Help with 3 phase star delta motor

Hi,

I've been sent on a job to investigate a fan not working correctly. After investigation found that the fan doesn't run in 'Auto' or 'High speed' and just shows 'trip' on the control panel. The fan will run in low speed.

I've replaced the belts on the fan as they were frayed. Opened the control box and have voltage where I should. Atleast in low speed.
For high speed, I've investigated the control panel and found that the contactor for high speed switches on and then switches off after a few seconds. See wiring diagram attached.
I'm limited to what I can test on this visit as I cannot isolate the control panel.

Any thoughts? I was thinking of replacing the high speed contactor, overload, auxiliary contact and relays X 2. That's what I've told the customer anyway!
I'm a bit rusty on 3phase star/delta!

James first off, there is no such animal as a Star Delta (SD) motor
SD is a method of starting a 3 phase Squirrel cage Induction motor, have you had any experience of SD starters?

Thanks for the great video. Apologies. I have little experience with motors in general.

Not a lot to go on there,any wiring diagram for fan,usually on lid,regarding high,low connections.Just spotted the delta & star symbols,it looks like,delta connections.
Other drawing shows control circuit.

That is a dual wound motor, assuming the obvious - have you actually I.R. tested them?

Did you watch the Main, star & delta contactors to see if they pull in sequence?

James Clyne said:

Thanks for the great video. Apologies. I have little experience with motors in general.

Click to expand...

If you would like more info on SD there are loads on You tube, much more informative than guys trying to explain. Not an easy subject if you are inexperienced with Motors, must admit I was a bit ---- with induction motors, got to know them very well, but that was in the dim and distant past.

Which trip lamp of the two lit? The 'slow' run trip or the 'high' run trip?

You are dealing with a 2 speed motor. It has two sets of windings. One set creates the rotating magnetic field for a slow (730rpm) induction motor and the other set for a fast (1470 rpm) induction motor.

For low speed it starts and runs in 'star'. For high speed it starts in star and then runs in delta.

Is the contactor which turns on and then off 'T' in the diagram?

The wiring diagram is attached to the original message. The top picture.

Ah no I haven't, what reading would I expect? I assumed that it was star delta so then assumed that the windings should be okay. Assumptions being the mother of all f**k ups! My bad.

Yes, so C1 low speed would run, the timer would then switch over to C2 after sequence however the contactor would illuminate the 'tripped' light. So obviously I was thinking overload or relay. The overload has been set correctly.

The two windings have their own overload relays and contactors so it is reasonable for one to work without the other. We infer that the fault is only on the high speed winding and control gear, and if the h/s trip light is lit then the h/s overload relay is tripping. Is this auto or manual reset? If you reset it manually, does it trip at the next startup? Does the motor turn at all during startup in h/s? Does it hum?

The windings or contactors could be at fault, but you say you have difficulty troubleshooting due to isolation concerns. One possible cause is a fault in the high-speed winding, with which it will run up OK in star but then trips the o/l when it goes into delta, or indeed would trip eventually in star. So as above the first test is of the high speed windings - IR and resistance. But if the motor does not turn at all, the contactors or wiring could also be at fault such that a phase is missing. I would check that all three phases are present at one end of the windings during star runup and that there is no voltage across the other ends at that time (i.e the S contactor is linking the star point correctly). This could equally be tested using a clamp meter on all six cables in turn, all of which should be carrying the same current. Can you do this test?

It is all starting to make sense.

I'm assuming T in the diagram is the sequence timer? There's only one trip light on the panel so I guess they're both linked through the two contactors.

It's hard to tell if the motor moves in high speed due to the fact the control panel is about 20 metres away. It's a manual reset, the motor will not auto reset if tripped. However after reset it won't start right away, I guess that's some form of protection for the motor?

As you say it could still potentially be the other winding. I managed to test the phases and checked rotation from a diagram online. It was fine in that respect but impossible to check whilst in high speed. Unless I clamped the outgoing cables in the panel, it's tight in there though!
The motor is fine in low speed and is running without any issues.

Yes T is the timer contact, its coil / control input is not shown though. Once the o/l relay is reset and the trip light goes out, the run light should come on along with the M and S contactors and the motor should immediately run up in star. If nothing is happening until the trip occurs, it sounds like the problem is there in star, before we even get to delta (does it transition - the S contactor opens and the D closes?). One phase may be missing from the M contactor, or one winding is not being linked to the star point by the S. This causes a singlephasing condition on the remaining two lines which trips the O/L. Or a genuine overload condition caused by one or more bad windings, although the motor would usually at least attempt to turn before the trip.

Another possibility is that the S contactor is never engaging, and the motor is being put directly on-line from standstill when the D contactor closes. This can be a bit vicious to the belts etc but wouldn't necessarily trip the o/l.

I am afraid there's not much more to guess from the symptoms you describe, without readings. I would be checking the resistances and voltages and probably have an answer within a minute, although that might entail doing things that are not technically considered 'safe working practice' despite the fact they get the correct answer with a minimum of poking around inside the MCC!

BTW, if you can clamp the three lines to the contactors, then you can check for a singlephasing condition or gross winding fault there, without having to access the ends of the windings individually. Obviously if two lines are carrying current and one isn't, it doesn't tell you where the open-circuit is but there probably is one. If all three are carrying different currents then most likely you have a winding fault.

LN's posts #9 and #11 - lucid advice all provided without a fee.

Great advice. Thanks guys. Will re attend tomorrow with a contactor, overload and relay and a lot more knowledge about the system. If replacing those don't help, I will get the motor re wound.

Before you do what you said in #13, could you disconnect the motor first, and substitute 3 pairs of 230V filament lamps wired in series, where each pair is connected as would be a fast winding. Then you can observe and test the correct functioning of the panel and contactors and do some IR and continuity tests on the fast windings of the motor.

The motor is in a duct so without removing it it's difficult to work on! IR and continuity sounds good once disconnected, will I be looking for any resistance in particular on the windings? What IR voltage do you recommend? 500V as no sensitive electronics? I've never done this in practice but remember doing it in college 10 years ago!