Controlling DC Shunt Motors: Techniques and Applications

Introduction to DC Shunt Motors

DC shunt motors are commonly used in various industrial applications due to their reliability and robustness. These motors are characterized by the connection of the field winding in parallel with the armature winding. The control of the speed of a DC shunt motor requires a deep understanding of its electrical characteristics and the methods available. This article provides a comprehensive guide on how to control a DC shunt motor, focusing on three main techniques: reverse polarity control, armature and field voltage control, and supply voltage control.

Controlling DC Shunt Motors Using Reverse Polarity

DC shunt motors are controlled by reversing either the polarity of the armature or the field winding, but not both simultaneously. To reverse the armature polarity, a switch or a reversible contactor circuit can be used, ensuring that the field winding polarity remains automatic control, a reversible contactor circuit can be employed to switch the polarity of the armature automatically.

Controlling DC Shunt Motors Using Armature and Field Voltage Control

The speed of a DC shunt motor can be controlled in several ways. Here are three common methods:

Armature Voltage Control

To control the speed by armature voltage, a variable rheostat can be connected in series with the armature winding. Another approach is to use a potential divider circuit. The speed variation using this method is limited to the rated value and above.

Varying the Field Excitation

The speed of a DC shunt motor is inversely proportional to the field flux, which is directly proportional to the field excitation current. By varying the field excitation, the speed can be controlled. A resistor can be placed in series with the field winding to achieve this. Speed variation below the rated value is not possible with this method.

Supply Voltage Control

The supply voltage to the motor itself can be varied to control the speed. This method requires better control over the supply voltage. By varying the supply voltage, speed variation is possible both above and below the rated value.

For better speed control, the supply voltage should be controlled using a Mosfet PWM circuit or SCRs, particularly for mains-powered applications. A Variac on the AC input is another reliable method to achieve variable voltage control.

Applications and Practical Examples

The techniques described above can be applied in various practical scenarios. For instance, in machine tool power feeds, the "rapid" mode can be achieved by switching the field voltage to a lower value, effectively providing a "high gear" mode. This is easily accomplished by using a half-wave rectified field supply for a motor designed to operate at mains voltage.

Conclusion

Controlling a DC shunt motor involves a thorough understanding of its electrical characteristics and the available control methods. By reversing the polarity, adjusting the armature or field voltage, or varying the supply voltage, the speed of the motor can be effectively controlled for a wide range of applications. Proper selection and implementation of these techniques can significantly enhance the performance and efficiency of DC shunt motors in industrial and other technical applications.

Frequently Asked Questions (FAQs)

Can both armature and field polarity be reversed simultaneously to control the speed of a DC shunt motor?

No, both armature and field polarity cannot be reversed simultaneously. Reversing either the armature or the field polarity is sufficient for speed control.

What is a Variac and how is it used in the context of DC shunt motor control?

A Variac is an adjustable voltage transformer that is used to provide variable AC voltage input to electrical equipment. In the context of DC shunt motor control, a Variac can be used to achieve variable voltage control, making it easier to control the speed of the motor.

Can a full-wave rectified field supply be used for a DC shunt motor?

Yes, a full-wave rectified field supply can be used for a DC shunt motor, but a half-wave rectified supply is often sufficient and more efficient for obtaining the desired speed control.