Page 6 - Industrial Technology Extra 7th September 2020

performance. Further, resistive heating in the motor that ripple current has such an impact on
winding is proportional to the square of current. performance and on the lifespan of the motor, how
Excessive ripple current will increase the heating in do we minimise it? It should be noted, first, that
the winding pack, so decreasing motor performance current ripple is maximum when the duty cycle is
and impacting on motor life. 50%, so the goal should be to run the motor away
For brushed DC motors that do not use iron from the 50% duty cycle.
laminations, the eddy current and hysteresis losses Further, as we have seen, ripple current is
in the magnetic circuit are directly proportional to directly linked to PWM frequency. It is certainly
the current ripple, so again excess current ripple advisable to keep the PWM frequency outside the
would reduce the overall performance of the motor. range of human hearing (20 Hz – 20 kHz) as a
Current ripple also has a direct impact on the current ripple in that frequency range may introduce
commutation of the motor. For precious metal noise during motor operation. But if we consider
commutation, electro-erosion of the brushes during that a benchmark target is to reduce current ripple
the current spikes can be a serious issue, with to less than 10% in order to optimise performance
electro-erosion being proportional to the square of and maximise motor life, this can mean a PWM
the effective current through the winding. Brush frequency range as high as 40-120 kHz.
wear already represents the dominant failure mode Portescap recommends engaging with a
in brushed DC motors, and increased electro- knowledgeable engineer to discuss the application
erosion will only accelerate their wear. specifics, as the interdependencies between the
For carbon brush commutation, high levels of requirements of the application, the performance
current ripple increase patina accumulation – the characteristics for the motor and the resulting
copper oxide layer formed on the commutator design or specification of the PWM drive are
surface of the carbon brush. While a thin film is complex. Getting a firm grasp on these
advantageous in improving the commutation and dependencies becomes even more important in
reducing the friction, as the film size increases the battery powered applications, where the
brush contact will deteriorate. At moderate to high requirement to maximise application performance
speeds this might not affect the motor performance comes hand in hand with the need to extend battery
significantly, it would certainly begin to have a life.
noticeable effect at lower operating speeds. Given www.portescap.com