Definition: PWM Drive

"A drive is the power electronics board which is used to control the speed of a motor. Drives may use various techniques to control speed.

A moderately low-cost form of drive for controlling DC motors makes use of one of several types of power transistors. These transistors are switched rapidly on and off in a technique known as Pulse Width Modulation (PWM). Hence the term PWM drive .

A transistor acts as a gate to allow a precise amount of current to flow to the motor. As this gate is rapidly switched on and off, the amount of current which flows is dependent upon the ratio between On time and Off time. The larger this ratio is, the more current will flow the faster the speed. The lower the ratio, the less current will flow the slower the speed. Since the transistor is pulsed, the technique is known as pulse width modulation. Fig. 10

Through feedback systems, this PWM technique can also precisely control the voltage allowed to generate the armature field of the motor, and thus the speed is controlled.

PWM drives are most commonly used to control AC motors, but the system is also used in many newer, more precise, DC drives controlling DC motors.

Copyright 1997, Brian L. Boley All Rights Reserved".

The Kit

The upgrade consists of a pre-wired module containing the PWM circuitry and a remote potentiometer to be mounted on the panel.

The Pitch Trim motor has a clutch that disengages the motor at the end of the stroke.

High motor speed is reguired to release the clutch.

Now that slow speed is available I designd a switch plate to be wired in series with the Hat Switch and Relay thus limiting the stroke of the lead screw and preventing locking at the end of the stroke.

An operator which attaches to the Pitch Trim over ride tube actuates the switch at each end of the stroke.

An installation and operating manual is provided detailing the set-up procedure to establish minimum speed.