Introduction of Morgan Chopper
The basic circuit for the Morgan chopper is shown in the figure A. The SCR T1 is main SCR whereas diode D1, capacitor C and saturable reactor SR makes commutation circuit for the SCR T1. The exciting current of the saturable reactor is neglected.
Power circuit diagram of Morgan Chopper
Working of Morgan Chopper
Charging of capacitor
When SCR T1
is switched off, the charging of capacitor is
done through (+)Vdc – C – SR – LOAD – Vdc (–)
path and saturable reactor is in positive saturation.
SCR T1 turned on
When SCR T1
is switched on, the voltage across capacitor appears
across saturable reactor and core flux flows from positive saturation to
negative saturation. The voltage across capacitor remains
constant until core flux reaches at negative saturation.
Discharging of capacitor
When
the core flux reaches negative saturation, the discharging of capacitor is
done through (+)C – T1 – SR – C(–) path. The capacitor and
saturable reactor makes resonance circuit and its discharge time is equal to π
√ LsC. The Ls is inductance after saturation.
Therefore the discharging time of capacitor is less than the charging time
of capacitor.
The voltage across
capacitor becomes – Vdc is very short time and this voltage
appears across saturable reactor. The discharging of capacitor is
done through SCR T1
and inductance after saturation in the reverse direction when the core flux
reaches in the positive saturation. As this discharging current passes
through SCR T1,
it will switched off SCR T1
and forward biased diode D1.
SCR T1 turned off
When SCR T1
is switched off, the load current passes through freewheeling
diode. The turn
on time of the SCR T1
is always constant due to constant time of core saturation and it depends upon
√ LsC. The average output voltage can
be changed by changing only output frequency. The turn
on time of the SCR is
equal to time for positive saturation to negative saturation and again positive
saturation of the saturable reactor.
Waveform of Morgan Chopper
Advantages of Saturable inductor over linear inductor
- The inductance of saturable reactor is low during capacitor charging time and SCR turn off time. The inductance of saturable reactor is higher during SCR turn on time.
- Low cost of circuit due to only one SCR.
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