Synchronous Motor and Induction Motor Mechanically Coupled

Why would you want to do this?

This was one of the interview question which was posed to my friend by a Panel member. The recruiter is India's leading Power sector company.

The mind boggles!

See post #15 (I think) below and your mind will no longer boggle.

Re: This was one of the interview question which was posed to my friend by a Panel member. The recruiter is India's leading Power sector company.

Because that is the case, I wonder if they were looking for more of an answer than just that they both rotate. Maybe the possibilities of the non-driven unit acting as a generator for power or kvars and such.

That's an excellent screening question! Anyone qualified to be in the interview could answer it without hesitation, anyone else should terminate the interview. Answering it here is a disservice to the industry.

Would teaching a class in the subject, which also would give away the answer, likewise be a disservice to the industry?

Not at all, and that is the point, anyone properly educated would have no problem answering. Along the same line, why do we constantly remind posters that this is not a homework cheat site?

Seems like a trick question to me. If the supply is connected to ONLY one or the other, then PWSlack's answer is the only right answer. Both would spin and the unconnected one would just be a rotating dead weight mass to the powered one in either case.

Maybe the purpose of that trick question would be to weed out the undesirables; flakes who would try to make statements about things they know nothing about, or brainiacs who will postulate endlessly based on unspecified criteria (something that people do in here all the time) but would not likely be very productive from an employment perspective.

Can some one tell me what happens when supply is given to both the machines?

"or brainiacs who will postulate endlessly based on unspecified criteria (something that people do in here all the time)" - That alone is worth the GA.

If supply is given to induction motor it will drive the synchronous motor which could be used to generate power at a non-standard frequency. If supply is given to syn motor

it will drive the induction motor at syn speed and if connected to a) mains will it generate power?.

b)a suitably rated capacitor will it generate power?.

the induction motor would have to be driven slightly above the sync speed to generate power. otherwise you get what PWSlack said

I am assuming that both the motors have the same number of poles.

1] On connecting the supply the induction motor will start rotating till it reaches near synchronous speed. The synchronous motor will not start as it has no self starting property.

2] On reaching near synchronous speed if supply is given to the rotor of synchronous motor the poles may get locked and the entire set up will run at synchronous speed.

3] The induction motor will behave like a induction generator.

S. V. IYer

GA

Maybe this is what was expected as an educated answer. It covers the topic and ensures a good appreciation from the examiner.

just saying that bothe will spin will certainly seem arrogant to the examiner!

LOL, I'm loving this game...

The term "Synchronous motor" as used in that question is not really a sufficient description to allow any meaningful postulation as to what it would do. PM Synchronous? Self starting synchronous with an amortisseur winding? Some sort of old synchronous motor without an amortisseur winding (I've never seen one but I know they exist)? How many poles in each (as previously mentioned)? Heck that statement didn't even clarify if both were connected to the same supply! There just is not enough REAL information in that simplistic statement to get to a conclusion without jumping there.

But one thing is for sure, an induction motor driven by a synchronous motor (of equal poles) will NOT become an induction generator if it is also connected to the line. It has to spin SUPER synchronous. How would the "synchronous motor" do that?

I still think the question is a trap, and people here are falling right in, even after being told it's a trap!

PS: Or, if it is NOT intentionally a trick question, they the person posing that question in the interview is not qualified to judge any answer given!

Re: But one thing is for sure, an induction motor driven by a synchronous motor (of equal poles) will NOT become an induction generator if it is also connected to the line. It has to spin SUPER synchronous. How would the "synchronous motor" do that?

As long as we're treating this as a game (and not as practical real world stuff), what if there is a difference in poles--say for example the synchronous motor is rated (has the number of poles) for 1200 rpm and the induction motor is rated for 900 rpm. (Probably not a good thing to try in real life, but ... Or some other mixture of speeds / poles so the speeds are still different but closer...

"As long as we're treating this as a game (and not as practical real world stuff), what if there is a difference in poles--say for example the synchronous motor is rated (has the number of poles) for 1200 rpm and the induction motor is rated for 900 rpm. (Probably not a good thing to try in real life, but ... Or some other mixture of speeds / poles so the speeds are still different but closer..."

Legit, and the induction motor would then generate, at least until one or the other overloads. But the idea of "different but closer" is limited until you get into high pole counts, which then has other issues.

Blimey, J Raef, a Californian quoting French - must be the "godless" influence! Noah Webster would have called them "damper windings"! I did think that, if an induction "pony" motor was coupled to a synchronous motor, it was not a "self-starting" synchronous motor. Maybe, in India, they buy up these old machines (without control panel or instruction manual) - and give the installation job to the "new starter" engineer - so it is important they know which machine breaker to close first......

Amortisseur is the "official" name in the Synchronous motor world, at least in my circles. Although I did take 6 years of Francais in middle and high school, lack of use has allowed those brain cells to almost completely atrophy. I think I could still ask for the location of the library and a toilet, and for sure I can comment on the snow being beautiful today and ask if you want to go skiing, but after that I'm lost.

Of course it's a trick question since as far as the synchronous motor goes some are self-starting (before I get slammed think in terms of the old analog clocks, they had enough starting torque to overcome the inertia of the hands and gear train). I suspect that the OP is a synopsis of the conversation, not verbatim.

So the conversation might have gone like this:

Interviewer: Assume we have two three phase motors of equal rating which are coupled together and there is an appropriate source to power them. What happens when each is energized individually?

Interviewee: In most cases the synchronous motor won't start because a synchronous motor can't develop enough torque at zero speed to overcome the combined inertia of both rotors unless it is equipped with some means of starting such as a controller, special windings or a pony motor to bring up to synchronous speed first. The induction motor will run up to synchronous speed or near synchronous speed if there is appreciable windage and/or bearing losses.

Short, simple and to the point; and as we can see by reviewing the multitude of answers given here, only about 5-10% of the respondents would have passed. There are many ways to continue to probe the depth of the interviewee's knowledge, as was suggested by some posters. Questions about attaching the load motor as a generator, synchronous condenser, attaching a mechanical load, etc. As the interviewer I would know immediately if I were talking to an Electrical Power Engineer, an Electrical Engineer or a wannabe.....

In a nut shell 5 to 10 %

GA

do u give the supply at the same timeor to one of them by turn?

The likely "real world" reason for mechanically joining the shafts of an induction and a synchronous motor is so that the induction motor can drive the s. m. up to speed. So one closes the induction motor contactor first, then the s.m. contactor - but only when it is going as fast as possible. Finally, DC is applied to the rotor of the s.m. to pull it into synchronism. Other sequences are possible, but of no practical use - unless one wants to show that electrical protection works, or how to burn-out a motor.

If I have read the question right it is so that the induction motor gets the synchronous motor up to speed so that it will run.

Just as in the old days of the automobile with the hand crank or the spinning of the airplanes propeller. Once the motors get up to speed the magneto (magnetic synchronous field) keep them running at speed until the field is broken (turned off).

Synchronous motors need a little help (motor Viagra) to get them up to speed and keep running.