motor and generator

[PG1995]

Hi

1: I'm confused between the terms "rotor" and "armature". I think an "armature" is that part to which electric current is applied. For example, in an induction motor we have windings on both stator and rotor but electric current is applied only to the stator so in this case the stator would be termed armature. Is that correct?

2: Many persons have commented that the motor shown in the video is not an induction motor; someone has called it a "universal motor". What kind of motor is it really? I don't see any brushes etc. but it still can't be an induction motor because there are only two poles (as far as I know an induction motor should have at least four poles). To help me with the query you only need to watch it from 00:30 to 01:22. I understand that the video is not a good one, I'm just curious.

Please keep in mind that I'm totally new to this machine stuff and in the past have only studied basic DC motor and generator so please excuse the complicated details. Thank you.

Regards
PG

 

[Mickster]

This may be of some help in regard to point 1:
https://books.google.co.uk/books?id...onepage&q=electric motor nomenclature&f=false

 

[cr0sh]

Hi

Hello - first off, take my reply with a grain of salt; I'm not a motor expert - my knowledge comes from just playing around with them...

1: I'm confused between the terms "rotor" and "armature". I think an "armature" is that part to which electric current is applied. For example, in an induction motor we have windings on both stator and rotor but electric current is applied only to the stator so in this case the stator would be termed armature. Is that correct?

The "rotor" is the part that "rotates" (notice the root/prefix of "rot"), the "stator" is the part that doesn't rotate - ie, it is "static" (notice the root/prefix of "stat"). As far as which is the "armature" and why? Hmm - based on this:

https://en.wikipedia.org/wiki/Armature_(electrical_engineering)

...It depends. Read the article for more info.

2: Many persons have commented that the motor shown in the video is not an induction motor; someone has called it a "universal motor". What kind of motor is it really? I don't see any brushes etc.

No - you can't see any brushes, but you clearly see a commutator. My first guess would be that it is a universal (or something similar) brushed motor. Now - I suppose it might be possible to turn a universal motor into an induction motor if you shorted the poles of the rotor coils/commutator, then applied just the right waveform/frequency to the field coils...hmm, dunno.

but it still can't be an induction motor because there are only two poles (as far as I know an induction motor should have at least four poles). To help me with the query you only need to watch it from 00:30 to 01:22. I understand that the video is not a good one, I'm just curious.

From what I have seen of induction motors (firsthand, and reading) is that the rotor consists of special laminations (aluminum/steel/copper?) into which the stator coils "induce" current (like an electrical transformer) in and this current in turn is repelled/attracted by the field set up by the stator. The stator coils usually, as you have noted, are multiple; but I don't think the number has any real bearing, though you generally see more than two coils, simply because to induce rotation you can't have only two, unless you have an external means to "kick" the rotation going - also, see for example Michael Faraday's simple electric motor/generator, which technically was a form of induction motor with a single "coil" in the rotor, and an external permanent magnet field.

Generally though, modern induction motors have that somewhat "solid looking" laminated rotor, though some (I think) have a rotor with coils (it's just that the coils are shorted together in a certain manner; like I said, it may be possible to turn a universal motor into an induction motor in some manner - maybe - which may be why the person in the video refers to the motor as an induction motor, though it clearly has a segmented commutator at one end of the rotor).

Please keep in mind that I'm totally new to this machine stuff and in the past have only studied basic DC motor and generator so please excuse the complicated details. Thank you.

No problem - I'm fully expecting someone to come in here and kick me in the butt for leading you astray with incorrect information. Here's hoping we both learn a thing or two about these machines!

 

[strantor]

Many persons have commented that the motor shown in the video is not an induction motor; someone has called it a "universal motor". What kind of motor is it really? I don't see any brushes etc.

I wouldn't call that guy an idiot, I'd call him a liar. He presents what is clearly a universal (brushed) motor and at time 5:00 says that it does not have any brushes. The brushes are obvious and can be seen in the video:
View attachment 64894

There is no possible way he took that motor apart and put it back together, making a tutorial on aq subject he knows nothing about, without ever seening those brushes. Why would he lie about such a thing? Maybe it's worse than I thought and he doesn't actually know what a brush is. In any case, he shouldn't be teaching. He actually call both parts the stator.

 

[KeepItSimpleStupid]

Yea, the rotor is the part that rotates. But as usual, there are gotcha's An Alternator for a car, applies current to the rotor and the stator has three phases that are rectified. There may be a slight residual magnetic field, but for the most part, this is supplied by the battery.

An induction motor won't have brushes.

Then there are brushless DC motors which don't have brushes either.

So, it all depends on context.

Rotor: The part the rotates
Stator: The static part (doesn't rotate)

Don't read ANYTHING else into it.

You also might find terms like:
Armature: The part that rotates
Field coil:
Shunt coil:

See: http://www.swcoils.com/Field-Coils.htm

 

[crutschow]

.............................
(as far as I know an induction motor should have at least four poles).

Don't know why you think that. An induction motor can have a minimum of two poles (1 pole pair). You can determine that by the no-load speed of the motor. A 2-pole motor will have a no-load speed slightly below synchronous of about 2900/3500 rpm (50/60Hz).

 

[cr0sh]

Don't know why you think that. An induction motor can have a minimum of two poles (1 pole pair). You can determine that by the no-load speed of the motor. A 2-pole motor will have a no-load speed slightly below synchronous of about 2900/3500 rpm (50/60Hz).

You know, you're absolutely right about that; I was thinking when I wrote my response about the cheap shaded-pole motors you see in fans, and they are induction motors with 2 poles, and they start up just fine without needing to be kicked over to get them to spin, obviously (something that has to do with the "shaded-pole" part which I don't understand well, but I know that it sets up a difference in phasing of the fields between the stator and rotor, and this difference causes the motor to spin, synchronously, of course)...

 

[Dean Huster]

The armature of an induction motor has current running through the windings, just not directly applied like the stator or field coil. The motor is a transformer and as the primary, the field induces a current into the winding on the armature. This current develops its own magnetic field which causes the rotational motion.

 

[cr0sh]

The armature of an induction motor has current running through the windings, just not directly applied like the stator or field coil. The motor is a transformer and as the primary, the field induces a current into the winding on the armature. This current develops its own magnetic field which causes the rotational motion.

Yeah - you could think of an induction motor as analogous to a transformer, with a primary (stator) that doesn't move, and a secondary (rotor/armature) that does move/turn...

 

[crutschow]

You know, you're absolutely right about that; I was thinking when I wrote my response about the cheap shaded-pole motors you see in fans, and they are induction motors with 2 poles, and they start up just fine without needing to be kicked over to get them to spin, obviously (something that has to do with the "shaded-pole" part which I don't understand well, but I know that it sets up a difference in phasing of the fields between the stator and rotor, and this difference causes the motor to spin, synchronously, of course)...

The purpose of the shaded pole is to delay the current through the shaded winding so that its magnetic field lags the field from the primary pole. This sets up a rotating field that drags the rotor along with it and starts the motor (3-phase motors automatically generate this rotating field and thus require no start winding). Without some mechanism to provide this phase change for the starting winding (shaded-pole, capacitors, etc.) the field of a single-phase motor just oscillates back and forth without any rotating component and the motor doesn't start (just hums).

 

[shortbus=]

the motor doesn't start (just hums).

Thought they did that because they forgot the words.

 

[PG1995]

Hi

My questions are about the relation between the number of poles, speed, and torque in an induction motor.

The speed and number of poles are inversely related. That is, if the number of poles are increased, the speed increases. In a three phase induction motor, the minimum number of poles is six so if the number of poles is doubled, i.e., 12, the speed would be reduced to half.

Question 1:
Would it take two electrical cycles to complete one mechanical cycle in case of 12 poles?

Question 2:
For this question, I would assume that it does take two electrical cycles to complete one mechanical cycle in case of 12 poles. Let's think of electrical cycle as a push therefore two electrical cycles per one mechanical cycle would mean two pushes per one mechanical cycle. So, in case of 12 poles there would be two pushes compared to one push in case of 6 poles. Am I right?

Thank you for your help.

 

[jpanhalt]

Each electrical cycle is a "push and pull". A 2-pole synchronous motor runs at 3600 rpm when connected to 60 Hz. A 12-pole motor would run at 1/6th of that or 600 rpm. See: https://www.theautomationstore.com/poles-rpm/

Very old thread, BTW.

 

[PG1995]

Thank you.

Okay. What I said might not be exactly correct. I mean that it's not that speed is exactly halved when number of poles is doubled. I should have rather said that the speed is reduced as the number of poles increased.

Could you please reply to my questions, 1 and 2?

Yes, that's an old thread but I thought that I should keep it in one place. Thanks.

 

[MaxHeadRoom78]

Most question have been answered, but in the case of the video, which is very poor at explaining the motor IMO, I am guessing that it is a Repulsion type Induction motor, this explains the confusion by some when they see a wound brushed armature on an AC motor that is not a series wound Universal type motor.
There are basically 3 type of repulsion motor, Repulsion start-Induction run, Plain Repulsion Run, and the Repulsion-Induction motor.
In repulsion start, the brushes are often lifted after the initial start up.
These motor run at the same rpm as a normal induction motor, somewhere just below syncronism.
A Universal or series wound motor operates in a runaway condition and rpm decided by load, friction etc.
Armature is usually refered to a brushed rotor as opposed to a non-wound rotor.
Max.

 

[PG1995]

Thanks.

But I was talking about the questions in post #12.

 

[MaxHeadRoom78]

Looks like you have the answer, the formulae is RPM = Freq of supply/(No. of poles/2).
Mechanical cycles per Electrical cycles (rotations).
Max.

 

[schmitt trigger]

What MaxHeadroom is talking about is what is called a synchronous speed, which applies only to synchronous motors.
For induction motors, there is a parameter called slip, which causes the actual rotor speed to be lower than the synchronous speed.

That is the reason a 4 pole induction motor operating from 60 Hz will not run at 1800 RPM, but at a lower speed, like 1725 or 1750 RPM.
Slip is specified at the motor's rated load.


For universal motors, this relationship does not apply. As previously mentioned, their speed is load dependent. At no load they may spin at dangerous speeds.

 

[MaxHeadRoom78]

There is also a Induction/synchronous motor, these were once popular in large air compressor or pumping station applications, i.e. up to and around 100hp range.
The motor has a induction style rotor with also a winding connected to two slip rings, the motor is ran up to speed as an induction motor, and the signal out of the slip rings detects when the slip has decreased to 5 to 7 cycles, at that point DC is fed into the slip rings and the motor comes up to synchronism.
Max.

 

[dr pepper]

Hi cruts,
Are you sure the magnetic shade delays current, I thought the shunt delayed a copy of the magnetic field to different part of the core.
I might be wrong though.