Reactive load refers to the load that is in consequence of the impedance or the capacitance of the load. Thus when a capacitive or inductive load is connected to a power source a current flows through the load which does not produce any active power consumption (kW). This charging current is called the reactive current (I .Sin φ).

The power caused by this current is called the reactive power. It is denoted by Q .

Q= V x I x cos φ

The reactive power is measured by kVAr (Reactive Kilo Volt ampere). The higher the value of the reactive load, the lower the power factor. For example, as a motor consumes more reactive power its power factor decreases.  This is evident from the following diagram.

As the reactive power decreases, the power factor increases.

Alternatively, the power factor is the ratio between the Active power, P and the Apparent Power, S

Cos φ = P/S


Tags , , , ,

In the earlier article, we saw what power factor is and how it is calculated. Now let us see why it is necessary to control the power factor. Power Factor Control refers to the reduction of the phase angle - the angle between the current and the voltage. As the power angle reduces, the power factor which is a cosine of the phase angle increases. It becomes closer to one. In the industry, around 80% of the power is inductive. This causes the current to lag behind the voltage resulting in a power factor that is less than one.

But how does this affect the power system? To understand this, we need to look at the the formula for power.

Active power = V x I x cos φ

If we are to increase the power factor, the current for a given value of kilowatts will be less resulting in a reduced loading of the system and reduced losses. This is the reason why the power factor is increased to a value closer to one.

Thus when the value of the power factor cos φ is less, more current is required to deliver the same amount of kilowatt. This increased loss will result in copper losses or I2R losses in the system. The conductors, cables will also be subjected to higher loading as they have to carry more current.

Let us take an example, say, a single phase motor with a rating of 100kW with a supply of 440V. Running this motor at a power factor of 0.5 will result in a current of 454.5A. However, running the motor at a power factor of 0.9 will result in a 252A only.

The reduction in current required is substantial.  This reduced current will also result in reduced loading of the source as power sources such as generators and transformers are rated in kVA

Power factor improvement is done by using capacitors, active power factor controllers and so on. We will discuss about them in the next article.


Tags , , , , ,

Power factor refers to the cosine of the angle between the voltage and the current. In AC circuits, the nature of the load determines the power factor. Power factor is a critical parameter in AC circuits as it determines the amount of current which goes into delivering a certain quantity of power.  Equipments which run at a lower power factor draw a high current for the same amount of load. What causes Power Factor All Electric loads can be categorized into three types, viz. Resistive, Inductive and Capacitive loads. Consider an AC voltage being applied across a simple resistor, the current drawn will be 'in phase' with the voltage.  That is, the current reaches maximum when the AC voltage reaches maximum and falls to the minimum when the current reaches minimum.
In a purely inductive circuit, the current lags behind the voltage by an angle of 90 degrees.  Thus the current is zero when the voltage is maximum and rises to the maximum when the voltage falls to zero.
In a capacitive circuit, the current leads the voltage by 90 degrees.
The angle between the current and the voltage is called the phase angle.  The cosine of the phase angle is called the power factor. In the next article we will see the relation between power factor and kW and why power factor control is necessary


Tags , , , , , ,

An exploded battery
Battery explosions are serious accidents which can cause severe injuries and burns to operating personnel and damage to equipment.  Explosions in batteries are caused by accumulation of gas inside the battery.  The gas is formed when the electrolyte(usually acid) gets electrolysed resulting in the formation of gas, usually hydrogen.  Hydrogen is extremely combustible.

This acculumulated gas sometimes shows up in the form of a bulge in the batteries.  These batteries should be replaced immediately.  These explosions may be triggered by a short circuit inside the battery or a sudden load which results in high current, such as jumpstarting a car.

You can prevent battery explosions by making sure that the batteries are charged only by the correct chargers provided with them.

Ensure that the battery connections are properly tightened.  This can prevent sparks due to loose contact.  

Avoid sparks or naked flames near the battery bank.

Ensure that no overcharging takes place.

Do not short circuit batteries.  Cellphone batteries should not be carried in your shirt or trouser pockets where small coins can short them.

Do not dispose off batteries in fire.

Batteries can also explode if they are suddenly subject to mechanical impact or deforming forces.

image courtesy: www.tractorshed.com


Tags , ,


The Rotor voltage drop test is used to identify the presence of shorted turns in the salient poles of the rotors of synchronous machines. The test works by measuring the impedance of each pole on the rotor.  If there are short-circuits in the rotor winding, the impedance of the rotor poles will be less.

The equipment required is relatively simple.  A fixed AC voltage is applied across each end of the rotor winding and the voltage drop across each pole of the rotor is measured. The voltage values are tabulated.  The average of the values is calculated.  Each individual value should be within a limit of 10% from the average value.  Any further variation would indicate shorted turns in that particular pole.

While the rotor voltage drop test is useful in identifying inter turn faults in the rotors, they are not considered comprehensive.  Since the rotor is a rotating object, there are shorts which may form only during running due the action of the centrifugal force on the rotor.


Tags , , , , , , , ,

Bulging in batteries usually indicates gas formation. Gas formation can occur due to overcharging or due to an internal short circuit in the battery. Short circuit causes arcing which can cause gas formation. Overcharging can be caused by improper battery charger settings which apply current even the battery has become fully charged.

Batteries which have bulges need to be replaced. These batteries  may not charge fully or may not hold charge. If the reason for the battery bulge is found to be overcharging, the charger also needs to be checked and the charge settings changed.


Tags , , , , , ,

The Di-electric Absorption Test is a test conducted on the insulation of the windings in electric equipment.  This test tells us about the cleanliness and the condition of the windings. 

The dielectric test is carried out using an insulation tester (Mega-ohm-meter).  The insulation test of the winding is conducted and the values noted after 1 minute.  The test is continued and the reading is noted after every minute.  Ten readings may be taken.

The readings thus taken are plotted on a graph with respect to time.  The curve obtained will look like curve A in the diagram.  This curve A indicates that the insulation resistance increases as time increases.  This is because the insulation  gets polarized as the test voltage is applied.

As the insulation gets polarized less and less charge carriers are available for the current.  This causes the resistance to increase.

If the curve is flat (Curve B) or drooping downwards, it indicates that the insulation is in bad condition and should be repaired.  


Tags , , , , ,

The Magnetic Balance test is conducted on Transformers to identify inter turn faults and magnetic imbalance.  The magnetic balance test is usually done on the star side of a transformer.  A two phase supply 440V is applied across two phases, say, 1U and 1V.  The phase W is kept open.  The voltage is then measured
between U-V and U-W.  The sum of these two voltages should give the applied voltage.  That is, 1U1W + 1V1W will be equal to 1U1V.

For instance, if the voltage applied is 440V between 1U1V, then the voltages obtained can be

1U1V = 1U1W  + 1V1W
440V =  260V +  180V

The voltages obtained  in the secondary will also be proportional to the voltages above.

This indicates that the transformer is magnetically balanced.  If there is any inter-turn short circuit that may result in the sum of the two voltages not being equal to the applied voltage.

The Magnetic balance test is only an indicative test for the transformer. Its results are not absolute.  It needs to be used in conjunction with other tests.


Tags , , ,

The Rotor Protection relay is used in synchronous motors and generators to identify the presence of an earth fault in the rotor winding.  While the winding in the rotor is insulated from the ground during normal operation, the Rotor is subjected to stresses due to vibration, heat, etc.  These stresses can cause the winding to give way in a particular place and the winding can get earthed.

While a single earthing in the winding is not immediately damaging.  It sets the stage for damage if a second failure should occur.  The second earthing can cause a short-circuit through the rotor causing extensive damage to the rotor and the winding.

The currents produced during a rotor earth fault can cause excessive vibration and disturb the magnetic balance inside the alternator.  These forces can cause the rotor shaft to become eccentric and in extreme cases cause bearing failure. 

Hence, it is necessary that any earthing in the rotor is detected at the earliest.

In slip ring rotors, carbon deposits on the slip rings may compromise the insulation resistance of the rotor.  Hence, the slip rings need to be inspected for any deposits.

The Rotor Earth Fault Protection Device consists of a current injection device which applies an AC voltage to the rotor winding by means of a slip ring fitted on the rotor.  The current is applied to the rotor through a coupling capacitor.  In the normal condition, the system is floating and the current flowing through the device is zero as the resistance is high. 

When a fault occurs, the current increases causing the relay to operate.  The relay can be configured for alarm or trip depending on the criticality.


Tags , , , , , , ,
Subscribe to: Posts (Atom)