AC versus BLDC Motors

Q. If AC induction motors are more common, and potentially cheaper than a BLDC motor, why would operators select a BLDC motor for a pump application?

A. Motors AC induction motors and BLDC motors are similar; the primary difference is in the construction of the rotor.

An AC induction motor does not have any magnets on the rotor; instead it has a series of laminations and winding. When 3-phase power is applied to the stator of the motor, a rotating magnetic field is generated. This rotating magnetic field creates a current flow in the rotor via induction. The rotor current creates its own magnet that interacts with the stator field and generates torque.

Most AC induction motors can be run directly off of AC power with no controller, but if variable speed is required—as is the case in many pump applications—this advantage is eliminated since a VFD must be installed between the AC power and the motor.

The VFD changes the speed of the motor by altering the frequency of the power provided to the motor. For example, a motor rated at 1800 rpm and 60 Hz can be slowed down to 900 rpm by running it at 30 Hz. Even with a VFD, industrial AC induction motors have a limited speed range of about 30 to 130 percent of rated speed. They are not optimal for delivering rated torque at very low speeds, or when stalled.

Alternatively, a BLDC motor replaces the windings on the rotor with a series of permanent magnets. These magnets create a magnetic field that interacts with the stator’s field and generates torque. However, rather than simply relying on the 3-phase power to generate a rotating magnetic field, a BLDC motor requires the stator’s magnetic field to be precisely controlled and aligned with the rotor position and its fixed magnets. The stator field is controlled by a device that is all but identical to a VFD used with an AC induction motor but with one additional input; a shaft encoder attached to the rotor is required to help the motor controller keep the rotor and stator fields in proper alignment. The precise control of the stator’s magnetic field allows complete control of the motor including speed, torque and acceleration. A BLDC motor can generate full torque at zero speed. The motors are usually smaller for any given power level, and the rotor with permanent magnets is lighter than a corresponding induction rotor. Both of these traits allow a BLDC motor to respond much faster to changing load condition.

There are several advantages and features that only a BLDC can offer, including: higher efficiency, precise control of torque and speed, lower rotor inertia and smaller size.

In addition to the lower inertia and better torque control—which allow the pump to respond faster to changes in demand, precise pressure control and the ability to “dead head” the pump while maintaining pressure—a BLDC motor’s inherently fast response allows the mechanical connection to be significantly simplified.

Both types of motors require a method to convert the rotary motion of the motor to the linear reciprocating motion of a positive displacement piston pump. An AC induction motor’s relatively constant speed operation and slow dynamic response requires a complicated mechanical mechanism to accomplish this. For example, a cam or yoke arrangement may be used. Oftentimes, these devices are two to three times larger than the actual motor. They also have wear points and bearings that can easily break or wear out, and require costly maintenance or replacement.

In an AC induction motor driven pump, the AC motor, gearbox and a rather large cam drive system are all separate, but are all required to convert the rotary motion to linear motion. The cam drive system has a multiple parts, which all experience constant wear.

In comparison, a paint circulation pump system may use a small BLDC, a two-stage gear reduction and a simple rack and pinion drive system to convert the rotary motion to linear motion.

To create the reciprocating action, the direction of rotation BLDC motor is simply reversed. With lower inertia and precise torque control, the BLDC motor makes this simple and efficient. In addition to pumps, this type of solution is common in other automation equipment such as ultra-precise high speed CNC machining equipment.

Originally published in the August 2015 issue.