Filter cap at dc motor input

On a sunny day (Tue, 20 Jul 2010 18:39:27 +0530) it happened "pimpom" wrote in :

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.

"pimpom" kirjoitti viestissä:i2476a$qif$ snipped-for-privacy@news.albasani.net...

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.

It's not the cap across the motor terminals per se that's causing trouble. As I said earlier, it runs fine and speed control works as expected as long as the motor frame is not grounded. (I use a slotted wheel optical tachometer for feedback and, with a counter, for duration control).

It's when the motor is mounted on the steel frame of the machine, connecting it to electrical ground, that things go wrong. The cap from the negative terminal (also MOSFET drain) to ground must be playing havoc with the switching output.

I'll consider that option. Thanks.

So tip your circuit upside down and pop in a P-channel MOSFET?

Grant.

I've thought of that as a possible solution, but for various reasons not a practical one in this case: 1) I don't have a P-channel MOSFET in stock (I can never remember to include it in my orders). 2) It's not locally available in my small town. Placing small orders from another city is not as easy in India as it is in places like the US. 3) Unless it's absolutely necessary, I want to avoid having to start the pcb design, construction and assembly all over again.

I live in a biggish small town, can get overnight deliveries from Sydney, about 1000km away :)

That's a really good reason. So you add inductor to isolate the cap, or rip cap out of motor?

I prefer leaving the motors alone, because a replacement down the track will cause same problem if 'they' forget to take out the cap.

Grant.

Judged by population, my town is not truly small with about

For the time being, I've gone the route of reducing the caps from

There's that, yes. I have not yet completely ruled out inserting a series inductor. OTOH, it's very unlikely that anyone else will work on the machine during its lifetime.

Ah. My country town is large (for Victoria, it's fourth largest in state) at around 100k population, but we're only 160km from Melbourne, so overnight from Sydney to Melbourne, then a two hour drive up to Bendigo, delivery here around lunchtime :) It's good.

A normal AM radio tuned off-station would tell you something, perhaps TV or shortwave Rx for higher frequencies? Compare with RF noise from other devices.

Then you're in control :)

Cheers, Grant.

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