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2ps
17-09-2011, 08:44 AM
What could cause there to be 240v between each phase and earth without there being 415v between any of the phases?

nike123
17-09-2011, 08:52 AM
Lack of 2 phases.

nike123
17-09-2011, 09:03 AM
With motor connected or not?

2ps
17-09-2011, 09:14 AM
power is fine into the contacter and contacts are pulled in. would this be a faulty contact/s?

monkey spanners
17-09-2011, 09:30 AM
Sounds like it's lost two phases, power from the third one will then feed back up to the break in the circuit as you describe. A faulty contractor or fuses could cause this, if you don't know how to test a contractor then change it and see. I would also change the fuses if they are replaceable as they could have been damaged and blow in the near future.
Why not take the old contractor apart and see how it works, this knowledge can be helpful in future for working out what's gone wrong and how.

chemi-cool
17-09-2011, 10:44 AM
You should check the resistance of each winding.
Disconnect te motor wires from the contactor and check for 415V between the three phases.

Sounds like faulty contactor, overload or bad electric supply.

paul_h
17-09-2011, 04:05 PM
power is fine into the contactor and contacts are pulled in. would this be a faulty contact/s?
So you have 415V at the contactor before it's pulled in, but only 240v out when the contactor is pulled in measured at the contactor outputs?
lack of 415v to the contactor before pulled in means fault supply (fuses, c/b or mains), lack of 415v at the contactor outputs after it has pulled in means faulty contactor if it measured fine supply side of it before pulling in..

cadwaladr
19-09-2011, 11:55 PM
i check power to neutral or another phase whichever is present not to earth,between two phases should read 380/440volts that is over in the uk,240volts per phase to neutral.

texas64
20-09-2011, 05:21 AM
Disconnect the motor from the contactor. Then, check applied power at the load side of the contactor, first each phase to ground, then across the phases. You may have a bad contactor or missing power. If you have the proper power, then your motor should operate.

SeanB
25-09-2011, 10:25 AM
You might find that there is a 2 pole contactor used to switch 2 of the 3 phases. Commonly used to cut costs. Check you have the 3 incoming phases to the contactor, and disconnect the motor and check you have voltage on all the outputs. If it is missing on one or more then change contactor, and check the removed one for burnt contacts. You will find some burning is normal, but if the contacts are badly pitted either the contactor is underrated or your motor is on the way out. Check when the motor is running that the phase currents are within 10% of each other, and note it and the supply voltage for each phase for future use. If the phase voltages are different by more than 10% you need to check why, you have probably got a loose connection or dying switch somewhere in the supply. If the supply is the same contact the utility, as they have to fix their problem before it kills your motors.

nike123
25-09-2011, 11:37 AM
If the phase voltages are different by more than 10% you need to check why, you have probably got a loose connection or dying switch somewhere in the supply. If the supply is the same contact the utility, as they have to fix their problem before it kills your motors.

Voltage unbalance should be below 3% deviation of average voltage, since voltage unbalance produce current unbalance of 6-10 times more of that.

http://www.pge.com/includes/docs/pdfs/mybusiness/customerservice/energystatus/powerquality/voltage_unbalance_rev2.pdf

Rajeev Prasad
25-09-2011, 12:21 PM
Can u post the circuit diagram on which you are working on for the problem. A 3 phase motor can have problem, if

a) Motor is faulty

b) Contactors in the line do not supply sufficient voltages due to loose connection, may be on line has got carbonised due to sparks and hence th problem or is creating heavy resistance due to which voltage drop is heavy

c) one phase is missing

d) someone would have connected similar phase at 2 poles in the contactor leading to double phasing and missing of one phase.

Remedy :

1. Check the motor windings with ground. If it shows contnuity...motor is faulty and hence the problem
2. If the motor windings do not show same resistance, it means there is break in the windings and thereby the problem
3. Take out connections from the contactor. Give seperate 230V, 1Phase supply to the coils of the contactor and see if all the poles get pulled properly and there is no chattering between the contacts of the poles.
4. Check the continuity between the two ends of the power wiring. If there is a break then also you have this problem.

But, the best way is to share the wiring diagram. Thank you!!...

SeanB
29-09-2011, 08:28 PM
Voltage unbalance should be below 3% deviation of average voltage, since voltage unbalance produce current unbalance of 6-10 times more of that.

http://www.pge.com/includes/docs/pdfs/mybusiness/customerservice/energystatus/powerquality/voltage_unbalance_rev2.pdf

I agree, but you will find often that the voltage between phases can vary, especially if there are heavy loads on each phase that are not well balanced. Allowing 10% max will show you that there is a problem, though it would be best to have all the phase voltages equal, but not all wiring is both close to the transformer and equally loaded on all phases. I have seen residential loads where one phase is showing 200A plus, but another is running 50A, the third being something inbetween. At least here we only have to deal with 3 wire plus neutral, not a split 2 phase system, with a 3 phase system thrown on top as well. So the supply will have either 3, 4 or 5 wires including a PE conductor. Depends if it is a single phase, 3 phase without a neutral or a 3 phase and neutral supply.