Elec 3 Revision

• A power transistor will remain on as long as sufficient base current is applied. 
• The thyristor must fall below the holding current value in order for it to completely turn off.
• In a power transistor, when the base current (Ib) is increased, keeping Vce constant, then Ic will increase.
• If V and I are the forward voltage and current of a diode, then the power loss across the diode equals P = I*V xsdc
• A thyristor SCR is a PNPN Device
• To maintain the conduction of an SCR when the gate signal is removed the anode requires a holding current.
• The diode is in reverse biase when the cathide is positive
• When an SCR is in reverse blocking mode, leakage current flows from cathode to anode
• The controlling parameter in a power transistor is Ic
• The controlling parameter in a MOFSET  is Vgs
• The average gate power dissipation for an SCR is 0.5 Watts, the voltage applied to the gate is Vg = 16V. Using P=I/V, the maximum value of current Ig for safe operation of the SCR is 0.031 Amps.
• Reverse breakdown occurs in a diode when the voltage is constant and current increases
• The latching current for an SCR is 120mA, a DC source of 200V is also connected from the SCR to an inductive load, L = 0.6H. Rearranging Farradays law for time, we get t = E/LI. Therefore, the minimum width of gate pulse required to turn the device on is 0.36ms
• A power transistor will remain on as long as sufficient base current is applied.
• An IGBT power transistor has very high input impedance
• The forward break-over voltage of a thyristor (SCR) is the anode-cathode voltage at which conduction starts with no gate signal applied.
• The controlling parameter in an IGBT transistor is Vge
• The gate characteristics of a thyristor (SCR) is a graph of Ig on the X axis and Vg on the Y axis.
• A diode is said to be forward when the ANODE is positive.
• To turn on a SCR thyristor, the gate triggering method is most suitable.
• An ideal power diode should have a high reverse breakdown voltage.
• In a 3 phase half wave diode rectifier using 3 diodes, a diode will conduct for 120° starting from 30°
• A three phase, six pulse full converter works as an ac – dc converter for firing angles in the range on 0° < a < 90°
• A three phase inverter requires a minimum of 6 switching devices
• In a three phase half wave rectifier, the primary side of the transformer is normally delta connected because delta connects help reduce the third harmonic
• 3 devices in a VSI 180° conduction mode inverter will conduct at any one time
• A three phase, three pulse controller thyristor converter has a firing angle for one of the SCRs set as 15'. This means the SCR would start conducting at 45'.
• When operating in 120° mode of operation, a three phase VSI inverter requires 6 steps
• A three phase, three pulse controlled thyristor converter T1 starts to conduct at 30'. T2 and T3 would then start to conduct at 150' and 270' - assuming a sequence of T1-T2-T3
• A three phase three pulse type controllers thyristor converter supplies a resisitive load with a<30°. For this arrangement each SCR device would conduct for 120° each cycle
• What is the peak value of phase voltage for a 3-phase VSI inverter with 180°conduction mode. The supply side consists of a constant dc voltage source of Vs. 2Vs/3
• Resistor loads do NOT produce harmonic currents.
• A power factor in a power system is the ratio of active power used by a circuit and the total apparent power supplied.
• Transformers and asynchronous induction motors have a lagging power factor.
• Within a DC system, DC has no harmonic components.
• A single phase motor is connected to 400V, 50Hz supply. The motor draws a current of 31.7A with a power factor of 0.7 lagging. The value of capacitance required in parallel with the motor to raise the power factor to 0.9 lag is 94.6 micro farads.
• Reactive power levels contribute to a low voltage and poor power factor of a system.
• Continuous and rapid variations in the load magnitude which can cause voltage variations is known as Flicker
• Total Harmonic Distortion (THD) is the equivalent rms value of the harmonic component divided by the rms value of the fundamental frequency
• Synchronous generators could be used to control the power factor of an electrical isolation