3 phase delta connected motor advice

[fullback]

I have a 3 phase brook crompton delta motor connected to a direct on line starter, it keeps blowing the mcb, any suggestions as to why this is happening?

 

[Darkwood]

serious lack of info....... a man last week was shouting down my street can you tell me why (get my drift).

Motor size.
Is it duel voltage.
What overload protection do you have.
What size and type mcb
What is the motor doing.
Where are you located (uk).
Is it tripping during start-up or during normal running.
Is this a recent problem, has the motor been ok with setup before or is it a new install.
.....................................................just to get the ball rolling.

 

[fullback]

Motor size- 7.5kw.
380 delta on the plate.
Overload protection- 14amps.
MCB type- 3. 32amp.
Plastic crusher.
Bristol.
Tripping during start-up.
A new problem as it is a second hand unit.
Electrician wired this unit up and thought it could be the MCB not being motor rated.

 

[SirKit Breaker]

Have you checked it is balanced, and done an Ins Res test? What about all the obvious culprits, loose connections, dodgy crimps etc etc.

 

[fullback]

What is the difference between a 3 phrase delta connection on a motor and a star connection?

 

[Darkwood]

Well just to start with, the most obvious is you require a 50amp type c mcb and cabling size to suit, Dol starting for this motor isn't advised unless your mains system is designed for it (doubtful), been a crusher i assume it spins an high innertia load to which will have a larger than norm run up time, suggestion.. upgrade to either star/delta start (only if motor suitable), soft start or vsd, if using vsd then seek manufacturers advice for mcb rating if star/delta 50amp (c) still applies due to spiking on changeover.
The last method that might be acceptable but your ELI reading must be met is a 20amp type (d) possible solution if close to origin of installation.

Judging by replies and questions this is well beyond your knowledge and suggest you get proffesional in this field.

All this above advice is given with the assumption motor and controls are ok and have been checked.

 

[Knobhead]

I don’t think our OP is an electrician so I don’t think we’re going to get much more info.
The O/L’s are set correctly is about the only useful thing I can glean from this.

The best course of action is get the electrician back to do a full set of IR (1000V) and CR tests to prove the motor. Mechanical loading could be a problem but without seeing the machine it’s just a guess.
The MCB could do with being 50A type C

 

[Marvo]

The voltage across the windings is different. If your motor's rated at 380 volts then leave it wired delta.

First thing to do as already suggested is the usual basic off-load tests. Do all 3 windings have continuity and do they pass an IR test?

Electrician wired this unit up and thought it could be the MCB not being motor rated.

From this quote I'm assuming your not a sparky so you'll need to get him back to finish what he's started. I doubt the 32A MCB is the problem unless it's starting under massive load in which case he'll need to install a star/delta starter.

The 32Amp MCB is there to protect the cable. The DOL starter is there to protect the motor. If the 32 A MCB is tripping before the thermal overload in the DOL starter I'm already suspecting this motor might be heading for the scrap bin.

 

[La Poste]

Just one quick question from me.

Why would a loose connection or dodgy crimp trip the MCB?

 

[Darkwood]

I don’t think our OP is an electrician so I don’t think we’re going to get much more info.
The O/L’s are set correctly is about the only useful thing I can glean from this.

The best course of action is get the electrician back to do a full set of IR (1000V) and CR tests to prove the motor. Mechanical loading could be a problem but without seeing the machine it’s just a guess.
The MCB could do with being 50A type C

You just edited that type (d) lol .... although i got the drift hes out of his depth hence a my advice, im sure my terminology will let him know it isnt as simple as....