motor calcs

[davelerave]

how do you take into account starting current when sizing cables for 3-phase motors

in general thanks

 

[pmac10]

in general you only need to size for full load amps, FLA on the nameplate, which can be @ 6 - 10 times less than starting current, any special application??

 

[davelerave]

a couple of 3-phase lifts in a garage( 2.2kw each ) and a 3-phase power washer(overload set at 9amp cant read nameplate)

runs about 10-20m max so was going to use 2.5

thought i should multiply FLA by 1.25 for motor calcs?

think i'm well ok but wanted to calculate anyhow

 

[pmac10]

for the lifts I=P/Square root 3* supply(400)*power factor(guess 0.85), works out about 3.7 then add together and add to 9A for your power washer. 2.5mm cable good for about 25A so youre good to go, youd need to check those figures against your regs, you might need to multiply fla by 1.25 for braked motors which lifts are, couldnt say for sure

 

[davelerave]

i'll be using 2.5 for 'each' motor

i think you need at least 80% of rated voltage during starting-(5-7 times FLA)

and then the standard 5% VD after that for FLA

 

[thepurple1]

your design would be for the fuse or mcb protecting the motor, so a 10Amp motor would need a 16 amp motor rated fuse or type c mcb your maximum earth fault loop reading would be lower than a "normal" fuse or mcb.
fitting a motor rated fuse will allow for the high starting current of the motor. always size your cables to the fuse size not the appliance on the end.

 

[davelerave]

yes i know all that but thanks anyway

i'm enquiring about VD on startup or starting KVA -the voltage at the motor terminals on startup

 

[VoltzElectrical]

I know nothing about motors, but i always thought it was the job of the motor starter to limit start up currents. Love to learn more on this subject though.

 

[Knobhead]

I know nothing about motors, but i always thought it was the job of the motor starter to limit start up currents. Love to learn more on this subject though.

Have a look on the Electrical calculations and induction motor control web site. Lots of motor and starter types covered. Some of them are weird and wonderful things that you’ll never come across in a life time. But the information is all there. I recently had to find out about Dual speed Dahlander motors, the information was there.

 

[Knobhead]

yes i know all that but thanks anyway

i'm enquiring about VD on startup or starting KVA -the voltage at the motor terminals on startup

Forget about VD at start up, you will end up running a cable 5 times bigger to compensate for it. Motors are designed to cope with it. So long as the cable can cope with the FLC +15% overload it will be OK. The MCB is there just to protect the cable and starter. But it has to handle the inrush current so a type C is in order, type D if it’s a biggish HE motor. The O/L’s in the starter will look after the motor.

 

[davelerave]

Forget about VD at start up, you will end up running a cable 5 times bigger to compensate for it. Motors are designed to cope with it. So long as the cable can cope with the FLC +15% overload it will be OK. The MCB is there just to protect the cable and starter. But it has to handle the inrush current so a type C is in order, type D if it’s a biggish HE motor. The O/L’s in the starter will look after the motor.

ya i got the voltage drop ,overload and c type etc.

but i thought there might be a ballpark figure used for vd at startup

i'm sure there's a maximum drop specified sometimes for large motors

 

[Knobhead]

This was just out of interest to myself and to show an apprentice I put a Fluke recording meter on the end of a 185mm cable feeding a 300hp DOL compressor. The volts dropped to near 300 with the start up, the MCCB tripped. The new motor was a HE so the starting current went up from 5X to 8X compared to the original motor. I had to fit a type D MCCB to get the bloody thing going!