Filter cap at dc motor input
in [Design]
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From: pimpom on 20 Jul 2010 09:09 I'm rebuilding an old machine for a local company by replacing all the electronics with my own design. It uses a 24V DC brushed motor with an integral reduction gear. The original drive circuit used a couple of SCRs for speed control and current was limited to 3.5A. My circuit uses high frequency PWM (~22kHz). (The motor control is only a small part of the overall design). My design worked nicely at home, but lost power drastically as soon as it's fitted on the machine. I traced this to a shunt capacitor at the motor input. I opened the motor and sure enough, there's an LC filter inside the gearbox housing. Like this: + --------L------- | | C | | --GND Motor | C | | | - --------L------- My circuit uses a MOSFET output stage with the motor floating between the drain and Vdd. The two caps are 1uF each. Should I reduce them to something like .01uF each or just omit them altogether?
From: Jan Panteltje on 20 Jul 2010 09:47 On a sunny day (Tue, 20 Jul 2010 18:39:27 +0530) it happened "pimpom" <pimpom(a)invalid.invalid> wrote in <i2476a$qif$1(a)news.albasani.net>: >I'm rebuilding an old machine for a local company by replacing >all the electronics with my own design. It uses a 24V DC brushed >motor with an integral reduction gear. The original drive circuit >used a couple of SCRs for speed control and current was limited >to 3.5A. My circuit uses high frequency PWM (~22kHz). (The motor >control is only a small part of the overall design). > >My design worked nicely at home, but lost power drastically as >soon as it's fitted on the machine. I traced this to a shunt >capacitor at the motor input. I opened the motor and sure enough, >there's an LC filter inside the gearbox housing. Like this: > > > + --------L------- > | | > C | > | > --GND Motor > | > C | > | | > - --------L------- > >My circuit uses a MOSFET output stage with the motor floating >between the drain and Vdd. The two caps are 1uF each. Should I >reduce them to something like .01uF each or just omit them >altogether? Those filters are there to reduce RFI originating from the brushes. I have a very similar setup here, and the RFI even causes bit errors on my sat receiver, and that is a 600mA motor (RFI up into the GHz range, sparks are BAD, as from those brushes). Simple caps help only very little. So removing the filter may cause problems. The right way would be perhaps to use a different kind of switcher, with a series L and filter capacitor, if you want to use the existing motor. So the motor sees pure DC, and the filter can be left intact.
From: pimpom on 20 Jul 2010 11:51 Jan Panteltje wrote: > On a sunny day (Tue, 20 Jul 2010 18:39:27 +0530) it happened > "pimpom" > <pimpom(a)invalid.invalid> wrote in > <i2476a$qif$1(a)news.albasani.net>: > >> I'm rebuilding an old machine for a local company by replacing >> all the electronics with my own design. It uses a 24V DC >> brushed >> motor with an integral reduction gear. The original drive >> circuit >> used a couple of SCRs for speed control and current was >> limited >> to 3.5A. My circuit uses high frequency PWM (~22kHz). (The >> motor >> control is only a small part of the overall design). >> >> My design worked nicely at home, but lost power drastically as >> soon as it's fitted on the machine. I traced this to a shunt >> capacitor at the motor input. I opened the motor and sure >> enough, >> there's an LC filter inside the gearbox housing. Like this: >> >> >> + --------L------- >> | | >> C | >> | >> --GND Motor >> | >> C | >> | | >> - --------L------- >> >> My circuit uses a MOSFET output stage with the motor floating >> between the drain and Vdd. The two caps are 1uF each. Should I >> reduce them to something like .01uF each or just omit them >> altogether? > > Those filters are there to reduce RFI originating from the > brushes. > I have a very similar setup here, and the RFI even causes bit > errors > on my sat receiver, > and that is a 600mA motor (RFI up into the GHz range, sparks > are BAD, > as from those brushes). Simple caps help only very little. > So removing the filter may cause problems. > The right way would be perhaps to use a different kind of > switcher, > with a series L and filter capacitor, if you want to use the > existing > motor. > So the motor sees pure DC, and the filter can be left intact. Thanks. Yes, an L-C filter is probably the best solution, and it may not even need changing the switcher design. The problem must have been caused by the cap between the negative terminal of the motor and ground as it essentially parallels the output transistor except for the (small) lead inductance.
From: E on 20 Jul 2010 11:56 "pimpom" <pimpom(a)invalid.invalid> kirjoitti viestiss�:i2476a$qif$1(a)news.albasani.net... > I'm rebuilding an old machine for a local company by replacing all the > electronics with my own design. It uses a 24V DC brushed motor with an > integral reduction gear. The original drive circuit used a couple of SCRs > for speed control and current was limited to 3.5A. My circuit uses high > frequency PWM (~22kHz). (The motor control is only a small part of the > overall design). > Try lower PWM frequency, like 50 Hz or whatever the original SCR circuit used. -ek
From: Cydrome Leader on 20 Jul 2010 12:03
E <invalid(a)invalid.invalid> wrote: > > "pimpom" <pimpom(a)invalid.invalid> kirjoitti > viestiss?:i2476a$qif$1(a)news.albasani.net... >> I'm rebuilding an old machine for a local company by replacing all the >> electronics with my own design. It uses a 24V DC brushed motor with an >> integral reduction gear. The original drive circuit used a couple of SCRs >> for speed control and current was limited to 3.5A. My circuit uses high >> frequency PWM (~22kHz). (The motor control is only a small part of the >> overall design). >> > > Try lower PWM frequency, like 50 Hz or whatever the original SCR > circuit used. > > -ek This sounds like the best idea. Even at 2 or 3kHz I ran into weirdness with motors that had capacitors across the leads. They would race instead of running slow though. |