FAQ on Electrical Power

 

What is Power?

Power is the capacity to do any work. For the same work, a less powerful man will take more time and a more powerful man will take less time. Power is the rate at which work is done.

 

What is work?

A body remains at rest unless it is moved by a force. The distance the body moves is a measure of work done. The amount of force required to move the body and to keep it moving depends on forces that resist movement, like friction. More friction means more work need to be done to move the body by the same distance. This may require higher force. Hence force multiplied by the distance gives a full measure of the work done. How quickly the body can be moved by a distance depends on how much force is exerted and for how long. If the force is just enough to overcome the friction, it will barely move. If the force is increased it will move faster. With the higher force the work is done faster. The capacity to exert this higher force over a period of time is known as power. Power is measured as the rate of work done.

 

What is electrical power?

Electrical power is rarely used directly as electrical power. It is always converted to: mechanical power in pumps and drills, heat power in furnaces and heaters and light power in lamps etc. Efficiency of conversion varies as per the devices. Raw electrical work is done by charges moving in an electrical field created by voltage (Electro Motive Force). The medium, like an electrical conductor, resists the movement of charges. This property of electrical conductivity is known as resistance. The amounts of charges that move overcoming this resistance represent the raw electrical work done. More voltage, more charges and more work. Hence voltage multiplied by amount of charges moving gives us total electrical work done. The electrical power is the rate of this work. Rate of movement of charges is known as electrical current. Now it is easy to see that raw electrical power is just voltage multiplied by current.

  

Ohm’s Law

In general, when Voltage(V) increases the Current(I) increases. For a particular conducting material, (eg) aluminum, I, the current is proportional to applied voltage, V.

                      i.e. V = kI.

This constant k is known as resistance (R), measured in ohms in the name of the scientist who found this law,

           Ohm’s law, V= I R.

Now we can express electrical power, P as,

            P =     VI     =      I2 R    =      V2/R      measured in Watts.

 

What is Direct Current and Alternate Current Electricity?

All naturally available forms of electricity are direct current.

(eg-1) Static electricity obtained by charging up certain materials. When an ebony rod is wiped vigorously by a silk cloth, the ebony rod gets charged up to a voltage. When touched to earth it discharges by passing a current to earth.

(eg-2) When a lightning strikes, a highly charged lower cloud arcs through to a high point on the earth, discharging a heavy current with a flash of arc.

Other man-made devices, like thermo-couples and batteries also provide an easy source of direct current. About Alternate Current Electricity we will see later.

 

What is a Generator?

For heavier applications we needed a more powerful source of electricity. Movement of an electrical conductor in a magnetic field was found to produce an electrical current in the conductor. This phenomena lead to the discovery of a device known as a dynamo. When a conducting coil of a dynamo was rotating between the poles of magnets, it was initially found to produce a current in alternating directions as they pass through north and south poles of the magnet. But it was converted to direct current by a device called split ring. This, in contact with metal brushes, eventually reversed the circuit for every half rotation of the coil mounted on the shaft, thus producing the unidirectional ‘direct current’. These dynamos were the earliest electricity generators, used in place of batteries, for heavier applications such as arc lamps and heaters.

 

What is a motor?

In a generator, ‘movement’ in a ‘magnetic field’ produced ‘electricity’. In a motor electricity’ in a ‘magnetic field’ produced ‘movement’. When a current was injected into a coil mounted between the poles of a magnet, the coil along with the shaft was found to rotate. The device, known as a DC motor, found major applications in traction and drives in factories, textile mills and transportation. More and more powerful DC generators were manufactured and DC grids were formed to supply different customers. Thomas Alva Edison was the great engineer who pioneered such grids, starting with New York City, way back in 1880. (Reference quoted).

 

Then, why we did not continue with DC?

When we started, we talked about ‘power’ and ‘work’. No matter how powerful you are, you exert less force for a smaller work and more force for a heavier work. Same is the case with electric power. We need lower voltage for smaller devices like bulbs and heaters, whereas we need higher voltage for electric traction and arc furnaces etc. Further for transmitting power from a generating station to a consumer substation, it would be preferable to use a much higher voltage. This will result in lower currents and higher efficiency of transmission. The DC electricity once generated at a specific voltage is not suitable for ‘transforming’ into another usable voltage.

 

What about AC?

One man, Nikola Tesla of Hungary, wondered why at all the naturally produced alternating current in a dynamo should be converted to DC?  He invented AC motors which could run on alternating current. In the process he faced a major problem of stalling of the motor. Hence it was not easy to promote the use of AC systems, though improved designs of AC generators were developed.

The famous technocrat, George Westinghouse of USA was not interested initially so much in electricity. He found no future in electricity business, unless one finds a way of transporting electricity through a large distance. But, he happened to see a piece of report about a device developed in Europe to transform AC voltages from one value to another. This gave him a spark of an idea to harness AC power for generating, transporting and utilizing at different user levels. This idea changed his entire career.  He bought up this device known as transformer, and made suitable design changes. In March 1886 Westinghouse Electric Company commissioned the first AC grid, in Massachusetts, USA. This led to several AC grids to be commissioned in USA.

 

 

What is 3-Phase and Single Phase AC?

The war against Edison’s DC grid was still on, not won. AC cannot win unless reliable AC motors were developed to power the drives of the factories and transports.

Mr. Tesla was delighted at the success of AC grid and promised Westinghouse he will give him the AC motors he needed. They entered into a pact to develop the AC motors. Mr. Tesla dealt a master stroke when he reversed the roles of stator and rotor of the motor. The magnets in the rotor were made to follow the alternating magnetic field generated by the current coils on the stator. Immediately he found the solution for stalling. What is required is only to find a way to produce a rotating magnetic field. The induced magnetism in the rotor will automatically try to follow the rotating magnetic field in the stator and in the process will try to rotate as fast as the rotating magnetic field. One sure way Tesla found, for creating the rotating magnetic field, was to think of a poly-phase AC windings which are out of phase with each other in a way to complement one another, (very much like pistons in the cylinders of the car). This finally led to AC induction motors and 3-phase AC generators. This made a large electrical network possible through devices called transformers. High voltage was used for long distance transmission of electricity and gradually lowering the voltages to lower levels for smaller networks and then to domestic level of 230V or 110V.  It is like carrying Rs 100,000, (a) as 100 nos. of Rs.1000 notes while traveling, and changing them to (b) 1000 No. of Rs 100 notes at the time of spending. Finally we have the 3Ph AC grids as existing now everywhere. For a normal domestic use one phase of this 3-Phase AC is used. All such single phase loads are distributed equitably among all the three phases to obtain a balanced 3-phase load.

(Reference quoted).

      

In the AC system when the current is alternating either way, how at all the power gets transferred to the device?

When you consider alternating currents, the input voltage is alternating between a positive and a negative voltage, as a sine wave, (normally) at a frequency of 50 or 60 cycles per second. Coming back to our discussion on electrical power, V*I is still the power, but in this case, it is an alternating power, or is it? Let us consider an AC circuit with a resistive load such a heater. As before I2R is the power consumed in the circuit. Though the current is alternating, it will still heat up the coil either way. The power also will continue to be transferred from AC power source to the heater, irrespective of alternating current. So the average power in the circuit will be R multiplied by the average of I2 over a cycle of the alternating current. This average of I2 over a cycle is knows as the Mean Square value. The square root of this current is known as the Root Mean Square current or IRMS. Same way, we can define a VRMS for the voltage wave form. Without going into rigours of mathematics, Power in a AC circuit with a resistive load, can still be expressed as:

 

           P =   VI    =    I2 R   =    V2/R      measured in Watts.

 

But however herein, V and I are RMS values of Voltage and Current.

For a pure sinusoidal waveform, RMS value = Peak Value/ √2

In an AC electrical system, when we refer to magnitudes of voltage and current, we normally refer only to there RMS values.

 

What is energy?

Work done over a period of time consumes energy of the source. A man spends his energy when he executes a manual work. When he is exhausted (of his energy) he is not able to work any more. Similarly when we use electricity we consume electrical energy and convert them to useful work in devices like heaters, motors etc. Electrical energy is measured as:

            Power x Time or         Voltage x current x time

Voltage is measured in Volts and Kilo Volts

Current is measured in Amps and milliamps

Power is measured in Volt-Amperes or Watts or Kilowatts

Energy is measured in Watt-hours or Units (Kilo Watt-Hours)

 

Reference:

THE GRID, by Phillip F. Schewe, Joseph Henry Press, Washington, D.C. (2007)

 

 

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