What is the frequency range for AC locomotive drives? At stall, I assume it would be the slip frequency of the traction motor. What is the stall freqency and what is the highest frequency at max. speed?
While I don't know the maximum frequency used, I would be surprised if it were much above 100 Hz. Slip frequency is very low in interest of high efficiency and reducing cooling needs. Starting torque is not an issue due to the variable frequency output of the inverter.
- Erik
erikem While I don't know the maximum frequency used, I would be surprised if it were much above 100 Hz. Slip frequency is very low in interest of high efficiency and reducing cooling needs. Starting torque is not an issue due to the variable frequency output of the inverter. - Erik
The working range for the Siemens Inverters used in EMD locomotives was 40 - 400 Hz. That is according to the shop manual.
Would that mean the magnetic field in the traction motor is always rotating at 600 RPM or more, even when the locomotive is just starting?
How could an AC motor always run at the same speed if the frequency varied? And why 600 RPM?
The AC motors on modern diesels and electrics are not true synchronous motors but hysterises non-synchrnous or some similar variation. If you look at the rotors you will see the horxzontal bars are not exactly horizontal, but slanted, and in fact some slippage from synchonous speed is necessary for torque to be developed. If the slippage is too great, torque is lost completely. This certtainly prevents overheating and burnout, but it does mean the computer control of frequency must be done very carefully.
There are multiple field poles, and the rotation speed of the magnetic field would be the inverter frequency divided by the number of poles.
Thanks beaulieu for your reply. Those were the numbers I was looking for. I assume that the GE AC inverters would operate over a similar range. Anyone know?
Bill
The SD60MAC demonstrators operated at rougly 100Hz at 70 mph. We had to test them at that speed to insure the wouldn't interfere with the cab signal track circuits that used 100Hz carrier.
-Don (Random stuff, mostly about trains - what else? http://blerfblog.blogspot.com/)
Don,
That makes sense, go much higher in frequency and you'll get dinged by skin effect and eddy currents. Most large motor practice is based on 50 and 60Hz designs, and it would make sense that the manufacturers stick with a frequency range they are comfortable with.
Efficiency concerns would suggest a slip frequency of 1Hz or less, so the inverter would have a minimum frequency of less than 1Hz. This is not a problem with a well designed PWM inverter.
The figures that I quoted were the GTO switching frequencies rather than the output frequency of the 3-phase power supply. Supplied 3-phase power to the traction motors is quoted at 110 Hz at 65 mph. The GTOs themselves are operating at their fastest switching rates at higher power outputs below 8 mph. At full power below 8 mph the inverter GTOs are in Asynchronous mode and will switch at 360 Hz. From 8 to 16 mph the Inverters function in a mode called Sinusoidal Switching, from 16 mph to 21 mph they are in Rectangular Pulse mode. Finally from 21 mph to 38 mph they function in Block Mode. Above that point you cannot maintain maximum motor torque and output voltage is at maximum. The control computer manipulates output voltage and frequency to control the induced current in the rotor. The modes vary in how the Inverter generates the output waveform, the width, and the amplitude.
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