Which DC motor type provides high starting torque and reduces with RPM?

Which DC motor type provides high starting torque and reduces with RPM?

• A. Compound Motor
• B. PM Motor
• C. Series Motor
• D. Shunt Motor

Answer: (C)

The main idea is how torque depends on flux and current. In a DC series motor, the field winding is in series with the armature, so the same current that drives the armature also creates the field. At startup, back electromotive force is almost zero, so the current is large, the flux is strong, and the motor develops a very large torque. As the motor speeds up, back EMF grows, the current falls, the flux weakens, and the torque decreases. This gives high starting torque that diminishes as RPM increases. Other types don’t show this sharp drop in torque with speed: permanent-magnet or shunt-field motors have fixed or more constant flux, so their starting torque is not as high and their torque behavior with speed is different.

The main idea is how torque depends on flux and current. In a DC series motor, the field winding is in series with the armature, so the same current that drives the armature also creates the field. At startup, back electromotive force is almost zero, so the current is large, the flux is strong, and the motor develops a very large torque. As the motor speeds up, back EMF grows, the current falls, the flux weakens, and the torque decreases. This gives high starting torque that diminishes as RPM increases. Other types don’t show this sharp drop in torque with speed: permanent-magnet or shunt-field motors have fixed or more constant flux, so their starting torque is not as high and their torque behavior with speed is different.