AC Spindal Motor

Well, this is judged by the motor's rated pullout torque and multiplying by the square of the fraction representing the actual voltage compared to theoretical constant torque value.

So, if you have pullout torque of 200%, and 400V 100Hz on a 400V 50Hz motor, then ½² = ¼,, so ¼ x 200% = 50% pullout same as constant power torque. Then, you are in a stall situation, as you say. If your motor is nearer 300% po, then you have a little in hand although the motor is less 'stiff' i.e. slips more on load increasingly above 50% speed.. Only fractional power motors seem very weak in the way you describe, in my experience.

I've seen plenty of spindle applications using this and also 87Hz connected motors to get the higher speeds and therefore a much wider speed range, because the higher torque this offers is vastly more than selecting a constant torque range machine. Even if you could only rely on, say 45% torque, at double the speed, this then would be 90% of the constant torque alternative increasing to 200% at half speed! When many applications need increasing torque at lower speeds (constant power), this seems a better 'fit' to me.

Maybe I misunderstand you but this seems to me to be a no-lose selection. Of course, you should take other mechanical considerations into account in your selection - e.g. balance needed, fan noise (consumption), frame size, bearings etc into account.

If the "soll" signal at the frequency drive input is about 1.6 volt, than this has nothing to see with the output frequency that the spindle motor gets.

Inside the frequency drive, there is software that can rescale the input signal.

Per example, if you have an input signal from a pick up sensor between lets say -2.0 and +1.5 volts dc, than it is perfectly possible to use this signal for a speed regulation between 30 and 40 Hz.

The actual output frequency you can only verify at the display of the drive or by measuring the RPM of the motor. (if you know the number of motor poles …)

The differency between a spindle motor and a normal motor:

- spindle motors are designed to hold the shaft torque constant, for a much wider frequency span. This means at 10 Hz you get the nominal output torque, but also at 200 Hz you get the nominal output torque.

- because your input voltage is limited to 400V, the output of a frequency drive is also limited to 400V. In a normal motor the ratio voltage/frequency is kept constant by the frequency drive, to obtain nominal output torque. This means at 400 V @ 50 Hz, but also 1600 V @ 200 Hz. Unfortunally the frequency drive will limit the output voltage to the input voltage (400 V in this example).

So above the 50 Hz the ratio voltage/frequency will decrease, and this results in a decrease of the motor flux and a decrease of the nominal output torque.

- The solution for this problem is to build a motor with a winding that is made for 100 V@50 Hz.. At 100V@50Hz the motor has its normal magnetical flux.

If I increase the output frequency of the drive to 150Hz, the output voltage will also increase to 300V, the ratio voltage / frequency will still be constant, AND the frequency drive can deliver the 300V output voltage because it is lower than the 400 V supply voltage.

- in this case the motor gives nominal output (shaft) torque at a speed that is 3 times the nominal motor speed. Because output power equals to speeds times torque, the motor output power @ 150Hz will 3 times the power @ 50Hz.

- Conclusion, at a higher frequency, on can get more power out of a smaller motor. This means on can make a smaller machine with the same performance.