Working on 3 phase

[Andy B]

I never work on 3 phase and haven`t thought about it since college many years ago.

I looked at a job for someone it has 4 x 3 phase motors rated at 7.5kw each. They need an SWA run to the panel supplying the pumps. What size SWA and MCB will be needed. Are they likely to need some type of delayed MCB. I don`t think I will do the job but would like to know.


Is public liability insurance much more expensive if working on 3 phase is included?

 

[Marvo]

More details would be good. What's in the control panel? Soft starts, VFD's star-delta starters?

 

[SirKit Breaker]

More details would be good. What's in the control panel? Soft starts, VFD's star-delta starters?

Such as length of cable run, and how it will be installed.
Your MCB will be determined by the load and motor type, so you could be needing type C or D.
As Marvo has said, more info needed, and no doubt you will get some expert advice. Although i have extensive 3 phase motor experience, the design side is not my thing, so i will leave it for someone more knowledgable to come along like Tony or Tel.

Cheers..........Howard

 

[windtech]

Its been awhile since i worked it out but Amps= WATTS÷(1.732 x VOLTS x PF) I = P÷(1.732 x E x PF)
i dont know the PF but with out it the motors run on 11A each give or take.
Such small motors wont be star/delta, maybe soft start but more like DOL. if its not soft start you will need type c or d mcbs, i would always use type d to be on the safe side

 

[windtech]

to work out the PF INPUT WATTS÷(1.732 x V x A)

 

[Knobhead]

Andy, send me a PM with your mail address and I’ll send you a copy of an Excel spreadsheet to work out motor currents. Other than that you don’t give enough information to go on.

A = ((KW/3)/(V/√3))*1000 where V = phase to phase

 

[sjhall]

Tony, would you be so kind as to send me that email too

[email protected]

Cheers

 

[Knobhead]

OK Sent

 

[telectrix]

Such as length of cable run, and how it will be installed.
Your MCB will be determined by the load and motor type, so you could be needing type C or D.
As Marvo has said, more info needed, and no doubt you will get some expert advice. Although i have extensive 3 phase motor experience, the design side is not my thing, so i will leave it for someone more knowledgable to come along like Tony or Tel.

Cheers..........Howard

you flatter me, howard. i used to have a fair knowledge of motors, maybe a poor second to tony, but over the passage of time , not working with them for many years, most has been forgotten. happy now to let the younger ones do all the head work and leave me to connect a to b with whatever bits are supplied. i can just about remember how to use a slip stick to convert hp to kw and work out the amps.

 

[sparkdog]

Public liability only a few pounds more for 3 phase.Mine is anyway.