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Discuss Thermal overload 3 phase to single phase pump in the Industrial Electrician Talk area at ElectriciansForums.net

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Boiler panel controlling a 3 phase grundfos pump. Pump was tripping thermal overload...
Pump replaced for a single phase grundfos by engineer etc...
I was asked to convert the control circuit to single phase which I did.
But still tripping on the thermal overload.
Set it slightly higher for now but anyone have any experience when converting etc... Also the original 3 phase pump may have been fine and the issue may have been the contactor thermal overload.
Cheers
 
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B

Bobster

The OL is seeing an imbalance. Loop the line through L1 and L2 on the OL and put the N through L3
 

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Some overloads such as the Allen Bradley electronic ones still wont work even with the live looped round and neutral through, they only work with a balanced 3 phase load across the overload. But it's most likely Robs solution will do the job.
 
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  • #4
Ye I did think that but the single phase pump will require a higher start up current as only working through 1 phase.
Will doing this definitely balance it or maybe need a higher rated overload.
The original problem seams it was the thermal overload possible fatigue.
 
B

Bobster

It will of course have a higher running current.

This needs to be checked also.
 
Three phase overload relays are designed to protect a three phase motor against single phasing and thermal overload.
Single phasing condition occurs if one of the phases is lost i.e. broken cable/lost phase leaving the motor running on two phases, the thermal overload sees this as an imbalance and consequently trips.
A thermal overload is when the motor draws more current than the rated full load current i.e. the motor is overloaded. Most modern thermal overloads should be set at the rated full load current of the motor for protection purposes.
If you use a three phase overload on a single phase motor you have to ensure all three thermal elements are connected in series, otherwise the overload will trip out on single phasing protection.
The rating of the overload should ideally be selected such that the full load current of the motor falls ideally half way within its range. i.e. Motor full load current 10A, overload range 8 to 12A, relay set at 10A
Most thermal overloads will quite happily withstand 6 or 7 times rated motor full load current for up to about 10 seconds (DOL starting).
 
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  • #7
I don't think an overload senses a lost phase? Rather thy the pump is over worked due to lost phase.
 
B

Bobster

I don't think an overload senses a lost phase? Rather thy the pump is over worked due to lost phase.
Most monitor phase balance. Try running a single phase motor passing line through L1 and Neutral through L2. Leave L3 with nothing in. Most will trip.
 
All motor starter overloads be they electronic or thermal have single phasing protection its a requirement of BS EN 60947-4-1
 
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