HW-K wrote:
On Sun, 17 Jul 2011 09:31:42 +0100, lid
(Adrian Tuddenham) wrote:
HW-K wrote:
On Thu, 14 Jul 2011 13:56:47 +0100,
lid (Adrian Tuddenham) wrote:
Adrian C wrote:
On 11/07/2011 18:09, Adrian Tuddenham wrote: Does anyone happen
to have the circuit diagram for a Lansing R16 forklift truck?
I've been asked to find an intermittent fault in the traction
motor control circuit.
[...]
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.
Have you verified that the motor brushes are in good order and
properly bedded ?
[...]
Step 1: Is the machine located on level ground/floor ?
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.)
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.
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.
Place suitable wood or metal supports under chassis adjacent to drv
wheel.
And build up wooden blocks under outer sides of machine to give greater
stability.
Remove jack/jacks.
Step 3: With battery disconnected inspect drv mtr commutator and brushes
integrity.
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).
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)
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).
--
~ Adrian Tuddenham ~
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