The terminal voltage Vt of the source connected to the armature is suddenly reversed keeping the field current unchanged.

The net voltage acting on the armature circuit becomes (Ea + Vt) and it will produce enormous reverse current, perhaps 50 times greater than the full-load armature current.

We must limit the reverse current by introducing a resistor in series with the reversing circuit.

When the machine reaches zero speed we must immediately open the armature circuit, otherwise it will begin to run in reverse direction.

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