HW-K wrote:

On Tue, 19 Jul 2011 09:33:50 +0100,
lid (Adrian Tuddenham) wrote:


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pedal is depressed, the magnet is withdrawn, reducing saturation and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.

http://www.freepatentsonline.com/3562616.html

http://www.freepatentsonline.com/3562616.pdf

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.


--
~ Adrian Tuddenham ~
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www.poppyrecords.co.uk