Tests of ATM

 

Extreme Load Tests

On

Electricity GOD - ATM

As per Hindu Mythology our body is made up of Panch Tatva (Five Elements) Akash(Space), Agni(Fire), Prithvi(Earth), Jal(Water) and Vayu(Air).  At the end of life, the body is disintegrated back to these basic elements.

Electricity also has similar pattern. Electricity is produced from basic source like Heat, Light, Wind, Water, Motion, Height, Waves, Tide, Fire, Etc.

Above basic forms of energies is converted to electrical energy by appropriate process. Electricity generating systems are identified according to source of energy and conversion process as Thermal Power Plant, Hydro Power Plant, Atomic Power Plant, Wind Power Plant, Solar Power Plant etc.

Human life has three stages Janma, Jivan and Nirvan. Janma is incarnation of life from Panch Tatva. Lord Brahma is considered as regulator of this stage. Jivan is life journey starting as infant and transform through various steps like baby, toddler, child, teenage, young, adult, elder etc. During the life span, human move to various places, meet others and have harmonious coexistence and interaction. Lord Vishnu is considered as regulator of this stage. Nirvan is extinguishing of life turning back to Panch Yatva. Lord Mahesh (Shiv) is considered as regulator of this stage. Functionally load Brahma, Vishnu and Mahesh are Generator, Operator and Destroyer of human life and known as GOD.

Electricity also has similar life pattern. Its existence starts with conversion from basic sources to electricity is known as Generation at power plant. Its life passes through transmission and distribution network with transmission and transformation to various forms, travel to various locations and meet others from different sources and operate in synchronism during the T & D stage. At the end electricity finally extinguish and converted to form required by user. These three stages are Generation, Transmission-Distribution and Utilization abbreviated as GTU.

There are numbers of equipments in above three stages of power system. But Alternator, Transformer and Motor (ATM) is the most significant equipment representing GTU stages generation, transmission-distribution and utilization.

Normally all electrical equipments undergo different types of test at various stages. But following pair of tests are common in all the above three by principal, objectives and treatment.

Power balance equation for any machine is known as under.

Power Input = Power Output + Power Losses.

Iron Loss and Copper Loss are significant losses whereas others losses like Dielectric, Stray, Friction and Windage losses are insignificant and ignored for generally study.

So above equation stands simplified as under

Power Input = Power Output + Iron loss + Cupper loss. 

The Equation further reduced when machine output is zero.

Power Input = Iron loss + Cupper loss. 

Of various test for different purpose, the following tests are to estimate significant losses by extreme loading without output.

A.     Extreme Minimum load means zero Load.

B.     Extreme Maximum load can be infinite load but it is not feasible for safety of equipment. So Maximum load is restricted to rated load.

Iron loss is in the magnetizing circuit and is proportional to square of applied voltage ( V2/X ) as it is across the supply in equivalent circuit.

Cupper Loss is in resistance of the coils and is proportional to square of load current ( I2R ) as it is in series with the load in equivalent circuit.

Ultimately above power balance equation with no output at applied voltage Va and load current Ia will be as under.

Power Input = Normal Iron Loss (Va/Vr)2 + Normal Cupper Loss (Ia/Ir)2

Where

Va is applied Voltage and Vr is rated voltage.

Ia is actual load current and Ir is rated load current.

General Setup for Equipment Performance Testing is as under.

Input Block

It has following three elements.

a.      Voltage Regulator   Rheostat in DC     OR      Variac in AC

b.      Power Measurement

           For DC supply -        DC Wattmeter     OR                                                                                                                         DC Voltmeter and DC Ammeter and P = V*I

            AC 1ϕ Supply –         AC Wattmeter OR

            AC Voltmeter, AC Ammeter and PF meter and P= V*I*PF

            AC 3ϕ Supply –        Three Phase Three Wire Wattmeter  OR

                                                Two AC Watt meters   P= W1+W2  OR

                                                 AC Voltmeter, AC Ammeter and PF meter   P= √3* V*I*PF

Two Watt Meters Method

            Current coil of W1 is connected in series in one of                 the phase.

Current coil of W2 is connected in series in other phase

Pressure coil of W1 and W2 is connected between respective phase and third phase.

W1 and W2 have unequal reading except for Unity power factor.

W1 has backward deflection at very low power factor. Leads of current coil or pressure coil have to be reveres to read the meter and assign negative sign.

c.       Mediator and Auxiliary Devices – Depends what, when and where required

Output Block

It has following three elements.

a.      Power Measurement   Similar to input block

b.      Loading Devices       Lamp Bank for Resistive Load – Unity Power Factor

                                                Chock Coil for Inductive Load – Lagging Power Factor

                                                Capacitor Bank for Capacitive Load – Leading Power Factor

                                                Single set for single phase load OR

Three sets connected in star for three phase load.

c.       Mediator and Auxiliary Devices – Depends what, when and where required.

Now

Input Output power format of above three representative equipments is as under.

Input

Equipment

Output

Electrical

Transformer

Electrical

Mechanical

Alternator

Electrical

Electrical

Motor

Mechanical

 

 

 

 

Test on Transformer  Input and Output are Electrical.

Test A    Condition -> Secondary circuit open.

Known as Open Circuit   OR   No load Test


 Voltage Applied. Va =Vr

Load side voltage is VL but current IL is Ia = 0

Output power = √3 × VL × IL × PF = 0

Pin = Pout + Piron (Va/Vr)2 + Pcupper (Ia/Ir)2

Pin = o + Piron (Vr/Vr)2 + Pcupper o/Ir)2

Power Input = Piron

Power input in this condition is mostly Iron Loss

Test B    Condition -> Secondary circuit shorted.    Known as Short Circuit Test

High current may flow and damage the machine. Therefore low voltage is applied to restrict load current to rated value.

Actual load current  Ia = Ir

Applied voltage Va is very small and therefore (Va)2 is negligible

Output load current is Ir but voltage across Short Circuited load VL = 0

So output = √3 × VL × IL × PF = 0

Pin = Pout + Piron (Va/Vr)2 + Pcupper (Ia/Ir)2

Pin = 0 + Piron (0/Vr)2 + Pcupper (Ir/Ir)2

Pin = Pcupper

Power input in this condition is mostly Cupper Loss

Test on Alternator Input is Mechanical but Output is Electrical.

Test A   Condition -> Similar to transformer.  Output Circuit open.

Known as Open Circuit   OR   No load Test

Input power is mechanical and is measured indirectly. Electric motor with known efficiency is used as prime mover. Power drawn by motor is electrical and is measured as usual. Multiplying it by its efficiency is output of motor and is the mechanical power input to alternator. Generator is operated as normal with rated speed and voltage but output circuit is kept open.

Power input in this condition is mostly Iron Loss as in case of transformer.

Test B  Condition -> Similar to transformer. Output circuit shorted. 

Known as Short Circuit Test

Under this condition load current will be very high at normal output voltage and it may damage the machine. Therefore output voltage is reduced by rheostat in excitation circuit to restrict load current to rated value.

Power input in this condition is mostly Cupper Loss as in case of transformer.

Test on Motor Input is Electrical but Output is Mechanical.

Input Electrical power is measured as usual.

Mechanical output power can be measured by dynamometer OR Friction Belt method. But measurement is not required as output power is kept zero as under.

Test A  Condition -> Normal voltage is applied to motor and allowed  free spin without any load connected.  Known No Load Test.

Motor runs at normal angular velocity ω (RPM). As motor is running freely, Torque T developed is insignificant and actual Load Current Ia = IL  is also negligible.

So T and Ia  taken as 0

In case of Leaner Motion

Work = Force × Distance

Work/time = Force × Distance/time

Power = Force ×Velocity

Similarly for Angular Motion

Power = Torque × Angular Velocity  i.e.  P= T × ω

Now,  Pin = Pout + Piron (Va/Vr)2 + Pcupper (Ia/Ir)2

Pin = T × ω + Piron (Va/Vr)2 + Pcupper (Ia/Ir)2  

Pin = 0 × ω + Piron (Vr/Vr)2 + Pcupper (0/Ir)2

Pin = Piron

Power input in this condition is mostly Iron Loss

Test B  Rotor of the motor is not allowed to spin is known as Blocked Rotor Test.

In this condition heavy current is expected and damage the motor.

Hence very voltage is applied to limit the current to full load.

In this condition angular velocity ω = 0 as rotor is blocked.

Actual current is rated current Ia =IL

Applied voltage Va  is  very small, hence (Va)2 is negligible.

Now  Pin = Pout + Piron (Va/Vr)2 + Pcupper (Ia/Ir)2

Pin = T × ω + Piron (Va/Vr)2 + Pcupper (Ir/Ir)2

Pin = T × 0 + Piron (0/Vr)2 + Pcupper (Ir/Ir)2

Pin = Pcupper

 Power input in this condition is mostly Cupper Loss

Conclusion

Above estimation of losses are approximate due to various assumptions as under

There is no Friction and Windage loss in transformer and block rotor condition of motor but it is ignored in other conditions also. These losses are comparatively meager but not zero.

Current during no load (open circuit) is very small but not zero. Copper loss is proportional to square of the current and hence ignored.

Supply voltage during short circuit (block rotor) is very low but not zero. Iron loss is proportional to square of the voltage and hence ignored.

Dielectric and Stray loss are also ignored.

Perfect estimation of all these losses is very complex exercise.

In addition to basic findings of losses as above, other parameters of machine can be derived graphically from above test data.

Fixed losses can also be estimated graphically by series of reading with different applied voltages.

Summary – Common Identical Test Conditions

Test A,  Min load -> No load, Open Circuit, Free Spinning but No Output. 

Test B, Max load -> Rated load, Short Circuit, Blocked Spin but No Output.