If there is no slip then the current in the rotor is zero, no matter what the current is in the stator. Rotor has a L/R ratio, and has an amount of slip that produces the maximum current, more than this slip and the current in this rotor drops off due to the rotor inductance. Talking about motors designed for 50 /60 Hz mains, they have to start at 100% slip, so this L/R ratio is made to allow a lot of slip so that the rotor current will be high at start, but this is a compromise with efficiency and rotor heat as to do this the rotor resistance is made high. A motor designed for inverter usage will have a low resistance rotor and less slip, so less heat. This may just be a function of the motor type in this case we are discussing, it may be a motor designed for good starting torque and low starting current. High stator starting current does not mean high starting torque, it is more likely to mean the opposite, not always though. There are many different rotor designs, some have two cages, one for starting and another for running.
-----Original Message----- From: EV [mailto:ev-boun...@lists.evdl.org] On Behalf Of Cor van de Water via EV Sent: 14 February, 2017 7:51 AM To: Electric Vehicle Discussion List Subject: Re: [EVDL] AC induction motor, rotor heating up.. I think it is possible to "over-drive" an AC induction motor, that is why commercial drives can be configured for an acceptable slip between electrical motor frequency and actual mechanical speed of the rotor. The higher the voltage on the AC motor, the higher the torque and the lower the slip, but also the higher the current in the rotor. Unnecessarily high current just wastes power and heats up the motor, without significant reducing the slip, so you can lower the motor voltage to the point where the rotor slip is just acceptable for the torque you are demanding from the motor. If you need higher torque or speed, then you will need to increase the voltage on the motor but in steady-state / low torque operation there is no need for full AC voltage so commercial drives can modulate their output to lower voltage based on acceptable slip / torque demand. It is also needed to increase the max RPM that the drive can run the motor, for a certain supply voltage The effect is similar to the "filed weakening" in DC motors. You see the SepEx motors also having an algorithm of how to apply enough field to deliver the work without constantly burning up the field coil. Hope this clarifies, _______________________________________________ UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub http://lists.evdl.org/listinfo.cgi/ev-evdl.org Read EVAngel's EV News at http://evdl.org/evln/ Please discuss EV drag racing at NEDRA (http://groups.yahoo.com/group/NEDRA)
