Selecting Universal Motor Controls

Inside the home, one will find a number of gadgets with the universal motor dominating. Mostly, these are used in high speed, low-cost motor applications, such as in power tools, vacuum cleaner, and countertop blenders. However, not all gadgets perform equally. For example, a bargain-basement blender may make a lot of noise when working. Others may be relatively quieter. While some products have a tendency to overheat, others run cool even if you load them over. Actually, the motor itself has little to do with the wide variation in performance. Mostly, the lifespan and performance of the universal motor depends on its drive circuitry.

If speed control is not necessary, gadgets have their universal motors simply connected to the AC mains or to the DC rail, as this is the most cost-effective method for driving the motor and letting it spin. However, the speed of the universal motor depends largely upon the voltage applied, and connecting it directly to the voltage source allows it to spin at its maximum speed at minimum load.

While connecting directly to the voltage source for maximum speed might suit power tools or a vacuum cleaner, other applications may require the speed of the shaft to vary. Designers accomplish these using subtractive measures, mostly by reducing the motor voltage. This helps to reduce the speed to a fraction of the maximum RPM.

One can power universal motors through either alternating voltage or direct voltage, with each approach having its own advantages and disadvantages. While DC control circuits tend to be more expensive than their AC counterparts are, the DC controls have the advantage of prolonging the motor life, and offer noticeably quieter operations, with improved efficiency.

Running a universal motor on alternating voltage and implementing a lowest-cost speed control entails feeding the motor varying amounts of the AC half-cycle. The cheapest open-loop method employs two semiconductor devices, a Triac triggered by a Diac, with a series RC network controlling the phase at which the Diac fires.

Closed-loop controls replace the Diac with a low-cost 8-bit micro-controller. Apart from offering improved control of the Triac, use of the micro-controller results in a closed-loop speed control, more sophisticated user interface, and a proprietary software-based design. By digitally monitoring the motor voltage and current under load, most applications are able to forego the use of a tachometer on the motor shaft for feedback.

Although the above makes for a very economical drive, the downside to using this approach results in a high current ripple, making the operation run fairly noisy. Dissipations in the Triac reduces the efficiency of the approach, with the thermal strain on the brushes ultimately reduces their life.

DC drives, on the other hand, take a different approach. Regulated DC power to the motor is pulse-width modulated, with the rate of rotation of the shaft being directly proportional to the duty cycle of the PWM waveform. When the energy supplied to the motor is low, it spins slower.

The DC drive has the advantage of higher efficiency, reduced noise, highly responsive speed regulation, prolonging the motor life. Overall, the application may use a smaller motor.