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?

 

[Marvo]

My intention wasn't to surprise you

....but unless you want nuisence tripping its good practice to follow their guides and works backwards to then weigh up cable sizing etc.

Yes, from a new design point of view I completely agree, start with the load and it's mode of operation and design backwards to the supply but the installation is already existing in this case so I would be very wary of replacing an existing 32A MCB with a 50 Amp device.

In this particular case the start current of a 7.5kW 4-pole motor should be under the magnetic trip range of the 32A MCB and the run current is under half of the MCB rating so I'm still not convinced the issue is the MCB size unless there's unusual circumstances such as high stop/start frequencies or high start torque requirements which may need a remedy that involves a completely different starting arrangement.

 

[Darkwood]

My intention wasn't to surprise you



Yes, from a new design point of view I completely agree, start with the load and it's mode of operation and design backwards to the supply but the installation is already existing in this case so I would be very wary of replacing an existing 32A MCB with a 50 Amp device.

In this particular case the start current of a 7.5kW 4-pole motor should be under the magnetic trip range of the 32A MCB and the run current is under half of the MCB rating so I'm still not convinced the issue is the MCB size unless there's unusual circumstances such as high stop/start frequencies or high start torque requirements which may need a remedy that involves a completely different starting arrangement.

OP explains he has just got said machine and explains its not an existing install so assume it been either connected to existing point or wired in new, either way it should have been design checked and not be on a 32amp mcb, regardless of time current curves for any mcb rating chosen; the manufacturers guidelines and advice would over-rule this and is easily accessible and for a 7.5kw motor i have 5 differing brands all giving same 50(c) in their recommendation so to re-iterate it shouldn't be on a 32(c), i note that sometimes you can get away with it like off load start-ups but its poor practice to try second guess this or rely on it.
I also note this is likely to be a high inertia load (im familiar with machine)with long run up time which deffo would cause issues with 32amp and even 50amp mcb's also VD needs to be taken into account on startup[which also effect cable sizing but this is where the calcs need to be done and then related to tech' support of mcb brand.

At the end of the day it could turn out to have a faulty field winding etc as none of us are there but it dosn't mean our discussion has any less importance and still the situe should be viewed by a competent electrical engineer.
If you read my early posts i already covered what you replied to me with stating a 50amp and clearly mentioning cable sizing issues, also 20amp (d) if ELI permits it, and high inertia load and possible solutions.

 

[Marvo]

I understood the OP had replaced the motor on a plastic crusher with a second hand delta-wired Brook Crompton unit and the tripping problem had started. Any advice I posted was on this understanding. I reread his posts and I'm still not 100% sure but I've probably misunderstood his thread.

 

[sysrq]

My intention wasn't to surprise you



Yes, from a new design point of view I completely agree, start with the load and it's mode of operation and design backwards to the supply but the installation is already existing in this case so I would be very wary of replacing an existing 32A MCB with a 50 Amp device.

In this particular case the start current of a 7.5kW 4-pole motor should be under the magnetic trip range of the 32A MCB and the run current is under half of the MCB rating so I'm still not convinced the issue is the MCB size unless there's unusual circumstances such as high stop/start frequencies or high start torque requirements which may need a remedy that involves a completely different starting arrangement.

What could be done if C20 MCB trips due to frequent reversing caused by stalled shaft on a Vecoplan VAZ 100/80 1994.
Eaton online selection tool suggests PKZM0-25 motor protection circuit breaker with a 150 Amp tripping current. C20 MCB has the same tripping current (want to keep it low enough due to thin field wiring and long cable runs).
Motor is stalling cause the owner wants to shred a really rigid plastic material. Might need a bigger shredder with sharper blades.
PKZM0-25 motor protection circuit breaker should withstand this kind of motor abuse.