Can transformers be used in parallel?

Figure: Transformers in Parallel

Single phase transformers can be used in parallel only when their impedances and voltages are equal. If unequal voltages are used, a circulating current exists in the closed network between the two transformers, which will cause excessive heating and result in a shorter life of the transformer. In addition, impedance values of each transformer must be within 7.5% of each other. For example: Transformer A has an impedance of 4%, transformer B which is to be parallel to A must have an impedance between the limits of 3.7% and 4.3%. When paralleling three phase transformers, the same precautions must be observed as listed above, plus the angular displacement and phasing between the two transformers must be identical.

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What is the difference between Electric Motor and Electric Generator?

Figure-1: Generator

Figure-2: Motor

Difference between Electric Motor and Electric Generator: 

• A generator converts mechanical energy into electrical energy, while motor converts mechanical energy into electrical energy. 
• In a generator, a shaft attached to the rotor is driven by a mechanical force and electric current is produced in the armature windings, while the shaft of a motor is driven by the magnetic forces developed between the armature and field; current has to be supplied to the armature winding. 
• Motors (generally a moving charge in a magnetic field) obey Fleming`s left-hand rule, while the generator obeys Fleming’s right-hand rule.

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Describe a Starter-Motor Circuit.

Figure 1: Typical Starter-Motor Circuit.

A Starter-Motor Circuit: 

Figure 1 shows a very simple diagram of an engine starter-motor circuit. If one day you turn on the ignition switch, push the start button, and nothing happens, this is the circuit diagram you will need. A complete engine wiring diagram would probably contain the same information, but it would also contain all of the meters, idiot lights, and alternator wiring, adding considerable visual confusion.

For the starter-motor circuit all you need to see are the:

• Battery switch 
• Fuse between battery switch common terminal and engine panel 
• On/Off switch 
• Momentary start switch
• Wire from starting switch to solenoid 
• Heavy positive cable from battery-select common terminal to solenoid 
• Heavy negative cable from battery negative terminal to engine negative terminal. 

If the engine won’t turn over, the problem probably lies somewhere in this diagram.

Source: Boatowner's Illustrated Electrical Handbook

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What is the difference between Electric Motor and Electric Generator?

Difference between Electric Motor and Electric Generator: 

• A generator converts mechanical energy to electrical energy, while motor converts mechanical energy to electrical energy. 
• In a generator, a shaft attached to the rotor is driven by a mechanical force and electric current is produced in the armature windings, while the shaft of a motor is driven by the magnetic forces developed between the armature and field; current has to be supplied to the armature winding. 
• Motors (generally a moving charge in a magnetic field) obey Fleming`s left-hand rule, while the generator obeys Fleming’s right-hand rule. 

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What is RMU (Ring Main Unit) and Its Use in Electrical Power Distribution System?

Ring Main Unit (RMU) is a 11 KV or 33 KV HT panel having 3 nos. of switches (Circuit Breakers or Isolators or LBS) that are 2 for incoming one for outgoing. It enables consumer use 2 sources of HT power in at the same metering point. It is a totally sealed, gas-insulated compact switchgear unit.

RMU is to be used two incoming with mechanical or electrical interlock and one out going  to the load generally but some times one incoming and two outgoings medium voltage supplies. In engineering distribution this is called flexible power supply. Now a day SF6 (Sulfer Hexa-Floride) circuit breakers going to be used. These are maintenance free breakers. 

Figure 1 – Outlook of a typical three-feeder 24 kV RMU unit

Figure 2- 11 KV 2 Incoming and 3 Outgoing RMU at Bangladesh Film Archive.

Ring Main Unit is used in a secondary distribution system. It is basically used for an uninterrupted power supply. Alongside, it also protects your secondary side transformer from the occasional transient currents. 

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What is a Resistor? What are the Types of Resistors? What are the Applications of Resistor?

What is a Resistor?

A resistor is a passive two-terminal electrical or electronic component that resists an electric current by producing a voltage drop between its terminals in accordance with Ohm's law. The electrical resistance is equal to the voltage drop across the resistor divided by the current through the resistor. 

Figure 1: A typical axial-lead resistor.

Figure 2: Two common schematic symbols of resistor.

In electronic circuits, resistors are used to reduce current flow, adjust signal levels, to divide voltages, bias active elements, and terminate transmission lines, among other uses. High-power resistors that can dissipate many watts of electrical power as heat may be used as part of motor controls, in power distribution systems, or as test loads for generators. Fixed resistors have resistances that only change slightly with temperature, time or operating voltage. Variable resistors can be used to adjust circuit elements (such as a volume control or a lamp dimmer), or as sensing devices for heat, light, humidity, force, or chemical activity.

Theory of Operation:

Ohm's law: The behavior of an ideal resistor is dictated by the relationship specified by Ohm's law:

             V = I/R

Ohm's law states that the voltage (V) across a resistor is proportional to the current (I), where the constant of proportionality is the resistance (R). For example, if a 300 ohm resistor is attached across the terminals of a 12 volt battery, then a current of 12 / 300 = 0.04 amperes flows through that resistor.

Practical resistors also have some inductance and capacitance which affect the relation between voltage and current in alternating current circuits.

The ohm (symbol: Ω) is the SI unit of electrical resistance, named after Georg Simon Ohm. An ohm is equivalent to a volt per ampere. Since resistors are specified and manufactured over a very large range of values, the derived units of milliohm (1 mΩ = 10^-3 Ω), kilohm (1 kΩ = 10³ Ω), and megohm (1 MΩ = 10⁶ Ω) are also in common usage. 

Figure 3: A few types of resistors.

Types of Resistors:

1.     Linear resistors.

                                 i.         Fixed resistors

a)    Led arrangement

b)    Carbon composition

c)     Carbon Pile

d)    Carbon film

e)    Printed carbon resistor

f)      Thick and thin film

g)    Metal film

h)    Metal oxide film

i)      Wire wound

j)      Foil resistor

k)    Ammeter shunt

l)      Grid resistor

m) Special verities

                                    ii.         Variable resistor

a)       Adjustable resistor

b)       Potentiometers

c)        Resistance and decade boxes

d)       Special devices.

2.     Non-linear resistors.

Applications of Resistors:

• In general, a resistor is used to create a known voltage-to-current ratio in an electric circuit. If the current in a circuit is known, then a resistor can be used to create a known potential difference proportional to that current. Conversely, if the potential difference between two points in a circuit is known, a resistor can be used to create a known current proportional to that difference.  
• Current-limiting. By placing a resistor in series with another component, such as a light-emitting diode, the current through that component is reduced to a known safe value.  
• A series resistor can be used for speed regulation of DC motors, such as used on locomotives and train sets.  
• An attenuator is a network of two or more resistors (a voltage divider) used to reduce the voltage of a signal.  
• A line terminator is a resistor at the end of a transmission line or daisy chain bus (such as in SCSI), designed to match impedance and hence minimize reflections of the signal.  
• All resistors dissipate heat. This is the principle behind electric heaters.  

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