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>I was going to let this go, but since this is like the 10th time I've seen >"MOV," I've just gotta ask.. What does MOV stand for? Metal Oxide Varistor. :-) David. _________________________________________________________________________ Get Your Private, Free E-mail from MSN Hotmail at http://www.hotmail.com.

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Metal Oxide Varisistor (not sure I spelled Varisistor correctly) looks like a small ceramic disc capacitor designed to conduct and dissipate energy above a certain rated voltage typically used to absorb small, brief high voltage spikes generated by back EMF of DC motors or typically used on 120vac circuits to absorb inductive spikes ----- Original Message ----- From: ffolkes <> To: <> Sent: Thursday, May 24, 2001 8:35 AM Subject: Re: [BasicX] Large Current Control (was: I've forgotten! Relays..) > I was going to let this go, but since this is like the 10th time I've seen > "MOV," I've just gotta ask.. What does MOV stand for? Thanks. > > TM > > At 09:25 AM 5/24/2001 -0500, you wrote: > > While we're on the topic ... > >what would be some typical component values for controlling a 12 v. > >25 amp motor with a contactor? I have an appropriately rated > >contactor but can just see the contacts arcing, possibly welding shut. > >What size diode, MOV, and RC snubber components are needed? > > > >1. >From discussions on other forums on this, the consensus is that you > >MUST put > > >a diode across the relay coil, " > > > >2. >Also, if you're switching any inductive loads (eg motors, solenoids > >etc.) > > >with the relay CONTACTS, you'll need to fit some suppression components > >Typically, this would consist of a MOV across the inductive load itself, > > > >3. >and one of those RC snubbers either in parallel with the MOV > > > >Thanks, > > > >John Piccirillo > > > > > > >

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>>what would be some typical component values for controlling a 12 v. >>25 amp motor with a contactor? I have an appropriately rated >>contactor but can just see the contacts arcing, possibly welding shut. >>What size diode, MOV, and RC snubber components are needed? > >12V? Is that AC or DC? DC. We're making an automous robot out of the chassis for a child's sidewalk car. Going up a 10 deg incline with a 100 lb load, each motor takes 12 amps at 12 volts. >Basically, to size the MOV, you need to consider the total ENERGY it will >have to handle (usually in Joules). This relates to the motor voltage and >current in some way that I don't really understand. Sounds like a MOV might get fried. >I have a nomograph for computing R/C values for snubbers but the "current" >scale tops out at 10A. Even at 10A, the capacitor recommended is 10uF. And, >at 10A, 12V, the resistance is so way below 1ohm that your wire connecting >to the motor is likely to be too much resistance! Doesn't sound like the values are critical. I suppose the basic idea is to provide an RC time constant of a a 100 ms or so with a big enough capacitor to store the back emf current and then bleed it back and forth through the motor winding and dissipate it. John Piccirillo

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[snip] > DC. We're making an automous robot out of the >chassis for a child's sidewalk car. Going up a 10 deg >incline with a 100 lb load, each motor takes 12 amps >at 12 volts. Wow! That's some "load"... Ok, well coz it's DC as opposed to AC that changes some things and leaves others the same. And because the voltage is rather lower than most snubbing circuit nomographs, there's not so much information available. But, I'll offer what knowledge I have in this arena. 1) The back emf is ALWAYS there,... it's related (proportional?) to motor speed only (not power/load). In fact, in an otherwise "open-loop" system, a clever motor-speed controller can sense the back-emf and use it to measure the rotational speed of the motor, regardless of loading conditions. 2) This means, inherently, that the worst case EMF will occur at full speed, and at low speed, the EMF is near zero. That's why, if you use dynamic braking, it's way more effective when the vehicle's moving faster, and becomes useless at very low speeds (unless you apply a small reverse-current to the motor, then it's still very effective :-). Try this. Obtain a relatively beefy 12V dc motor. Short the leads. Try to turn the shaft (by hand!) slowly, it sort of feels like the motor's turning in honey or molasses... Now try to spin it fast, ... see (feel) the difference? You're playing with (back) EMF entirely (since the motor's shorted, there's no other source of voltage!). 3) I don't remember the energy equations here (for spinning motors), but given that a 22mm diameter MOV (about $1.50 worth of component) can dump 100's (or maybe even 200's) of Joules maybe a MOV isn't entirely out of the question. (that same 22mm dia. part will clip transient currents of over 4000A). To put some sense on that Joule figure, in a flash capacitor in a camera, Energy = 1/2 * C * V^2, so if it's a 680uF/300V flash cap, you get E = 0.5 * 0.00068 * 300 * 300, or about 31 Joules. If I remember rightly, the defibrillator machine in hospitals ("Clear!!") has an upper limit on it's controls of about 150 to 200 Joules (which they rarely, if ever, use!). So a MOV that can dump 150+ Joules is a pretty beefy thing. That said, however, they're not designed for "repetitive" dumps of that magnitude. They're typically designed for infrequent dumps (power surges, spikes, etc.) with plenty of time between them. The other hassle with MOV's is that their over-voltage is several (dozen!) percent above their running voltage. The "knee" of the curve is quite well rounded. Therefore a theoretically 12V MOV might allow up to 18V or more across the load,... However, it will nicely clip the 30V+ back-emf kicks. > Doesn't sound like the values are critical. I suppose the basic idea is >to provide >an RC time constant of a a 100 ms or so with a big enough capacitor to >store the >back emf current and then bleed it back and forth through the motor winding >and >dissipate it. The biggest issue in your case is what you're intending to do. Are you trying to limit radiated emissions? Are you trying to solve a problem (that already exists) where every time you reverse a motor (or turn one off), your CPU crashes? Are you trying to prevent your switching device (relay?) from contact-welding? Limiting the RF emissions could be as simple as sticking a 100V (or better) 0.1uF "bypass" cap across the motor terminals. Using a solid-state (semiconductor) control switch lends itself very nicely to solving the other problems. Have you considered a power MosFET based switch instead of a relay. MosFETs these days have Drain current ratings up to 50A or so, they're easy enough to drive and even easier to parallel up (no balancing resistors required in the load circuit). One distinct advantage of a MosFET based swtich is that you can use PWM to control the motor speed without sacrificing torque at low speed. Essentially you can use the full torque available at 12V for a brief burst, and use the duty cycle to control the speed. That's what they use on most if not all golf-carts, wheelchairs, etc. etc. Regards, David. _________________________________________________________________________ Get Your Private, Free E-mail from MSN Hotmail at http://www.hotmail.com.

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