Electric Bench

Posted on 24th October 2008 by admin in Power Tools

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Electric Bench

I have a 24 volt d.c. (2 12 v lead acid batteries) power source for an electric …?

scooter i built. It dosnt have any wiring except for the wiring i put in which is as such>> 1st battery positive to switch-switch to to red motor-1st battery negative to 2nd battery positive-2nd battery negative to to black motor. Its a 400 watt electric motor. Obviously, i put the switch in temporarly on a bench test because the surge by turning on at max speed , something will break. I would imagine the speed controller(throttle) would go where the switch would go. And a actual switch would probley be good to have prior to the speed control. Can somebody give me an idead in detail to what kind of throttle control i need and where it would go in detail.Do i need a relay switch?

Dear Mr. Mouse,

It sounds like you know a little about what you are doing. When you close the switch, you will get full speed from your motor. You have things tied down pretty well? To control this thing you need a way to change the voltage. Three possibilities are common. First, I will assume that it is a shunt wound motor. If it is a series wound motor, then you need a load on it. A series wound motor will speed up until it comes apart if it has no load. Your finger on the switch is a good idea. A shunt wound motor will develop “back EMF”. It acts like a generator except that the current that it generates is in the opposite direction. When it (almost) cancels the current from the battery , the motor reaches its maximum speed. AS the motor has more load, it will draw more current.

The oldest method is to add Resistance to the circuit. A rheostat or variable resister will work, so will a series of resisters with a sliding switch . It is simple, cheap, and the resisters waste a lot of your electricity from the batteries.

The second method is to tap the batteries in different places to get different voltages. If you had a two way switch between the two batteries, one put only one battery on line, the other position put both batteries on line, then you have two speeds. Tap more than two places, you have several steps. The only thing is that the battery that you use most will discharge first. If you use all of the batteries most of the time, then you will probably all right.

The third way is Pulse Power. Electric trains use this method sometimes. The controller feeds full voltage to the motor, but in little slices. If the slices are bigger, the motor goes faster. It works best, but is also most expensive.

The old Baker Electric car used a Click-Clack switching mechanism to change the batteries from series to parallel to vary the voltage. It worked well when it worked, but was complex and prone to break. It did discharge the batteries evenly without to much waste.

My suggestion is first get (buy or borrow) an automobile battery charger (ten amp maybe) and try the motor on six volts, then twelve. You might try one of your batteries at twelve volts. If the motor is already installed in the scooter, prop it up so you can test it. Make sure it is clamped or tied down.

Your motor at twenty four volts is only about one half horsepower. At twelve, you might get one fourth horsepower, A lawn mower is usually maybe two horsepower. May the creator smile upon you.