Address Given on the Safety of Electric Vehicles to the Electric Vehicle Association 18

ADDRESS GIVEN ON THE SAFETY OF ELECTRIC VEHICLES TO THE ELECTRIC VEHICLE ASSOCIATION 18 OCTOBER 1988

Britain leads the world in electric vehicles both for industrial and road usage. We have a greater population of electric vehicles per head of population than any other country in the world.

Britain’s knowledge of electric vehicles goes back many years and from the early days of resistance control systems. Vehicles have now acquired much more sophisticated electronic control systems or S.C.R. or thyristor and the latest transistorised circuits.

These electronic systems are designed to achieve smooth step less control to full acceleration, greater conversion of battery power into vehicle drive with less heat loss on the systems and generally reduce wear and tear of drive trains. The emphasis with these new systems is less maintenance for with contractors closing without any arcing contactors tips should not need replacing, with less resistance in the circuit there is consequently less heat loss and as a result there is less decay of resistance, cables and other components in the circuit.

It all adds up to a rosy picture, fit an electronic controller, forget about the maintenance and forget about the training because the system needs no maintenance. During my extensive travel throughout the U.K a disturbing picture has been emerging of some vehicles behaving in a peculiar way i.e.

they have taken off of their own accord.

On the first occasion this was brought to my notice, the driver says that he left the vehicle in neutral, went to deliver his milk and on his return found the vehicle down the street stuck up against a tree, the vehicle was returned to the depot for examination, was repaired and then made ready for return to its’ round. It was loaded on a lorry, manoeuvred into position for transportation by selection of reverse and then it literally jumped off the back of the lorry. Subsequent examination revealed stray voltages on the vehicle which, through water saturation, were causing the vehicle to behave erratically. I thought this incident alarming but in isolation was not to be treated with too much concern. The general cause of the fault could be diagnosed and rectified. It therefore, came as a great surprise to me to learn of other incidents of a similar type.

On another occasion a driver had collected his vehicle uncoupled it from the charger and was driving the vehicle around the yard to collect his load of milk, in his words “The vehicle just took off”

He was unable to eject himself from the vehicle because the doors were shut, with his foot on the brake to try to restrain the vehicle he was unable to reach the battery plug to disconnect the power. The only way the vehicle could be stopped was for the back drive wheels to be jacked off the floor.

The driver’s only salvation that morning is that he had remembered to wear his brown trousers. The driver was adamant that the forward/reverse switch on the vehicle was in neutral.

On another occasion a milkman only recently brought into the industry was half way through his round and was delivering to the door when his float disappeared down the road damaging a number of cars in its path. It finished hard up against a garden wall. A milkman from another dairy who was off duty saw what happened, got into the vehicle, put it in reverse and it careered off in the opposite direction damaging another line of cars in its path. The fire service arrived to attend the incident and proceeded to chop their way through the battery cables and interconnecting straps on the battery to render the vehicle safe. No one realised they could achieve the same objective by simply lifting the battery plug clamp.

It is not my intention to relate chapter and verse of all the incidents of this type which have happened and been brought to my attention, suffice it to say that it gives a great deal of concern that it should be happening on electric vehicles, especially where they are being used on public roads in direct contact with the public.

So what can be done to prevent this type of fault outlined earlier from happening in the future, If we can’t understand the problems we won’t appreciate the dangers.

First, we need to be aware of how thyristors and transistor systems operate and to do this we may have to draw on the experience gained by other users.

I have worked in the forklift truck industry for many years prior to joining this industry and this type of fault has never been reported in the forklift truck type of vehicle.

That is except in the early days following the introduction of electronic systems onto forklifts. There were instances where peculiar faults were occurring. It was reported occasionally by drivers that when they sounded the horn on the vehicle, the vehicle would suddenly lurch forward.

Following investigation into this type of fault it was found that the coil within the horn when it was oscillating could generate very high voltages, something in excess of 1000 volts.

This peak voltage was being bounced around the circuit and either this voltage or the induced electromagnet field in the control cables was sufficient to trigger the main S.C.R. thyristor, causing it to ignore the instructions from the control system and sending the S.C.R. into full conduction.

Effective measures were taken to separate the horn wiring from the electronic system control cables. Added to this the horn was properly suppressed by producing a path for the back E.M.F. retained in the coil to be gradually dissipated. Quite clearly, if these measures were found necessary on forklifts then they should be equally applied to milk floats. When installing an S.C.R. or thyristor system the horn wiring should be separate from control wiring and as an added precaution the horn itself should be properly suppressed.

It is, therefore, incumbent upon anyone undertaking a conversation from resistance control to electronic control to ensure that this type of action is carried out. With the introduction of further electrical features onto electric vehicles such as battery discharge indicators, hour meters and electronic beacons we must be aware that these units are capable of generating small peaked pulsed voltages and therefore must be properly suppressed to prevent their stray voltages randomly running around control circuitry.

Suppression of any electric feature capable of the production of a small peaked voltage can be achieved in a variety of ways. The most common way is for a diode to be fitted across the device in opposition to the current flow so that any E.M.F. retained in the device is gradually dissipated.

Where doubt exists regarding whether to suppress or not the supplier of the equipment should be asked for their recommendation - DO NOT ASSUME THAT A DIODE WILL SUFFICE.

Where are their other differences other than suppression between the operation of forklifts and electric road vehicles which could precipitate the types of runaway conditions that have occurred?

The environment they work in must play a part. The majority of electric forklift trucks are purchased for work within a factory or warehouse, the environmental conditions within these working areas are generally stable where temperature and humidity are concerned and therefore have less degrading effect upon the control systems.

Road vehicles work in totally different conditions. Given the vagaries of English weather it is almost possible to get all four seasons in one day. These temperature and humidity changes create moisture around the controller which together with deposits of dust provide tracking paths for small amounts of electric current which could have a potentially disruptive effect on the circuit.

It therefore must be emphasised that regular period checks must be carried out on the control system and preventative measures, taken to alleviate any possibility of the build up of dirt or moisture within the system.

When carrying out mechanical servicing we advise and request the engineers carrying out these mechanical checks to regularly inspect vehicle systems.

For instance, we regularly carry out inspection of the braking system removing brake drums, inspecting shoes for wear, cleaning out the system and replacing any worn or fragile parts.

This procedure is also applied to handbrake systems, greasing moving parts and lubricating cables. This maintenance is also applied to other mechanical features such as steering axles, road springs and drive axles. The maintenance is designed to ensure that the vehicle is mechanically sound and to ensure that the vehicle works more efficiently and hence prevent breakdowns in service.

So why an electrical system should be treated any differently from these mechanical systems. It also has mechanical functions of contractors dropping in and out, of brushes sliding in brush holders and forward and reverse mechanisms. The electrical system accumulates dirt, dust and other contaminants just as other systems do and therefore they should be subject to a similar regular maintenance schedule.

Contactors should be regularly inspected for free movement. Examination of a contactors operation in an unladen state with the battery disconnected by mechanically moving the contactor in and out will reveal any inhibition of the contactors operation due to the build up of dirt and it will also reveal if the wiping or knuckling action of the contactor is properly adjusted.

The most dangerous period for any contactor is during the opening and closing procedure and any prolongation of these periods can lead to excessive arcing causing the contactor to weld.

Suppression of contactors are also important.

Regular checks should be carried out on all interconnections within the control and power circuitry.

These checks will reveal any loose connections. Loose connections create high resistance joints and if left to fester will eventually lead to even greater heat and a potential fire hazard.

Regular examinations of this type will also uncover any corroded joints causing loss of performance particularly when it effects power circuitry or battery terminations.

Clearly, there is a need for regular inspection and examination of the complete electrical control system whether it is a resistance control system or an electronic control system.

Electronic systems have been used on electric vehicles for at least 25 years.

These systems have been gradually refined and improved technically and now possess a reliability which earlier panels did not have.

Now people expect, quite rightly, to fit an electronic control system and if it does not fail within the first few weeks of operation then it will generally give the right maintenance and care operate trouble free for a considerable time.

Then why do systems fail?

There are no moving parts except for contactors (which close without arcing) and interlock micro-switches within the circuit.

Thyristors, diodes capacitors and the other components in the circuit are inanimate objects simply passing electricity, so why should they fail?

When a vehicle is being driven the control system is constantly being switched on and off very rapidly. Diodes and thyristors in operation produce heat and this heat is dissipated into heat sinks with the object of transferring this heat as quickly as possible away from the device. This constant raising and lowering of the temperature, the repetitive switching on and off produces a fatigue effect and gradually the device succumbs to the fatigue and fails.

Obviously, the life of the controller or to be more precise the components within it will depend upon the work it has to do, the amount of cooling it receives and the preventative maintenance it receives during its service life.

If we mount the controller in a position where it receives adequate cooling but is exposed to high concentration of dirt and moisture this will adversely affect the controllers operation and could lead to the type of fault that was highlighted earlier in this report.

If the controller is mounted in an airtight box it won’t receive the correct amount of cooling and premature failure of components can occur, so there has to be a happy medium. Keeping the controller cool and clean should be our first objective. This is one of the first benefits from understanding electronic controllers. If we fit the thing properly we can expect the unit to function without any niggling faults created by moisture, temperature, suppression or dirt accumulation.

What other benefits can be derived from understanding the principles of operation of an electronic controller? Understanding the operation will enable you to differentiate between abnormal and normal faults.

I have attended a number of vehicles where the operators has said that the “The vehicle was working alright this morning, when I first started. When I got half way round it started jumping. I left it for a bit and tried it again and it then seemed to work okay”

When you arrive on site to inspect the vehicle the problem is no easier to understand for the vehicle appears to function normally and does not exhibit the type of fault described by the operator.

So what can be wrong with a vehicle which functions normally one minute and in the next minute behaves in such a fashion as to operate the fail safe system.

The fail safe system built into the controller is there to detect any malfunction of the controller. In the event of a fault occurring which could render the vehicle in a dangerous condition, the fail safe will operate and open the forward of reverse contactor. For example, if the main thyristor went into a short circuit or if the commutation circuit was unable to switch off the main thyristor, then this would precipitate the fail safe system into operation. So we appear to have a vehicle which on occasions triggers the fail safe system.

If the vehicle functions normally you have to assume that the electronic system is working correctly and it is, therefore, an abnormal fault, which is creating the problem.

The first step is to render the vehicle safe by either disconnecting the battery or if you need the battery connected for checking voltages, get the drive wheels off the floor.

Always err on the side of safety.

Abnormal faults are in most cases produced by abnormal conditions and in the majority of cases these are caused by the ingress of water. High pressure hoses or steam cleaners should never be used around electronic controllers if any cleaning is necessary in the cab or wherever the controller is situated it should be carried out with and airline or brush. Water creates corrosion and corrosion creates faults.

In two cases where the controller had behaved erratically, high pressure hoses had been used to clean the cab. The jet of water was forced between the mating faces of the plug on the footswitch and remained trapped. This moisture was sufficient to trigger the fail safe system.

Understanding electronic systems will assist in the diagnosing of faults. It will allow you to distinguish a fault of the controller, or a fault created by abnormal means.

Electronic systems are not new, during the last few years more and more controllers have been brought onto the market. When we install a controller we do it with a view to improving the operation of the vehicle, we do it so that we don’t have to change those heavy copper contactor tips and to relieve the stress of the drive train caused by staccato operation of the resistance control system. So how do we select the controller?

There is a need for electronic controllers to have a minimum standard of performance, by this mean that the controller should exhibit similar function for example:

Plugging or Plug Braking

Plugging control is the ability to select the opposite direction of the one you are travelling in whilst the vehicle is in motion. Without the operator lifting his foot from the accelerator pedal, he can select the opposite direction. The vehicle would continue its’ original mode but would gradually come to a stop before accelerating away in the opposite direction.

Temperature Control

In the event of overheating of the controller the system would detect the rise in temperature and limit the power available to the controller.

Current Limit

A predetermined current limit, determined by the controller’s manufacturer should be set. This current limit would not be adjustable and this would therefore prevent the controller from being overloaded.

Fail safe System

As pointed out earlier, this system to detect any malfunction of the controller which would render the vehicle unsafe to operate, when a fault of this magnitude occurs the controller should be able to close itself down.

Forward/Reverse Contactors or Single Line Contactor

The system should contain either forward or reverse contactors or a single acting line contactor as a mechanical break between the semi-conductor and battery. This would put an air gap into the system so that the semi-conductor device is not he only unit that needs to be activated before the vehicle can be driven.