Electronic Design

Bob's Mailbox

Dear Bob:
About your reference to coffee timers (Electronic Design, Jan. 22, p. 127): we too have been threatened by our security guards for leaving our coffee pots on. So, I purchased a couple of timers for coffee pots to use both at home and here at work. I got the kind with the plastic inserts you can move around to set the on and off times. Then, I completely removed the "on" inserts and now just use the "off" inserts. That way, I have to manually turn the pot on, and the timer turns the pot off.

I tried to convince our security guards that this system was highly reliable, but their regulations state that timers on coffee pots aren't allowed. My solution was to put the timer in a place where the guard couldn't see it—end of problem, end of the nasty notes.
Chuck Neal
via e-mail

I have to say, that's a cute and elegant scheme! I like it! (Not that my timer is bad.) I'll have to think of some cases where my scheme does things better than yours (if there are any instances). But yours probably works better on a holiday.—RAP

Hello Bob:
I noted in your column that although you had satisfied your need for a coffeepot timer, you had to "wire it up." I've had a timer on my copier for years, which was made by Intermatic. You can set the on/off times for each day of the week. It's called their Time All Scheduler, Model SB911 (Rated 15A, 1/3 hp). Once plugged in, you only need to set it, plug in your appliance(s), and program it by moving sliding actuators.

By the way, because I'm involved in the electrical product safety compliance trade (www.safetylink.com), be certain that the coffeepot you're using isn't "Rated for Household Use Only"—as are nearly all appliances available in the mass marketplace. You will typically find this notice adjacent to the safety agency's markings.

It's not unusual, in my experience, to find home-rated appliances in restaurants, offices, church kitchens, and other workplace environments. These appliances aren't rated for commercial duty and shouldn't be employed in the workplace; to do so is a misuse of such appliances. Electrical appliances used in a workplace should be rated for commercial usage and bear the mark of a nationally recognized testing laboratory (NRTL), among which the more familiar are UL, ETL, and CSA.
Art Michael
via e-mail

Ours is an industrial-rated BUNN-omatic and is marked UL.—RAP

Howdy Bob:
I started out as an electrical power engineer at the University of Idaho. I have been working for the Navy for about 25 years and I learned analog engineering on the job, with a lot of help from your design articles. I did pass the P.E. exam a while back and haven't used it much, but I have kept an eye on the electric industry. We have been blessed with relatively cheap energy, until now.

Some of my friends here at China Lake studied ways to reduce energy consumption of fractional horsepower motors back in the Carter Administration energy crisis, but their solutions were ignored when the crisis passed. They published "Naval Weapons Center Technical Memorandum 4552" in July 1981. It showed their investigations and recommendations to retrofit fractional horsepower motors (1/3 through 3/4 hp) and turn them into two-phase motors by using an ac capacitor across the start (speed) switch.

This way, the start winding is used as a second-phase winding during running. At stop, the switch is closed and the capacitor is safely shorted. When the motor gets rolling, the switch opens and the capacitor phase shifts the line current to use the start winding to help push the rotor. It takes about 35 µF for a 1/3-hp motor, about 45 µF for 1/2 hp, and about 55 µF for a 3/4 hp. Again, these are ac caps with at least a 350-V ac rating. I got my caps from C & H Sales in Pasadena, Calif.

I'm going to retrofit my private well pressure pumps and my evaporative cooler motors so that the "wheel of fortune" doesn't gig me any more than necessary. Of course, the wiring has to be done safely, although the savings are on the order of 20%, if all is done properly. The motor pulleys need to be properly sized, and the minimum current point must be determined with a clamp-on ammeter, as the capacitor values are tried.

A second benefit is that the run winding with the start winding actually runs cooler than just the run winding alone. That should help the motor last longer. This TM 4552 isn't available in electronic form at this time, but I can send a copy to you with my added notes, if you're interested. (Note: this isn't a simple power-factor correction, but a conversion of the motor into a two-phase unit.)
Ed Tipler
via e-mail

Hello Ed. This seems kind of neat. It sounds like the sort of thing that is ignored. Yes, please send it to me.—RAP

All for now. / Comments invited!
RAP / Robert A. Pease / Engineer
[email protected]—or:

Mail Stop D2597A
National Semiconductor
P.O. Box 58090
Santa Clara, CA 95052-8090

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