Learn more about brushless permanent magnet motors.
There is no good reason to be scared of brushless permanent magnet motors, despite the fact they represent perhaps the biggest evolution of the electric motor to occur in more than 100 years.
First let’s address some language issues. You may have heard these motors referred to as electronically commutated, brushless direct, brushless alternating current and/or variable speed motors. No reason to be confused—they all describe the same thing: a brushless DC motor used in AC induction motor applications.
Applications for brushless permanent magnet motors seem to grow every day. Markets currently served with the BPM include residential and commercial HVAC, home appliances including dishwashers, washer-dryers, refrigerators, and window air conditioning units. You can also add booster pumps, air compressors, fan coil units and exercise equipment to that list
How it works: Understanding BPM requires understanding AC Induction
The basic principle of the AC induction motor has not changed significantly since Tesla’s original patent 125 years ago. With a standard three-phase AC induction motor, the rotor is the internal (rotating) component and the stator is the external (stationary) motor component. AC power is fed into the windings on the stator. As the AC sine wave moves up and down it changes the polarity of the windings and creates a rotating field. The rotor consists of a series of bars that are all connected at the ends of the rotor. The rotor and the stator are separated by a thin gap of air. The current passes from the electrical field of the stator through the air gap and into the rotor. This current that has been “induced” into the rotor circulates in the bars of the rotor creating a rotating magnetic field. As the magnetic field in the stator rotates, the rotor follows behind the magnetic creating torque. The fact that the rotor must “follow behind” the magnetic field of the stator is critical to the function of all AC induction motors and is referred to as “rotor slip.” The more the rotor slips (thus decreasing speed), more current is induced and is the function that creates torque.
Though there have been vast improvements in materials and design, AC induction motors still suffer in several aspects. AC induction motors tend to have lower efficiencies than BPM due to the circulating current losses in the rotor and losses in the stator unless expensive higher efficiency components are used. AC induction motors also tend to be more difficult to control than BPM due to non linear torque-speed characteristics caused by the complex relationship between the rotating magnetic field of the stator and the induced magnetic field of the rotor.
The efficiency of induction motors is generally low due to current circulating losses in the rotor and the losses in the stator—unless high efficiency (more expensive designs) are chosen.
Induction motors are more difficult to control, due to their non linear speed-torque characteristics. This is due to the complex relationship between the rotating magnetic field in the stator and the induced magnetic field in the rotor. As such they are more difficult to control with electronics and intelligent devices such as DSPs (Digital Signal Processors).
Both BPM and AC induction motors physically look similar and can have stator structure. Both motor types can use similar electronic drives using three phase modulating inverters. The primary differences are the rotors and the inverter control. A BPM motor requires an electronic drive with some method of sensing absolute position, while an AC induction motor electronic drive requires only a speed sensor. Also, unlike the AC induction motor, the brushless permanent magnet (BPM) is a synchronous electric motor, with the performance characteristics more like a DC brushed motor. Permanent magnets are used on the rotor instead of an induced wound field. The permanent magnets generate a DC magnetic field. This magnetic field enters the stator and interacts with currents flowing in the windings to produce a torque interaction. It is necessary that both the magnitude and polarity of the stator windings be continuously varied in a precise method using an electronic control. Because the rotor does not slip and is synchronous with the stator winding a simplified control can be utilized as compared to an AC induction motor.
One of the basic fundamentals of the AC induction motor is rotor slip, hence the term asynchronous motor. As stated earlier, rotor slip is the term used to refer to the fact that AC induction rotor has to rotate at a lower rpm than the synchronous speed set by the magnetic field of the stator. The induced losses, created by the torque generating rotor current, can increase dramatically if the slip is more than a few percent. The BPM motor, on the other hand, operates in a synchronous mode. There are no slip-induced losses. Simply adjusting the voltage and current applied to the motor results in maximum efficiency over a wide operating range, regardless of changes in speed and torque.
Traditionally, the largest drawback of BPM motors was the fact that they will not operate with AC voltage connected directly to the motor. For years electronic drives were expensive and because they were designed for any and all applications, were complicated to set up. There are now multiple U.S. manufacturers who supply permanent magnet motors with integrated electronic drives that are as easy to install as a standard AC induction motor, yet offer higher efficiency and are easier to control torque and speed than an AC induction motor.
The current generation of BPM motors has greatly increased ease of use while lowering the cost of the motor drive system. As an example:
In a furnace blower retrofit, one energy service contractor was able to retrofit an entire residential complex by replacing the original ½ horsepower AC induction motors with ½ horsepower integrated BPMs. The original motor was simple to replace because the integrated BPM was a direct replacement and used the original mounting bracket and wiring harness. Testing was done before and after on several motors from several brands of furnaces. The average watt savings of the BPM compared to the AC Induction was 152 watts at a full speed of approximately 1,200 rpm, to 248 watts at 800 rpm. BPM motors can bring about huge energy savings for end consumers.
So the bottom line is this: real cost savings and increased energy efficiencies can be realized by transitioning from AC induction motors to brushless permanent magnet motors. And never has it been so easy to do so.