How is reversing a DC motor accomplished?

How is reversing a DC motor accomplished?

• A. Reverse speed by increasing voltage
• B. Short-circuit the armature
• C. Reverse polarity or field in armature but not both
• D. Reversing rotor direction

Answer: (C)

The direction a DC motor turns is set by the relationship between the armature current and the magnetic field. The torque the motor develops is proportional to the field flux and the armature current. If you reverse the current in the armature, you reverse the torque, so the motor spins the other way. If you reverse the polarity of the field, the direction of the field flux changes, which also reverses the torque. But reversing both the armature current and the field flips both factors and the torque ends up in the same direction as before, so the rotation doesn’t change. So, to reverse the motor’s rotation, you change the polarity in one path: either the armature or the field, but not both. Increasing voltage changes speed, not direction; short-circuiting the armature is dangerous and not a control method; and simply “reversing rotor direction” isn’t a valid control approach because rotation direction is determined by the relative polarities of current and field, not by some separate rotor setting.

The direction a DC motor turns is set by the relationship between the armature current and the magnetic field. The torque the motor develops is proportional to the field flux and the armature current. If you reverse the current in the armature, you reverse the torque, so the motor spins the other way. If you reverse the polarity of the field, the direction of the field flux changes, which also reverses the torque. But reversing both the armature current and the field flips both factors and the torque ends up in the same direction as before, so the rotation doesn’t change.

So, to reverse the motor’s rotation, you change the polarity in one path: either the armature or the field, but not both. Increasing voltage changes speed, not direction; short-circuiting the armature is dangerous and not a control method; and simply “reversing rotor direction” isn’t a valid control approach because rotation direction is determined by the relative polarities of current and field, not by some separate rotor setting.