Model trains and slot cars use voltage level control, the simplest method of all. The theory is simple: The more voltage you apply to a motor, the faster it spins. A model train's controller has a variable resistor connected to the speed knob. The more you turn the knob toward a faster speed, the lower the resistance in the controller, which in turn allows higher and higher voltage levels to reach the motor. While this is a quick and easy method, it has inherent deficiencies such as the loss of motor torque at low voltage levels. Overcome these deficiencies requires the use of other methods for large industrial motor control.
Duty cycle modulation (DCM) is a more efficient method of control, since torque does not dissipate at low revolutions per minute (rpm). To understand DCM, picture a motor hooked up to an on-off switch and a power supply. If you just turn on the switch, it's called a 100 percent duty cycle, since the switch is on 100 percent of the time. Flipping the switch on for half a second and off for another half second creates a 50 percent duty cycle, since the switch is on only 50 percent of the time. If you are able to tun the switch on for only a third of a second, you have created a 33 percent duty cycle. The lower the percentage in the duty cycle, the slower the motor turns but without loss of power. Special motor controllers can vary the duty cycle from 1 to 100 percent. You turn a knob to vary the duty cycle time.
Pulse width modulation (PWM) refers simply to how long you leave the on-off switch on. If you leave the switch on for one second, the motor will run for one second. If you leave the switch on for one minute, the motor will run for one minute. In industrial automation systems, this simple technique controls complex machinery. For example, a motor is connected to a conveyor belt carrying a bottle, which has to be stopped and filled at a certain point. The engineer determines the conveyor travels at 1 foot per second and the conveyor has to stop at 5 feet of travel. Therefore, the pulse lasts for five seconds. When the pulse stops, the conveyor stops. PWM motor controllers are usually computerized and can vary the pulse width by microseconds.
Sometimes, you want the motor to stop instantly. Large electric motors coast to a stop when you turn off their power. If a part laying on a conveyor has to stop instantly at a certain point, a dynamic brake is applied to the motor via electricity, as opposed to a mechanical brake. When a motor is coasting, it turns into a generator. If the leads of the motor touch each other, the motor will short out. The motor backfeeds the generated voltage, which is opposite in polarity. Therefore, the motor stops instantly. Picture a motor connected to a battery. When you want the motor to stop, take out the battery and reverse the polarity for just a quick moment. The motor tries to reverse. Before it can have a chance to reverse, disconnect the battery. While shorting out the leads can work, it is extremely hard on a motor. Placing a resistor between the leads during the dynamic braking phase limits the current of the backfed voltage.