COIL SUPPRESSION
Part II
Suppression networks can be found, typically, in three different locations. Some of the contactor coils have the suppression networks built into the coil itself, this is true of the General Electric EV-1 type coils. If this is the case you will notice a "+" sign marked on the coil designating that the coil is polarized meaning it must be connected properly with respect to positive and negative. The coil suppression may also be located within the main control card, such as the later GE EV-100 - EV-100LX and ZX controls with internal coil drivers. In both of these cases no coil suppression should ever be placed externally across the coil. The final location would be directly, externally placed across the coil itself. This, obviously, is the best method because you can visually inspect and replace the network yourself. If any other method is used you will have to replace either the control card or the coil itself, both options are much more costly.
Now what kind of failures can this simple circuit have on such complex systems? There are three different ways a suppression network can fail, first it can become leaky and allow small amounts of current to flow through it. This kind of failure will allow current to split between the coil and the suppression network. With current going through the suppressor it may take current away from the coil and reduce its magnetic strength. If the field strength is reduced the tips may separate when the vehicle crosses a bump creating tip bouncing which no control system likes to see. The additional current path may also allow more current to flow in the circuit. A leaky suppressor means that it has reduced in resistance, or in the case of the diode/resistor combination (as seen in Figure 1) the diode is allowing current to flow backwards through the diode. This additional current path and additional current can overheat and damage coil driver devices, internal or external. If you have a system which frequently burns up driver modules, such as the GE EV-1 style, chances are you have a leaky suppression network. To verify this you can measure the current flowing through the contactor coils. Systems differ, depending upon the coil resistance, typically a 36 volt coil will draw approximately 1.5 amps and a 24 volt coil will draw approximately 2 amps.
The most catastrophic and easiest failure to diagnose would be a shorted suppressor. Since the only resistive element in a contactor circuit is the coil, if the suppressor shorts across the coil you end up basically shorting battery positive to battery negative. This is fortunately where the fuse is supposed to blow open, assuming no one improperly replaced it with an incorrect value. This surge of current will also flow through any coil driver module, external or internal, and damage or reduce the life of that driver significantly.
An open suppression network can cause several failures depending upon the coil it is meant to suppress. Coils used for pump or power steering contactors which are not suppressed may cause interference with the main control cards, especially those using microprocessors which are notoriously susceptible to noise pulses. Depending on the control system an open suppressor across the 1A/Bypass coil can cause single or double trips. If the stored energy within the coil has no direct path to release, such as with an open network, it will actually remain within the coil and allow the coil to remain magnetized and energized for a longer period than normal. The General Electric system monitors the 1A/Bypass tips for release time, in order to detect a welded condition which would cause a dangerous full speed situation. So even if the 1A/Bypass coil remains closed for a split second longer than it should the control card will see this as a fault and de-energize the directional contactor for safety purposes.
A failure we recently experienced was an open network on a directional coil involving an EV-1 control panel. It was an intermittent failure but as we cycled the accelerator to energize and de-energize the directional coil we acquired a severe release of the contactor opening with a spark. Fortunately the fault occurred only in the forward direction and that helped to eliminate most of the circuitry. The fault was a suppressor which had been physically damaged and broken apart.
Sometimes the worst failures to solve are not defective components, but components replaced incorrectly. The early General Electric EV-100 systems use external suppression networks, consisting of a diode/resistor combination, which mounts on the side of the coils. There are many different networks with several different part numbers. We experienced a failure with a single trip, or PMT, when the vehicle de-energized 1A/Bypass. The vehicle would operate normally through the speed range and into 1A/Bypass, but when we de-energized 1A/Bypass to go back into SCR speed the directional contactor would dropout then pickup again and continue to run normally. The failure was an incorrect suppression network. Typically the auxiliary coils (1A, pump, power steering) require a G11 or G12 network which uses a lower resistance value than the directional coil networks. In this case someone had used a directional coil suppression network in place of an auxiliary coil network. This additional resistance allowed the 1A/Bypass coil to remain energized a split second longer than normal. This additional time was seen by the control card as a welded 1A/Bypass contactor and the system would dropout the directional contactor and reset itself for a second look. By the time the directions reset the 1A/Bypass contactor had opened and the system was allowed to function as normal.
As we have seen a simple one or two component electronic device can not only be very important to vehicle operation but sometimes cause unusual failures. External suppression networks can be visually checked. Remember not to overlook those components replaced by someone who has worked on a vehicle before you. Sometimes the most important question, before servicing an electric forklift is "has anyone worked on the vehicle since it failed" and if so "what was done or replaced". We always have a tendency to immediately blame those complex parts such as the main control cards, don't overlook the simple things.
BACK TO:
Robert Meyers
8/06/97
rmeyer32@paonline.com