bogie_engineer The simple answer is that there is no fear of a runaway motor during a wheelslip; it runs at whatever the controlled speed of the applied frequency creates. So it can be pushed beyond what the DC wheelslip system can control. Dave
The simple answer is that there is no fear of a runaway motor during a wheelslip; it runs at whatever the controlled speed of the applied frequency creates. So it can be pushed beyond what the DC wheelslip system can control.
Dave
Far be it for me to dispute someone "on the ground" during the transition from DC to AC tractions motors, but the AEM-7 electric locomotives derived from Swedish design had a control system for DC motors allowing heretofore unheard levels of tractive effort as a fraction of axle weight. Whereas the control system for an advanced wheel-slip control is simpler for an AC motor, it is still possible for a DC motor?
Where I see the AC motor as winning out is 1) dispensing with the commutator to transmit electric power to the rotor. The AC induction motor transfers electric power to the rotor through contact-free electromagnetic induction, hence, induction motor. The commutator can required maintenance and repair, can "flash over", and had required cutting back on motor current (hence tractive effort) when the wheels are bouncing on crossovers or other trackwork. 2) the rotor of an induction motor replaces many turns of wire that need to be insulated with "shorting bars" or a "squirrel cage" of heavy copper bars embedded in steel. This means the rotor of an AC motor is rugged mechanically and thermally, allowing a rotor geared for low-speed lugging to not fly apart when run at higher RPMs than the more mechanically and electrically complicated DC motor rotor. The rotor of an AC motor is much less susceptable to damage from heat at high tractive effort that damages the insulation in a DC motor. Finally, 3) AC induction motors with variable frequency electronic "drives" are much more energy efficient than DC motors, especially at low speed and high torque, which combined with the simpler design of the AC motor rotor, further gives them the advantage of producing high torque at low rotation speeds in drag service without shortening the life of the motor.
I read that Norfolk Southern was a holdout keeping with DC motors, where for some time DC locomotives were less expensive per locomotive unit, where for some time after the introduction of AC units, you had to run drag freights that lugged locomotives for them to pay, or at least in the opinion of NS management.
If GM "killed the electric car", what am I doing standing next to an EV-1, a half a block from the WSOR tracks?
To embellish a bit on what Dave wrote: AC induction motors are not quite constant speed, as the speed will drop a bit with increasing load - this is why a 4 pole induction motor is rated at a bit over 1700 rpm instead of the 1800 rpm for a synchronous motor, with high efficiency motors running closer to 1800 rpm than a low efficiency motor. The upshot is that it doesn't take much slip to cause a dramatic reduction in generated torque, which inherently limits the slipping. The adhesion control system acts to fine tune motor control to get the last bit of adhesion from a slow creep form of wheelslip.
A DC series motor needs to be experience a much larger amount of slipping before the torque falls off (this is where the "fear of a ruaway motor" comes from for DC series motors). This means that the adhesion control system must act quickly to prevent a high seed wheelslip, however the inductance of the field and armature windings will limit how fast the wheelslip control can act.
Some of the advantage of an AC motor over the DC series motor can be had by separately exciting the field windings as was done on the orignal AEM-7's.
Most of the advantages of AC traction motors are clear cut.
The simpler traction motors that lack features like the brushes and commutators found in DC motors means less maintenance, less potential or perhaps even complete elimination of the risk for flashovers and ground relay faults, etc. Easy for the average railfan such as myself to grasp that this all results in power that's more durable and reliable, cheaper to maintain, spends less time out of service, etc.
And thanks to the thermal load that they can handle, AC traction eliminates short time ratings. That means that they can work harder for extended periods of time all while visiting the shop less frequently than DC motored power does. Also easy for railfans to wrap their head around.
But what I personally don't understand and have never seen explained, is what perhaps is their biggest advantage over DC traction. Can someone explain to a layman like myself, what it is about AC motors that enables AC traction equipped locomotives to have increased adhesion compared to their DC motored counterparts?
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