Motor slip

[winnie]

slip = current? :?

For a common 'squirrel cage induction motor', when the rotor is rotating in perfect synchronism with that magnetic field, than it 'sees' a DC field. No current is induced in the rotor, and there is no rotor magnetic field to interact with the stator field to produce torque.

As the rotor slows down, the 'sees' an AC magnetic field. Current is induced in the rotor, which produces a rotor magnetic field, which allows torque production.

'Slip' is just one way of describing the difference between rotor speed and synchronous speed. It goes from 0 (synchronous speed with zero torque production) to 1 (locked rotor). (Actually slip can range from below zero to above 1...below zero means a 'plugging' motor, meaning one spinning against the drive direction. Above 1 means a motor operating as a generator. Normal operating region is very near 1.)

The greater the slip the greater the induced rotor current, at least in the limit of less than 'breakdown' torque. Motor current consumption goes up as rotor speed goes down, but I don't know if rotor current goes up monotonically.

-Jon

 

[Jraef]

When I teach my class, I explain slip this way. The stator is a set of electromagnets created by coils of wire in the steel frame. The magnets created in the stator have expanding magnetic fields that then cut the conductors of the rotor bars and induces current to flow in them. The flow of current in the rotor bars, combined with the steel core, becomes another electromagnet. The repulsion interaction of those opposing magnetic fields is what creates torque. I explain slip as the resulting lag in the creation of the electromagnetic field in the rotor, because it has to be CREATED by the stator magnets FIRST.

 

[Sahib]

The repulsion interaction of those opposing magnetic fields is what creates torque.

The rotor magnetic filed tends to align with stator magnetic field. So there is an attraction and not repulsion between the two and that creates the torque.

 

[Strathead]

160309-1956 EST

Strathead:

I don't understand what you were trying to say.



.

Gar, I don't profess to be an expert on motors, but I do remember from my theory training a synchronous motor is a specific kind of motor. So I took to Google and this came up right away. http://www.electrical4u.com/synchronous-motor-working-principle/

In the first paragraph he says, "Among 3 phase induction motors and synchronous motors are more widely used"

He goes on to say that a synchronous motor is a motor designed such that the rotating magnetic field and the rotor rotate at the same speed and it is a fixed speed motor. I vaguely remember that it can't run slower or faster than the field, or it just fails to have any torque, so I felt it important to point out what I did originally. We are not talking about synchronous motors here.

 

[gar]

160310-0911 EST

Strathead:

The article you referenced is really not very clear on a number of points. It appears to be written in English by a translator or a person that grew up in a non-English speaking country.

But separately in your post you said

So a synchronous motor is different than a motor running at synchronous speed.

This made no sense. Why would you imply that a synchronous motor does not run at synchronous speed?

.

 

[dkidd]

That one had me scratching my head. :?

 

[Strathead]

160310-0911 EST

Strathead:

The article you referenced is really not very clear on a number of points. It appears to be written in English by a translator or a person that grew up in a non-English speaking country.

But separately in your post you said This made no sense. Why would you imply that a synchronous motor does not run at synchronous speed?

.

No, I implied that a synchronous motor is a specific kind of motor and when we are talking about the slip of a motor we are not talking about synchronous motors, because in fact, they actually do run at synchronous speed. Shortly before post another post used the term synchronous motor to reference an induction motor running at a theoretical synchronous speed. I was pointing out they are not the same thing. The best (but not great) analogy I can come up with is if we threw in a shunt motor in to the discussion. It isn't germane to the discussion just like a synchronous motor isn't. Or another way to put it, synchronous motor, shunt motor, wire wound motor, and permanent core motors refer to different styles, designs or types of motors that have different construction and function characteristics.

Does that make sense to you?

I only referenced that article because it was the first one on Google that referred to synchronous motors and induction motors in the same early paragraph. Are you denying that a synchronous motor is not the same thing as an induction motor?

 

[mivey]

Are you denying that a synchronous motor is not the same thing as an induction motor?

LOL, I don't see that ever happening with gar. Your wording was just a little confusing. When I read it I just thought you left out "non" due to a typo. Its clear what you meant now.

 

[gar]

160310-1029 EST

Strathead:

A synchronous motor is not an induction motor, but both have the same kind of stator. Their rotors are what make a big difference in the characteristics and performance of the motors.

A synchronous motor runs at synchronous speed with or without friction.

A synchronous motor always runs at synchronous speed (except that instantaneous speed can slightly fluctuate) until torque load is great enough to cause breakaway.

An induction motor can never run at synchronous speed because of friction and losses.

.

 

[Besoeker]

In the first paragraph he says, "Among 3 phase induction motors and synchronous motors are more widely used"

In my experience, cage (asynchronous) induction motors are far more widely used.

 

[kwired]

No, I implied that a synchronous motor is a specific kind of motor and when we are talking about the slip of a motor we are not talking about synchronous motors, because in fact, they actually do run at synchronous speed. Shortly before post another post used the term synchronous motor to reference an induction motor running at a theoretical synchronous speed. I was pointing out they are not the same thing. The best (but not great) analogy I can come up with is if we threw in a shunt motor in to the discussion. It isn't germane to the discussion just like a synchronous motor isn't. Or another way to put it, synchronous motor, shunt motor, wire wound motor, and permanent core motors refer to different styles, designs or types of motors that have different construction and function characteristics.

Does that make sense to you?

I only referenced that article because it was the first one on Google that referred to synchronous motors and induction motors in the same early paragraph. Are you denying that a synchronous motor is not the same thing as an induction motor?

A pure induction motor has no electrical connection between the stator and rotor and only one of them is connected to an outside source. One or the other (stator/rotor) has it's current induced into it by the magnetic field produced by the other.

 

[Strathead]

160310-1029 EST

Strathead:

A synchronous motor is not an induction motor, but both have the same kind of stator. Their rotors are what make a big difference in the characteristics and performance of the motors.

A synchronous motor runs at synchronous speed with or without friction.

A synchronous motor always runs at synchronous speed (except that instantaneous speed can slightly fluctuate) until torque load is great enough to cause breakaway.

An induction motor can never run at synchronous speed because of friction and losses.

.

I sort of know that. Refer to post #7 and the answer to post #7 which is post #9. These two are what prompted me to make the statement in the first place. I have reread my post and it still doesn't confuse me. And it still reads as clarification.

For future reference I wrote, "So a synchronous motor is different than a motor running at synchronous speed." What part of that is confusing and how else should it be said. Another analogy, WE could be talking about the candy "Orange slices" if someone came in in the middle and thought is was slices of an orange clarification would be called for. I don't believe the OP understood the difference.

 

[dkidd]

I wrote, "So a synchronous motor is different than a motor running at synchronous speed." What part of that is confusing

A synchronous motor is a motor.

It runs at synchronous speed.

That's not different than "a motor running at synchronous speed".

 

[mivey]

For future reference I wrote, "So a synchronous motor is different than a motor running at synchronous speed." What part of that is confusing and how else should it be said.

You have this minor problem:

160310-1029 EST
An induction motor can never run at synchronous speed because of friction and losses.

 

[dkidd]

OOPS

 

[Sahib]

Are you denying that a synchronous motor is not the same thing as an induction motor?

No but synchronous motor can work as an induction motor as it also has a squirrel cage winding in rotor. It only to be ascertained how long it may work that way.

 

[Strathead]

A synchronous motor is a motor.

It runs at synchronous speed.

That's not different than "a motor running at synchronous speed".

But they are not synonymous. An apple is a fruit, but a fruit is not necessarily an apple.

a synchronous motor is a motor, but a motor is not necessarily synchronous.

 

[Strathead]

You have this minor problem:

I don't have a minor problem and this is very frustrating. Read the thread. It was directly discussing the theoretical issue of an induction motor running at synchronous speed. That is exactly why I felt the need to make my statement in the first place. I am not heavy on motor theory, but I know just enough to understand when I need to stop and do research on it. The intention wasn't to get in to the entire theory of eddy currents, magnetism and motor theory, it was to inform the OP that synchronous motors exist and the discussion to that point wasn't applicable to them.

 

[dkidd]

but a motor is not necessarily synchronous.

But the assumption was made that the discussion was limited to asynchronous induction motors. That limitation was never stated.

 

[Strathead]

But the assumption was made that the discussion was limited to asynchronous induction motors. That limitation was never stated.

That doesn't jive with this statement that you made "So synchronous motors can't produce any torque? :?" If the discussion was only about asynchronous motors by assumption then that statement wouldn't have been made. Because it was in response to a statement that an asynchronous motor theoretically running at synchronous speed would produce no torque. So you are the one who dispelled the "assumption that the discussion was limited to asynchronous induction motors in the first place, and the sole reason I said what I did.

The OP was respectfully coming here to ask a very basic question, "What is motor slip." So I am pretty sure he didn't understand the difference between asynchronous and synchronous.