Large motors and RCD/MCB problem

[N_Cook]

Luckily I'm not directly involved with the installation of these, its
subcontractor's problem , just interested bystander, in what would be
done in the electrical world to cure this problem.
A batch of local houses as part ofa marine flood protection scheme have
had sumps and pumps installed,one per house, about 2KW motors
surprisingly. One of the residents tested one setup , with buckets of
water to trigger the pump. It cut out at the mini "consumer unit"
installed utility side of the normal consumer unit. All the others are
likely to do the same. I assume it is large inductor , current inrush
problem, like the notorious spin-cycle of washing machines. Inductive
unbalancing the RCD or short duration over-current of the MCB.
In my sort of electronics world, for torroidal transformers an NTC
thermistor would be included to avoid down-rating RCD or uprating MCB.
What would they do in the electrical world, anything like NTC thermistors?

 

[Phil Allison]

N_Cook wrote:

Luckily I'm not directly involved with the installation of these, its
subcontractor's problem , just interested bystander, in what would be
done in the electrical world to cure this problem.
A batch of local houses as part ofa marine flood protection scheme have
had sumps and pumps installed,one per house, about 2KW motors
surprisingly. One of the residents tested one setup , with buckets of
water to trigger the pump. It cut out at the mini "consumer unit"
installed utility side of the normal consumer unit. All the others are
likely to do the same. I assume it is large inductor , current inrush
problem, like the notorious spin-cycle of washing machines. Inductive
unbalancing the RCD or short duration over-current of the MCB.
In my sort of electronics world, for torroidal transformers an NTC
thermistor would be included to avoid down-rating RCD or uprating MCB.
What would they do in the electrical world, anything like NTC thermistors?

** MCBs come with various rating for the tripping time, known as B, C and D curve.

The C curve is the one usually chosen for motor start applications, it allows a current of up to 10 times the nominal rating to pass for about 5 seconds.

The problem is not the same as with large transformers ( ie core saturation due to DC component ) but simply that the motor is stalled when power is first applied and takes time to spin up.



.... Phil