Unsure if this is required by the slant-bar hysteristes type. I think this type is self starting, and always runs slower than the rotating field, with greater load = greater lag in speed.
daveklepper Unsure if this is required by the slant-bar hysteristes type. I think this type is self starting, and always runs slower than the rotating field, with greater load = greater lag in speed.
The speed of this type of motor does not vary significantly with load within the capacity of the motor and power supply, a few RPM at most. Yes, it does turn a little slower than synchronous speed and it does fall off a few RPM from no load to full load.
When BN was testing the first AC SD60M they broke a pinion, removing the motor load. The motor continued to spin at the same RPM instead of bird nesting like a DC motor would have and they didn't even realize the pinion had failed.
Again this kind of drive has nothing to do with the drive system on a GG1 or any other electric or diesel locomotive produced prior to the early 90's.
Of course the motor without load will not go faster than the speed of rotation of the field. And the speed of rotation of the field is controlled by the frequency of the inverters. My impression is that the slant-bar version can be started from standstill, by deternmining the direction of field rotaton with a very low frequency start.
CastroPussSorry, but it is still not clear how locomotive AC dynamic braking process works, particularly with respect to the alternator, dynamic braking resistors, inverters, and DC Link capacitor. Only two aspects are (partially) clear at this time - 1) In order for the AC traction motors to generate power in dynamic braking mode, some 'excitation' power has to be applied to the traction motors stators. This excitation power either originates from the alternator, and/or from the DC Link capacitor. 2) Braking power generated by the traction motors is dissipated in the dynamic braking resistors. Again, more questions. 1) What supplies excitation power to the traction motors during dynamic braking? Is it from the alternator? Or is it from the DC Link capacitor? Or is it some combination of both the alternator and the DC Link capacitor? If a combination, how does that work? 2) As excitation power is, presumably, still being sent to the traction motors at the same time, how does the generated power from the traction motors while in dynamic braking mode get back to the dynamic braking resistors? The excitation power going to the traction motors would surely conflict with the generated power somewhere (the inverters?) before the generated power could reach the dynamic braking resistors? And wouldn't the generated power tend to charge the DC Link capacitors also? The status (either on, or off, or what?) of the alternator, dynamic braking resistors, DC Link capacitors, and inverters during (different phases of the) dynamic braking process would be helpful.
The easiest way of thinking about this is that the stator windings make a rotating magnetic field, driven by the invertor. If this field rotates faster than the rotor it will magnetically try to speed it up, taking power from the inverter, if the field rotates slower it tries to slow the rotor down, this generates power that feeds back in to the invertor. Unless it can feed that back to the supply (i.e. electric loco) the d.c. bus voltage rises, the resistors are switched in to dump that excess energy to keep the voltage under control. If the field is cut off the motor will just coast, with no electrical breaking.
There is another way to brake an a.c. motor that is to put d.c. into the windings this can cause a heavy braking action and prolonged use will overheat the motor. It is often used to stop machinery quickly in an emergency.
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