Start A Free Spinning Motor

What type of motor?

A motor will only generate back-emf if there is a magnetic field within it with which to create a back-emf. Permanent-magnet motors will be fine, though other types may vary.

Is this a real-world situation or something academic? What size motor? What type of supply?

A motor, not connected to the supply yet, is free spinning: What makes it spin?

It will generated some back emf to the stator windings, won't it? What kind of motor is it? Need ratings also! Note: If there is no magnetic field, there will be no emf.

Any safety concerns? Such as?

If the motor is started, say when it is free spinning at the half rated speed in the right direction, how to est. the starting current? And, in the opposite direction? What kind of motor is it? How much inertia is involved?

We would love to help but you must be more specific. Otherwise, this looks a lot like a "think it through" type of homework problem.

Sorry guys for not providing the details but his is not a "think it thro" problem.

The motor drives a ship's propeller. When the propeller is not driven by the motor and ship is make way (by other propulsion units), the motor will spin freely in the direction depending on the pitch and which way the ship is moving.

It is a squ.cage induction motor, 4160 V, 152 A, 900 KW, 0.86 pf at 1784 rpm. No info about the inertia.

If the connections at the motor need to be checked, with the supply isolated, while the motor is free spinning, the back emf, if any, could be a safety issue or not?

Thank you.

Is this an azipod drive system? I don't think back EMF would be the issue, but the motor being turned into a generator. I think the question should be. Can a driven squirrel cage induction motor, become a generator?

I would check with the supplier...but it should be easy enough to check with a multimeter...or perhaps you would feel safer with one of these...

http://www.sears.com/shc/s/p_10153_12605_SPM2798655702P?sid=IDx20101019x00001a&ci_src=14110944&ci_sku=SPM2798655702

useful information. To reduce delays next time, please post all relevant information in the original posting.

An induction motor derives its excitation from the supply, once you have opened the breaker the voltage will rapidly decay to zero. To be safe I would short the leads together and ground them (depending upon the ship's electrical system), then do your tests.

A round-rotor squirrel has its residual magnetism spread evenly across the surface of the rotor so there is no identifiable pole structure. A salient pole wound-rotor induction motor on the other hand, may have sufficient residual magnetism to create a varying magnetic field that generate a voltage that might be sensed by certain types of instrumentation.

As far as starting with partial rotation: if the motor is turning at half speed in the "right" direction you should have no problem, the inrush current will be roughly half of the normal starting current; however, I would advise against doing it if the propeller is turning the shaft at half speed in the "wrong" direction since the slip frequency will be 150% of the normal starting and so will the starting current. Unless the motor is designed for that type of duty everybody on board will feel the effects. As others have said, the inertia of the prop and the shaft plus the effect of the pitch could be problematic.

Thank you. My text books do not have this kind of info.!

Your explanations correspond to what I have observed but I do not have the knowledge to interpret them.

The drive motors have star-delta starter which can be in manual or auto mode.

When starting the spinning motor (not very often), sometimes it did cause the motor winding high temp. alarm. Probably the motor was spinning backward before the start then.

We are not as sensitive to the vibrations from the drive motors during start up, not only because of their location but also we are so used to the heavy impacts from closing the breakers (located in the control room) every time these motors start; these impacts shake the whole control room.

DWong

If you are getting motor heating alarms on a star-delta started motor, that could be an indication that your transition time is too long. It's a delicate balance; too soon and you might as well not have it, too late and you stop accelerating and heat up the motor needlessly. Proper setup dictates that you observe a normal start and set the transition rime to correspond with the motor being at 80-90% speed. Most people don't know that and many instruction manuals fail to mention it. That's why I NEVER recommend star-delta starting, and I think it should be outlawed on any motor where the load changes from start to start. For a few more dollars (or whatever your denomination is) solid state out performs it every time.

Thanks guru Jraef,

It cleared some of my concepts too..

:) If you can send any link of the literature that describes this phenomenon, that would be of help more.

Best regards.