AC vs. DC Question

General discussion about locomotives, rolling stock, and equipment

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Re: AC vs. DC Question

Postby Pneudyne » Wed Jan 18, 2017 1:23 am

Sepex DC motors have been used on electric locomotives since the 1970s. A typical operating mode was an initial phase of fixed maximum excitation, constant current acceleration to around 40% of maximum speed, referred to as the baseline speed, then constant armature voltage with steadily decreasing excitation to around 80% of maximum speed, and finally proportional excitation to maximum speed. The first phase is essentially constant torque (constant tractive effort), the second phase is constant power, and the third phase is steadily decreasing power. During the first and second phases the motors operate in shunt mode. During the third phase they emulate series motors. Fine control of motor excitation is individual, to ensure proper load sharing.

The diesel-electric locomotive case is somewhat more complicated, in that the locomotive as a whole operates at constant power at above say 10 mile/h, and both motor armature voltage control and excitation control would need to account for this.

Anyway that the sepex motors operate in shunt mode over much of the speed range should confer slip resistance properties similar to those of AC polyphase induction motors. In electric locomotives at least, slip control can be done by a combination of clamping the field and reducing armature voltage.

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Re: AC vs. DC Question

Postby Jay Potter » Wed Jan 18, 2017 3:52 am

I'm having some difficulty differentiating between wheel slip issues and creep issues in this discussion; and so I'll offer some comments that I hope will be generally relevant.

I view creep as being basically controlled slippage. Before creep control was developed in the 1980s, the objective of locomotive traction control systems was to detect slippage and stop it. The objective of creep control is to allow a limited amount of slippage to occur because it increases adhesion above the level that would exist if there were no slippage at all.

I might be mistaken; but it would surprise me if creep could be regulated as precisely by varying the volume of DC current as it can by varying the frequency of AC current.

Precise creep regulation is important because the slower a locomotive's speed, the more difficult it is to regulate creep. This is because creep rate is a percentage of wheel speed. So the slower a locomotive is moving, the smaller (i.e. more precise) the creep adjustments will have to be. Also, the slower a locomotive is moving, the greater the risk of stalling the train. For example, if a train that is moving at eight miles per hour loses some adhesion and slows down, its traction control system will have a greater opportunity to regain adhesion before speed drops to zero than it would have if the train had been moving at only two miles per hour.

As Allen has pointed out several times, one advantage of AC traction locomotives is that they can continue to operate at slow speeds (i.e. where tractive effort is a function of adhesion, not of speed) for far greater periods of time than DC traction locomotives can operate without derating themselves in order to protect their traction motors from thermal damage. That's part of the theory behind AC-traction locomotives that have six axles but only four traction motors. Not only can the four AC motors produce the same total amount of tractive effort that six DC motors could produce; but also they can continue to produce those amounts of (low speed) tractive effort for longer periods of time than the DC-traction locomotives could produce comparable amounts.

I believe that the preceding factors explain why DC-traction locomotives having a separate adhesion control system for each axle have not seen much service in this country.
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