As the gate current begins to flow from gate to cathode with the application of gate signal. Gate current has a non uniform distribution of current density over the cathode surface. Distribution of current density is much higher near the gate. The density decreaseas the distance from the gate increases. So anode current flows in a narrow region near gate where gate current densities are highest. From the beginning of rise time the anode current starts spreading itself. The anode current spread at a rate of 0.1mm/sec. The
spreading anode current requires some time if the rise time is not sufficient then the anode current cannot spread over the entire region of cathode. Now a large anode current is applied and also a large anode current flowing through the SCR. As a result turn on losses is high. As these losses occur over a small conducting region so local hot spots may form and it may damage the device.
During the triggering process of the thyristor from forward blocking state to forward conduction state through the gate signal, by applying the gate signal (voltage between gate and cathode) increases the minority carrier density in the p-layer and thereby facilitate the reverse break over of the junction J2 and thyristor starts conducting. Higher the magnitude of the gate current pulse, lesser is the time required to inject the charge and turning on the scr. By controlling the amount of charge we can control the turning on time of the scr.
Different losses that occur are
a)Forward conduction losses during conduction of the thyristor
b)Loss due to leakage current during forward and reverse blocking.
c)Power loss at gate or Gate triggering loss.
d)Switching losses at turn-on and turn-off.
Knee point voltage is calculated for electrical Current transformers and is very important factor to choose a CT. It is the voltage at which a CT gets saturated.(CT-current transformer).
It reduces the harmonics and it also reduces sparking and arching across the mechanical switch so that it reduces the voltage spike seen in a inductive load.
Advantages :
1. Fast Switching Characterstics than Mosfet, BJT, IGBT
2. Low cost
3. Higher Accurate.
Uninterrupt power supply is mainly use for short time . means according to ups VA it gives backup. ups is also two types : on line and offline . online ups having high volt and amp for long time backup with with high dc voltage.but ups start with 12v dc with 7 amp. but inverter is startwith 12v,24,dc to 36v dc and 120amp to 180amp battery with long time backup.
If their is high power factor, i.e if the power factor is close to one:
a)Losses in form of heat will be reduced,
b)Cable becomes less bulky and easy to carry, and very cheap to afford, &
c)It also reduces over heating of tranformers.
(1). The main advantage of using the star delta starter is reduction of current during the starting of the motor.Starting current is reduced to 3-4 times Of current of Direct online starting.
(2). Hence the starting current is reduced , the voltage drops during the starting of motor in systems are reduced.
Forward Resistance: Resistance offered in a diode circuit, when it is forward biased, is called forward-resistance.
• DC or Static Resistance: DC resistance can be explained as the ratio of the dc-voltage across the diode to the direct current flowing through it.
• AC or Dynamic Resistance: It can be defined as the reciprocal of the slope of the forward characteristic of the diode. It is the resistance offered by a diode to the changing forward current.
