FOUR CONDITIONS FOR PARALLEL OPERATION OF 2 OR MORE AC GENERATORS.
- The voltage rating must be equal.
- The polarity must be equal
- The phases must be equal
- The running frequency must be equal
C] THE PRINCIPLE OF OPERATION OF AC SYNCHRONOUS MACHINE
The principle of operation of AC synchronous machine is one which follows the fact that they are machines in which the rotor moves at a speed which bears a constant relationship to the frequency of the current in the armature winding. In other words, the speed of the rotating flux is the same as that of the rotor speed. As a motor, the shaft speed must remain constant Irrespective of the load, provided that the supply frequency remains constant as a generator.
MENTION THE COMPONENTS PART OF AN INDUCTION MOTOR (5 marks)
The components parts of an induction motor includes, the frame, rotor and Stator winding.
(a) FRAME: This consist of craft iron inform of rolled steel. It does not serve any magnetic purposes as the yoke does in a d.c machine but supplying mechanical protection and support for the stator winding and arrangements for ventilation and guard apparatus.
(b) ROTOR: Is the field or exciter winding of the machine. It may be wound rotor type for smaller machine, a squared cage rotor ( where the rotor winding is made up of cylindrical frame of coppers) for larger machines ends by brass or copper. In case of cage rotor types, the protection and blades of the cage, help to provide cooling systems to bolts and nuts, installation and beerings, while in larger machines, temperature saving transducers such as transistors, others are slip rings and brushes.
(c) STATOR WINDING: These are 3-symmetrically placed windings which may be connected in either STAR or DELTA. They are either fitted into the slotted lamination or encapsulated, and is fed from or supply by 3-phase supply. This forms the armature of the machine.
(d) SLIP: The speed of the rotor relative to that of the rotating flux is Known as the slip OR can be defined as the difference in speed, between the speed of the stator flux and the speed of the rotor.
(e) SPLIT-PHASE INDUCTION MOTOR: To take a single phase induction motor, self-starting a rotating field can be reduced from a single phase supply. The winding is seperated into two, running or main winding and starting or auxillary winding. The auxiliary angle is placed in an angle approximately 90⁰.
ADVANTAGES OF 3- PHASE MOTOR. [5 marks]
- It is robust.
- The maintenance required is very minimal.
- It’s a trouble free machine.
- The power factor is very good.
- It doesn’t require the use of commutator.
- It’s the most common machine usually used in industries.
- The efficiency is of a greater one.
CHARACTERISTICS OF TRANSFORMER OIL. [3 marks]1) An insulant.
2) A coolant.
3) Has viscosity.
4) Has dielectric strength.
5) Must not contain sulphur or its compounds.
6) low sludye formation.
7) Acidity must be known.
8) As fire point [>200⁰c].
short note on SHORT CIRCUIT TEST
A variable voltage is connected to primary of the transformer. The voltage is varied until the rated full load current, i2 flows in the secondary. The voltage at which the full load current flows is the short circuit voltage [Vsc]. The current observed by the ammeter A1 is primary rated current at full load. The wattmeter (W) records the total full load losses, ammeter A2 measures secondary current on full load.
THE TWO MAJOR LOSSES APPLICABLE TO MACHINE
1) CONSTANT LOSSES: is of two parts
→ Mechanical losses include friction and windage losses.
→ Iron or core losses include Eddy current and hysteresis losses.
2) COPPER LOSSES:
→ Represented in the winding of a machine, including series, shunt and armature losses.
C) The three (3) major energy conversions Wf = ½Li²
- Magnetic field energy = B = flux/Area
- Force of alignment, F = B²A/2μ⁰
- Force of Attraction = Fe = -½L² – dL/dx
STATE THE ENERGY BALANCE EQUATION AS APPLICABLE TO ALL SITUATION OF ELECTROMAGNETIC MACHINES. [7 marks]
Energy balance equation
A Machine accept energy in two forms either through mechanical or electrical from its terminal system. We take energy input to be positive. As the function of the mechanical energy conversion terminal input will normally be negative(-ve) of the total energy inputs, some converted, some stored and the rest dissipated in loss.
Therefore, (Elect Energy input) + (Mech Energy input)
= (Store field energy) + ( stored mech energy) + Dissipation
Comparable relations apply to energy changes and to energy rates (i.e power)
Energy changes: dwe + dwm = dwf + dws + de
Energy rate = Pe + Pm = dwf/dt + dws/dt + P
So energy rate equation becomes
Pe + Pm = P
TYPES OF SINGLE PHASE OF TRANSFORMER
There are three (3) distinct types of construction;
- Core type transformer
- Shell type transformer
- Berry type or spiral type transformer
DISTINGUISH BETWEEN CORE TYPE TRANSFORMER AND SHELL TYPE TRANSFORMER. ( 6 marks)
In CORE TYPE, the windings covers a considerable part of the core
where as in SHELL TYPE transformer; the core surrounds a considerable amount or portion of the windings.
THREE REASONS WHY STARTER IS NEEDED IN INDUCTION MOTOR
A) starters are needed in other to start high horse power induction motor due to the following reasons;
- Starting currents are 4-7 times the full load current and when switched directly on-line.
- The heavy starting current causes voltage drop, so we need starter to avoid voltage to drop
- High voltage drops results in obtionables, so starter should be used to avoid it.
List three (3) types of transformer according to use. (3 marks)
1. Power transformer
2. Distribution transformer
3. Transmission transformer
Explain briefly the working Principle of a transformer. (3 marks)
When two independent coils are wound on the opposite limbs of a common magnetic circuit of the rectangular shape of soft iron laminated core and an alternating voltage is applied to winding 1 which is known as primary winding and alternating flux is produced in the magnetic circuit, links with another.
According to it, the transformer may be step up and step down. If the secondary windings of transformer has more turns than the primary winding. It is called step up transformer. This is used whenever it’s necessary to increase the voltage, that is if the voltage is 220Volts and we want 310Volts. Then a step up transformer of 220 to 310 will be used in the circuit depending upon the requirement of current also.
Types of transformer
1. According to construction
2. According to the system of winding used in the transformer
3. According to the system of cooling of the transformer.
4. According to use of power distribution