Star delta testing help needed

[mickfred]

How can you say it's not the starter? I have seen motors that have been stuck in star either because of a failed timer or even a welded star contactor and boy does the motor run hot!

if my thinking right the current (in star) of each phase runs across two sets of wiring doubling the resistance and reducing the current therefore why would it be getting hot

 

[Andy-1960]

if my thinking right the current (in star) of each phase runs across two sets of wiring doubling the resistance and reducing the current therefore why would it be getting hot

Because of the load on the motor, it cannot deliver the required power, torque and will not achieve its maximum running speed when running in star hence the motor will be overloaded.
When the motor is connected in star, the motor will be effectivly rated at approximately a third of its motor plate rating, so a 7.5kw rated motor will have an output of about 2.5kw. So if the motor is being asked to deliver 7.5kw of power but only capable of delivering 2.5kw it will effectively be overloaded and get rather warm! The control system will not normally detect this as usually the overload relay is shorted out by the star contactor during start up.

 

[mickfred]

Fair enough it's still not the starter

 

[Andy-1960]

Fair enough it's still not the starter

Lol.. it may not be, you may be right but worth a check all the same.

 

[mickfred]

Okay check it switches over and when it does check what i said (god , i really hope it not the starter )

 

[ipf]

Fair enough it's still not the starter

Just an example.
One panel I installed over 30 years ago controlling 4 x 15kw motors via s/d starters. In all those years the motors have all been changed just once with the odd bearing change, whilst I would say that at least 25 contactors have needed changing with contact damage due to load.

 

[Kj14]

It's switching over and it will run for a while sometimes a few hours and then trip. It's running a vac pump. Just need to know how to test winding from the panel as the motor box is not easily accessible.

 

[Pete999]

Are you talking IR or resistance testing

It's switching over and it will run for a while sometimes a few hours and then trip. It's running a vac pump. Just need to know how to test winding from the panel as the motor box is not easily accessible.

Are you talking IR or winding resistance testing

 

[Deleted member 72666]

It could be a multitude of things, does the motor sound ok? rumbling or knocking noises could indicate bearings/ alignment problems, check the supply voltage at the panel (line to line and line to earth) then from there visually check for bad/burning connections ( does the panel smell of burning when first opened up?) then carry out all the dead testing checking contact resistance on all poles of the contactors not forgetting to check the resistance of the overloads themselves. Check that the resistance of the motor windings from the panel are balanced if not check at the motor terminals.

 

[Deleted member 72666]

I've just seen that it's a vac pump, when these start to fail they can become very noisey (sticking or broken blades) and at the same time take longer to reach the preset vacuum (this can checked by asking the operator if they have noticed any changes).

 

[David Prosser]

I'm going for mechanical fault (or lack of maintenance) on the vacuum pump. Oil and filter candles need changing. Just a complete guess but as I can't see it, hear it, or test it I'm just having fun guessing.

 

[Kj14]

Are you talking IR or resistance testing

Are you talking IR or winding resistance testing

Winding resistance from the panel

 

[Pete999]

Winding resistance from the panel

At which contactors? you will need to identify the 6 cables going to the motor

 

[Kj14]

That's what I need to know I can see the star contactor and the delta contactor

 

[Deleted member 72666]

Check at the main contactor and the delta contactor not forgetting the o/l unit. Identify the windings U1 start and finish U2, V1 & V2 and finally W1 & W2, these should have the same value, don't be surprised at the very low impedance readings. Also check for leakage to earth from U, V and W. Don't forget to check the input voltage.
If all above are present and correct including what was said in #24, then it's almost certain a mechanical problem.