Many types of electric motors are used today, but the two most important are the ac induction motor and the permanent magnet (PM) brushless synchronous motor, also known as the brushless dc or ac servo motor. These two types of motors have different rotor configurations and operate very differently. Design tools like CD-Adapco’s SPEED help motor design engineers optimize both types’ performance more speedily and accurately than ever before.
We trace the ac induction motor’s roots to its invention by Nikola Tesla in 1883 and Mikhail Dolivo-Dobrovolsky’s “squirrel cage” three-phase electrical motor in 1888. The brushless PM synchronous motor has a more recent history. It began in 1956 when H.D. Brailsford developed a hybrid PM dc rotor with spring contacts that flew out to permit true brushless running.
The next dc-motor improvement came in the form of a solid-state speed-control product in 1960. The final invention was the development of higher-temperature Hall devices for electronic commutation in the mid 1960s. NASA and the U.S. government played a major role in brushless PM motor development in the 1960s and 1970s through many government-funded projects.
AC Induction Motors
Induction motors come in two popular forms: single phase and three phase. Both types operate on an alternating current and create a rotating magnetic field. Figure 1 shows a cross section of an induction motor with the internal rotor and the external stator. AC motors employ copper or aluminum bars for the rotor and copper magnet wire in the stator in prescribed distributed winding patterns. Figure 1 also shows 28 rotor and 24 stator slots, one of many available combinations.
The stator rotating current field induces currents in the rotor’s shorted copper or aluminum turns or bars. Induced magnetic fields interact to produce rotor motion and rotor rotation, producing both torque and speed.
Induced rotor current never quite catches up with the stator current, and we apply the word “slip” to describe the phase difference between the two currents. Slip increases as the load increases, creating more torque until the decreasing rotor speed reaches a point on the torque-versus-speed curve where the slip value becomes too large and the rotor speed breaks down to its zero speed or stall position.
AC induction motors can achieve sufficient torque with a constant ac power excitation until it reaches the breakdown point (Fig. 2a). Nonlinear torque and speed performance arises when drive is a constant voltage and frequency, such as 50 Hz or 60 Hz. Changing the R2 rotor resistance (rotor slot configuration) also impacts the motor’s torque-speed output curve. Unfortunately as the starting torque performance increases by increasing the R2 rotor resistance, the copper losses increase and the power efficiency decreases.
With a constant input frequency of 50 Hz or 60 Hz, the stator current levels remain almost constant over the normal torque-developing region of the nonlinear torque-speed curve. There’s a great waste of power and efficiency until reaching the rated point. Starting torque is usually lower than the rated or breakdown torque, which requires some help from the electronic drive to reach rated torque levels. The induction motor curves are quite nonlinear.
Important developments for ac induction motors, emerging in the early 1970s, include the inverter drive plus a number of new control strategies ranging from volts per hertz (V/Hz) to vector control. Variable frequency drives using a changing input frequency (a V/Hz drive) significantly improves ac-motor power efficiency over a wide speed range and reduces internal heating caused by the stator winding. Figure 2b shows a family of torque-speed curves with a varying frequency in 6-Hz steps up to 60 Hz. Start-torque capability for the variable-frequency drive significantly improves over the constant drive beginning at 12 Hz.
Brushless PM (DC) Motors
While the PM motor is also a brushless motor, along with the induction squirrel-cage motor, the interaction between the two rotating magnetic fields is quite different. Figure 3 displays the permanent magnets located on the rotor and the stator holding the copper winding. Figure 3 also shows a four-pole rotor with ferrite or rare earth magnets located on the rotor and a 12-slot stator configuration.
Stator windings turn on and off in a predetermined sequence to ensure continuous rotation in a process called electronic commutation. A magnetic field in the rotor is in synch with the stator switched windings and associated stator magnetic field. Additionally, BLPM motor frequency depends on the motor’s Hall-device system or equivalent rotor-switching frequency, determined by rotor position.
The motor’s back electromagnetic field (EMF) can be measured to acquire its torque-versus-speed curve when driven by a voltage. Figure 4 illustrates the linear nature of the torque-versus-speed curve. This feature allows the motor to develop a starting torque up to four times higher than its rated torque for fast acceleration conditions, depending on the saturation of the magnetic circuit.
One outstanding BLPM motor benefit is the linear nature of the motor’s torque-speed curve when the magnets reside on the rotor’s surface. More complicated designs bury the magnets inside the rotor hub’s structure and change the shape of the motor’s torque-versus-speed curve to achieve a wider, constant-power speed range. Additionally, the most popular configurations for BLPM motors are either delta-winding or wye-winding configurations.
Differences Between AC Induction And BLPM Motors
There are two major classes of applications in precision motion control: speed and position controlled motion systems. Single-phase ac induction motors are very popular in driving many appliances at constant and variable speeds. But the need to significantly increase energy and power efficiency, particularly in pumps, fans, and blowers in commercial and industrial applications, requires the use of a variable frequency drive (VFD) to improve torque and speed performance.
Induction motors with vector-controlled inverters are popular for achieving higher power efficiencies, particularly in power applications greater than 3 kW that require a wide speed range. It is a cost-effective solution with some interesting limitations.
Emerging around 1982, the first high-volume applications for BLPM motors were precision-speed spindle drivers in hard-disk drives specifying less than 0.1% speed regulation. Today’s most popular applications are position-based servo motors with feedback devices, resolvers, or encoders.
BLPM motors are synchronous components that energize the proper stator windings by measuring rotor position via a feedback device. The feedback device provides the necessary precision to accurately and quickly stop the motor’s final position. BLPM motors produce much more starting or acceleration torque to move almost spontaneously from one position to another. A typical application is milling machine table work where pieces need to be moved to a new position very quickly for further machining.
In terms of performance, the BLPM motor performance generates a linear torque-speed profile in a VFD and/or a constant torque versus speed profile in a current-controlled drive (Fig. 4). The motor’s load current provides an excellent method for directly measuring its torque load for any given application. Basically put, the motor’s torque is a linear representation of the motor’s current.
Since there are no heat (I2R) losses in the BLPM rotor, it intrinsically exhibits higher power efficiencies than the ac-induction motor. If this is true, why use an ac-induction motor at all? Why is the ac-induction motor so popular in a wide range of speed-based applications? The answer is quite simple: it is a more cost-effective solution, particularly in most constant and variable-speed applications. The BLPM motor will achieve the highest starting torque of any motor type, surpassing any equivalent sized ac-induction motor.
The BLPM motor reigns supreme in positioning servo applications, although recent pricing spikes in the popular rare earth neodymium magnet materials have spurred new looks at other motor types. The impact of new government efficiency requirements in household appliances has induced many motor manufacturers to evaluate BLPM motors—or as they call them, electronically controlled motors (ECMs)—because of their higher power efficiency ratings. The next five years may be tumultuous for both motor types as they impinge on each other’s market territories and applications.