Page 4 - Industrial Technology Extra 7th September 2020

ELECTRIC MOTORS...

Controlling brushed DC motors

with pulsed width modulation

With speed proportional to applied voltage, the coreless brushed DC motor can be

simply driven using a continuous linear regulation power supply. But is this always
the best solution? Dr Sunil Kedia, Core Market NPD Manager at Portescap, looks at
the benefits of using a PWM drive as an alternative and examines the optimal
frequency, current ripple and lifespan considerations
rom robotics and industrial automation to PWM with a motor – can be minimised, allowing
electrical appliances and even toys, there the motor to be optimally driven. The improved
are many applications for brushed DC efficiency of the PWM drive reduces the heating of
Fminiature motors that demand driving the electronic components and, in battery powered
motor at more than one load point or through applications, increases battery life.
specific load cycles. While this can be achieved The argument for turning to PWM control, then,
with continuous linear regulation power supplies, is a strong one. However, there are a number of
these tend to be inefficient and bulky – two design considerations to take into account when
characteristics which are particular disadvantages using a PWM drive with brushed DC motors, with
in battery powered equipment. parameters such as PWM frequency and duty cycle
One alternative is pulse width modulation all having an impact on the performance and
(PWM) voltage regulation. Here the input power to lifespan of the motor.
the motor is continually turned on and off at a high
operating frequency: the combination of the coil Design considerations
inductance and the motor inertia serves to smooth The brushed DC motor can be simply modelled as a
out the speed, such that the motor behaves as if it series resistor/inductor (RL) circuit. For any such
is seeing a pure DC voltage. The motor speed is circuit, when voltage is applied across it, the current
defined by the duty cycle – the ratio of the on time rises on a curve towards to steady state value. And
to the off time of the applied voltage. when the voltage is removed, the current follows an
Control via PWM voltage regulation addresses inverse of the curve towards zero. The time constant
the two key drawbacks of continuous linear voltage of the RL circuit defines the maximum rate of
regulation, with compact drives that are highly change of the applied voltage in the circuit.
efficient. Output torque can be more precisely When using PWM to drive the motor, the current
controlled and, with correct design, any resulting across the motor rises and falls with every period of
eddy current effects – an inherent trade-off of using the PWM. Ignoring the back EMF of the motor, the

September 2020 • INDUSTRIAL TECHNOLOGY EXTRA • p4