DYNAMIC BRAKES

[Greenhornet]

HELP: HOW IS DYNAMIC BRAKEING ACHIEVED IN THE "AC" LOCOMOTIVE MOTORS? DO THE ELECTRIC MOTORS HAVE AN ELECTRIC EXCITION WINDINGS IN THE ARMATURES, OR ARE THE ARMATURES MULTI-POLE PERMANENT MAGNETS?

[timz]

Neither. Maybe somebody will explain how an AC induction motor works.

[M636C]

I spent some time looking at the explanation of traction and dynamic braking on the SD70 ACe. Firstly, the SD 70 ACe uses a three phase alternator coupled to the diesel engine to generate the power. This is rectified into DC at around 1000 volts from where it is fed to two inverters which convert it to variable voltage variable frequency alternating current which is supplied to three traction motors from each inverter. The induction motors have no windings on the armature. These are called "squirrel cage" motors because they consist of a set of metal bars arranged in a cylinder that vaguely resemble the bars of the circular cage wheel used by small rodents. The stator surrounding the armature carries all the windings and when supplied with an alternating current creates a rotating magnetic field whose speed is dependent on the number of windings and the frequency of the current. In these locomotives the motors, and the current supplied is three phase but the frequency from the inverters can vary from zero to a high frequency as required to suit the desired motor speed. The rotating magnetic field induce magnetic fields in the rotor, (hence "induction" motor) which rotates at a speed determined by the AC frequency and develops power dependent on the voltage and the resulting current in the motor. OK, that explains the operation as a motor. If the armature is driven, as it is by the weight of the train during dynamic braking, and the appropriate frequency suitable for the speed the motor is turning at is provided by the inverters to the stator windings, the motor will act as a three phase generator and this current will be converted by the inverters to DC at up to 1000 volts, just as is supplied by the alternator through the rectifier. This DC current is fed to the dynamic brake resistors exactly as it is in a DC locomotive and provides a load to the traction motors acting as generators, and this slows the train. So the SD70 ACe can be viewed as a DC locomotive as far as the inverters both in power and dynamic braking. Beyond the inverters the AC is varying in frequency and voltage (and direction) as required by the motors acting as motors or generators. Is that clear? M636C

[Deggesty]

Yes, I understand the system much better, now. 

M636C

If the armature is driven, as it is by the weight of the train during dynamic braking,

Wouldn't "velocity and weight," and not "weight" be the proper expression? A train standing still has weight, but a moving train has both velocity and weight, and dynamic braking is used to reduce the velocity.

Johnny

[tleary01]

Another way to look at AC drive motoring or dynamic brake is the term “phase shift” which is the degree to which the stator current frequency is either in an advancing  angle to the induced rotor current frequency = motoring, or is in a retarding angle = dynamic brake.  The rotor sensors of an AC traction motor produce a signal for RPM and direction of rotation.  Motoring is tractive effort while rotation is in the commanded direction, dynamic brake retarding effort while rotation is also in the commanded direction.  There is a third condition where rotation is opposite the commanded direction but the phase angle is attempting to change the rotation to the commanded direction.  The term for this is “rollback” and can occur when the train brakes are released while on a grade and the locomotives are pulled back while attempting to motor forward. 

 

When first made SD70MAC locomotives could not pull out of a rollback, instead they would switch to dynamic brake in an attempt to provide some retardation if the train continued to roll backward.  The train would have to be stopped (if possible) and helpers  added to get the train moving forward.  GE AC locomotives from the beginning could pull out of a rollback.  When Union Pacific was negotiating for the purchase of the SD90 it was made clear that the rollback feature was required.  Once EMD developed the software the rollback feature was applied to all EMD AC locomotives. 

 

The rollback feature is particularly important for distributed power operation.  The remote locomotives may be more than a mile away from the leading locomotives where they and a significant portion of the train may be on a different grade than the front portion of the train.  DPU does not indicate to the engineer what direction the remote consist is moving, he has to depend on the remote consist to provide its full tractive effort to move the entire train forward even if at the moment the remote consist may actually be rolling backward.

 

The way that the rollback mode works is that the locomotive will motor in the commanded direction even if it is actually moving in the opposite direction up to 4.5 MPH.  If the backward moving locomotive exceeds 4.5 mph it will then switch to full dynamic brake.  The rollback feature also provides for the traction motors to be “plugged” if an emergency situation occurs up to 4.5 MPH.