October 2020

36 n PRECISION ENGINEERING AND MOTION CONTROL October 2020 www.drivesncontrols.com Should you use PWM to control brushed DCmotors ? T he enduring popularity of core- less brushed DC motors comes from their combination of simple design, fast transient responses, negligible iron losses, and ease of control. From robotics and industrial automation to electrical appliances, and even toys, there are many applications for miniature brushed DC motors that require the motor to be driven at more than one load point or through specific load cycles. While this can be achieved using continuous linear regulation power supplies, these tend to be inefficient and bulky – two characteristics which are particular drawbacks in battery- powered equipment. One alternative is to use pulse width modulation (PWM) to regulate the voltage. Here the input power to the motor is turned on and off continually at a high frequency. The combination of the coil inductance and the motor inertia smoothes the speed, and the motor behaves as if it is seeing a pure DC voltage. The motor speed is defined by the duty cycle – the ratio of the on-time to the off-time of the applied voltage. Control via PWM voltage regulation addresses the two key limitations of continuous linear voltage regulation resulting in compact drives that are highly efficient. Output torque can be controlled more precisely and, with correct design, any resulting eddy current effects – an inherent trade-off of using PWM – can be minimised, allowing the motor to be driven in an optimal way. The improved efficiency of the PWM drive reduces the heating of the electronic components and, in battery- powered applications, prolongs the battery life. The argument for turning to PWM control, then, is a strong one. However, there are several design considerations to take into account when using a PWM drive with brushed DC motors. Parameters such as the PWM frequency and duty cycle can have an impact on the performance and lifespan of the motor. Design considerations The brushed DC motor can be simply modelled as a series resistor/inductor (RL) circuit. For any such circuit, when a voltage is applied across it, the current rises on a curve towards to steady-state value. And when the voltage is removed, the current follows an inverse of the curve towards zero. The time constant of the RL circuit defines the maximum rate of change of the applied voltage in the circuit. When using PWM to drive a motor, the current across the motor rises and falls with every PWM cycle. Ignoring the back-EMF of the motor, the current rise is a function of the motor inductance and total resistance. Ideal design intuition is to choose the PWM frequency that allows sufficient time for the current to reach its steady state for each cycle. But this is not always the right approach. If the PWM frequency is increased beyond this threshold value, there is insufficient time for the current to reach its steady state, and the current oscillates between two non-steady-state values, giving rise to current ripple. This ripple is proportional to the applied frequency and as the frequency of the PWM is increased further, the ripple effect reduces to an acceptable level. This current ripple has several detrimental impacts on motor performance, including non-linear torque behaviour, because output torque is proportional to current. Hence, the frequency is chosen so that the torque ripple doesn’t impact the performance of the application. Furthermore, resistive heating in the motor winding is proportional to the square Brushed DC motors can be driven simply using linear power supplies, with their speed being proportional to the applied voltage. But is this always the best approach? Dr Sunil Kedia, new product development manager at Portescap, looks at the benefits of using PWM drives instead. Core-less brushed DC motors have become more popular due to their combination of simple design, fast transient responses, and ease of control.