For a permanent magnet direct current (PMDC) motor, a variation in load generally causes a change in speed. If, for a given application, the desired speed under load is located halfway down the motor curve, then a reduction in the applied load will cause the motor to accelerate to much higher speeds. The result is usually higher motor noise and greater wear rates on components like the bearings, brushes, and commutator.
One practical example is a vacuum cleaner that uses a roller brush. There, it is desirable to keep the brush from running at high speeds when the cleaning load is reduced, which happens when moving from carpet to hard floors.
To prevent the motor from overspeeding, designers generally turn to a motor controller. Indeed, numerous types of controllers are available that can achieve this objective, but most require a feedback signal from detection devices such as Hall sensors or other signal generators, all of which complicate the construction of the motor.
There is, however, at least one method of controlling a DC motor’s speed that eschews the need for integrating Hall sensors or other signal-generating devices into the motor. Developed by Johnson Electric, the controller uses a simple and cost-effective system that is external to the motor, and requires neither a velocity dependent signal nor any added components or features to complicate the motor’s construction.
To understand how the controller works, consider that a PMDC motor behaves largely in a linear manner; that is, the current drawn to meet a load is determined by the motor constant, which is a function of the motor’s construction. Specifically, the motor constant is set by the motor’s magnetic field strengths and the armature winding.
Consider, as well, the simple equation that governs the PMDC motor’s behavior:
Input Voltage = Regenerated Voltage + Voltage drop across the motor circuit resistance
The regenerated voltage is determined by the motor speed and the motor constant. The voltage drop across the motor circuit resistance is proportional to the current being drawn, and is based on the motor’s operating load and the motor constant. Thus, for a given load, the motor speed is a function of the applied voltage.
To keep the speed constant under different loads, then, it is necessary only to vary the input voltage in a way that keeps the motor’s current demand in line with varying load levels. In this way, because the motor constant is fixed, it is possible, by determining the input voltage needed, to drive the motor at a regulated speed for every possible load within the motor’s performance limits.
In operation, the sensorless controller measures the voltage across a known resistance in the electric circuit feeding the motor. As current is drawn by the motor, it passes through this fixed resistance, developing a proportional voltage drop across it. The controller then compares the voltage drop and, using a digital circuit, sets a pulse width for the input voltage that creates the average voltage needed to drive the motor at a particular pre-determined speed. In practice, the controller maintains the motor velocity within five percent of a desired speed, preventing the motor from increasing its speed as the load decreases.
More information is available by contacting Johnson Electric North America Inc., 10 Progress Dr., Shelton, CT 06484; calling 203-447-5362; or visiting www.johnsonelectric.com
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