COIL SUPPRESSION
Part I
Today's forklift controls are becoming more and more advanced with glamorous features and microprocessor controls. As these controls change, we still see the simple items that seem to remain the same. Hidden behind the glamor and advancements are the electronic components that we all seem to ignore, yet can cause some of the most unusual failures. It is because they are so simple that we forget that they actually perform a very important function. Many of the "COIL SUPPRESSION" devices remain unchanged from 20 years ago. Lets take a look at what they do, where they are and what happens when they fail.
Coil suppressors are obviously found around the magnetic coils used for direction, 1A/Bypass, Field Weakening, pump, power steering and any other contactor used to control high current devices with low control currents. Forklift contactors can be considered as high current relays or solenoids. They allow us to control very high currents, by opening and closing the power tips, with the use of low control currents, energizing and de-energizing the magnetic coil. When we apply voltage (which allows current to flow) to the contactor coil, we build up a magnetic field within the coil. This magnetic field pulls the contactor armature down, along with the contactor tips, and engages both tips to allow high currents to flow through them.
The contactor coil is made up of hundreds possibly thousands of windings of insulated wire. These windings, as current flows through them, saturate and, somewhat like a small capacitor, store a charge within these windings. Think of an electrical coil as compression spring. As you push to compress the spring you are applying energy to the spring. Now that you have the spring compressed what happens if you release the pressure being applied? The spring will release the energy stored and fly across the room. A magnetic coil has similar characteristics - it stores energy while energy is being supplied and it also releases that energy when the voltage or current is removed. Like the compression spring the coil releases its energy all at once in one swift moment. This release of energy creates a very large voltage spike which could cause damage or interfere with the operation of other parts in the control system. This is where our coil suppressor comes into play. The coil suppressor's job is to suppress that high voltage spike.
There are a couple of methods used to suppress these high voltage pulses. The most common is a resistor/diode combination which is placed across the magnetic coil, see Figure 1. Notice that the diode is mounted so it is reverse-biased, meaning the cathode or negative side is placed toward battery positive. This arrangement keeps the normal current used to energize the coil from passing through the suppression network. When the energy is released from the coil, after the battery positive source is removed, the spike which is generated is actually a negative going pulse, opposite of the positive voltage being supplied. Just as the compression spring has an opposite reaction when released, compress the spring inward and the release generates energy outward. This negative going pulse is allowed to pass easily through the diode/resistor combination and is shunted or directed to ground.
Another method is the varistor, which is also placed across the coil. A varistor looks like a small round loli-pop without the stick. Colors of varistors vary but are usually black or red. It is a semiconductor device which is rated for a certain voltage. At the rated voltage the internal resistance of the varistor reduces dramatically and allows the high voltage spike to pass easily through it, usually to ground potential. A 36 volt system may use a varistor rated at 100 volts, this means at 36 volts the varistor resistance is very high, which will not allow the coil current to pass through the varistor during normal operating conditions. When the coil is de-energized and the stored coil energy is released this high voltage spike is allowed to flow through the varistor, this voltage spike will reach the rated voltage of the varistor and pass through easily.
Even though these suppression networks are relatively simple, their specific ratings can be critical. The varistor must be rated so that the high voltage spikes will easily pass through. If the varistor is rated too high it will not allow all of the spike to pass, on the other hand, if the rating is too low it may allow some of the normal coil current to flow through during normal operation. In the diode/resistor combination the value of the resistor is the critical part. If the resistance value is too high it will not allow the energy to be released easily or quickly. Different suppression networks may use resistance values from 10 to 400 ohms depending on the circuit.
Understanding the vehicle operating system is certainly the key to any successful service adventure, and I do mean adventure. Suppression networks are very simple and are basically the same in most vehicles you will service. Next time we will conclude our discussion with where they are located and what kind of failures they will cause if they become open, shorted, leaky or simply replaced incorrectly.
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6/13/97
rmeyer32@paonline.com