Rc Network To Filter Brush Noise From Universal Motor?

[DaveD]

I have a 40 or 50 year old electric hand-held scraper for scraping (flattening) lathe and mill bed ways. Its cord was frayed, and so I decided to replace it. On opening the motor housing, I found an ancient cylindrical two-terminal component tied across the brushes. It had died, one of the terminal wires was loose of the cylinder body. The cylinder was marked with values at birth, but they have faded into obscurity.

 

I'm thinking that the placement of the device on this universal brushed motor makes no sense for any sort of starting feature. I vaguely recall that sometimes there's a little RC net designed to filter brush/commutator switching noise on DC motors, and maybe the same holds true for universals?

 

The motor was built by Bosch. It's nameplate says 220 Volts, 140 Watts, and 'n1000' which I believe is the germanic way of writing RPM=1000.

 

If it's not too much bother, I would like to ask 2 questions:

 

1. If it is an RC network, does anyone have suggested values for R and C on a motor of this size?

2. I just spent a couple minutes thinking about how to build a low-pass filter that has only two terminals. I'm a little stymied. Three terminals, and I'm good to go. How would I wire an individual resistor and capacitor?

 

I'm missing something, and I'd sure appreciate it if someone would point me in the right direction!

 

Thanks!

Dave

 

[marke]

Hello DaveD

 

Welcome to the forum

 

I expect that the "device" was just a plain old capacitor installed to reduce the radiated noise down the cable from the arcing brushes. Any series resistor would reduce the effectiveness of the noise suppression, so I would suggest that you find a 100nF 250VAC X rated capacitor and try that.

Phillips used to have a 330 series capacitor that would do the job, but that has been sold so I can not give you another good reference at this stage.

 

Bet regards,

[DaveD]

Welcome to the forum

 

I expect that the "device" was just a plain old capacitor installed to reduce the radiated noise down the cable from the arcing brushes. Any series resistor would reduce the effectiveness of the noise suppression, so I would suggest that you find a 100nF 250VAC X rated capacitor and try that.

 

Hello Mark,

 

Thank you for the kind welcome! It's a pleasure to be here; I have perused a number of the topics already and found them very interesting and informative.

 

I appreciate your response to my problem. It seems I was making things out to be harder than they were!

 

I can come close to the capacitor spec you mention: a Cornell-Dubilier CD10FD101J03, which is a dipped mica cap rated 100 pf at 500 V, with a max temperature of 150 C. Does this seem like an appropriate choice to you?

 

I don't know what an "X rated" capacitor is. It must be either a classification I'm not familiar with, or perhaps a very naughty capacitor. :-) Would you please enlighten me on this as well?

 

Thank you for the help!

Dave

 

[marke]

Hello Dave

 

I expect that the dipped mica cap has a DC voltage rating of 500V. I do not expect that this capacitor would be suitable for this application, but I could be wrong.

 

I would normally expect a metalized Polypropylene or a metalized polyethylene capacitor would be used.

The capacitor must have at least 250VAC rating which can commonly equate to greater than 1000VDC.

One advantage of the X rated capacitor is that it would generally be self healing in that it does not develop a short when it fails internally. The metalization around the insulation breakdown is vapourized, reducing the capacitance but not failing to a short circuit.

X rated capacitors and Y rated capacitors are designed and rated for across the line useage, commonly for supression purposes.

 

I would suggest that 100pf is too small a value. I suggested 100nf which is 1000 times larger than 100pf. 100nf = 0.1uF

 

Best regards,

[AB2005]

Hi DavedD

 

Read the topic in the following link. At page 25, there are formulas for calculating the capacitor value. It may help you for calculating the right capacitor for your application.

 

http://164.54.212.3/control_systems_manuals/D405_wiring.pdf

 

Best Rrgards

[DaveD]

I would suggest that 100pf is too small a value. I suggested 100nf which is 1000 times larger than 100pf. 100nf = 0.1uF

 

Mark,

 

Thank you very much for checking me! 3 orders of magnitude error, and that was just on copying a note to my book. Imagine what would have been the result had I actually needed to calculate something!

 

Digikey seems to carry a nice assortment of suppression capacitors. I chose one in their "X2" series: Epcos p/n B32921A2104M, "0.1uF, 305 VAC, EMI Suppression". It is one of the metalized polypropylene self-healing ones you reccommended.

 

I also wanted to give you a pointer to a really great paper I found from TI on snubber circuits:

http://focus.ti.com/lit/an/slup100/slup100.pdf

 

AB2005, thank you for the pointer to the manual. I read the part you referenced carefully, and learned several things from it.

 

Thank you both for your help!

 

Best Regards,

Dave

 

 

[AB2005]

Hello Dave

 

Thank you for providing a useful literature on snubber circuits.

 

[marke]

Hello Dave

 

You need to be aware that there are two different applications for capacitors in switching circuits.

Snubber circuits are used to reduce the rate of rise of voltage and overshoot when a switch opens, and are connected across the switch, be it a relay, or a solid state switch. Snubber circuits include a series resistance to reduce resonance problems and to limit the peak currents.

Suppression circuits are across the supply and are there to provide a very low impedance to any high frequency signals. These do not include series resistance as that will increase the high frequency impedance and reduce the effectiveness.

 

In your application, the capacitor is across the supply, and is therefore a suppression capacitor, not a snubber. The snubber information is not relevant to this application.

If the circuit was across the brush to commutator connection, it would then be a snubber, bu it is not practical to make such a connection.

 

Best regards,