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I have a few DC motors of 1.5 V and 9 V and need them for some applications. These motors run at a certain speed (rpm).

But I need to slow them down. Will it be a good idea to connect a resistor in series with the motor and the battery in order to reduce the motor speed?

Will doing so create any bad effects to the motor? I know there are other methods I can employ to reduce the rpm of the motor such as using gears or a PWM circuit, but I'm interested in knowing whether using resistor is a good idea or not.

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Using gears is always best because that is the method that makes available the highest percentage to the motor power capability. Since power is torque multiplied by speed, keeping most of the motor's power capability increases torque capability while reducing speed.

Pulse width or pulse amplitude modulation is second best. That can preserve the highest torque capability while reducing operating speed and power capability. The best way to maintain torque capability is to have an inner control loop that regulates torque by regulating current. An outer control loop regulates speed and provides the current reference. Because a switching regulator is used, there is less power wasted than would be wasted by a linear control technique or series resistance.

A series resistor can certainly be used. It can be a variable resistance (rheostat) or one or more fixed resistors. With a series resistor, the wasted power is directly proportional to the percentage speed reduction. Since there is no current control, adding resistance increases the speed change that results from any load change. The speed variation due to load variation increases in proportion to the amount of speed reduction. There is also speed variation caused by resistance change due to resistor temperature variation.

Additional Considerations

If not only "slowing down," but also variable speed is required, it is best to select gearing for the proper maximum speed and then use electronic control to provide variation. Larger DC motors are seldom used for fixed speed operation, but electronic control or series resistance may be required just for starting a larger DC motor to avoid excessive starting current.

For very small DC motors, series resistance may be perfectly adequate in some situations depending on various factors such as power source, duty cycle, cost analysis, maximum speed reduction, load stability, etc.

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The speed of a permanent magnet DC motor is primarily determined by voltage. The problem with using a series resistor is that its voltage drop is proportional to current.

If the motor draws constant current then the resistor will drop a constant voltage and the motor will run at a fixed (lower) speed. However the motor will draw more current when starting up and as the load increases, causing higher voltage drop and reducing speed even more. If it is driving something whose loading increases as speed increases (eg. a propeller) the speed will eventually stabilize, but if the load is variable then speed regulation will be poor.

If the resistance is increased too much the motor may stall under heavy load, or not even start up if the initial load is too much for it. This is good for some applications because the resistor limits torque and protects the motor from being damaged by high current, but if you want to maintain relatively constant speed under varying load then it's bad.

If you want to reduce rpm without compromising speed regulation then use a voltage regulator or PWM controller to lower the motor voltage. If you also want higher torque then use a gearbox (which increases torque by the same proportion as it reduces shaft rpm). Most small DC motors are designed to run at high rpm, so for applications that need low rpm a gearbox is usually the best option.

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    \$\begingroup\$ PWM is/was the approach taken by more sophisticated model train controllers. (Less sophisticated ones used a wirewound rheostat). \$\endgroup\$ – Rich Mar 17 '17 at 0:01
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You CAN use a resistor, but understand all you are doing is dumping power out the resistor to drop the voltage to the motor.

If you want to go really slow, the resistor method will probably cause the motor to stall way before you reach your desired RPM.

Using PWM ensures you get pulses of full torque, which allows you to drive the motor to really slow speeds.

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    \$\begingroup\$ As well as wasting energy, the resistor may prevent the moor starting at all, especially under load, because a motor needs much higher current when starting than it does once it's reached speed. So it's a REALLY bad way to slow a motor. \$\endgroup\$ – Brian Drummond Mar 16 '17 at 21:13
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Yes, using resistors were the first method of controlling motor speed. Back in the early days of electricity, street trolleys were driven by electricity and the speed controlling was done by moving mercury in/out of tungsten pipes. The more mercury filled the pipes the faster the cart was running. The speed of a carousel was also controlled by a bucket of salty water, deepening electrodes in it. Back those days electric wires were kept as straight as they could not to loose high speed electrons by missing the curves and leaving the conductors forever. :)

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This assumes you've just got a little motor such as is common in battery-powered toys or fans, running at low voltage, and you just want to slow it down a little. It also assumes that you really want a cheap and cheerful solution rather than a control loop, and are prepared to tolerate some inefficiency.

One or more series diodes may be used to drop the voltage the motor sees while still allowing it to draw more current under load than a resistor.

This may be too much for the 1.5V motor (though it may be possible with a careful choice of diode taking into account the forward voltage drop at the current you will use) but works well in the 3--12V range, for example for slowing PC fans to make them quieter.

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Simplifying matters to some degree, the motor speed is proportional to the voltage applied to it, while the torque is proportional to the current. A series resistor will only limit that voltage reliably when the current is kept constant, meaning the motor is running at more or less constant load. If this is your case, a rheostatic speed control can be a viable solution (lots of toy slot cars use exactly that): enter image description here

If your load will vary significantly, so will the current. As the voltage drop on the resistor is proportional to the current, you will observe that the motor speed gradually drops as the load increases. This behavior is not harmful to the motor, but it may be not what you want.

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