Synchronous Motor Grounding

In a perfect world, nothing. 53 2E 4D 2E

In a perfect world?????

In any world, this is not done. This is a Synchronous Motor and not a Generator (even though it is reversible...). Since running as a motor, you might have a short circuit condition to ground and the protection system will trip OFF.

The Star point, as amotor, is left alone and not grounded.

In the real world, many different things may happen, especially if there are any faults between any phase(s) and ground. Unfortunately some of them may not become obvious until you attach the ground. It's time to test the integrity of your insulation and grounding system before you attach the ground.

Why do you want to ground the middle (could also be called neutral by some people....) point of a star connected motor?

There is no rhyme, reason or need to connect it to ground.

Exactly. The load is after all supposed to be balanced through phases so no current to ground. But all that in my dull "perfect world" Anonymous #1

Um, er, <...current to ground...> usually constitutes something called a "fault".

There is no absolute need to be <...balanced through phases...>, for that is what the <...neutral...> conductor is for.

it depends on capacity of the motor and type of protections used.Normally squirrel cage induction motor also star connected. There neutral never be earthed.

The winding coil resistances for the 3phase motor are supposed to be the same.This is a symmetrical load,and under a symmetrical load,no current is expected to flow in the star point which also form your neutral.No need to ground the star(neutral point) since the motor does not need neutral to start and run,but ground the body of the motor separately(protective earthing).If you ground the body of the motor from the star winding neutral point(protective earth and neutral-PE),there is no problem,as long as the windings resistances remain same(ie symmetrical).but if the winding resistances(loads) are not symmetrical,there would be flow of current in the neutral point and this may not be healthy to the motor and may also endanger person's life if u don't have earth fault protection relay in the circuit.You may tap neutral from your star point if your supply network has no neutral and or transformer for your control circuit.

Just follow usual practice of separate grounding of motors,as this is not house wiring.

Actually current does flow through the middle connection (the neutral point you mentioned, but should be called the "common" point on a 3 phase motor) even with balanced or unbalanced loads of each field winding, otherwise you would have no magnetic field in the first place to turn the motor. As each phase's voltage rises, falls and reverses, 120° apart, so that the motor rotor is "dragged" along with it.....

Just look at the way the fields are connected like here for example from a website I found with Google http://bldc.wdfiles.com/local--files/bldc-and-8051/Three_phase_BLDC_internal_diagram.jpg :-

This says it all and shows with arrows the current flows at various positions of the phases.....

I believe you are mixing up the way a (real) neutral point works when using single phase loads on a 3 phase mains supply......that is more similar to what you wrote......

Have a great day anyway.....

As is always the case, things are not so cut and dried.

The question no one has asked is... is the source grounded? If the source has a neutral and it is grounded and you ground the star point of the motor, you end up with multiple return paths for fault current to take back to the source.

If the source does not have a grounded star point and you ground the star point on your motor, the motor case and frame now becomes the ground point for the whole three phase system and any fault current from anywhere in the system could and would flow back to the source through the frame of the motor possibly raising the frame to a voltage potential relative to another local ground point..

In either case, these are less then desirable situations. Do not ground the star point of the star winding.

In an ideal world, (a) circuit protection device(s) would operate to disconnect the fault.