Best way to connect up air compressor

Tbob27 · Sep 22, 2021

Hi,

I'm looking for some guidance from more experienced hands. Have a friend who just bought a 2.2kw air compressor which is obviously blowing the fuse in the plug due to the start-up current. Manufacturers information suggests a 30a single phase motor DOL for the power rating whilst the person who sold it said it needs a 15a fused spur.

The compressor won't be installed on its own circuit with a type c circuit breaker so wondering what the best route to take given a fused spur off the ring is limited to 13a also? Someone suggested an unfused spur to a 16a single phase 2P+E connector and plug like you see at caravan parks which could cope with the start up current but any guidance much appreciated.

pc1966 · Sep 23, 2021

Based on the 30A fuse suggestion then it looks like a 32A C-curve MCB and matching circuit would be needed, either hard-wired to an isolator or a 32A commando style plug/socket (and of course RCD/RCBO use in that case, also if buried cables < 50mm, etc).

Tbob27 · Sep 23, 2021

pc1966 said:
Based on the 30A fuse suggestion then it looks like a 32A C-curve MCB and matching circuit would be needed, either hard-wired to an isolator or a 32A commando style plug/socket (and of course RCD/RCBO use in that case, also if buried cables < 50mm, etc).

got it, thanks

freddo · Sep 23, 2021

Could use a chunky VSD for less violent starting, though maybe not as chunky as this one that was connected to a 550KW fan motor...

ElectroChem · Sep 24, 2021

freddo said:
Could use a chunky VSD for less violent starting, though maybe not as chunky as this one that was connected to a 550KW fan motor...

Very nice. MV or LV?

marconi · Sep 24, 2021

SIngle phase motor driven air compressors like the one you mention most often use capacitor start and capacitor run type induction motors. For these motors, in order to have sufficient starting torque to quickly run up to speed without labouring the voltage at the motor terminals must not drop much. A consequence of too much voltage drop along the supply to the motor is a longer start time which will stress the start (and run capacitors) leading to reduced life. It is clear when they have failed when the motor fails to start and the overload trip operates through high stalled current - often the windings are damaged too because they have overheated.

As a for instance, my brother-in-law has a 2.2kW compressor in his workshop which was regularly failing. It was fed by a 30 to 40m run of doubled up 4mm2 singles - solution add in a third single for both L and N. If I remember correctly fed from a 32A type B mcb but would need to confirm this. I advised a new run of 16mm2 singles but we decided to see how triple 4mm2 singles performed first.

Do you volt drop analysis and calculations carefully. I cannot remember what the allowable voltage drop is for a CSCR motor driving an air compressor- someone will know. I think my BIL's run suffered 235 to 210V on start-up. (Of course some of this external due to Ze and he is fed by a pole transformer some 200m away).

See this:

Maximum voltage drop limit - Electrical Installation Guide - https://www.electrical-installation.org/enwiki/Maximum_voltage_drop_limit