QC Notes Lecture_8 Aramco Standard Electrical and Instrument Question Answers Part_5

Transformer is one kinds of electro static device which transformer energy from winding to another winding by electromagnetic induction effect without changing frequency.

Motors working with voltage 3.3KV and above are HT motors. This motor normally used for high loads are the operating current can be reduced. Normally these motors are connected. These motors are having some special cooling arrangements
like forced air cooling, condenser type cooling in which water is circulated to cool the temperature inside. Instead of a ball bearing, sleeve bearing are also used. For cooling the bearing, separate oil is used.

1.     Instantaneous over current: Settled normally 10 times the rated current. Operates only during short circuit.

2.     Thermal overload relay: Set normally at 110% rated operates at motor overload.

3.     Locked rotor prolonged start protection: Set normally at 125% the rated operates when the load got jammed. Set at normal starting time 2 Seconds.

4.     Negative phase sequence: Set normally at 25% the rated operates when phase unbalance occurs and single phasing. (One phase failed).

5.     Earth Fault: Set normally at 20% the rated operates when fault occurs between phase and earth.

Over Current relay, Ground fault relay: These are intended for detecting faults in the electric system connected with the Transformer. A fault in the Transformer results in the function of these relays, which is always accompanied by the actuation of protective devices of the Transformer.

Illumination: The illumination is defined as the luminous flux received per unit area.

Overload Relay Setting: Overload protection given for almost all motors. The overload relay setting will be of 110% in
general.

Starting Current: Starting current of a induction motor generally will be 6 times the rated current.

Single Phase: One of the supply lines gates disconnected due to rupturing of fuse or open circuit in one of the
three supply connection. In such cases the motor continues to run on a single phase supply of 440V. If the rotor is locked to its rated full load, it will draw excessive currents on single phasing. The windings get overheated and damage is caused.

Megger Value: The insulation resistance value of the winding of the motor shall be checked periodically during preventive maintenance. For 3.3 KV motor the IR value shall be checked with 1 KV Megger.

For 11 KV motor the IR value shall be checked with 5 KV Megger.

Winding Resistance: The winding resistance of the motor shall be checked during preventive maintenance with the bridge.
The winding resistance of the three windings should be equal.

Earthing:Generally earthing means non-current carrying metallic part of electrical equips should be earthed to avoid danger to human life.

Grid Resistance: Grid resistance means in a industry or building the no. of earth pits joined parallel together to get
a minimum earth resistance value and the value should be less than 1.0 Ohms acceptable in the industry.

Pit Resistance: Individual pit resistance can vary according to the solid resistance and the value can be 5
Ohms or more than that. But to get a minimum value all the pits should connect in parallel as grid.

Earth Pits: Earth pits are generally used by using charcoal, salt and G. I. pipe of 3 meters in length and 40 mm diameter.

Another type of earth pits is plate earth electrode and mesh type electrode with copper or GI strip brought out.

Another type in gulf generally used is the solid rod of copper driven straightway into to earth to get minimum value by driving to a depth of max 6 meters.

Earth Megger: For measuring the earth resistance earth Megger is used and the diagram given below how to measure the earth resistance.

Safe Earth Resistance: Generally earth resistance of the total grid should be less than 1.0 Ohms which is acceptable
for safe operation in industries.

Ammeter: Load current is measured through ammeter only. In AC lower range are directly can be read
through ammeter kept in series. For larger current transformer are uses for measuring the current. In DC Power current is measured through a shunt resistance.

Wattmeter: For measuring the power consumed by the circuit (any type of feeder) wattmeter is used and the unit is in KWH.

Avometer: This AVO meter is used for checking the electrical circuits measuring voltage, current, resistance and electronic components etc.

Shunt Resistance: In DC circuit is measured through this shunt resistance.

Transducer: They are used to change one varying parameter into other form energy. Like voltage to current to pneumatic signals etc.