Every day on the subway, I can see big signs by the track telling the drivers when to engage and disengage 'weak field'. It's where the line goes from being suburban to dense urban.
Using a lower field on a DC motor decreases torque and decreases back EMF permitting higher speed operation. This I understand, and its clear why weak-field mode would need to be engaged for suburban operation. The top speed would be limited otherwise.
But what about acceleration. I'd assumed that it would worsen acceleration, by reducing torque for a specified rotor current. But, after speaking to a driver, they basically said, the acceleration is fine - you just need to give it an extra couple of notches on the speed control, and that they'd use weak-field all the way if it was allowed.
As you've guessed - operational procedures are that it isn't allowed. In fact, the weak field switch when activated puts up a big bright yellow/black striped flag in the cab window, so that supervisors can make sure that the drivers have the switch set correctly.
Obviously, the limited top speed at full field, means that failing to engage it, means the trains may not run fast enough to meet timetables.
But what's the problem with leaving weak-field mode on for the urban sections?
I'm guessing it's probably due to motor and power stress, and energy efficiency - but I'm just guessing. Any ideas?
Using a lower field on a DC motor decreases torque and decreases back EMF permitting higher speed operation. This I understand, and its clear why weak-field mode would need to be engaged for suburban operation. The top speed would be limited otherwise.
But what about acceleration. I'd assumed that it would worsen acceleration, by reducing torque for a specified rotor current. But, after speaking to a driver, they basically said, the acceleration is fine - you just need to give it an extra couple of notches on the speed control, and that they'd use weak-field all the way if it was allowed.
As you've guessed - operational procedures are that it isn't allowed. In fact, the weak field switch when activated puts up a big bright yellow/black striped flag in the cab window, so that supervisors can make sure that the drivers have the switch set correctly.
Obviously, the limited top speed at full field, means that failing to engage it, means the trains may not run fast enough to meet timetables.
But what's the problem with leaving weak-field mode on for the urban sections?
I'm guessing it's probably due to motor and power stress, and energy efficiency - but I'm just guessing. Any ideas?
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