DC Electrical Systems For Forklift Trucks
If you drive an electric fork lift have you ever wondered why it is that there are no "gears"? The more you press the accelerator, the faster the truck goes.
Electric trucks are driven by a large battery called a traction battery. These range in voltage from as little as 24 volts for a small hand pallet truck up to 80 volts for the biggest electric counterbalanced machine. Since a battery produces only direct current (DC), it is not possible to have a direct connection between the battery and the motor since if that was done the truck would travel at top speed all of the time!
The reason that the more you put your foot down the faster it goes is actually quite easy to see and is made possible by an electronic component which is a member of the transistor family. It is called a Silicon Controlled Rectifier or SCR for short and many people also call it a Thyristor. A SCR behaves like a valve with a tap on it to use a water analogy. The more the tap is turned on the more water comes out of the pipe. So it is with an SCR. By using a fairly complicated electronic circuit to control the SCR it is possible to make the SCR fire "pulses" of electricity at the drive motor for a fraction of a second at a time.
Looking at the graph on the left, let's assume that the truck battery is 80 volts. If this voltage is applied to the drive motor the truck will travel at full speed
Using an SCR it is possible to apply the full 80 volts to the motor but only in bursts that last for a fraction of a second. Assume that the blue lines on the graph indicate when the voltage is applied and in the grey areas zero volts are applied. Using simple figures we can see that: If each burst of voltage lasts for, (say), one tenth of a second then for nine tenths of each second the voltage will be zero. These "bursts of voltage" get averaged out and the result is that only 10% of the original battery voltage is actually applied to the drive motor. If this were an 80 volt truck only 8 volts would be supplied effectively and it's unlikely that the truck would even move.
As the truck operator puts his/her foot down the pulses of electricity get more frequent and the voltage peaks get closer together. By the time the operator has pressed the accelerator half way to the floor, the blue area above would be the same as the grey area and we would have half the voltage or 40 volts applied to the drive motor.
It can be seen that continuing to press on the accelerator causes the pulses to get ever closer together until eventually the grey area on the graph disappears leaving a blue area which represents the full 80 volts available from the battery. At this point the truck is doing it's maximum top speed.
In practice this all happens extremely quickly, hundreds or even thousands of times per second. I have used one second here to simply illustrate the point. Also there are quite a lot of electronics causing all of this in the background but the net result of this activity is simply to make the SCR supply the correct bursts of electricity at the correct time.
Obviously the electronic control panel on a fork lift truck does much more than this but this is the essence of how the traction system works. It should also be noted that on some truck there is a switch called a bypass contactor. As the name suggests this bypasses the system outlined above. The truck manufacturer decides all this for us but typically when the bursts of electricity described above get to 90 per cent of the full voltage, the bypass contactor shuts, bypasses this system and applies full voltage to the battery. This is less wasteful of battery energy at or near full speed than it would be if it went through the circuit we have here. Many other refinements and protection such as fuses etc are in the truck's electronic control panel which should only be accessed and serviced by a suitably qualified engineer.