DC MOTOR.

what power source? sammmm

-------- Your question is simply too vague to be answered. What kind of DC motor - shunt, series or separately excited or permanent magnet field ? Application - i.e. what is the motor driving (as this will affect the speed reduction you will get)? What speed reduction is wanted at full load? (speed at no load may not be much different from that with no rheostat. ) There is no cut and dried answer.

in advance.

Oh, come on.

12 volts, 10 amps? DC motor (not ac/dc)

Just a WAG but a 1.2 ohm resistance would have that motor running at about

1/2 speed. That's ASSuming it's providing the same torque.

To a "zero order" approximattion, the torgue is proportional to the current and the speed it proportional to the voltage (at the motor0.

He could borrow a trick used for multi-speel operation of small motors in automotive application and use a 4 position switch to give 0,1,2,3 (where 3 is full speed) control. Low speed would have two resistors in series, medium would have one, and full speed would have direct connection to the motor.

To WAG things: use 1 ohm resistors. Experiment to find real values that will give the speeds you want. But with fans, for example, the running torque is ABOUT proportional to the square of the shaft speed. So, half speed, you only need 3 amps to supply the torque and only 6 volt to supply the speed. The resistor needs to drop 6 volts at 3 amps or 2 ohms.

If a rheostat is used, he might be able to use a 2 or 3 ohm model. The power rating? Maybe 10 watts to be on the safe side. If he can get it specified, he should ensure that the rheostat can also handle the the 10 amps within it's power rating.

-------------- Recheck this. As you have done it correctly for your fan example, I ASSume this is a slip.

I did start to give an example similar to what you have done but then I realised that the type of DC motor and the speed- torque characteristics of the load can throw such estimates out the window. In addition the range of control wanted would be handy to know for choosing a rheostat. Overcautious- maybe.

Given that -the assumptions that you have made are the most likely but without further information one can't be sure.

Assuming that the field is constant (permanent magnet or connected on the supply side of the resistor) at 1.2 ohms- the "no load" speed would be nearly the same as it was without the resistor and at rated current, the speed would be 0 as the rotor voltage would be 0. This is not half the original speed at this current (and torque). At 5A the speed would be a bit less than half the speed without the resistor (at the same torque). At 2.5A the speed would be 75% of the original. This does not take into account the load's speed torque characteristic as you have done below.

If it is a series motor the results would generally be the same -half speed at 5A, 75% speed at 2.5A and lets avoid no load.

If the motor has a wound shunt field and is such that some expertise would be needed to separate field and armature connections, then use of an external rheostat would not be effective (i.e. at 5A the speed would be near normal but the torque would be halved)- hence the desire for more knowledge of the motor and application.

As for what you have said below- no problems -assuming the motor is as you assume.

The motor has permanent magnets and the load is not constant, becouse the motor shake a food dispenser for animals and some times it has more food then others. Thanks for your help.

D>

OK then what Gilmer suggested is in line. If you want a minimum speed at rated current then set the rheostat to give this. If the desired minimum speed is 25% of nominal at 10A then there will be a voltage drop across the resistor of 9V which requires 0.9 ohms and a power dissipation capability of

90 (say 100 + watts to be safe). The rheostat must handle 10A at all settings. However at part load, a higher resistance will be needed so a higher resistance rheostat would be needed. Don't expect constant speed as load varies. Some playing around as suggested by Gilmer would be worth while. It might be worthwhile looking into electronic controllers which will be more efficient and can control speed as desired.