In a series DC motor, the armature winding and the field winding are connected in series, and the currents through them are equal, since the armature winding and the field winding of a DC parallel motor are connected in parallel. The current in a parallel motor is divided into two parts: the current through the armature and the current through the field winding, and the total current is the sum of the two parts. The structure of a DC shunt motor is the same as that of a DC motor, it contains all the basic components including the stator (field winding), rotor (also called armature) and commutator.
Stator/Parallel Winding
The input power is supplied to the fixed element of the motor, that is, the parallel winding. The shunt winding consists of several turns of winding on the coil. Since the number of turns consists of thinner wires, the size of the parallel winding is quite small. Unlike the wire diameters in the windings of a series motor, the parallel windings in this motor cannot carry very large currents.
Rotor/armature
The armature, commonly referred to as the "rotor", handles the shaft load, and it has a thicker wire diameter that can support higher currents. When the motor is starting or running at low speed, high current flows through the armature. As the speed of the motor increases, the armature generates a counter-electromagnetic force, which acts against the current in the armature.
commutator
Devices such as commutators and brushes provide current from the static field windings to the rotor, and torque in a motor is created by the interaction of the magnetic fields of the windings and armature.
working principle
When voltage is supplied to a parallel DC motor, it produces very low current due to the high resistance of the parallel winding, and the high number of turns of the parallel winding helps to create a strong magnetic field. The armature draws a high current, resulting in a high magnetic field. When the magnetic fields of the armature and parallel windings interact, the motor starts to spin. As the magnetic field increases, the rotational torque increases, resulting in an increase in the speed of the motor.
Parallel DC motors have a feedback mechanism that controls the speed, and when the armature rotates in the magnetic field, a current is generated. This electromotive force is generated in the opposite direction, thereby limiting the armature current. Therefore, the current through the armature is reduced and the speed of the motor can also self-regulate. Parallel windings, due to their thin wire construction, cannot withstand the high starting currents of series motors, so parallel motors are used to handle small shaft loads that initially only require low torque.
Motor speed
In a series motor, the speed depends entirely on the shaft load, and in a series motor, the load is inversely proportional to the armature speed. If the load is high, the armature will rotate at low speed. If the load is low, the armature speed will increase. The speed of the armature is infinite or uncontrolled at no load.
Unlike series motors, the speed of parallel motors is independent of shaft load, and as the motor load increases, the speed of the motor will momentarily slow down. Slowing down reduces the back EMF, which increases the current in the armature branch, which results in an increase in motor speed. On the other hand, if the load is reduced, the motor speed will momentarily rise, which in turn will increase the back EMF, thereby reducing the current flowing to the motor. Gradually, the motor will slow down. Therefore, the DC parallel motor is able to maintain a constant speed regardless of load changes. Due to this feature, the motor is used in automotive and industrial applications where precise motor speed is required.
Motor speed control
The speed of a DC shunt motor can be controlled in two ways:
By changing the current supplied to the rotor
By changing the current supplied to the stator
Since the voltage around the rotor and stator is the same, the speed of the motor can be controlled by controlling the current through the stator or rotor, changing its resistance is generally controlled using a thyristor. The resistance of the parallel winding and the armature branch can be increased or decreased by connecting a varistor in series. Since the current handled by the armature is much higher than that of the field winding, the varistor that controls the current in the armature branch is quite large, which is what is in the field winding. Reasons why current-controlled rheostats are preferred.
The shunt field current can change the speed of the motor by 10-20%, and as the current through the parallel windings increases, the speed of the rotor increases, creating a higher back EMF to maintain an equivalent reduction in armature current. Conversely, by reducing the current through the parallel windings, the speed of the motor can be reduced.
When a parallel DC motor is run at a voltage lower than its rated voltage, its speed also decreases, but this makes the parallel DC motor inefficient and has a tendency to overload and overheat. Generally speaking, electric motors have a rated speed in units of speed and rated voltage. When a parallel DC motor is below its full voltage, its torque is reduced, therefore, it is recommended not to operate the motor below the specified rated voltage.
in conclusion
Due to their automatic speed regulation capability, DC shunt motors are ideal for applications requiring precise speed regulation, they cannot produce high starting torques, so the load at start must be small. Applications that meet these standards and are suitable for DC shunt motors include machine tools (such as lathes and grinders) and industrial equipment (such as fans and compressors), centrifugal pumps, elevators, looms, lathes, blowers, fans, conveyors, spinning machines Wait.
