Variable frequency drives (VFDs) are not used in DC motors because DC motors already have a natural speed control and VFDs are designed to control AC motors. Additionally, DC motors require a different type of controller, known as a DC drive, to effectively control their speed and torque.
Variable frequency drives (VFDs) have become a popular choice for a wide range of industries because of their ability to control and regulate the speed and torque of motors. These drives are primarily used to manage the performance of AC induction motors that are used in a wide variety of applications. However, VFDs are not used with direct current (DC) motors, and do you know why?
The fundamental difference between AC and DC motors
- The key to understanding why VFDs are not used for DC Motor control is to recognize the essential difference between AC and DC motors.
- Alternating current (AC) motors operate according to the principle of a rotating magnetic field, and the synchronous speed of the motor depends on the frequency of the supply voltage and the number of stator poles. The VFD can control the motor speed by changing the frequency of the supply voltage.
- A direct current (DC) motor uses a constant voltage source to generate a continuous magnetic field. The speed of a DC motor is proportional to the supply voltage and inversely proportional to the load torque.
- A comparison of the two motors shows that VFDs aim to control the speed of AC motors by varying the frequency of the supply voltage and thus the synchronous speed of the motor. However, this speed control method is not suitable for DC motors because they use a constant voltage and the speed depends on the magnitude of the supply voltage rather than the frequency of the supply voltage.
- In addition, VFDs use complex algorithms to modulate the voltage and frequency applied to the motor in a synchronized manner. This process is called pulse width modulation (PWM) and is tailored for AC motor control. Applying a VFD to a DC motor not only fails to regulate its speed, but can also damage the motor due to improper application of voltage and frequency.
VDF controls the speed of the motor, mainly by changing the voltage frequency. The speed of the DC motor depends on the voltage, so using VFD in DC motor control is not only inefficient but also may lead to motor damage.