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rynorshine
20-11-2006, 01:36 PM
Hi Guys,

I recently installed 5 units of dual inverter KX4 system from MHI. These are 24Hp outdoor units running on 380 V, 3 phase, 60 Hz power supply.

MHI engineer informed me that the circuit breaker size should be 200A while my electrical consultant's advice was to use a 75A breaker. MHI replied that the change in the inverters frequency (60 to 120Hz) at full load triggers the rise in current requirement.

I dont knwo the relationship between current and harmonics / frequency but that would mean a 1000A main circuit breaker for me. Waah! but here's the catch, MHI only required a 14mm2 wire to the 200A.

Can anyone enlighten me on this dillema?

Thanks,

Rynor

NoNickName
20-11-2006, 01:55 PM
24 HP (or are they TR?) equals to 18kW
18000/400/1.73 = 26A. MRA should be in the range of 35 to 40A.
40 x 5 = 200A. The 200A breaker is the one.
Please note that if the frequency doubles, so do the amps.

rynorshine
21-11-2006, 02:36 AM
That was helpful.

When the system frequency doubles, it also sends harmonic feedback to the power supply, right? I am just concerned that eventually the AC will affect the power supply's frequency and damage sensitive equipment like LAN servers connected to the same power supply panel.

Thanks,

RYnor

NoNickName
21-11-2006, 08:46 AM
It is forbidden to commission a system without making sure it is CE compliant. It is CE compliant if it is, among the rest, compliant to RFI (incoming and outgoing).
As such there *SHOULDN'T* be harmonics flying around. Of course YMMV.
Oops... I assumed you were european.