Alstom SHPM 101, Руководство по обслуживанию

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SERVICE MANUAL R5888C
QUADRAMHO Chapter 2
Page 32 of 74
case by removing six screws and releasing two plugs. Upon replacing the
backpack the silicon rubber dust sealant must be renewed.
Three versions of the power supply are available, for nominal input voltage ratings
48/54, 110/125 and 220/250V dc. These versions have operating ranges of
37.5 – 60, 87.5 – 137.5 and 175 – 275V dc respectively. All three versions
produce regulated output voltage rails of +24V, +12V and +5V at a current of up
to 1A, and –12V at a current of up to –0.5A. Any output can be short-circuited for
a brief time with no resultant power supply damage.
The operation of the unit is now described with the aid of block diagram,
Figure 28.
The dc supply from the secure station battery, is used as a power source for the
production of the isolated, smooth and regulated internal supply rails. The dc is
firstly passed through a filtering section which attenuates electrical noise and
voltage spikes and ensures that Quadramho is immune to interference generated
by other equipment connected to the station battery. It also prevents the power
supply from transmitting interference to this same equipment.
The filtered voltage is sensed by the voltage detector and when this exceeds a
minimum value the internal voltage rail of the power supply electronics is
energised. This eliminates the danger of power supply maloperation for input
voltages less than the minimum operating voltages given above.
The “feed forward” principle is used in the pulse width modulator section to
produce a 40kHz square wave with a mark to space ratio (duty factor) which
varies inversely with the filtered supply voltage. This ensures that the power supply
output voltages remain relatively constant despite differing or changing input
supply voltages. The switching transistor is driven “on” and “off’ by the pulse width
modulator to energise the primary of the transformer with current from the filtered
voltage supply. The transformer electrically isolates the station supply from the relay
electronics (2kV for 1 minute) and transforms the primary voltage to a level suitable
for the outputs. Screens are incorporated on the transformer to render the relay
insensitive to common mode interference between the station battery supply and
the relay case/ground.
Pulsed voltage waveforms produced at each of the transformer secondaries are
smoothed by L – C low pass filters to produce near constant dc voltages.
It is then necessary to regulate these voltages since small variations in them occur
due to ripple, input voltage variation and load regulation. This is accomplished by
the use of solid state regulator devices which also feature overcurrent and thermal
overload protection.
If some part of the power supply fails, such that a large current is drawn from the
station battery, then a fusible resistor, connected in series with the station battery
voltage supply, operates and disconnects the battery voltage. Before replacing or
repairing this device the fault resulting in its operation must be investigated.