With energy conservation as a key focus globally, efficiency in motor designs is a compelling concern. With governmental regulations urging higher levels of efficiency, motor drives are becoming more sophisticated to meet this efficiency challenge. This paper will discuss motor drives, industry trends and solutions that make it easier for designers to reduce wasted energy, increase reliability, lower component count and contribute to a greener world.
Range of motors in the industry
For nearly 100 years, induction motors have been primarily used in the appliance designs. Even variable frequency drives use induction motors as well. Some of the newer appliances are using more efficient, more compact and lighter motors. These new kinds of appliance motors can effectively be divided into the brushless DC motor family and the switched reluctance motor family.
Brushless DC motors with Variable Frequency Drives are found in many common household appliances. These efficient and versatile motors have high torque-to-weight ratios. With the soaring cost of energy, the industry is witnessing a renewed interest in brushless DC motors. The main factor preventing the widespread adoption of these motors in more appliances has been cost and the overall complexity of drive design.
Switched reluctance motors have been popular in appliances such as vacuum cleaners and hand tools where the motor noise and torque ripple are less of a concern. Switched reluctance motors feature high torque and high speed operation at a very competitive price point.
Both the Brushless DC motors and the Switched Reluctance Motors use a microcontroller or DSP to synthesize waveforms that are then amplified using power switches such as Power MOSFETs or IGBTs.
A Review of Drives
Variable Frequency Drives can be designed in a number of different ways. The most popular low frequency drive scheme for a typical three phase motor is a trapezoidal waveform drive.
Trapezoidal waveform control method and real test waveform
Click on image to enlarge.
If higher efficiency and performance are required, a PWM approach is used to produce a sinusoidal waveform. For further improvements in efficiency, a Space Vector Modulation scheme can be used.
Three-phase synchronous motors with permanent magnets come in two popular variants: the sinusoidal PM synchronous motor and the trapezoidal BLDC motor. The sinusoidal PM synchronous motor is very similar to the trapezoidal BLDC (electronically commuted) motor. There are two main differences:
Motor construction -- Shape of BEMF inducted-voltage sinusoidal (PM synchronous) motor versus trapezoidal (BLDC) motor
Control -- Shape of the control voltage " three-phase sinusoidal (all three phases connected at one time) versus rectangular six-step commutation (one phase is non-conducting at any time)