Try a power tap switch with a couple of small ohmic value resistors in series with the motor, like this (view in fixed font or M$ Notepad):
| ___ ___ ___ | .--o-|___|-o-|___|-o-|___|-o---. | | | R | R | R | | | | | | .--' | | | | | '-. | | | | | | | | | / \\ |+12V| '------o o o o | ( M ) | --- ^ SW1 | \\_/ | - | SP4T | | | | '--------o-------' | | | | | | | | '------------------------------' | (created by AACircuit v1.28.6 beta 04/19/05
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Let's say you've got a 12VDC, 250W motor (20A). If you got three 0.1 ohm, 50 watt resistors, each would drop about 2V when switched into the circuit. That would mean 4 different speeds. If you go this route, be sure to get tap switches that are made to switch inductive loads, and are rated for the maximum current expected. Not cheap.
You could also try to find a real honker of a power rheostat to do this job. Again, using the above example, you'd want a rheostat between 0.5 ohms and 1 ohm, with a maximum current rating of 20 amps. This, while simple, is going to be really expensive.
If you look into the cost, limited control, wasted battery power, excess heat and lack of convenience of these two brute force methods, you can start to see why PWM speed control of DC motors is the method of choice.
An important, but often overlooked feature of a DC motor speed controller is that it has current limiting, which will save you energy, and wear and tear on your machinery. Directly switching lead acid batteries into a stationary electric motor sees a huge current surge until the motor speeds up to counter this with its generated back EMF.
Just a note of caution if you want to buy a commercial controller: DC motors are often used industrially, specifically because of the ability to control speed. But these are typically run from AC line power using controllers that are more like glorified lamp dimmers to provide PWM control. They are fairly inexpensive (I got one for a 1/3 HP motor for under $50), so you might be tempted. But this type won't work from a straight DC source.
Best regards,
Bob Masta dqatechATdaqartaDOTcom D A Q A R T A Data AcQuisition And Real-Time Analysis
So any time an electric scooter motor is not running up to speed, the motor speed controller will keep it from sucking current. That's important for my application.
Thanks to all of the replies, educational as usual.
What does the speed controller connect to besides the battery and the motor? Where can I get that information, or does that data come with the controller? Anyway, thanks for the clue.
No, the controller will limit maximum current draw to what its specification states.
Sorry, there are two things here. When you are running at half throttle and steady speed, the pulse width modulation (PWM) will be applying an effective voltage to the motor slightly more than the back EMF of the motor. This voltage will be pushing a current through the motor such that the product of this current and the net voltage will be the gross power cunsumption of the motor which is needed to overcome the resistances to motion of the vehicle. Is this clear?
I have several cheap controllers (~$10US) here that limit current to
20 amps, I think. When you twist the throttle (or push the thumb lever) and the motor is stationary and connected mechanically to the drive wheel, the contoller will only supply up to 20 amps. If you connected the battery directly, you might get 100 amps flowing which will heat your motor, crucify your brushes, and apply much torque to your drive components. Your batteries will also suffer and all in all, your machine will wear out prematurely. With a controller, you can effectively apply from zero amps, to 20 amps (or whatever the max current your controller will pass) so you can take off gently until you get some motor revs going, and have some reasonable back voltage to self limit the current draw, and your machine will last a lot longer. Controllers, are cheap and effective, and that is coming from a luddite who likes things simple and manually controlled :)
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