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....

 

[Knobhead]

Just one quick question from me.

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

Unbalanced load draws more curent on the two good phases.

 

[Darkwood]

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?



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.

I must say Marvo i totally supprised by your answer, may i point out the mcb is under-rated for the starting current of a 7.5kw motor, plus the extra duty of it been a plastic crusher (high inertia drum) may even require further allowances, a standard load motor of this size would require a mcb of 50(c) direct or star/delter, and if the ELI can be met a 20A type(d), although the mcb is there to protect from short circuit it still have to allow the start up inrush through without tripping, the overload on the contactor has already got this allowance.

 

[Marvo]

Unbalanced load draws more curent on the two good phases.

this should trip the DOL starter first, MCB's are very tolerant to smooth overload currents, the thermal overload should hopefully be faster off the blocks for unbalanced loads.

 

[Marvo]

I must say Marvo i totally supprised by your answer, may i point out the mcb is under-rated for the starting current of a 7.5kw motor,......

I would always size the MCB according to the cable it's supplying. With an AC3 or AC4 load I might spec a slower curve to build in some tolerance for start currents especially if there are very frequent stop/starts involved. If the cable size allows you could install a 50Amp MCB.

With a 10 horse motor (assuming 4-pole) a 32 Amp MCB should be more than adequate. If the machine is direct drive with high starting torque requirements then DOL is'nt going to cut mustard.

 

[Darkwood]

I would always size the MCB according to the cable it's supplying. With an AC3 or AC4 load I might spec a slower curve to build in some tolerance for start currents especially if there are very frequent stop/starts involved. If the cable size allows you could install a 50Amp MCB.

With a 10 horse motor (assuming 4-pole) a 32 Amp MCB should be more than adequate. If the machine is direct drive with high starting torque requirements then DOL is'nt going to cut mustard.

again im still supprised, this is one of the most common mistakes when selected mcb's for motor start-ups, always look-up mcb tech' charts with regard to motor size etc or talk direct to tech, i have the books and have quoted the sizes from Schneider's mcb's and motor rating charts and most brands out there are similar in values given, the mcb ratings given will under normal conditions be larger than you would normally think but unless you want nuisence tripping its good practice to follow their guides and works backwards to then weigh up cable sizing etc.