Since electric motors are essential for running your business, the question of which is better, variable-speed vs. fixed-speed motors, comes up a lot. While these electric motors use similar components to convert electrical energy into motion, each type has been designed and built to serve uniquely different purposes. While most would assume that a variable-speed motor would be the default choice, the correct answer depends on the application requirements discussed below.
Fixed-Speed Electric Motors
Sometimes called a constant-speed motor, a fixed-speed motor typically combines an induction motor with a clutch and brake through a power-on mechanism. When maintaining a constant speed is essential to performance or quality, fixed-speed motors are tough to beat. Their simpler design requires fewer moving parts, which improves reliability and lowers repair costs.
The most common examples of fixed-speed motors include.
- Brush DC electric motors
- AC squirrel cage electric motors
- Brushless DC electric motors
- Wound rotor electric motors
- Servo electric motors
Fixed-speed motors are ideal for repetitive start-stop operations as well as frequent reverses. The ability to deliver reliable and predictable power makes them essential for automated processes and running machines across dozens of manufacturing and assembly facilities. Industrial machinery typically uses fixed-speed motors for conveyors, compressors, pumps, wood shaping, and cooling fans. Commercial applications include grinders, saws, drills, and air conditioning units.
AC Fixed-Speed Motor Options
Include synchronous motors operating at the power supply’s synchronized frequency and low-slip induction motors designed to minimize slip during start-up or braking.
DC Fixed-Speed Motor Options
DC motors with parallel windings are well-suited for applications requiring consistent and predictable rotational speed. Brushless DC motors, while not typically fixed-speed motors, can be electronically controlled to provide constant speed levels.
Variable-Speed Electric Motors
For these electric motors, the speed is controlled by the frequency of the alternating current (AC) supplied. Because of this, variable-speed motors are called variable-frequency drives (VFDs). In addition to providing greater control and flexibility, running a VFD at a lower speed can provide up to 90% efficiency to lower utility bills month after month. The most common uses for variable speed motors include elevators, conveyor systems, irrigation and pumping applications, and cranes.
Adding a Variable Frequency Drive
Most motors made today are “inverter rated,” meaning that you can add a variable frequency drive to improve performance and reduce energy consumption. Check the motor nameplate for motors over 5-10 years old and see if it includes the term “CT/VT” (Constant Torque/Variable Torque) on the tag. The term “PWM” indicates that the motor is rated for a Pulse Width Modulation drive option. Inverter-rated motors are designed to deal with voltage spikes and run at lower speeds without the risk of overheating.
A VFD can be added to the following types of inverter-rated electric motors.
- AC Synchronous Permanent Magnet Motors
- AC Synchronous Brushless Motors
- AC Synchronous Wound Rotor Motor
- AC Synchronous Squirrel Cage Motors
The following motor types are NOT inverter-rated for adding a VFD unit.
- Pre-EPAct Motors (pre-1992): Can only be run with a VFD if they are VFD rated, have Class F insulation or higher, and the VFD has a constant torque ratio of no higher than 2:1.
- EPAct Motors: With a 2:1 constant torque rating and a 4:1 to 10:1 variable torque rating. Additionally, fractional and 56-frame motors are not VFD rated.
- NEMA Premium Motors: With a 4:1 to 20:1 constant torque rating and a 10:1 to 20:1 variable torque rating.
VFDs can cause voltage spikes within the motor windings, so ensure that the electric motor insulation is rated at Class F or higher before adding a VFD unit. Additionally, as the extra voltage builds up on the motor shaft, it can cause the lubrication to break down and damage the bearing. This potential damage can be avoided by installing insulated bearings and shaft grounding rings to the motor when adding a VFD.
Types of Variable Frequency Drives
A VFD converts AC power to DC before sending out a DC pulse that acts like an AC sine wave to control the motor speed. The most common and cost-effective VFD is the Voltage/Hertz option, which manages the voltage and frequency to provide speed control. Vector Control VFDs are much more complex but provide precise control across the entire speed range while providing holding torque (full torque at zero speed).
As you can see, fixed- and variable-speed motors and VFDs can improve your facility’s efficiency and production. While this discussion covers the fundamental differences between the two, due to the complexity of most commercial applications, the first step of the decision-making process should involve a consultation and site inspection. As part of our commitment to keep you up and running, the Hi-Speed Team of fixed- and variable-speed motors experts is ready to answer your questions or set up an initial consultation and inspection at your convenience.
Based on your interest in variable and fixed-speed electric motors, here are some other articles that you might find helpful:
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