DC Motor Controls

DC Motor Controls

DC Motor Controls

            Motor controls are used to start and stop motors during operation. Motors draw excess starting current which can damage electrical circuits if not controlled. Motor starting control circuits act to reduce the starting current. Motor braking involves converting the motor to a generator using the accumulated kinetic energy. The generated current acts against the current from the power supply thus reducing its effect on the motor. This is a paper on the starting and stopping controls of a DC motor.

DC Motor starting circuit

The emf (E) across the motor is composed of the back emf and the emf across the armature i.e.  E = Eb + IaRa. When the motor is not running, there is no back emf which means all current in the circuit is the armature current. This is the amount of current which will act as the starting current in the motor (DC Electro-Craft Corporation, 1972). The high initial current can cause several electrical damages such as blowing fuses and damaging the commutator brushes and the armature windings. The current can also cause high torque which can cause mechanical damage such as dislodging the armature.

The motor starting circuit controls the starting current by running it through a resistor in series with the motor. A time-delay relay is used to momentarily shunt the current around the resistor before the contactor is energized. When the relay gets energized, it closes the contacts and effectively shorting out the resistor. A smaller relay controls the operation of main relay. Its operation depends on the charging rate of an electrolytic capacitor. A potentiometer is used to control the charging rate of the capacitor and in turn the opening and closing of both relays (Sokira & Jaffe, 1990).

When the switch is at the “on” position, current flow simultaneously flows through the armature windings and the electrolytic capacitor. The capacitor charges up till it attains enough energy to actuate the smaller relay. After the small relay closes its contacts, the bigger relay is steadily energized till several amperes are running through it enough to close its contacts through electromagnetic induction. When the main relay closes its contacts, the series resistor is cut out and the motor runs at full speed (Electro-Craft Corporation, 1977).

DC Motor braking

DC motors are stopped through method of motor braking referred to as dynamic braking. The concept of dynamic braking is hinged on the principle of eddy currents. Eddy currents are swirling currents caused by a changing magnetic field. According Lenz’s law, the eddy currents cause a magnetic field which opposes the change causing it. For this to happen, electrons must swirl in a plane perpendicular to the magnetic field. When eddy currents oppose the change causing them, they cause loss of kinetic energy. Kinetic energy is dissipated as heat due to the eddy currents resistance (Electro-Craft Corporation, 1977).

The braking method is dynamic because the braking force reduces as the speed of rotation reduces. The interaction between applied magnetic field and the field set up the eddy currents slows down the armature rotation speed. The faster the speed of rotation, the stronger the opposing effect.  As rotation slows down, the opposing force decreases linearly, this produces a smooth stopping effect (DC Electro-Craft Corporation, 1972).

During the dynamic braking of a DC motor, the field windings are still connected to the current supply while the armature is disconnected from the supply terminal and connected to a high resistance. The motor now acts as an electric generator converting the kinetic energy of the armature rotation to electrical energy. Eddy currents are produced which oppose the rotation of the armature bringing it to a stop.

 

References

DC Electro-Craft Corporation. (1972). DC motors speed controls servo systems: An engineering     handbook. Hopkins, Minn: Electro-Craft Corporation.

Electro-Craft Corporation. (1977). DC motors, speed controls, servo systems: An engineering     handbook. Oxford, Eng: Pergamon Press.

Sokira, T. J., & Jaffe, W. (1990). Brushless dc motors: Electronics commutation and controls.          Blue Ridge Summit, PA: Tab Books.

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