In the last post of this sequence, we talked about carts that still used resistors to control the speed of the cart but didn’t have a solenoid and its circuitry for each resistor. It was a big improvement from the maintenance point of view over their predecessors. This month, we’ll talk about the next step in the evolution, which was to get rid of the energy consuming resistors altogether.
As we discussed in the first two posts of this series, the resistors slow the motor down by taking some of the energy that would be available to the motor and dissipating it in the form of heat. By controlling the voltage to the motor with an electronic speed controller, we eliminate using the energy to heat up the resistors. That means the battery pack will be able to supply the motor with energy for a longer period of time. There will, of course, be less time required on the charger and a longer range of driving time for the cart.
The method employed by the electronic controller to vary the voltage to the motor is called Pulse Width Modulation (PWM) and is a much more efficient method than heating up resistors. In the book Electronic Golf Cart Repair 101 (and a half), I spend a lot of time and effort to explain PWM in detail. If you really want to get into the nitty gritty of PWM, I highly recommend reading the book. Links to Amazon, where the book can be purchased, is at the bottom of this post. But for now, we’ll talk in general terms to get a picture of how PWM works.
Let’s say we had our golf cart motor connected directly to the battery pack (36 volts). No resistors, motor controller or anything else in the circuit. Just as soon as the circuit was made complete, the motor would begin to spin. It would continue to gain speed, right up toward its top speed. The top speed would only be limited to the design of the motor and how fast it is rated to go at the 36 volts by design. How much current it required to maintain the top speed would, of course, be proportional to the load (how much the cart weighs, how much the driver weighs, how level is the terrain, etc.)
But now let’s pretend that we placed a toggle switch in the circuit to where we could turn the motor on or off with a flip of the switch. Now remember, that this is just an imaginary experiment. If you tried to do this in “real life”, it would be a disaster for the motor, the battery pack, the switch and certainly for the driver of the cart.
As the switch is first turned to the on position, the cart would lunge forward as the motor starts to speed up toward top speed. Then as the switch is turned off, the cart would start to coast to a stand-still.
Now, let’s say that we decided to turn the switch on for one second and then off for one second and repeat the process for several seconds. The cart would lunge forward trying to reach top speed but before it ever got there, the switch would be turned off and the cart would start to coast. In the first two second period of time, the voltage to the motor would average 18 volts (36 volts for one second and 0 volts for one second) even though 18 volts was never sustained for any length of time. The average speed of the cart would be something that depended on the load, the specifications of the motor, etc. but it would be somewhere between zero and whatever the top speed of the cart was designed to be. As the process is repeated, the cart would finally (after getting over its initial attempt to get going) average out to some speed, even though it never really sustained any specific speed for any length of time.
But now, let’s say that we could flip the switch back and forth several times a second. Now the travel of the cart would smooth out a bit, in that it never was in one position long enough to get toward 0 or full speed. If we kept the switch in the on position twice as long as in the off position for a certain period of time, we would speed up the cart by increasing the average voltage to the motor during that period. Well, that is what an electronic speed controller does. It pulses the motor with bursts of energy proportional to the speed desired by the cart’s driver. The faster the driver wants to go, the wider the pulses of voltage to the motor are (pulse width). With the motor speed controller, this switching or pulsing happens at a rate of around 12 thousand times per second, so there is no noticeable lunging of the cart. It reacts to the average of the voltage to the motor for any given period of time. That is how pulse width modulation works. Now, of course, the controller must have some important information from the driver in order to determine just how wide to make its pulses of energy to the motor, and that is what we will discuss in our next post in the continuation of the study of electric golf cart evolution. See you then, Ron.