Why is using a variable resistor in parallel with a motor not the best way of controlling its speed?

Answer 1

By connecting a resistor in parallel to an electric motor to control its speed is bad practice. One would then be using the internal resistance of your source to reduce the voltage that drive the motor. In some cases it is not only insufficient but also dangerous because you might short circuit or overload your source.

In very low power applications with small motors, having a series resistor will work just fine (resistor in line with the motor)

With higher power AC circuits a choking coil may be used. Then you rely on the inductance of the coil to increase impedance without converting all the unused energy into heat.

The problem with resistors are that if your motor need to run on 6V in order to draw 1 ampere to produce 6 watt power and you obtained that by using a 12V supply with a resistor in series. Then the supply will deliver 12V x 1A = 12W and the motor will use 6V x 1A = 6W, what happens to the other 6W? it is dissipated by the resistor, the resistor will convert that unused energy into heat. This method is not very energy efficient and can become difficult to keep the resistor cool.

Ideally when larger motors are used. The use of PWM (Pulse Width Modulation) becomes essential. PWM allow you to switch the motor on and off at high frequency. When the power is on and no resistance is in series. Then all energy goes to the motor to be effectively used resulting in high efficiency. Then, when the supply is in off state, then no power is consumed at all. This will also result in very low energy loss. Thus, regardless whether the power is on or off there is always very little energy wasted.

If 12W is delivered when the supply is on and 0W is delivered when it is off. Should the pulse be on 50% of the time and off for the other 50% of the time. We will call it a 50% duty cycle (d.c.). The average power delivered will be 6W since it only deliver 12W half of the time. But since the 12W and 0W periods are both low energy loss states due to low resistance. The average 6W will also be delivered at high efficiency and almost no extra energy will be wasted on heat.

It makes sense, when you want to reduce the speed of a 100kW motor. Then using a resistor with a 50 000 W rating, baring in mind that 50 000W may be dissipated as heat. Then the "cooling towers" required to keep the resistor cool will be a bit absurd. Paying for an extra 50 000 W, what is not used will not do your business any good either.