Motor Overload

[whazza22]

At college I was ask to set the overload of a three-phase motor. I used P=V x I

Do you divide the motors wattage by 3 (as its three-phase) then divide by 230 or do you divide by 400?

 

[Guest123]

The overload protection is always set slightly higher than the FLC of the associated motor usually in the region of 10 - 12%.

What figures did you have to work with???

 

[whazza22]

its for my AM2 so not sure on wattage of motor as it varies. Just after some advice as I have to set the overload in exam

 

[Guest123]

The FLC should be stamped on the motors rating plate, multiply this figure by 110% and set your overload to that.

 

[whazza22]

Ok thanks. What if its not printed, do i use the p=v x I equation?

 

[sean123]

You don't need to set the overload protection for the am2

 

[Guest123]

Ok thanks. What if its not printed, do i use the p=v x I equation?

It's the current rating of the motor (amps) you need to find before you can set the overload, as overloads are rated in amps.

 

[whazza22]

You do need to do it for AM2, its in the booklet and was told on prep day. Tutor said you have to work it out by dividing the wattage by 3 and then dividing that by the voltage to find current.

 

[Guest123]

For 3phase current the calculation is......

Watts / (V x 1.732 x PF).

 

[davelerave]

divide by efficiency as well - if memory serves

calculations don't always match the actual 'FLA' anyhow

 

[silva.foxx]

When in DOL: Always set the motor overload to motor Full Load Amps, as stamped/etched on nameplate.

When in star-delta: overload over all then to motor FLA, overload in line side to motor only then 58% motor FLA

Never failed me in 16 years of machine control! Rather the overload trip than motor fail catastrophically. Nuisance tripping very unlikely unless motor rated too close to load.

 

[davelerave]

ya

some of the manual starters are 90%FLA

i'm dubious about settings above FLA even though there may be service factor

 

[pmac10]

im with silva fox on this one, set to FLA, or even better set to FLA and carry out running current test, then set slightly above your readings, ive never seen a serviceable motor run anywhere near the FLA on the nameplate in normal operation.

 

[controlman]

Back in the good old days you put 10 to 12 % on top. But not now. Modern overloads have this factor built in
and I agree with the efficiency

 

[controlman]

Just out of interest
The designers do a fancy calculation to calculate the fan or pump size, then add 10% (in case of error)
Thats why one of reasons you hardly see a motor doing its full duty
Unless there has been additions to the load

 

[whazza22]

Can you use I = P/V to work out the FLA for a three phase motor if it isn't printed? If so, do you divide the power by 3 because its 3 phase?

 

[pmac10]

transpose power=v*a*sq rt 3*pf

 

[sparkymark1]

i have got 0.63ohms written on my motor for the am2 and ive been told i need to work the overload setting out by a formula what do i do, i have to do it tomorrow

 

[jojo devera]

how to recconectwye delta motor with out tag

 

[thepurple1]

D.O.L is 110% of the FLC and star delta is about 60% of the FLA

 

[Guest123]

Nice to know it's not just me that does this.

 

[johnboy6083]

no its not, i was taught the same also. I was taught to measure current with clampmeter, and then set to 110% of that.

 

[davelerave]

no its not, i was taught the same also. I was taught to measure current with clampmeter, and then set to 110% of that.

if you set according to FLA the windings should be protected


but the motor may be oversized and run well under FLA -so a lower setting could be used as long as nuisance is avoided