Electric motors play a crucial role in modern technology in every sector. To decrease motor energy consumption in energy-consuming devices and optimize their efficiency, manufacturers have deployed drive-technologies for modifying motors’ speed. Such technologies include variable frequency drives and variable speed drives for controlling motors in variable conditions.
Electric Motor Speed Control
Two methodologies to control electrical motors are:
- Variable Speed Drives (VSDs)
VSDs change a motor’s speed by changing input voltage. They can be used with DC as well as AC motors.
- Variable Frequency Drives (VFDs)
VFDs also control a motor’s speed; however, they do so by modulating the frequency and voltage. However, VFDs can be used only with AC motors.
VSDs vs. VFDs: Understanding the Difference
- Variable Speed Drives (VSDs)
Variable speed drives help in supplying specific voltage and amperage to a motor. If the supplied power is in AC form, variable speed drives use a rectifier circuit for converting the alternating current to direct current, a pre-specified amperage, and voltage that the drive is able to adjust. Changing or modulating the voltage of the direct current changes a motor’s speed. A direct current motor VSD may consist of a simple circuit, making them less expensive than variable speed drives.
Variable speed drives change an equipment’s speed while allowing the electric motor to operate at full speed. In alternating current motors, AC passes via distributed stator winding for creating a magnetic field which is used for driving a shaft. Alternating current motors drive machines such as pumps, compressors, and fans at a constant speed. They are also found in HVAC systems.
The torque and rotational speed of an alternating current motor are usually determined by the supply’s voltage and frequency. As the electricity supply is constant, the motor’s speed remains constant. However, if the varying speed of a motor is required, then a variable speed drive would prove to be effective.
Let’s take up the example of an HVAC fan. When the fan’s speed needs to be decreased, it can easily be controlled for slowing down the speed, reducing the flow of energy, and therefore, reducing overall costs and energy consumption.
- Variable Frequency Drives (VFDs)
Variable frequency drives control the voltage and frequency of an alternating current signal. Adjusting an AC motor’s frequency controls the motor’s speed while modulating the voltage changes the torque which is provided by the motor. Additionally, VFDs are used for controlling electric motors’ slow-down and start-up speed and for preventing overcurrent in an electric motor.
These devices are effective at increasing the efficiency of a motor in different alternating current motor applications, wherein controlling the speed is important. VFDs may even be needed in applications where conservation of energy is a priority.
VFDs are capable of varying the speed control at start up, during running, and when it stops. A standard alternating current has a specified power factor and efficiency.
When operated on a variable frequency drive, the supply of power to the motor includes a reasonable harmonic content which doesn’t work; however, it adds up to the motor loss, diminishing the motor’s efficiency. As the speed reduces, the condition worsens further.
Variable frequency drives are usually equipped with bypass starter schemes for enabling a pump when this drive fails. Several VFDs are equipped with the feature of air conditioning for maintaining a safe temperature for operations. A few designs require customized harmonic filters for meeting harmonic distortion limits. Such solutions come at an additional hardware cost. Furthermore, there is a substantial cost for installing all such equipment or designing additional space for new facilities. While calculating the system’s presumed efficiency, the additional power required for operating the additional hardware is usually ignored.
Both VFDs and VSDs can decrease the unnecessary power consumption of an electric motor, optimizing the operation and efficiency of systems that utilize electric motors. However, though both of them rely on the manipulation of power that a motor receives, VSDs and VFDs have critical differences. Therefore, before making your final decision, it is advisable to consult an expert professional who has extensive knowledge about speed drives and practical experience in the electric field.
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