(a) With reference to water level control system, with the aid of sketches, how remote control of boiler water is achieved?
(b) Explain the function of transmitter and control valve?
(a)
- In three elements control system, measured variables are steam flow, feed water flow and boiler water level.
- In balance situation, steam flow equal to feed water flow and these two signals are compared in differential relay.
- Any deviation between desired and actual boiler water level, and deviation between steam flow and feed water flow result in controller action to adjust feed water control valve.
- A sudden increase in steam demand results in deviation signal from differential relay and give an output signal to open feed water control valve.
- For reduction in steam demand gives an output signal to close feed water control valve and thus avoiding shrinkage effects.
- Any change in feed water pressure result in feed water control valve movement to correct change before boiler water level affected.
(b) Function of Transmitter
1. It detects the water level in the drum and send signal to the digital controller.
2. A positive change of water level in boiler alters level transmitter coil‘s inductance value, causing an imbalance in the system.
3. Signal converted and sent to control valve
Control valve
1. Control valve works depend on the signal given by controller and after done the job,
Sends a feedback signal to controller for confirmation.
2. Valve opening is depending on supply air with the signal given by controller.
3. Level controller ( P+I controller) transmits an electrical signal to pneumatic positioner to position and adjusts position of control valve.
Q Describe Carbon Pile Automatic Voltage Regulator with aid of simple circuit and sketches.
Carbon Pile AVR
- Carbon pile resistance, varied by pressure changes, controls current flow through exciter shunt field.
- Pressure is, applied by springs and, relieved by magnetic field strength of electromagnetic coil
- Current for electromagnetic coil supplied from Alternator output to switchboard.
- AVR is designed, change Alternator voltage, due to load, change effect electromagnetic coil’s strength and alters carbon pile resistance.
- When Alternator voltage is low, spring exerts greater pressure & carbon pile resistance become low, more current flow through exciter shunt field & increase output voltage.
- When Alternator voltage is high, electromagnetic coil relieves pressure & carbon pile resistance become high, less current flow through exciter shunt field & decrease output voltage.(Strength of electromagnetic coil relieves spring pressure on carbon pile)
(a) Explain the construction and operation principle of single phase transformer.
(b) Explain the transformer losses.
(a) Construction
- Consists essentially two coils, wound on a closed magnetic circuit.
- Having low reluctance and high silicon content laminated steel core.
- Primary and secondary coils placed side by side.
- Two coils are insulated from each other and steel core.
- Core and windings assembled in suitable container.
- There are two main types of windings (1) concentric cylindrical and (2) sandwich.
(1) Concentric cylindrical windings
- Coils are circular in shape.
- Low voltage windings placed nearest to core
- In small transformers, each layer separated by small paper.
- In large transformers, each winding placed on a separate former.
(2) Sandwich windings
Two windings are placed in the alternate layers.
Operation principle
- Coil connected to AC supply called primary, another coil called secondary.
- If AC current passed through, primary coil produces change of flux, some of flux link secondary coil.
- Change of current in primary cause change in the flux linking secondary.
- Produced self-induced emf in primary, mutually induced emf on secondary.
- If the circuit is completed a current will flow to load.
(b) Transformer Losses
Total power loss is a combination of different losses.
Transformer is static machines, no friction and windage losses.
(1) The iron losses; (hysteresis and eddy current losses)
(2) The copper losses;
(1) Iron losses (hysteresis and eddy current losses)
- When core subjected to alternating flux, material becomes magnetized.
- Hysteresis loss takes place molecular fraction of core material.
- Hysteresis loss minize by high silicon content laminated steel core.
- Eddy currents induced in cores due to flow of eddy current.
- Eddy current minimized by using steel laminations insulated by light coat of varnish or by an oxide layer on the surface.
- Core flux independent of load, iron losses constant.
(2) Copper loss
- Variable loss, varies as the square of the load current.
- Directly proportional to (current)² and also proportional (KVA output) ² .
- This loss called copper loss or I² R loss, occurs in primary and secondary windings.
Describe the following with the aid of sketches
(a) Bi-metal type thermometer
(b) Wax element temperature control valve
(c) Differential pressure cell
(d) Strain gauge torsion meter
(a) Bi-metal type thermometer
- Bimetallic trip make two different metals.
- Typical metals use an iron-nickel alloy, firmly bonded together.
- When temperature changes, different amounts of expansion occur in the two metals, causing bending or twisting of strip.
- The coiling or uncoiling of helix as temperature change cause movement of pointer fitted free end of bimetallic stip.
- Temperature range is -30˚ to +550˚C.
(b) Wax element temperature control valve
- Has a copper capsule contain, wax expansion varies with temperature.
- Movement is transmitted by diaphragm, plunger and linkage to alter position of shutter in valve body.
- Temperature rise causes shutter, open ‘To Heat Exchanger’ and close ‘by pass’.
- This valve suited to “mixing” or “bypass” conditions.
- Temperature range about 10˚C, fail-safe inherent in design.
(c) Differential pressure cell
- Two bellows mounted back to back on plate diving pressure chamber into two halves.
- Bellows connected internally by a rod through division plate and filled with inert gas or viscous fluid.
- Each side of pressure chamber connected high pressure and low pressure.
- If one pressure becomes higher than other, a differential pressure applied across bellows assembly, move towards lower pressure.
- The rod transmits this movement to outside of pressure chamber through gland.
(d) Strain gauge torsion meter
- A torsion meter is designed to measure proportional twisting of shaft that transmit power.
- The more power transmits, greater twist torque.
- Four strain gauges mounted on shaft, two axially and two radially to form a Wheatstone bridge.
- Twisting of shaft results in a proportional change of resistance in each of strain gauges.
- By combining an electronic indication of shaft revolution, a direct indication of transmitted KW power produce
Q Describe the steam temperature control system with the aid of diagram?
Steam temperature control system
- To avoid damage to the metallic structure of the steam turbine (due to thermal stresses),
- Necessary to control temperature of the high-pressure superheated steam.
- Stems from the boiler’s drum is directed to the primary super-heater lead to secondary super-heater
- Outlet temperature of secondary super-heater is measured and transmitted to a PID controller.
- Any deviation from the desired value, signal, send to summing relay where another input signal (steam flow signal) is received.
- Steam temperature falls due to increased steam flow or increase load condition control of valves by a split-control mode
- Attemperator’s inlet valve will shut and the by-pass valve will open.
- Steam is directed to the secondary super-heater.
- Opposite case steam temperature rises due to reduced steam flow or load condition.
- Steam temperature always maintained at a constant value by adjusting both valves in opposite.
Q Describe the engine lubricating oil temperature control with aid of system diagram?
Lubricating oil temperature control
Method of temperature control uses a single measuring device located at the oil inlet.
- Use single measuring device, locate at oil inlet to engine.
- Low level alarm on L.O drain tank.
- High differential pressure alarm fit across duplex filter.
- Pressure switch locate after pump, provide automatic start-up of stand by pump in low pressure.
- A 3-way valve provide oil supply line to the cooler by-passing.
- The cooler provided with a full flow of sea water which is not controlled by system.
- On the L.O inlet to engine, temperature sensing element provide signal to (P+I) controller.
- The controller provided with a desired value, any deviation between desire value and input signal, send output signal to 3-way control valve.
- If temperature low, more oil bypass cooler, temperature will increase.
- If temperature high, less oil by-passed, temperature will fall.
Regarding with control engineering;
(a) Explain the current signal of about 4 to 20 mA.
(b) Explain about A/D converter (Analog to Digital converter)
(a) 4~20 mA current signal
4 ~ 20 mA current loops commonly used for analog signaling in many instrumentation systems.
(a) 4 mA represents ‘0’ percent of measurement
(b) 20 mA represents 100 percent
(c) 12 mA represents 50 percent and so on.
- 4-20 mA current signal easily converted to 1 ~5 V indicating instruments.
- figure shown is 4 ~ 20 mA current loop with 2-wire transmitter.
- A transmitter converts a physical property such as temperature, humidity or pressure into an electrical signal.
A simple 250-ohm precision resistor is connected in series with circuit which converts current signal to a voltage. Receiver resistor will produce
(a) 1 volt drop at 4 mA and
(b) 5 volt drop at 20 mA, etc:
- Power supplies for 2-wire transmitters must always be DC.
- For 4 ~ 20 mA loop with 2-wire transmitters, common power supply voltages are 12, 15, 24 and 36 VDC.
- For 4 ~ 20 mA loop using 3-wire transmitters, power supply can be either AC or DC.
- The most common AC power supply is the 24 VAC control transformer.
(b) A/D converter
- An analog-to-digital converter (ADC, A/D, or A to D) is a device that converts a continuous physical quality (usually voltage) to a digital number that represents the quantity’s amplitude.
- The conversion involves quantization of the input, so it necessarily introduces a small amount of error.
- Instead of doing a single conversion, an ADC often performs the conversions
- (“samples” the input) periodically.
- The result is a sequence of digital values that have been converted from a continuous-time and continuous-amplitude analog signal to a discrete-time and discrete-amplitude digital signal.
