DC Motor Control

DC Motor Control

DC Motor Starting

When power is first applied to a DC motor, a high current can flow in the armature. To limit the damaging effect this sudden high current can have, power can be applied gradually.

Fig. 34 DC motor starting

The current-limiting device can be electromechanical, such as timed contactors which switch resistors out of the circuit, or solid state, such as a circuit which electronically limits the current flowing while the motor starts.

DC Motor Direction Control

The direction of rotation of a DC motor is changed by reversing the relative directions of the armature and stator magnetic fields. For the wound stator motor, this is done by reversing the voltage across the field winding. For the permanent magnet motor, the rotor voltage is reversed. Voltage is reversed using relays or contactors.

Fig. 35 Simplified direction control circuit

When relay K2 is energized, the polarity of the voltage supplying the motor is reversed.

DC Motor Speed Control

The speed of a DC motor can be controlled using Open loop or Closed loop techniques.

Open Loop Techniques

Open loop techniques for DC Motor speed control include Field Weakening and Armature Resistance control.

Field weakening involves placing a current limiting device such as a variable resistor in series with the field winding. This causes the motor to speed up, so long as the armature voltage remains constant. There is a limit to the amount the field may be weakened.

Fig. 36 Field Weakening

Armature resistance control involves increasing the amount of resistance in the armature circuit. This causes a decrease in motor speed.

Fig. 37 Armature resistance control

Closed Loop Techniques

Closed loop control of DC motor speed involves using a servo system. This allows precise speed control under varying load conditions.

Fig. 38 Closed loop speed control of a DC motor

DC Motor Stopping

The simplest way to stop a DC motor is to remove power and let it coast to a stop. When this method is unsuitable, dynamic braking or mechanical braking methods may be used.

Dynamic Braking - If power is removed from the armature winding, a voltage still appears across it. This voltage is induced due to the magnetic field of the shunt winding. If the armature winding is now connected across a resistor, current flows through it. This current flows through the winding in the opposite direction to normal, as if the motor was being reversed. The motor quickly stops.

Fig. 39 Dynamic Braking circuit

Dynamic braking is only effective while the motor is turning. As soon as the motor stops, there is no induced current in the armature winding to produce a reversing force.

Mechanical Braking - a mechanical brake can be fitted around the shaft of the motor. This brake is actuated by an electromagnet. The brake may be made to be electrically held open or closed against the force of a return spring. Most brakes are considered fail safe i.e. spring applied, with electro-magnetic or hydraulic release.