12 volt motor delay? - PeachParts Mercedes-Benz Forum

dieselmania · #1 06-26-2006, 12:06 AM

I am working on a project where I need to delay the starting of a 12 volt fan motor by a second or two after power is applied. It would have to be rated at 30 amps. hope some of the electrical gurus here can help me find one, I'm not having much luck on Google.

Jeremy5848 · #2 06-26-2006, 01:19 AM

One source is Mouser Electronics. See their catalog at http://www.mouser.com/catalog/626/1367.pdf. The Omron H3CR-A SERIES: 1/16 DIN, ANALOG-SET, SOLID-STATE TIMER ($62.25 each) is available to operate on 12 Volts DC. It will handle a maximum of 5 Amps so you would have to use a relay to handle your 30 Amp load. For that great a load, it would be wise to use a capacitor or other supressive element across the load terminals or the inductive kick from the motor, when the power is turned off, will cause arcing across the contacts and will destroy them in a short time.

There are probably industrial delay timers that would handle such a load directly but they would be much larger, would be unlikely to operate from 12 Volts DC, and would be much more expensive.

There are also timers available in the surplus market that might do what you want, with or without an external relay to handle the load. Meeting your exact requirements (12 Volt DC operation, time delay of a couple of seconds, 30 Amp load) would be difficult.

Another possibility would be a pure delay circuit (instead of a more generic timer circuit), which could be built from a capacitor, a transistor, and a couple of resistors. That circuit could then actuate a relay to switch the 30 Amp load. You would have to build such a device yourself or find a technician to do it for you. You might be able to find such a person by asking at a local electronics shop in your area; such a place might stock a timer that would do what you want.

dieselmania · #3 06-26-2006, 11:01 PM

Jeremy, thanks for the link & info. I am trying to drive two fans @10 amps each. They each use a 30 amp fuse and a 30 amp relay, for the initial startup current. I could use the delay you linked before one of the 30 amp relays.
Do you think a 10 amp motor will fry the relay contacts if I don't use a cap? How would I size the cap anyway?

The reason I want the delay is to avoid simultanious starting which would be quite a sudden draw on my charging system.

Jeremy5848 · #4 06-27-2006, 01:05 AM

Thanks for the additional details. Your relays and fuses will be fine. As long as the fuse and the wiring can handle the inductive load of a starting DC motor (current drawn is maximum at stall, that is, when the motor starts), the charging system won't mind. Bear in mind that, any time the load exceeds the charging ability of the alternator, the excess comes from the battery. You are "overdrawing" the charging system but the battery is there to prevent problems -- in the short term only! The charging system must be able to keep up with the load while the motors are running. That depends on the size alternator in the vehicle. Make sure your alternator can keep up.

With each motor on a separate circuit and fuse, all I would be concerned about would be the wires. Make sure they can handle the load without excess voltage drop. I'd use at least 14 gauge, which is rated at 12 to 15 Amps, for each motor. Choosing too small a wire gauge is a "penny wise, pound foolish" error that we all have made. Too small a gauge cheats the motor and risks not only damaging the motor but, in extreme cases, starting a fire.

The right capacitor size is a function of the inductance of the motor, which is not easy to find out. You can easily experiment by buying some inexpensive electrolytic capacitors from a Radio Shack or other electronics store. Plain aluminum electrolytic caps will do fine; don't let the salesman sell you up to tantalum (expensive) or to an AC motor starting capacitor, which is a completely different animal. Get a series of values, for example, 1, 10, 100, and 1000 microfarads, at 100 volts (at least). Even though the motor is running on only 12 Volts, the back EMF generated when the relay contacts open will be much greater (that's why it causes sparks).

Try starting and stopping the motor with the relay and observe the sparking. Then put a capacitor across the relay contacts, with the (+) wire of the capacitor on the relay contact that is connected to the (+) battery terminal and the (-) wire of the capacitor connected to the other relay contact (sorry if you already know this). Try different values to see which one makes the least sparking and use that value. If the relay doesn't spark much without a capacitor you can get along without one but experiment and see.

Jeremy

Pete Burton · #5 06-27-2006, 08:18 AM

buy a simple time delay RELAY.

dieselmania · #6 06-27-2006, 11:04 PM

Thanks a bunch Jeremy, you are a wealth of electronics info...a field in which I need improvement for sure.

The wiring is 10 guage copper which came with the fans. The fuseholders are also on 10 guage pigtails. I will try them without the caps first. I guess even if the relay contacts fry after time they are just $7.00 plug in relays anyway.

Jeremy5848 · #7 06-27-2006, 11:35 PM

No point in you and I becoming experts when we can find them here so easily! We all contribute as we can.

Pete's point is well made. A simple time delay relay may even be able to handle the 10 Amp load of the fan.

However, I as I mentioned earlier, I don't think you have to worry so long as your alternator can keep up with the load over the long haul. And yes, it may just be easier to get a new relay occasionally. Keep an eye on the contacts, though. If they get burned, the fan won't work well.

A true horror story: the contacts in the pressure switch of my home water system became burned and pitted over many years. Early in the morning of September 25, 2002, the system "called" for water but the well pump wouldn't run because the points had so much resistance that enough current wouldn't flow. So the pressure never built up and the switch kept calling for the pump to run. The circuit breaker didn't pop because the current never exceeded its limit.

The resistance of the points generated heat, which increased the resistance and generated more heat, until, about 5:00 A.M., the pressure switch caught fire. A smoke alarm went off and I put out the fire with an extinguisher and called 911 to back me up. Little damage was done beyond the pressure switch and an adjacent plastic slop sink. We were lucky that it happened while we were home.

You will not have that problem with your fans, of course, but do keep an eye on those relay contacts.

Jeremy