotating field, allowing the motor to start with no extra equipment and run more efficiently than a similar single-phase motor." Induction motor - Wikipedia, the free encyclopedia

In a SINGLE POLE 3 phase motor the rotor makes a complete revolution 50/60 times per second at at the nameplate 50/60Hz. under load, induction motors run a bit slower.

In a TWO POLE motor, the armature would only make a complete revolution 25/30 times per second.
 
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Trust me reality is different than what wikipedia says. Their animation is wrong by showing motors to have an odd number of poles. :(

All induction motors have equal numbers of poles per phase. I have spent most of my life working on electric motors of one type or another and have done a number of rewinds as well. So trust me I know this stuff.

If you still dont believe me just stop by a electric motor shop and ask them how many poles there are in a common electric motor or just take one apart and look for yourself. ;)
 
Trust me reality is different than what wikipedia says. Their animation is wrong by showing motors to have an odd number of poles. :(

All induction motors have equal numbers of poles per phase. I have spent most of my life working on electric motors of one type or another and have done a number of rewinds as well. So trust me I know this stuff.

If you still dont believe me just stop by a electric motor shop and ask them how many poles there are in a common electric motor or just take one apart and look for yourself. ;)

they are correct for a 3 phase motor... there are 3 coils for each pole in a 3PH, which just happens to be the topic ... NOT DC or 1PH!
 
they are correct for a 3 phase motor... there are 3 coils for each pole in a 3PH, which just happens to be the topic ... NOT DC or 1PH!


The animation they have is wrong for any type of motor. There are three pairs of coils for each phase in an AC induction motor not just three coils as they show it. One creates relative north and one creates the relative south 180 degrees opposite of it for each half of the sine wave of each individual phase.

I have rewound single and three phase motors before and when making the new windings they are always made in pairs and installed as pairs because regardless of how may poles per phase or how may phases a motor has it still has to have them all set up as pairs. They always have two coils that set opposite of each other.

If you have ever rewound an AC induction motor you would understand what I am talking about regarding induction motors always having an even number of poles and never odd numbers.
 
Sounds like some people consider each pair to be a pole.
 
There is a north magnetic pole and there is a south magnetic pole. Can't have one without the other in magnetics (unless you find the elusive, hypothetical magnetic monopole the physicists are searching for). So it seems the minimum would be a 2-pole (or one pole-pair) motor, regardless of the number of phases.
 
Sounds like some people consider each pair to be a pole.

Yes some do but its incorrect and it even worse when they use an correct referance source to back themselves up.
The industry standard refers to electric motors by individual pole count per phase. A two pole single phase motor has a synchronous speed of 3600 RPM at 60 Hz just the same as a two pole three phase motor does. The 3600 RPM three phase motor has more winding sets because of the two additional phase inputs but each phase itself still only has the two individual poles so the motor get specified as a two pole motor.

If you went to purchase an electric motor it would be specified by an even number pole configuration such as 2 pole, 4 pole, 6 pole, 8 pole and so on but never as an odd number.

If someone wants to sound knowledgeable about something they first need to learn and use the correct terminology that relates to that particular field and then be able to actually use it correctly when describing or explaining something to others who do work and have experience in that field as well. But if someone wants to look like an idiot or worse then all they need to do is use the wrong terminology and continue to try and tell those who do work in that field that they are the ones who are wrong and dont know what they are talking about. ;)
 
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Yes some do but its incorrect and it even worse when they use an correct referance source to back themselves up.
The industry standard refers to electric motors by individual pole count per phase. A two pole single phase motor has a synchronous speed of 3600 RPM at 60 Hz just the same as a two pole three phase motor does. The 3600 RPM three phase motor has more winding sets because of the two additional phase inputs but each phase itself still only has the two individual poles so the motor get specified as a two pole motor. )

Fine... while it is possible I admit that it is not practical to make an odd number due to non optimal magnetic usage. My field is electronics and I do some with motors, and could care less about terminology accept for communicating concepts...
 
Fine... while it is possible I admit that it is not practical to make an odd number due to non optimal magnetic usage. My field is electronics and I do some with motors, and could care less about terminology accept for communicating concepts...
Please re-read the previous posts. There is no such thing as an odd number of poles. It has nothing to do with non-optimal magnetic usage. All poles come in pairs.

Most of this thread has been an argument over correct terminology.
 
Please re-read the previous posts. There is no such thing as an odd number of poles. It has nothing to do with non-optimal magnetic usage. All poles come in pairs.

Most of this thread has been an argument over correct terminology.

and in 3 phase the sum of 3 fields is 0, it is quite possible to have a 3PH motor with only 3 coils since the sum is 0. now, are you going to call that a 6 pole motor?

from a previous post: "the equation for the RPM is 120f/p" now this would say it is a 2 pole motor since said motor goes 3600 RPM and 60Hz, or am i mistaken?

as for me i think it makes far more sense to say a single pole motor does 1 full mechanical revolution for 1 full electrical revolution. it certainly make more sense for an AC motor that whose speed is related to the AC, normally line, frequency
 
Fine... while it is possible I admit that it is not practical to make an odd number due to non optimal magnetic usage. My field is electronics and I do some with motors, and could care less about terminology accept for communicating concepts...

Thats sort of the whole issue of communicating knowledge. If you didn't care to bother yourself with the task of learning the correct terminology then you also clearly did not bother to learn the actual information relating to the subject as well. Which means you basically dont have anything worth passing along that is accurate or useful.

When you have knowledge of a subject the correct terminology and descriptions come automatically at a level that anyone else with similar knowledge of that subject will see and be able to understand and then back up as well. ;)
Thats why those who really do know the subject can easily spot who's trying to fake it. :D

Ever wonder how the pseudo science goofs always manage to get singled out almost instantly and then immediately catch an unending hell for what they are proposing? :rolleyes:
 
hi there,

from what i know there are 3 factors which determine the speed of a 3phase induction motor which are:
i) frequency
ii) number of poles
iii) resistances of the windings.


As for premise (ii), the lower the number of poles, the faster the speed of the motor. Why is that so? Shouldn't the speed increase as there are more magnetic fields in the stator thus producing more e.m.f to drive the rotor?


help is greatly appreciated ;)

thanks!

This App. Note should get you started on at least one approach.

https://www.electro-tech-online.com/custompdfs/2010/03/00843a.pdf
 
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and in 3 phase the sum of 3 fields is 0, it is quite possible to have a 3PH motor with only 3 coils since the sum is 0. now, are you going to call that a 6 pole motor?
The 3 phase sum of 3 fields = 0? What does that mean in practice? The motor doesn't run? The minimum number of coils for a practical (one that actually runs) 3 phase motor is 2 per phase. That is called a 2-pole (per phase) motor.

as for me i think it makes far more sense to say a single pole motor does 1 full mechanical revolution for 1 full electrical revolution. it certainly make more sense for an AC motor that whose speed is related to the AC, normally line, frequency
That may make more sense to you but not to the rest of us. (I suppose it's beating a dead horse to again say that there's no such thing as a single pole motor.)
 
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thank you all for the overwhelming responses;) this has somehow benefited me greatly

I have a question. I read that the higher the resistance of the rotor, the lower the speed. of the rotor. Is this due to the fact that as the resistance gets higher, the current in the rotor is lower and thus results in a lower speed due to the electromagnetic induction?

Some sites say that a higher resistance results in a higher torque and a higher torque results in a lower speed. Why is that so?

Thanks again:)
 
Speed of an ACIM is directly related to the frequency of the AC solely. Obviously, if you do not have enough torque to run the load, it could slow down, but it's still 'wanting' to run at it's full speed. Reducing the current (increasing resistance of the windings) does not affect speed of the motor, only torque.

I'd like to see these sites that say higher resistance is higher torque?
 
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