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[Mowersman]

Hi all
Basically looking to see if anyone can help diagnose a problem with a 3 phase motor I have. It is a fairly small motor, 0.12kw, 4 pole. When connected to an inverter, as soon as it is driven over 50hz, it falls flat on its face, not even managing to increase soeed totally unloaded. I have tried it on 3 different inverters all with the same result. Testing the largest of these inverters on another motor and it behaves normally, running just fine right up to 100hz (4 pole moter)
Cheers
Andrew

[rosinante]

Is it connected in star or delta, is it a dual voltage motor ?
Chris

[Mowersman]

Hi Chris. It is a dual voltage motor, 220V in delta, 440v in star. Currently wired up as delta. I have checked the resistance of each winding and they are all the same.
Andrew

[winchman]

In general its not wise to drive normal motors over 50Hz.
If you look deep in the inverters programme it may be set to shutdown above 50Hz but you have ran another motor so that looks on?
Have you any fault codes?
Is this driven using a single phase supply I assume so as its wired in delta?
Have you tried an earth insulation test?

[andrewR]

I'm assuming this is a squirrel-cage motor. Eddy current and Hysteresis losses in the magnetic material of the rotor/stator will increase as frequency is increased, which can increase the current draw without causing useful torque. I'd expect this to result in an increased heat generation as frequency is increased beyond the rated 50/60 Hz, but probably only in a proportional relationship, not as a quite sudden torque drop-off. It does effectively set the upper frequency limit for motor and transformer frequency though. It's not clear to me why torque would reduce quite so suddenly, though.

Just to be clear, is your inverter 3-phase, as well as the motor, i.e. it's not a single-phase inverter driving a multi-phase motor with capacitors to phase-adjust the other windings?

[Mowersman]

Hi all
It is a single phase 240v in, three phase 220v out inverter, the main one I wanted to run it on was a 0.45kw max output, but I have also tried it on a 0.55kw and 2.2kw output units. The max frequency on 2 of them was set at 100hz, one at 65hz.
The general consensus seems to be that a 4 pole motor is generally ok to be run up to around double speed, around 2800/3000rpm, obviously its life will be less at this speed, though its unlikely to spend any real time running that fast, its there if needed.
The motors on my mill and lathe are both set to a 65hz max frequency and whilst spending very little time at that speed, run fine and have plenty of torque, this motor I'm having issues with would manage to get to 55hz, over that would start to slow and by 65hz would be struggling to even turn, nothing connected to it. I stripped it down last night, its all very clean inside, no nasty smells or signs of burnt windings.  Its a Teco motor, so named brand and 2009 build.
Andrew

[andrewR]

Just hypothesising here. Your squirrel-cage rotor is made of laminated steel, with the aluminium/copper ladder (the squirrel cage) embedded in it. For it to work as an induction motor the laminated steel on the rotor should not itself be magnetised - the force comes by the induction of massive currents in the squirrel cage (aluminium/copper) by the stator, which causes a magnetic field interaction between rotor and stator and causes the torque. The currents in the squirrel cage are caused by the "slip" between the rotating magnetic field created by the stator and the rotor speed. So normally when you run it at full power, with 50 Hz applied, it spins at slightly less than 50*60/2=1500 rpm, i.e. 1400 rpm, which creates a 100*2/60 = 3.3 Hz electrical "slip" frequency and the currents in the rotor are 3.3 Hz (and huge).

But, if your rotor happened to get permanently magnetised somehow, then it would turn into a hybrid between an induction machine and a synchronous machine. In a synchronous machine, the torque is not proportional to "slip" but proportional to the angle between stator magnetic field and rotor position. In this case, useful torque is only created when the slip is zero and the rotor moves exactly in synchronism with the stator magnetic field. It's just possible that if this has happened to your rotor, then as you try to drive it fast, it starts slipping (due to friction) and the useful torque drops to zero. I've never heard of this happening, but perhaps the rotor is accidentally (permanently) magnetised somehow.

Its common in synchronous machine to include squirrel cage windings to aid in starting, and to introduce damping to reduce rotor oscillations. But, I never heard of anyone deliberately introducing a permanent-magnet (synchronous) machine behaviour into an induction machine.

This might not be whats going on in your machine, but its a remote possibility.

Andrew.