Silicon Controlled Rectifier (SCR)

A Silicon Controlled Rectifier (SCR) is a four-layer, three-terminal semiconductor switching device belonging to the thyristor family. Its four semiconductor layers are arranged in the PNPN configuration. The three terminals are Anode (A), Cathode (K), and Gate (G).

An SCR is a unidirectional device and is mainly used for switching, rectification, and controlling electrical power. It is normally triggered by applying a small current to the gate terminal.

symbol:

Modes of Operation

An SCR has three main modes of operation:

1. Forward Blocking Mode (OFF state): The SCR is forward biased but remains in the non-conducting state.
2. Forward Conducting Mode (ON state): When a suitable gate current is applied, the SCR is triggered and conducts current from anode to cathode. Once turned ON, it remains conducting until the current falls below the holding current.
3. Reverse Blocking Mode (OFF state): When reverse biased, the SCR blocks the flow of current, apart from a small leakage current.

Breakover Voltage of SCR

The breakover voltage (VBOV_{BO}) is defined as the minimum anode-to-cathode voltage at which an SCR changes from the OFF state (high impedance) to the ON state (low impedance) without any gate current being applied.

When the anode-to-cathode voltage exceeds the breakover voltage, the SCR turns ON automatically due to avalanche breakdown, even though the gate has not been triggered.

• Applications: SCRs are widely used in motor speed control, light dimming, and controlled rectifier circuits.

Given:

Armature voltage, $$V = 480\ V$$

Armature current, $$I_a = 110\ A$$

Armature circuit resistance, $$R_a = 0.2\ \Omega$$

Number of poles, $$P = 6$$

Flux per pole, $$\Phi = 0.05\ Wb$$

Number of armature conductors, $$Z = 864$$

Lap-connected armature winding, therefore number of parallel paths, $$A = P = 6$$

(i) Speed of the Motor

For a DC motor:

$$V = E_b + I_aR_a$$

Therefore, the back EMF is:

$$E_b = V-I_aR_a$$

$$E_b=480-(110\times0.2)$$

$$E_b=480-22=458\ V$$

The EMF equation of a DC machine is:

$$E_b=\frac{P\Phi ZN}{60A}$$

For a lap winding, $$A=P$$. Therefore:

$$458=\frac{6\times0.05\times864\times N}{60\times6}$$

Since 6 cancels:

$$458=\frac{0.05\times864\times N}{60}$$

Therefore:

$$N=\frac{458\times60}{0.05\times864}$$

$$N=636.02\ rpm$$

Therefore:

$$N\approx636\ rpm$$

(ii) Gross Torque Developed by the Armature

The gross mechanical power developed by the armature is:

$$P_g=E_bI_a$$

The torque is given by:

$$T_g=\frac{60P_g}{2\pi N}$$

Therefore:

$$T_g=\frac{60E_bI_a}{2\pi N}$$

Substituting the values:

$$T_g=\frac{60\times458\times110}{2\pi\times636.02}$$

$$T_g\approx754.3\ N\cdot m$$

The same result can be obtained directly from the torque equation:

$$T_g=\frac{PZ\Phi I_a}{2\pi A}$$

Substituting:

$$T_g=\frac{6\times864\times0.05\times110}{2\pi\times6}$$

$$T_g\approx754.3\ N\cdot m$$

Therefore:

$$T_g\approx754\ N\cdot m$$

Final Answers

Speed of the motor = $$636\ rpm$$

Gross torque developed by the armature = $$754\ N\cdot m$$