Brushless dc motors: More power from a smaller package

Today’s brushless dc motors and gearmotors combine many of the best features of both ac and dc drive systems. Just like ac motors, brushless dc motors eliminate brush maintenance, dust and brush-generated electromagnetic interference. In addition, brushless dc motors provide a much wider speed range than inverter-driven ac motors. Brushless dc motors also run more quietly than their brush-type counterparts. Moreover, brushless motor construction makes the motors more thermally efficient, resulting in greater power from a smaller package.

Taken together, these benefits equate to longer life for a brushless motor than for a comparable permanent magnet dc motor. Compared to inverter-driven ac motors, brushless dc motors provide the same long-standing performance advantages of their brush-type counterparts:

• Higher starting torque Predictable performance (a linear speed-torque curve)
• Wider speed range
• Ability to run from a wide range of power supply voltages.

Because brushless dc motors are constructed with magnets, bearings, laminations, shields and processes similar to many widely available PMDC motors, they typically compare favorably in cost to PMDC motors. With the cost of controls continuing to go down, some OEMs are even finding it cost effective to swap brushless dc systems for older brush-type designs (see Brushless dc motors become increasingly common).

Designers are also increasingly taking advantage of the special performance capabilities inherent in the brushless dc design to substitute them for more costly high-end servomotors. Inverter-driven ac motors have also been used to replace older brush-type designs. However they lack the wide speed range of brushless dc motors.

AC induction motors were the earliest motors to be commercially available. Typically, these motors have cast aluminum rotors and windings on the outside that induce current in the rotor to create the electromagnetic fields necessary for rotor movement. As electronics and magnet technology progressed, brush-type PMDC motors appeared. These motors reverse the design of ac motors – the windings are on the rotor (called the armature) and permanent magnets or field windings are on the outside.

The brushes are pieces of carbon-copper composite graphite that rub on a portion of the rotor, called the commutator, to electrically connect it to the power source. The commutator segments are located so that as the rotor turns, current flows in the proper direction in the rotor winding to keep the motor going in the desired direction.

Brushless dc motors are a combination of the two. Winding construction in a brushless dc motor is similar to that of a three-phase ac motor. The major difference between ac and brushless dc motors is in rotor construction: the rotor consists of magnetized permanent magnet segments. In addition, the brushless dc motor requires position sensors such as encoders or Hall-effect sensor devices. These sensors provide electrical signals the control uses to sequentially energize the three-phase windings to produce maximum rotor torque and desired direction of rotation.

Motion-control applications run the gamut from fans to machine tools. The best type of motor is often obvious. For example, an inexpensive ac induction motor normally drives a fixed-speed fan. Conversely, a high-speed profiler normally requires a high-performance multi-axis servo control system. Brushless dc motors can often be used for applications that fall somewhere between those two extremes. In general, brushless dc motors can frequently be used for velocity and positioning applications.

A velocity application is one in which the motor rotates to drive the load; the stopping position is not of major importance. Examples include fans, centrifuges and continuously-running conveyors. These applications often use conventional brushless dc motors because of their high-speed capability, high power relative to size and low maintenance.

In a positioning application, the motor rotates to drive the load from point A to point B. Examples include garage door openers, supermarket check stands, parts elevators and pick-and-place robots. The first two examples often use conventional ac motors and limit switches or photoelectric sensors to control stopping position. Position tolerance is usually around ¼ inch. A conventional PMDC motor could be used on the parts elevator if rapid acceleration and deceleration are not requirements. But they are, a brushless dc motor is the better choice.

With the recent development of cost-effective brushless dc motors with integrated drive electronics, and with the cost of brushless dc controllers continually falling, brushless dc motors will appear in more applications that once may have used conventional PMDC gearmotors, brush-type servomotors and ac inverter-duty motors.