Forklift truck circuit

I know someone in the repair side of their UK operation if you could get more on the symptoms, I'm assuming no schematic would be available , external to the co. I only see him weekends.

The hydraulics and their associated electrics appear to work properly but the traction system became intermittent and has now failed altogether. The relays on the electronic control board are working in response to the foot pedals, but the main traction power contactors on the +ve busbar do not operate. The contactors will operate when power is applied directly to their operating coils, but still the thyristors don't switch on.

I have already found several potential problems, including intermittent connections in the 50-way ribbon cable to the electronics board. A proper circuit diagram would be so helpful for tracing the path of the non-working circuits.

Any help would be much appreciated

Be careful with these things.

German Forklift Training Video (funny)

It may be bit slow at the start. Jump about 2mins in and the sick mayhem starts...

;-)

Thanks for that, you've really cheered me up :-(

I was already being very cautious. Before I did anything I measured the battery voltage: 48v, and had a look at the main fuse: 500 A, - and worked out that one slip of the spanner could involve me in a lot of very hot metal.

It weighs about 4 tonnes, so I'm also very aware of what might happen if the speed contoller chopper circuits turn out to have have failed S/C and I accidentally close the motor contactor.

I've now managed to borrow the service manual. Although it doesn't give a full circuit diagram, it gives individual diagrams of the sub-sections, so I ought to be able to work out what is going on (or failing to go on).

Thanks for offering to help.

Have you verified that the motor brushes are in good order and properly bedded ?

the

he's on his hols this week as it happens.

Not yet. I haven't even succeeded in getting the motor contactor to close.

There are a few stray wires that need putting back in their correct places before I can do anything more. The circuit incorporates lots of interlocks which won't allow anything to operate if there is a fault in the poower control gear.

The manual (which I have now been able to borrow) says that the wheels ought to be jacked clear of the floor before testing the traction circuits. The electronics problem has temporarily become a background issue and some heavy engineering is now required (the truck weighs 4 tonnes).

Step 1: Is the machine located on level ground/floor ?

Chock front wheels to prevent rolling forward.

Step 2: Use suitable vehicle jacks to lift drive wheel approx 30 mm clear of floor.

Two robust car jacks placed strategically would probably suffice.

Place suitable wood or metal supports under chassis adjacent to drv wheel.

Remove jack/jacks.

Step 3: With battery disconnected inspect drv mtr commutator and brushes integrity.

Other considerations can now be safely assessed, including command logic manual and closure of drv contactor at lowest speed selection. ( i.e. no fwd or rev command input)

HTH

It is now - after using my car to bulldoze it into the warehouse. (We couldn't get it to move at first, then discovered that the hydraulics had run down and allowed the fork tines to dig into the tarmac.)

My car jack had insufficient lifting capacity. We used a length of steel joist as a lever to increase the lifting capacity of the jack, one side at a time. Even with the handbrake firmly on, the machine over-rode its chocks at the first attempt and we had to re-position the lever. No damage resulted because we had anticipated that possibility and had taken suitable precaustions.

And build up wooden blocks under outer sides of machine to give greater stability.

In-wheel motors with integral gearbox, very difficult to inspect. We temporarily ignored this step as there had been no indication of motor trouble (the machine had two motors and it was unlikely that both had failed simultaneously).

Broken microswitch replaced in speed controller. Supply wire to 'direction' contactors replaced onto correct PCB tag.

All relays and contactors now operating, but the thyristors are not being triggered. Injecting a small amount of noise into the base of the drive transistor of the 'ON' thyristor will trigger the thyristor; then both wheel motors run for a moment before the 'direction' contactor opens and kills the circuit.

That ilustrates one of the safety features of this machine: the 'direction' contactor is fed from a reservoir capacitor which is fed via. a diode from the anode of the 'ON' thyristor. If the 'ON' thyristor stays conducting for too long, the contactor drops out. As long as normal ON/OFF commutation takes place, the contactor will stay in.

The conclusion is that neither the 'ON' nor the 'OFF' thyristor is being triggered properly. I have traced the circuit of their drive stages and run the board up on a bench supply; both drivers appear to be working satisfactorily but are not receiving any triggering signals from the timing circuit.

So far, I have not yet managed to fathom the timing circuit. According to the manual it is a closed-loop system which monitors the motor current and the 'throttle' position. A small three-limbed saturable reactor forms the basis of the sensor system. The centre limb carries a coil whose inductance controls the timing circuit; one outer limb is gapped and closed by a permananet magnet on the throttle shaft; the other outer limb is magnetically coupled to the motor supply cable.

At low throttle settings. the magnet saturates the centre limb, the coil inductance is low and the motor 'ON' pulses are short. As the throttle pedal is depressed, the magnet is withdrawn, reducing saturation and causing the inductance of the centre limb to increase. This lengthens the 'ON' pulses until the increase of motor current returns the saturation to a median value. At full throttle, after the motor has attained full speed, neither the magnet nor the motor current is sufficient to give saturation, the inductance is high and the 'ON' pulses reach their maximum length.

I have not yet discovered how the inductance is detected and used to control the timing of the pulses. It is a simple two-wire circuit and the coil resistance at DC reads 4 ohms (the manual doesn't indicate what value it should be).

FYI.

Title: CONTROL SYSTEMS FOR ELECTRIC MOTORS

United States Patent 3562616

Abstract: A motor drive circuit in which a direct current motor is fed with pulses of current by means of a controlled rectifier connected as a power switch between a power source and the motor. The conduction of the rectifier is controlled by a potential divider one element of which is a flux-responsive resistor adjacent a conductor of armature current. The divider is energized with a constant voltage thereby avoiding use of a free-running pulse generator as a source of trigger pulses for the rectifier. A coil adjacent the resistor provides additional flux through the resistor for controlling the pulse rate of the motor.

regards

Very similar, but it doesn't use a saturable reactor to sense the motor current.

My problem was the detail of the electronics which sensed the change in inductance due to saturation. I still haven't fathomed it; but whilst following the circuit I found a S/C transistor and replaced it. The board suddenly burst into life, so I put it back in the truck and everything is working properly again.

The truck owner is happy - thanks to everyone who helped.