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Pseudo Bipolar PWM Switching in DC Motor Braking

IP.com Disclosure Number: IPCOM000052216D
Original Publication Date: 1981-May-01
Included in the Prior Art Database: 2005-Feb-11
Document File: 3 page(s) / 50K

Publishing Venue

IBM

Related People

Green, TA: AUTHOR

Abstract

In braking a DC motor from high velocities, the deceleration rate is often difficult to control. Initially, the motor back emf (BEMF) may be high, causing excessive deceleration, while at low velocities, the BEMF alone will cause insufficient deceleration. Thus, a motor driver is required which will oppose the BEMF at high velocities while adding to it at lower velocities. Therefore, a bipolar-type driver is advantageous.

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Pseudo Bipolar PWM Switching in DC Motor Braking

In braking a DC motor from high velocities, the deceleration rate is often difficult to control. Initially, the motor back emf (BEMF) may be high, causing excessive deceleration, while at low velocities, the BEMF alone will cause insufficient deceleration. Thus, a motor driver is required which will oppose the BEMF at high velocities while adding to it at lower velocities. Therefore, a bipolar- type driver is advantageous.

Typically, bipolar pulse-width modulated (PWM) drivers are limited in frequency by the delay time (i.e., 20 us for inexpensive devices) which must be imposed when switching from forward to reverse mode to prevent a momentary short across the power supply. Those drivers with inexpensive devices are not usable in a 20 KHz PWM driver (period =50 mu s). During DC motor braking, however, bipolar drive may be obtained from a standard H-drive configuration by actively switching only transistors T2 and T3 (Fig. 1). Transistors Tl and T3 are PNP type, while transistors T2 and T4 are NPN type.

Referring now to Fig. 1, assume that the motor M is initially running forward with current IF (i.e., transistors T1 and T4 have been switching in limited unipolar mode). To brake, transistors T1 and T4 must be turned off and a brake current IB established in the opposite direction. Assume that a constant deceleration corresponding to IB = 3 A is desired. If the BEMF is initially 12 V, then the PWM must initially supply -6 V to the motor in order to maintain IB = 3 A. If the BEMF at a lower velocity is 2 V, then the PWM must supply +4 V to the motor.

Thus, it is necessary for the drive to be bipolar. This may be accomplished, however, by switching transistors T2 and T3 simultaneously and utilizing commutating diodes Dl and D4. Without motor generator action, this switching would produce the driver voltage to the motor shown in Fig. 2.

For a duty cycle less than Sbi in Fig. 2, current delivered through transistors T2 and T3 during the "on" cycle is negated by reverse charging through diode;

D1 and D4 during the "off" cycle so that the average voltage is zero. If motor generator...