Morgan Chopper - Engineering Notes

Morgan Chopper :

Circuit diagram of Morgan Chopper

• The circuit diagram is as shown in figure.
• This circuit has only one thyristor.
• The special feature of this circuit is the saturable reactor (SR).
• The commutation circuit is formed by the capacitor, saturable reactor SR and the diode D1.
• Assume  that the exciting current of the saturable reactor is negligibly small.
• Load current is assumed to be constant to simplify the analysis of the chopper.

About Saturable  Reactor:

• It is a special type of inductor in which the magnetic core is deliberately saturated by some external means.
• With the change in the direction of current, the saturation polarity is also changed i.e, if for one direction of current the saturable  reactor is in positive saturation then for another direction of current  it will be in negative saturation. This is obvious from the hysteresis loop of a magnetic material.
• For any inductor, it has very high inductance during unsaturated condition and has very low inductance during saturated condition.

Circuit Operation of Morgan Chopper :

Morgan chopper waveforms

• Before turning on the thyristor T1, load is flowing through free wheeling diode. In this period capacitor voltage is Vs. Capacitor is charged through this voltage by the source through Vs - C - SR - load - Vs. 
• During this period saturable reactor is placed in positive saturation position and for current passing through SR from left to right, SR will be in positive saturation.
• When T1 is turned on, load current shifts from free wheeling diode to the main SCR. In this period the capacitor voltage will appear across SR. With this applied voltage, SR will drive itself into negative saturation from the positive saturation.
• When SR is negative saturated its inductance will fall and discharge of capacitor will occur in a very short time.
• Now the capacitor is negatively charged and this voltage is applied across SR. With this applied voltage SR drive into positive saturation from the negative saturation.
• Exciting current of SR is very small, so capacitor holds its voltage till SR drives into saturation.
• When SR is saturated (positive saturation) its inductance will fall and discharge of capacitor will occur in a very short time during the negative current flow through SCR, SCR will turn off. After turn off of SCR, the discharge current flows through the diode D1.
• After turn off of SCR the load current shifts again to free wheeling diode.
• For the next cycle same sequence of steps are repeated.