How does speed control work in an electric golf cart? — Stephen, Pittsboro, N.C.
Marshall Brain Answered:
A golf cart is a pretty simple device. It has a set of batteries (usually six) connected in series. They drive a DC motor.
In the simplest possible speed controller, the accelerator pedal is just a big switch. When you press the accelerator pedal, it connects the batteries to the motor. If you want to go slow, you tap the pedal every few seconds. If you want to go faster you tap it more often. If you hold the pedal down, the batteries have a straight connection to the motor and you go as fast as possible.
Tapping like that gets old very fast, and it is hard on the motor. So newer gold carts use a PWM (pulse width modulation) controller like those found in electric cars. In a PWM controller, a computer sends pulses of power to the motor thousands of times per second. Very short pulses cause the motor to go slowly. Long pulses cause it to go fast.
In the past, PWM controllers were prohibitively expensive. So older golf carts use resistors instead. Power flows from the battery through a big resistor. The resistor turns some of the battery power to heat. The remainder goes to the motor, causing to motor to turn more slowly than it would with full power. There are usually two or three resistors, and solenoids choose different combinations to give different speeds. You can see the resistors and solenoids in this video:
In this schematic you can see different taps on a single resistor. Solenoids chose the right tap. It’s a very crude technique that wastes lots of battery power, but it is simple, cheap and reliable.