induction motor

[Vic]

One practical use for applying power to the stator without the rotor is to quickly heat up the motor windings.

Years ago I worked in a motor rewind shop. When we had a rush rewind job and didn't have enough time to do the normal varnish dip and bake, we would insulate the windings with an insulating epoxy. We would stand the stator vertically and apply a low voltage to the windings with an autotransformer starter connected to a low line voltage. The power would be applied in about 5 second bursts and the winding temperature checked between the bursts. The winding would quickly heat up and when hot, the epoxy would be slowly pored over the winding. The heat would thin out the epoxy and allow it to flow through the winding and slots. The heated epoxy would then cure quickly.

As others have already stated, the current is very high without the rotor in the motor, probably even higher than locked rotor current. It's not a good idea to apply line voltage to the stator, even for a few seconds, without the rotor. We would routinely use the ball bearing rotating in the stator to check the winding connections before insulating a new winding, but always at a low voltage.

[iant]

good point Vic

[drspanda]

Nice discussion...different views...My view is follows..

Stator winding would draw a starting current at t=0+ and this could become normal only when rotor emf builds up.

As there is no rotor the above would not happen and stator winding will get burnt. I have done practically.

Ok now I am rising one more for discussion.

Suppose induction motor is coupled to dc motor. induction motor is allowed to run at no load (just its rotor and dc motor rotor). say it is running at 1440rpm. Now induction motor current is no-load current. Then dc motor is started with slowly increasing its voltage so as to get speed of 1500.
What shall be the value of Induction motor current?

But if we refer to current vs slip curve in any standard books we find current to be nearly equal to no-load value. But exact reason is not found. As the rotor emf dies rotor current is zero its mmf is zero..motor should draw high current.

[iant]

sorry but I am definately not interrested in further discussions on hyperthetical motor configuration with EMF (electro motive force)
on a PLC site.
A stator can burn out - with no rotor, due to heat I am suprised that you actually did this????
The reason you would do this at full voltage for an extended lenght of time could only be for destructive purposes...

I will be unsuscribing from this post - not interested in it any more

[rejoe.koshy]

Whre did you come accross such a situation??....
Anyways, to answer to your question...the question is incomplete!!!..when you start the dc motor, in which direction does it start moving?? in the same direction as that of the induction motor or in the reverse direction.??
The answer will depend on this given fact!!!

[leitmotif]

He is using the DC motor to provide torque to meet the torque demand in the AC motor from bearing friction and windage thus to have it running at synchronous RPM ie no slip. AC motor current will be minimal ie just that to supply the magnetic field approxomately 10 to 20 percent of full load current if I recall correct.

Dan Bentler

[DamianInRochester]

Originally Posted by iant

sorry but I am definately not interrested in further discussions on hyperthetical motor configuration with EMF (electro motive force)
on a PLC site.
A stator can burn out - with no rotor, due to heat I am suprised that you actually did this????
The reason you would do this at full voltage for an extended lenght of time could only be for destructive purposes...

I will be unsuscribing from this post - not interested in it any more

What is the point to respond to a question only in the regard of how "not interested" you are in the question? Seriously?

Why is this a stupid question? He is basically just asking what happens if you run a stadard AC machine at synchronous speed. This can happen often on winding/unwinding machines where motors transition from being driving to over-driven.

Faraday once was asked what practical purpose the rotating magnetic field he discovered served. He said nothing, it was just a curiosity. I guess this made him an idiot, thinking about things and asking questions for no reason other than wanting to know the answer.

Edison didn't think much of Tesla's three phase rotating field ideas either until Tesla went out on his own kicked the pants off him.

It is GOOD to ask these types of questions because it demonstrates a better and deeper understanding of the world we are trying to control. The best controls guys are the ones who have this deeper understanding.

I have a feeling that many who have knocked the OP for asking these questions do so primarily becauase he has made them feel uncomfortable about their own understanding.


In answer to the question, basically Dan summed it up quite well. Find any good book on electrical AC machines and you will find a graph of torque v.s. speed. You will see that as you overdrive the motor from the slip region to the synchronous speed your torque will bottom out. If you continue beyond that point you will enter the region where the AC motor begins to act like a generator. The point at drving it synchronous corresponds to least torque, it also corresponds to the point where current draw will be at a minimum. As the slip approaches zero the modeled rotor resistance approaches infinity and magnetizing reactance of the rotor is the primary component.

As for the practicality of the original question, some of the constants used in the mathmatical models for the motors are determined by doing such tests and taking measurements.

[jvdcande]

I totally agree Damian. I have the feeling too many PLC programmers today are just programmers. They don't even have a clue what equipment they are controlling with the PLC. When I started out, all programmers were once electromechanical technicians. They knew the difference between a monostable and a bistable valve. So they knew exactly how to control these. Nowadays, too many programmers have to ask "what's a valve".

My point: it's not only good to ask these kind of questions, it is an essential part of becoming a good programmer.

Kind regards,

[DamianInRochester]

Originally Posted by jvdcande

I totally agree Damian. I have the feeling too many PLC programmers today are just programmers. They don't even have a clue what equipment they are controlling with the PLC. When I started out, all programmers were once electromechanical technicians. They knew the difference between a monostable and a bistable valve. So they knew exactly how to control these. Nowadays, too many programmers have to ask "what's a valve".

My point: it's not only good to ask these kind of questions, it is an essential part of becoming a good programmer.

Kind regards,

A company I worked for about 15 years ago hired hired a PLC programmer one time that was let go by a larger coporation that had been doing some R&D work at our facility. Our management decided to hire him and make him a technician with the belief that along with his age 45-55 that he would bring a lot of experience to the table.

One day we were starting up a small dial machine with a regular old 2 wire 90VDC permanent magnet motor. We powered it up and as luck would have it the motor was running reverse of the direction we desired. We asked him to go ahead and swap the leads on the motor. He said sure.

Now silly me, here I was thinking that there was only one logical way to swap the leads. As soon as he finished he looked up at us and said "OK, done". He had taken the two wires coming out of the drive and wired them together. Then the two wires going into the motor and wired them together. At first I though he was just joking around with us, but it soon became clear that he honestly had no clue what he had done was wrong. I asked him how the power was getting to the motor and he got defensive and insisted that he did exactly as was asked and "swapped the leads"

He didn't last long. It soon became clear he didn't know much of anything about nearly everything. But the thing that always bothered me was how this guy lasted for so many years as a PLC programmer and had virtually no understanding of the equipment he was controlling. How do these people slip through the cracks?

My beginnings were in motion control and Servo/Stepper sytems. I got into PLCs because I was often hired to write the communication and interface schemes between the PLCs and the Motion Controllers. This was often because the PLC programmers they had neither had any understanding of servo, nor did they appear to want to learn. I could never understand this mentality. I always wanted to know everything about the equipment I was using.

It is one thing not to want to yourself, but entirely another to ridicule others because they do.

[Lancie1]

Hmmm...There was a nation-wide power outage in India last weel. I wonder if Pallavi finally pulled the stators out of a bunch of motors?

What surprised me was that the government was blamed by most Indians. It seems that in India, the government owns the power comapanies. Can you imagine a better recipe for creating a disaster? Putting any vital function in the hand of politicians is a good way to insure its eventual failure. There is no profit motive to keep the system repaired and upgraded.

Oh, wait, I forgot, we are in the process of doing the same thing with our health-care system. We are in the same boat as the Indians.

[Mickey]

Originally Posted by Lancie1

Hmmm...health-care system. We are in the same boat as the Indians.

And not enough fingers to plug the leaking holes.

[leitmotif]

Originally Posted by Lancie1

Hmmm...There was a nation-wide power outage in India last weel. I wonder if Pallavi finally pulled the stators out of a bunch of motors?

What surprised me was that the government was blamed by most Indians. It seems that in India, the government owns the power comapanies. Can you imagine a better recipe for creating a disaster? Putting any vital function in the hand of politicians is a good way to insure its eventual failure. There is no profit motive to keep the system repaired and upgraded.

Oh, wait, I forgot, we are in the process of doing the same thing with our health-care system. We are in the same boat as the Indians.

Hmm here in NW many utilities are publicly owned
Seattle City and Tacoma City Light both city owned
Snohomish, Grays Harbor PUD are both public owned. Many others here in Wa and in Ore too.

I do agree with the statement letting the politicians too close to utilities is not a good thing. Seattle City lost a court case when they tinkered too much with City Light. The nice thing about utilities is they have ratepayers who will not tolerate the politician games - ask City of Seattle about it.

Not all Axxholes are politicians and
not all politioians are Axxholes.

Dan Bentler

[Lancie1]

I was mostly talking about the power producers. Most power generating plants in the US are owned by private companies. Washington Public Power (WPPSS) in your neck of the woods, and Tennessee Valley Authority (TVA) in Tennessee are two big exceptions in the US. These are examples of how NOT to run a power production company. You can't blame the politicians running these bureaucracies. They are humans and do what is best for themselves, which means keeping the status of being a government-owned agency.

TVA reformed itself in the 1980s when Ronald Reagan threatened to sell it to private companies. There is nothing like the threat of having to stand on your own feet to get a bureaucrat working. It actually started a program to pay off some of its debt and become self-sufficient. Now 20 years later it is mostly self-funding from power-generating revenues. Like most power generators, it has massive debt funded by selling bonds.

[jvdcande]

Private companies generating the power isn't everything. Here in Europe EDF (Electricité de France), a French electricity productor, is bying up all the production facilities they can get. As a result electricity here in Belgium is twice as expensive as a couple of years agoo. And that's all because the European council decided that there was a lack of competition when the individual governments provided power to their population. It sure was better in the old days.

[leitmotif]

Originally Posted by Lancie1

I was mostly talking about the power producers. Most power generating plants in the US are owned by private companies. Washington Public Power (WPPSS) in your neck of the woods, and Tennessee Valley Authority (TVA) in Tennessee are two big exceptions in the US. These are examples of how NOT to run a power production company. You can't blame the politicians running these bureaucracies. They are humans and do what is best for themselves, which means keeping the status of being a government-owned agency.

TVA reformed itself in the 1980s when Ronald Reagan threatened to sell it to private companies. There is nothing like the threat of having to stand on your own feet to get a bureaucrat working. It actually started a program to pay off some of its debt and become self-sufficient. Now 20 years later it is mostly self-funding from power-generating revenues. Like most power generators, it has massive debt funded by selling bonds.

WPPSS (aka WHOOPS) was a private consortium which included "govmt entities". Seattle City Light decided to stay out of it.

TVA is comparable to Bonnevile Power Administration. BPA if I understand it correctly does not run any generation - they are just a distribution outfit. Many govmt agencies operate dams on the Columbia. Bur of Reclamation operates Grand Coulee which is the largest generation plant in USA.

Like TVA dams Columbia River dams are put in for flood control. In addition they provide irrigation (the prime driver behind Grand Coulee) generation and deep navigation capability.

More than likely the US govmt was the only outfit that could put in these systems. They seem to have done a decent job so far.

Dan Bentler