How do u measure current passing thru a 3 phase 420v motor? do u measure current from each phase and add them?

Question

I am actually trying to measure the current using an amprobe meter. But the current passing through a single phase is just 1.5 amps to 2 amps. so i am confused if a 3 hp motor will use just 1.5 to 2 amps.

Answer 1

Your measuring technique is correct. A single leg of the 3phase should be equal in all three phases. Do not add them.

However, your motor is probably not loaded down to draw the 8amps you are expecting.

Full Load mtr rating is what the capability of the motor is when loaded.

Answer 2

Presumably the motor is not loaded at 1.5 to 2 A. This sounds reasonable. The full load of a 460v, 3HP motor is around 4.8A so I think you may be looking at the nameplate incorrectly. If there are two numbers for current, this is for the two voltages you could use (Wye or Delta). My guess is that you are looking at the current for 240v operation.

All 3 currents should be fairly well balanced if the motor is good and the supply voltage is healthy.

Answer 3

star or delta

Answer 4

One lead is not the same as a single phase. The reading you took with the CT is the current draw to the motor. Each leg will vary somewhat. Full load amps on a 480 volt, 3 phase motor will be approx. 5 amps. If you get an accurate reading of 1.5-2.0 amps it means you are using approx. 1.5 HP. Rule of thumb is 1.25 amps per HP @ 480 volts.

The motor is not under full load. This is not an unusual situation.

Answer 5

It may be possible that could be the no load current.