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Getting more out of your motor

Motor driver ICs integrate more functions. By Bob Christie.

With the increasing focus on communications and computing, the humble electric motor is often overlooked. It has been estimated that in excess of one third of all electricity generated is converted to mechanical energy by electric motors. While a large part of this is by ac induction motors, an increasing amount is by smaller, lower voltage motors driven from a dc supply. The average pc, for example, will have about ten dc motors for functions such as moving air or driving hard disks and cds. When printers, scanners and games controllers are taken into account, this number can double.
dc motors fall into two general categories; brush motors, which rely on mechanical commutation; and brushless motors, which use electronic commutation. All electric motors can be controlled by simply varying the applied voltage, but, for improved efficiency and control, pulse width modulation is often used to control the motor current and thus the torque and speed. A simple linear control system using a variable resistance element in series with the motor will dissipate more power in the controller than in the motor, unless the motor is running close to its maximum output. To avoid this inefficiency, pulse width modulation (pwm) switches the full supply voltage across the motor, avoiding resistive losses, and relies on the inductance of the motor to filter the applied waveform and average the motor current.

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Vanessa Knivett

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