Does PWM harm DC motor ? - Page 1
Forum section: electronics / beginners

author: boo9
author_rank: Contributor
timestamp: March 14, 2015, 07:01:22 pm
content: Does the PWM method of controling the speed of a DC motor causes adverse effects to the motor ?What are the pros of of chaning the V level over PWM when controling the speed of the motor ?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 15, 2015, 03:59:49 am
content: Harm? No. It may cause extra heating though. Which will reduce the available power within ratings, or may harm it if used at ratings.Very low frequency PWM (< kHz) also causes torque pulses which will increase mechanical wear.PWM with an electrical filter before the motor is indistinguishable from normal DC, and won't cut into ratings.Tim

author: Rerouter
author_rank: Super Contributor
timestamp: March 15, 2015, 04:37:23 am
content: pro of PWM, is higher torque at lower speeds, as the motor is being turned by the full voltage, just for a smaller fraction of the timecon is its better if you try and avoid the audible range, otherwise you can have a clearly audible hum at the main frequency or a harmonic,pro, you will waste less heat in your switching / pass elementcon, more thought required on minimizing radiated and emitted noise from the circuit

author: joeqsmith
author_rank: Super Contributor
timestamp: March 15, 2015, 05:29:57 am
content: Brush or brushless?

author: boo9
author_rank: Contributor
timestamp: March 15, 2015, 10:06:50 am
content: The motor uses brushes.

author: BradC
author_rank: Super Contributor
timestamp: March 15, 2015, 10:45:14 am
content: I don't know, but I've always wondered. Is there likely to be any longevity impact to the brushes and commutator due to extra or excess sparking that you don't get with a standard DC drive?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 15, 2015, 11:26:59 am
content: I wouldn't imagine brush wear is any worse than normal conditions at the same RMS/peak current. Which won't be very much above average or rated current for a modest PWM.Tim

author: joeqsmith
author_rank: Super Contributor
timestamp: March 15, 2015, 04:04:38 pm
content: Quote from: boo9 on March 15, 2015, 10:06:50 amIt's an interesting question.Seems I remember having ran into a problem with reduced brush life but wouldn't bank on it. I did not run the tests and there may have been something else going on. The system would have only spun the motor in one direction. Suspect it would have been a single driver, using low side FET and using the FET to control the flyback as well. I later designed a variable switch mode supply for it and had no problems.

author: Mechatrommer
author_rank: Super Contributor
timestamp: March 15, 2015, 10:32:37 pm
content: put desired capacitor between motor terminals?

author: rs20
author_rank: Super Contributor
timestamp: March 15, 2015, 10:49:29 pm
content: Quote from: Mechatrommer on March 15, 2015, 10:32:37 pmAn H-bridge is very low impedance, so putting a capacitor across its output is just going to stress the H-bridge? So as a way of smoothing PWM to a DC-ish voltage, it's a big no-no. As a way to absorb little voltage spikes from the motor, maaaybe?Quote from: Rerouter on March 15, 2015, 04:37:23 amCitation needed? I'm very curious about this. Is torque related to the square of the applied voltage or something?