The AC induction motor has dominated appliance applications as the prime-mover for many decades. Most of these applications used relays for on-off control of split-capacitor AC induction motors. Mechanical transmissions provided changes in speed and direction. However, many modern appliances, including clothes washers, HVAC, refrigeration appliances, and others now take advantage of variable-speed motor drives to improve their functionality and energy efficiency – all while improving performance, reliability, and cost.

An additional benefit of electronically controlled, variable-speed motion control is that it generally enables improved energy efficiency compared to previous mechanical methods. This combination of better control and improved energy efficiency is driving a rapidly increasing adoption rate of variable-speed motor drives in appliance applications.

Appliance designers have a number of commonly used motors, drives, and control systems from which to choose, so it is essential to understand their differences before specifying a system for  a particular appliance application.

AC induction motor

Enlarge this picture Fig. 2. Volts/Hz induction motor control.

The AC induction motor (ACIM) has been used for more than a century, and continues to survive today because of its simple, rugged construction and low-cost. An outer cylinder of steel laminations wound with copper wire comprises the stator of the ACIM. The rotor is also a cylindrical steel lamination stack, typically with cast aluminum rotor bars and end caps that form the one-turn windings of the rotor. Transformer action between the stator and rotor induces current in the rotor that creates its magnetic poles. The interaction between the rotating stator field and the rotor field result in the motor’s torque-versus-speed characteristic shown in Fig 1.

In order to provide the effect of a rotating magnetic field, the winding is arranged into two- or three-phased windings in the stator. For traditional fixed-frequency line-powered ACIM, one winding is connected through a capacitor to shift the phase-angle of its current relative to the other winding. This fixed-speed arrangement is simple and involves no active electronics, but cannot provide any effective speed or torque control. The motor is run at full magnetization current at all times. As a result, its energy efficiency at lower torque levels is compromised, because the motor flux-level is always set at the highest level necessary for full-power operation.  So the overall reason that ACIM motor efficiency is not high is that the stator must carry the burden of higher circulating Volt-Amperes to induce rotor magnetization.

ACIM drive and control

Enlarge this picture Fig. 3. Permanent magnet motor driven by voltage source inverter.

The ACIM is an asynchronous machine, meaning that the mechanical rotation is not synchronous with the electrical rotation: there is some slip (a few percent) necessary to induce the rotor current. For applications that do not require precise velocity control, a simple open-loop variable-frequency inverter can power the ACIM with variable speed. Because of the transformer action within the ACIM, the open-loop frequency control method must keep the ratio of frequency and voltage constant to avoid magnetic saturation and the resultant high currents. This control method is often called “volts per Hertz” control because of the fixed ratio. An example of such a system is shown in Fig. 2. These variable-speed methods apply to 3-phase ACIM, not to the fixed-speed split capacitor types mentioned previously.

Better control of ACIM speed and torque is possible by applying vector control methods, but these do not provide good low-speed performance unless an expensive shaft encoder is used for rotor position feedback. Moreover, the overall efficiency of the ACIM does not measure-up to what a permanent magnet machine can achieve.

Permanent magnet motors

Enlarge this picture Fig. 4. Ideal trapezoidal commutation.

In contrast to the ACIM, permanent magnet machines do not rely on induced AC currents to provide rotor magnetization. Fixed permanent magnets on the rotor take care of that task. As a result, permanent magnet motors use less overall current for a given torque, and provide superior performance down to zero speed, unlike the ACIM. This also means that permanent magnet motors have higher electrical efficiency than induction motors. Permanent magnet motors are synchronous, so there is no slip between the electrical and mechanical rotation speed.

One of the most commonly used permanent magnet motors is the brushless DC (BLDC) motor.  Its name is a bit of a misnomer, as it is not truly a DC machine. The name was applied more than 30 years ago when it was compared to the brush-type DC motor for its similar speed-torque characteristic. At issue was mechanical commutation (brushes and commutator) versus electrical commutation. So the BLDC motor behaves like a DC (brush) motor as opposed to an AC (induction) motor, but the applied winding voltages are really AC for both.

Trapezoidal vs. sinusoidal commutation

Enlarge this picture Fig. 5. Sinusoidal commutation.

Both trapezoidal six-step (BLDC) and sinusoidally commutated motors (permanent magnet AC or PMAC) most commonly have three stator windings arranged in a 3-wire WYE configuration as shown in Fig. 3. Both motor types are also driven from the same 3-phase Voltage-Source Inverter (VSI) topology also shown in Fig. 3. Phase-currents for both types of motors are controlled by pulse-width modulation of the applied voltage on each phase. Both BLDC and PMAC motors clearly have a lot in common. A casual observer might not even see any difference in the overall construction of these two types of motors.

The difference is in the “torque function” of each motor type, and how current is driven into the windings. If each motor type is spun, and the resulting back EMF examined on an oscilloscope, it would be seen that the BLDC motor has a flat-topped trapezoidal voltage, and the PMAC motor has a sinusoidal shape. In order to derive smooth torque from these motors, the applied current should be the same shape as the back EMF.

In the case of the BLDC motor, current is applied to only two of the three windings at a time.  During one commutation interval, a steady current is injected into one motor lead, returning through a second lead – and the third lead is open. (It can be used for position estimate by measuring zero-cross). As the motor shaft rotates, the next set of windings is energized, sequencing through all six possible combinations (thus the name six-step) – at which point the motor has gone through one electrical revolution (fractional mechanical revolution depending on how many winding “poles” the motor has). An important implication of this commutation method is that only two of the six inverter switches are conducting at any time, and only two of the three windings carry current at any time. These two factors affect distribution of thermal loads in both the inverter and the motor. Trapezoidal commutation is depicted in Fig. 4.

In contrast, for a sinusoidally commutated PMAC motor, each of the three phases of the inverter is modulating (PWM) all the time, driving three out-of-phase sinusoidal currents into each winding. Sinusoidal commutation is depicted in Fig. 5. From an application standpoint, the biggest difference between trapezoidal and sinusoidal commutation is that sinusoidal commutation delivers absolutely smooth torque at any rotational angle or speed. Torque disturbances due to commutation, combined with magnetostriction in the stator generate acoustical commutation noise in BLDC motors. This effect is absent in sinusoidally commutated PMAC motors, making them virtually silent (except for possible PWM noise). Quiet operation is often a very important consideration for appliances.

Controlling synchronous motors

Enlarge this picture Fig. 6. Sensorless PMAC controller.

Based on the previous paragraph, it may seem that sinusoidal commutation would always be the preferred choice because of the smooth torque and lack of commutation noise. However, there is an additional control burden to consider as well. Simply put, trapezoidal control is simpler to implement than sinusoidal control. This added complexity must be considered in the decision of which motor and control type is best for an application.

For BLDC motors, a straightforward PWM current loop can control winding current, while commutation is handled by position sensors (Hall-effect or shaft encoder) or position estimate from winding voltage (from zero-crossing of unused winding). The current loop (which equals torque loop) is then controlled by a velocity control loop, and even an outer position control loop. Top speed of BLDC motor drive is reached when the motor back-EMF reaches the drive bus voltage. At that operating point, the drive cannot deliver enough current to increase motor speed, even at 100 percent duty-cycle.

Sinusoidal commutation offers some additional benefits. Simple open-loop variable-frequency sinusoidal drive control is used for ACIM, but not for permanent magnet synchronous motors.  PMAC motors are driven with closed-loop sinusoidal current control. Maximum performance and efficiency is obtained with field-oriented control. Field orientation requires rotor position information, either through direct measurement (encoder) or estimation (sensorless). Motor currents are modeled in a two-axis format with orthogonal direct (D) and quadrature (Q) axis components. The D-axis vector controls the magnetization of the motor, independently of the Q-axis (torque-producing) vector. In normal operation of a PMAC motor, the D-axis command is zero, since the permanent magnets provide the necessary flux-levels. The Q-axis current is then the controlled parameter used to close the torque-loop.

For extended speed-range of a PMAC motor, the D-axis command can be set to reduce the motor flux (essentially opposing the flux generated by the permanent magnets). Reduced flux results in a reduced motor constant (both the speed constant Ke, and the torque constant Kt). So the net effect is to enable a higher motor top-speed from the same bus voltage. This method, known as Field Weakening, can practically extend the motor top speed to 3 times higher than without field weakening – at the direct expense of torque. In other words, more speed but lower torque – just like a mechanical gear-ratio. Field weakening can be particularly useful in a clothes washer drive for example: high torque at low-speed is needed for the washing cycle, but high-speed at low torque is needed for best water extraction during the spin-cycle.

An example of a complete sensorless PMAC motor drive control system is shown in Fig. 6. This controller uses motor current and voltage feedback to accurately estimate rotor position, eliminating the need for any expensive position sensors or shaft encoders. This complete control system is implemented as a dedicated hardware engine using a low-cost digital process.   For more information, email: epersso1@irf.com

Acknowledgements: The author wishes to thank Nicholas Nagel at Illinois Institute of Technology for providing some of the figures.