We've been troubleshooting a problem here at work and we're kinda stuck so I thought about asking for some opinion from you guys. We have a 30kW 240V 3-phase motor with a rated current of 109A that is driven by a Toshiba VF-A7 inverter. The inverter trips a few seconds after startup with an over-torque error - and current soars above 150A - when with load, though we feel the mechanical parts driven by the motor isn't really that hard to move manually. So we're trying to determine if the problem is the inverter or the motor. If we determine they are both alright then our problem might really be mechanical in nature.

That isn't what I want to ask about though - I just thought I had to say it so you'd understand the situation. Anyway, when we run the motor without load, we get a current of around 38A. The voltage supplied is around 105V at this point. Would this be normal? In any case, what would the minimum current we could measure from such a motor running without load at that voltage? Is there a way to calculate that?

I ended up working as an all-around machine technician but I'm no electrical engineer, so when it comes to formulas and stuff, I'm somewhat clueless. I hope someone can shed some light for me. Thanks!

Edit: Whoops! I originally accidentally wrote 150V for the supplied voltage to the motor. It's actually 105V.

Final Edit: The problem was confirmed to be mechanical and we were finally able to fix it. Though turning the machine manually was easy at first (hence we didn't think it was a mechanical problem), but after several more turns, we found out that it was getting more difficult to turn.

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    \$\begingroup\$ When you measured 38A at 105V, what was the drive frequency? \$\endgroup\$ – Bruce Abbott Dec 1 '14 at 8:51
  • \$\begingroup\$ I'm not sure about the running frequency. We found the problem though - it was mechanical. There was some parts of the machine that was getting hard to turn which is why we didn't have a hard time turning it manually at first. No wonder the amps was so high when running at full load. \$\endgroup\$ – DeVilFisCh Dec 2 '14 at 7:37

There is an old rule of thumb that no load current (NLA) is about one third of full load current (FLA). So \$NLA = \frac{1}{3}*FLA\$. As I said, this is just a rule of thumb. As the number of poles goes up, the ratio of NLA to FLA will also go up. The NLA/FLA ratio also varies inversely with motor power.

Your NLA/FLA ratio is 39 A / 109 A = 0.35, which is pretty close to 1/3. Assuming you have a 4 pole motor and given it's power of 30 kW, I'd say that is pretty typical.

  • \$\begingroup\$ It may just be a rule of thumb but it should be a useful thing to know when we troubleshoot. We also found the problem to be mechanical in nature yesterday after noticing that turning the machine manually had some parts that would get very difficult to turn so the motor and inverter has been confirmed to be fine. Thanks for the answer. \$\endgroup\$ – DeVilFisCh Dec 2 '14 at 7:41

just a hunch.

I guess the no load current is too high.

if you have another similar motor please check to verify.

  • \$\begingroup\$ Unfortunately, we don't have another motor to check. Fortunately, we have confirmed that the motor is still good as the actual problem was mechanical in nature. Thanks for the answer, regardless. \$\endgroup\$ – DeVilFisCh Dec 2 '14 at 7:43

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