October 2020

Mechanical Status Monitor Statu s & wear detection (2-Level) Compact & reliable Micron accuracy microprecision.ch For Safety Brakes C M Y CM MY CY CMY K MP-Banner3-Printv1.pdf 3 27/03/2020 16:30:45 of the current. Excessive ripple current will increase the heating in the winding pack, thus decreasing motor performance and impacting on motor life. For brushed DC motors that do not use iron laminations, the eddy current and hysteresis losses in the magnetic circuit are proportional to the current ripple, so again excess current ripple would reduce the overall performance of the motor. Current ripple also has a direct impact on the commutation of the motor. For precious metal commutation, electro-erosion of the brushes during the current spikes can be a serious issue and is proportional to the square of the effective current through the winding. Brush wear already represents the main failure mode in brushed DC motors, and increased electro- erosion will only accelerate their wear. Accumulating patina For carbon brush commutation, high levels of current ripple increase accumulation of patina – the layer of copper oxide that forms on the commutator surface of the carbon brush. While a thin film is advantageous because it improves the commutation and reduces the friction, as the film gets thicker, the brush contact will deteriorate. At moderate to high speeds, this might not affect the motor performance significantly, but it would certainly start to have a noticeable effect at lower operating speeds. Given that ripple current has such an impact on performance and on the lifespan of the motor, how can we minimise it? It should be noted, first, that current ripple is at a maximum when the duty cycle is 50%, so the goal should be to run the motor away from a 50% duty cycle. As we have seen, ripple current is directly linked to PWM frequency. It is certainly advisable to keep the PWM frequency outside the range of human hearing (20Hz–20kHz) because a current ripple in that region may introduce noise when the motor is running. But if we consider that a benchmark target is to reduce current ripple to less than 10% to optimise performance and maximise motor life, this can mean a PWM frequency range as high as 40-120kHz. Portescap recommends that you discuss your application with a knowledgeable engineer, because the interdependencies between the requirements of the application, the performance characteristics for the motor, and the resulting design or specification of the PWM drive, are complex. Getting a firm grasp on these dependencies becomes even more important in battery-powered applications, where the requirement to maximise application performance goes hand-in-hand with the need to extend battery life. n As the current oscillates between two non-steady state values, it gives rise to current ripple which is proportional to the applied frequency.