High current DC motor control question - Page 1
Forum section: electronics / projects

author: TheGorf
author_rank: Newbie
timestamp: December 15, 2023, 09:42:15 pm
content: hmmm it appears that my account here previously disappeard so I've lost all my history.So, a popular approach to variable DC motor control used in treadmills seems to be to use SCR's on the AC side then regulate that output to DC. I'm curious if someone has any insight into what the value of this is? My hunch is that it's a manufacturing cost trade off. That it's easier to clip off the AC waveform and rectify it meaning no transformer or doing any DC switching for PWM.I've been using treadmill motors scavenged from Fleabay to do some single-phase AC motor conversion in tooling for folks. For myself I've been building my own power supplies. But in looking at these onboard controllers, the SCR method seems to be the go-to. Am I missing something that makes them better for this than PWM DC switching?

author: johansen
author_rank: Super Contributor
timestamp: December 15, 2023, 10:35:31 pm
content: well its far cheaper and it works well.the scr/triac is fault tolerant. a surge will just cause it to conduct, if the current is within its limits, the fuse will blow, not the scr.

author: Terry Bites
author_rank: Super Contributor
timestamp: December 17, 2023, 12:30:33 pm
content: Unless you're buiding down to a price, take the easy way out.Big fat SCRs are cheap. You can get a 50A continuous for $5.I'd measure the motor resistance and get a thyristor that can manage that peak with room to spare.Lazy I know.

author: mjkuwp
author_rank: Unknown
timestamp: December 18, 2023, 01:18:50 pm
content: are you looking at the absolute bottom end of the treadmill market? like stuff that will barely do the job and last a few months? I'd guess this is not better for the job, but just the cheapest. it is far more normal in that industry to rectify the AC to DC and then do PWM control with speed feedback from a speed sensor.does the SCR design reduce the need for or reduce the size of the capacitor bank? ( also cost )

author: David Hess
author_rank: Super Contributor
timestamp: December 18, 2023, 03:19:26 pm
content: Quote from: mjkuwp on December 18, 2023, 01:18:50 pmThe SCR design does not require a capacitor bank at all. Rectified but unfiltered DC drives the DC motor directly through a choke, with the inductance of the motor providing more smoothing.The advantage is that SCRs and TRIACs are cheaper than MOSFETs and IGBTs. They can only operate at low frequencies and require commutation to shut off, but that is a perfect match for a line frequency AC application.I am surprised they are not using series or shunt wound DC motors, but maybe they are and it is not apparent.

author: johansen
author_rank: Super Contributor
timestamp: December 18, 2023, 06:44:41 pm
content: Quote from: David Hess on December 18, 2023, 03:19:26 pmI've never seen a shunt or series wound treadmill motor. ceramic magnets by volume are cheaper than iron and copper.anyhow, you do need a choke for most ferrite brushed motors, the inductance of the motor is not high. in any case, the choke only needs to be about 1 pound per hp for good enough filtering to the point you won't really feel the 120hz ripple on a treadmill. may treadmills have a small inductor.. about 2 inches on a side.my 300 pound lathe resonated like a tuning fork without an inductor.