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Uprating wiring for motor starting? - trip problems

Wivsi

Hi,

I have a customer who is having problems starting a 3ph fan. The fan is rated at 5.5kw, 10a run current. They're running from a 16a socket. The fan is DOL started. It is tripping a D16 breaker on their board. With a D32 breaker the motor will run, but is underperforming. The fan runs fine at another location, so something is not quite right locally. The supply voltage is 415 between each pair.

My uninformed thinking is that, while their 16a socket is fine for the motor run current, the start current which is up in the 80a range is potentially too high for their wiring so the motor is never getting enough power to get up to speed. I suspect that uprating their socket to a 32a with a 4mm run will solve the problem. But I would like to have a sensible (standards based) reason for suggesting this - rather than just suggesting changing it and hoping.

I'm hoping for some expert input to steer the customers electrician in the right direction.  I'm just providing some informal input to help them track down the issue (and am not a sparks) but no one seems to be able to solve the problem at the moment so thought I'd put it out here. Any advice appreciated.

TIA!

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  • 0

    The excessive inrush current can induce a voltage sag in the supply circuit, which can impair the performance of the motor. Power factor correction can help reduce the current demand, installed locally at the motor to minimize the reactive power. What is the cross-sectional area of the circuit conductors? Also, does the DOL starter have a motor brake relay ? 

  • 0 in reply to AMK

    It strikes me that if up rating the cable is to 4 mm², it may be inadequate in terms of voltage drop. Just a thought!

  • 0 in reply to AMK

    This does sound like the way my thoughts were going. I can take a look into power factor correction… not something I know enough about!

    CSA is I think 2.5 all the way…. Supply cable to the 16a socket, then 2.5 to the breaker. Total cable run is probably 10m including both the supply cable, and the run from socket to board. 

  • 0 in reply to Chris Pearson

    Hi…only really punting the idea of increasing cable size as a possible solution… I’m worried about voltage drop but don’t have any real reason to believe upping it would solve the issue! But if I can find a reason….

  • 0 in reply to Wivsi

    OK so 20 mA/A/m with 2.5 mm² cable gives 0.02 x 80 x 10 = 16 V = 7% of the line to earth voltage.

    Even if that impaired the starting capability, wouldn't the motor speed up eventually because as it speeds up, the current would drop?

  • 0 in reply to Chris Pearson

    I agree, really going with 0.8 P.F, 10 metre run, 5500 W, you could use a 1.5mm  single cores 70  degree (4D1) ref method B, cg of 1, assuming 0.35 ohms at DB, you should be able to use a C16 A MCB. DOL starter - say 6.5 x 9.92 = 65 A starting current for about 4 seconds

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  • 0 in reply to Chris Pearson

    I agree, really going with 0.8 P.F, 10 metre run, 5500 W, you could use a 1.5mm  single cores 70  degree (4D1) ref method B, cg of 1, assuming 0.35 ohms at DB, you should be able to use a C16 A MCB. DOL starter - say 6.5 x 9.92 = 65 A starting current for about 4 seconds

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