Electrical Equipment and Devices

WEEK : MOTOR SPEED SYSTEMS: METAL / THYRATRON, MERCURY POOL

Definition. A rectifying device is a circuit element that will conduct current in one direction only

Rectifier is an integral assembly of a rectifying device and suitable power supply and control equipment for changing alternating current to continuous current.

Types.
1. Metal Rectifiers
a. Copper Oxide
b. Selenium
2. Thyratron
3. Mercury Pool
4. Mechanical

Copper Rectifiers

Description. A copper rectifier consists of a copper disc one (1) inch in diameter and heated to form a Copper Oxide (CuO) on their surface. This is not a linear device. Its resistance losses necessiate large heat dissipation. Used in control circuits unsuitable in large amounts of power.

Specifications Operates at 45 degrees Centigrade or less. Voltage ranges from 5 to 11 volts. Efficiency ranges 65 to 75 %. Voltage Regualtion ranges from 25 to 35 %.

Selenium Rectifiers

align= Description the cell consists of an iron or aluminum disc, one side of which is coated with selenium. Used in electroplating.

Specifications Operates at 75 degrees Centigrade or less.
Voltage ranges from 100 volts or less.
Efficiency ranges 75 to 85 %.
Power ranges from 50 to 100 kW.

Thyratron

Description a three-element tube containing a heated cathode, a plate and a starting grid.

align= Specifications Current rating at 50 A operating at a thousand volts.

Operation Space inside the housing contains mercury vapor 3 to 10 millions of meter pressure. Cathode is heated by A and switch S2 is open, S1 is closed. Accumulation of negative charge on grid will prevent flow of plate current. Closing S2 to point a will reverse the negative charge from the grid and allow electrons to from the cathode to the plate or vice versa. This current flow will cause ionization by collision in the gas and opening S2 or changing it to b will not stop the plate current. Plate current is interrupted by opening plate circuit, or makinf R very high or reducing E to a valueless than voltage drop in the tube.

Mercury Pool

align= Description a mercury-arc rectifier with two anodes, a mercury pool cathode and a starting anode, enclosed in a glass housing. The upper portion is used for condensing the vapor. Used on series street lighting and on battery charging.

Specifications Voltage rating ranges from 50 to 400 Volts..

Operation: Tilting the tube to the left will close the circuit S and C. Back to vertical position will cause arc at surface C. Without the reactor, the arc will extinguish each time the anode voltage approaches zero.

Mechanical Rectifier

Description work on the principle of synchronuous switching of DC current to AC power supply. See Converters.