Current Measurements

[neelam29]

hi friends,

I want to measure current consumed by a 3-Phase 220v motor. I can do AC current measurements using CT(Current transformers) but i just want to know do i need to install only one CT or do i need 3 CT's one for each phase. What calculations are involved in finding out the net result.

Is it :- Itotal = (Sqrt(3)) * Iphase

Thanks in advance.

With best regards,
Garg

 

[Hero999]

Is the motor a balanced load?

I assume it probably is, therefore you only need one current transformer.

Is phase angle important?
Do you want the power consumption as well as the current consumption?
If so then, I recommend adding a power meter too.

 

[Joe McGivern]

current measurement

3 phase motors will have the current in each phase pretty close to equal , if not, there will usually be a problem and eventually failuire due to overheat.

I am more familiar with 3 phase 400 volt motors and the rule of thumb I use is 2.2 amps /Kw/phase. With 220 volt 3phase the current will be approximately double that of 400 volt 3 phase.


Again, the motors I am familiar with will have only one phase measured. This method you use for depends on the type of ammeter you opt for.

It can be a direct reading ammeter in which case the current measured is taken through the ammeter, or in the indirect type the CT is matched to the ammeter and the current in the ammeter is a sample of that flowing in the supply to the motor.

Hope this helps.

JMcG

 

[Sceadwian]

The important thing a current meter on a single phase can't measure is the fault condition of the phases being out of ballance, which can happen for a great number of different reasons. The more asumptions you make, the more a simple error will screw up your results.

 

[Hero999]

You can monitor for faults and measure current using two current transformers.

Put one phase thorugh one transformer and all three phases through another transformer. The transformer with all three phases should measure nothing unless there's a fault and the other transformer should just measure current.

 

[RODALCO]

3 Ø motor

Hero999 One thing to add there, that you will only pick up an earth fault that way.
The CT won't pick up a phase to phase fault The three currents measured will still cancel each other out in the sensing CT.

Generally the moror currents are measured with one ammeter either direct or via a CT for large motors.
Cheers
Raymond