Single Vs. Variable Speed Motor

Single speed motors operate at a constant rate and a set level of torque. They are commonly found in household appliances such as washers, dryers and vacuum cleaners. Variable speed motors can function at multiple speeds and torque levels, and are common in ventilation systems, conveyor belts and elevators.

Frequency

While different types of variable speed motors alter speed in different ways, one of the most common is the variable frequency electronic drive (VFD). According to the "Energy Efficiency Manual," these alter the motor speed by changing the frequency of the current powering the device. Single speed motors do not alter frequency in this manner.

Current

The most common single speed motors use AC (alternating current) power. This is the type of current that comes out of a standard wall socket. In contrast, variable speed motors often use DC (direct current) power, which comes from batteries.

Power

According to Oddparts.com, single speed motors are more cost effective in devices requiring more than 10 horsepower. Variable speed motors are common in applications requiring less than 10 horsepower, though they are often required in more powerful devices.

References

• Oddparts.com: Overview of Motor Types Tutorial
• Energy Institute Press: Energy Efficiency Manual: Note 36: Variable-Speed Motors and Drives