3 Phase Motor - Drawing 1.5amps difference between phases.?

Question

I have a 5.5kW 3 phase motor (415v) on a vacuum pump.
We have measured the amps on each phase with a clamp meter and have found two phases are around 9 amps whilst the third is measuring 10.6 amps. The motor is configured in Star (one strap across three of the terminals).

Is this difference enough to cause a problem?
What would the problem be? Noisy, alternating speed??
The motor name plate states amps as 13.5. Will this be full load current? Typically how many amps would you expect a motor to draw if the full load current was 13.5 amps.

Any help very much appreciated as we are going to begin dismantling the vacuum pump any minute now and don't want to if the motor is suspect. Still got Christmas shopping to do / start!

Thanks in advance.

No other equipment running.

Clamp meter readings consistent - even with varying locations.

The three voltages are 429, 429 and 424. Would this account for the difference in amps?

I have checked each phase resistance and they are all the same.

Answer 1

The voltage imbalance is what is causing the current inbalance. The extra 1.6 amps isn't enough to worry about.

Motor nameplates tend to overstate the actual amperes that it will draw under normal operation. Don't worry about it claiming 13.5 amps and you only measuring 9 - 10.6 A.

In theory, a 5.5 kW motor with a lagging power factor of 0.85 amps would draw 5.5 kW ÷ 0.85 = 6.47 kVA. At 415 V 3Ø, each leg would draw 6.47 kVA ÷ .415 kV ÷ √3 = 9 amps.

Another possibility is that your meter is not a true RMS meter and you're picking up harmonics on the system.

Answer 2

The single phase motor will have probably no more than 4 leads coming from the windings. A 3 phase motor will have 6 or more. If the wires are marked with an A B or C it's 3 phase. This is usually a crimped ring around the wire. It could be a dual voltage motor with different ways to hook it up for either 230 V 3 phase or 480 V 3 phase. If single phase, it will be a 240 Volt.

Answer 3

I should think in a star wound where each winding appears twice in each current any motor winding fault would show up in two readings. So either one of the windings is reading low or the extra current is not due to the motor. You could try changing over two phases and see what that does to the readings (REMEMBER THE MOTOR WILL THEN GO IN REVERSE) so you would change twice for the same direction. Best to run the motor off load. But anyway if the motor runs ok and dose not overheat then it is probably ok.

Answer 4

Its not a huge difference. Is there any other equipment running off that phase ? eg a controller, lamps?

Are the 3 voltages all the same?

Are your measurements repeatable at slightly different locations on each phase ? Clamps are often unreliable.

The 13.5A figure is probably a worst case running level, not a typical figure. Its to ensure that users provide sufficient current, not marginally enough.

edit: I don't think the voltage difference (~1%) could produce that current difference. (~16%). There should be some reason for tthe difference. It might still just be a measurement problem.

Answer 5

if voltage is the same on all phases and only the motor load is connected.All currents should be the same.If there is a differance like u have it indicates the motor may have an earth leakage and will eventually burn out