Just checking you follow these numbers for AC induction motors…
Full Load (normal) torque Nm - the torque the motor will supply continuously in an industrial situation. ~ 73Nm on an 11kW 4 pole motor
Locked rotor torque - usually given as the ratio to full load torque if the motor were started DOL (direct on line) to 50Hz mains with rotor locked (not rotating). Basically irrelevant when using a VFD in an EV.
BTW it is deliberately kept low in design so DOL is possible without having to ring the power station before starting motor.
Maybe 140Nm on above motor.
The motor is 100% lost sync in this mode so the current (usually some large number) is not representative of anything to do with EV operation.
As I said, applies to DOL starting.
One fact that comes from it though is that the motor is designed to carry such a large current for up to 60 seconds without damage. It will be hot in the windings (150deg C) and expect a rest after ! Such is the industrial world !
Break down or pull out torque is the max torque that the motor can supply before ‘losing the plot’ i.e. losing sync with supply frequency.
Usually given as a ratio, typically 3 times the normal torque at rated voltage and frequency.
Around 220Nm on above motor.
Note this is at normal voltage and frequency. When operating on a VFD with load dependent (not fixed) V/F (voltage to frequency ratio) this is a variable up to nearly 5 x at low frequencies provided the controller has the current capacity.
Bargain on at least 3 x at synchronous revs (1500RPM)
Induction motor slip. At no load the rotor rotates at the magnetic field speed of the stator (supply frequency) 1500RPM on 4 pole motor at 50Hz. As the load is increased the rotor speed slips by a few percent so a magnetic field can be generated in the rotor to pull it along.
Most motor manufacturers have similar offerings (each has the best !).
You need to check kW for size and weight as these vary.
Some variation in pull out torque as well. (more is better)
Limiting voltage on the motor is the point of magnetic saturation.
Premium efficiency motors are less sensitive to over-voltage than standard efficiency motors.
2 and 4 pole motors are less sensitive to overvoltage than 6 or 8 pole.
Around 500V at 50Hz seems to be the limiting voltage on the above 11kW (old) motor that I have.
Remember this would be 1000V at 100Hz. (not practical from battery pack or motor winding insulation point of view)
500V is around 20% over normal and this gives torque increase of 40% from most data I have seen. This seems to correspond to the Danfoss controller’s overall 466% indication. Remember this is a low revs.
Does any of that help ?
