Overfluxing in Transformers

The transformer works on the principle of mutual induction between the primary and secondary windings. The induction is caused by the constantly varying magnetic flux that links the two windings. The flux density in the windings is directly proportional to the induced voltage and inversely proportional to the frequency and the number of turns in the winding.

Magnetic Flux α Voltage/Frequency

Overfluxing is a dangerous situation in which the magnetic flux density increases to extremely high levels. The high flux density can induce excessive eddy currents in the windings and in other conductive parts inside the transformers. The heat generated by these eddy currents can damage the windings and the insulation. The high flux density also causes magnetostriction inside the transformer core and produces noise. The powerful magnetostrictive forces can also cause damage. The winding temperatures may also increase due to the heat produced.

The magnetic flux density is dependent on the current flowing through the primary windings in a transformer. This current is dependent on the voltage applied across the windings and the winding impedance. The impedance is dependent on the frequency of the applied voltage. If the nominal voltage is applied at a reduced frequency, the low inductive reactance will cause a higher current to flow through the windings.

Overfluxing is usually encountered in Transformers which are directly connected to the generator. It usually occurs when the generator is being started or stopped. As the rpm of the generator and consequently the frequency of the power falls, the same system voltage induces a higher magnetic flux. Modern Automatic Voltage regulators are equipped with V/Hz limiters which limit the voltage in accordance with the frequency.

Overfluxing can be prevented by the use of a Overfluxing relay. An overfluxing is an adaptation of an overvoltage relay. The PT voltage is connected across a resistor and a capacitor in series. The voltage sensing relay is connected across the capacitor. The relay operates in the event of an overfluxing and isolates the transformer

What are opto-coupler relays?

Optocoupler relays are relays in which the changeover of contacts is effected by the switching on or off of a light source which is linked to a SCR or a TRIAC. The SCR or the TRIAC is switched on or off when the LED is switched unlike conventional relays, where it is done electromagnetically.

These relays are faster than electromagnetic relays. More importantly, the provide isolation between the control and the power circuits.

These relays do not have any moving parts which can deteriorate due to arcing or operational wear. However, they are expensive over conventional relays and hence still find limited application.

Another advantage of the Solid state relay is that it can open an AC circuit when the sinewave crosses zero. This ensures that the back-emf is minimum and this ensures that there are no voltage kicks in the opposite direction when the circuit is open due to inductance. This is because the Triac or the SCR used in the solid state relays conduct current till the waveform reaches the zero point even after the optocoupler LED has been switched off. This prevents premature failure of the contacts.

The Advantages of Optocoupler relays include

• Smaller Size
• Faster Response time
• Noiseless operations as there is no mechanical movement of the contacts.
• Optocoupler relays can withstand high vibration compared to conventional relays
• They do not generate Electromagnetic radiation as there is no coils to be energized.

However, they also have some disadvantages.

• They are more expensive
• They generate heat and require special heat-sinking fixtures.
• They cannot switch on very low currents
• When Solid state relays fail, they fail in the "closed position". In this situation, the machine which is connected will continue to be in the operating condition and there will be difficulty in isolating it. Electromagnetic relays usually fail in the "open" position.

Zero-Switching of Transformers

When inductive loads such as transformers or motors are switched on, a sudden rush of current into the winding is observed. This is known as the inrush current. Typically, the inrush current is around 5 to 6 times of the rated value. This is due to the absence of the back-emf when the winding when the power is first applied. This sudden surge of current causes disturbances in the system voltage and sometimes spurious operation of relays. The high current also causes stress on the windings of the machines.

There are a number of methods to address this problem. Adding resistors in series to the winding and then gradually taking them out of circuit is one option. Another option is the use of softstarters which raise the terminal voltage of the machines gradually. Zero crossover switching is one method of addressing the issue of high inrush current when switching on inductive equipment. The method involves the use of Static Relays consisting of devices such as SCR or TRIACs. These devices switch on when the sinewave crosses the zero point so that the voltage is gradually increased. This, in some circumstances, reduce the inrush current.

However, this methods has its downsides too. There have been reports that if the zero cross over switching is carried out on a core that is already saturated from previous operation, extremely high currents can result.

Apart from inductive loads, inrush currents is also observed in resistive loads such as filament lamps. In filament lamps the resistance when the filament is in the cold, that is, switched off condition is lesser than the resistance when the lamp is in the switched on condition. This is due to the positive temperature co-efficient of resistance. When the lamp is switched on from the cold condition, there is a high surge of current which continues till the temperature of the resistance increases. Zero-crossover switching in resistive loads can ensure a smoother increase in current value to the steady-state condition.

This ensures a smoother increase in the current to the steady-state value.

What is the difference between AC and DC relays?

AC and DC relays work on the same principle, that of, electromagnetic induction. However, there are some differences in construction. DC relays have something known as the freewheel diode which acts to discharge the emf built in the inductance when the coil is de-energized. (Click here to read about the phenomenon of freewheeling.) AC relays have cores which are laminated to prevent losses due to eddy current heating.

Another more conspicuous difference between a DC relay and an AC relay is presence of the Shading Coil. In AC relays, the alternating current supply changes direction about 100 times a second. At each instance, when the sine wave passes through zero, the current flowing through the coil becomes zero. This results in a loss of magnetism for a few milliseconds. When this happens about 100 times a second, the repeated drop and pickup of the coil produces a noise known as chattering. This also leads to the making and breaking of the relay contacts leading to disturbances in the connected electric circuits.

A shading coil is a coil with high remanence. thus when the magnetism of the coil collapses when the current becomes zero. The shading coil still retains the magnetism. Thus, ensuring that the contacts do not drop off.

AutoTransformers

Autotransformers are transformers which contain only one winding unlike two windings in the conventional transformer. The same winding, therefore, serves as the primary and secondary windings.

Autotransformers are advantageous over normal transformers as they are cheaper. Autotransformers are used generally for voltage conversion of equipments from one voltage to another such as from 110V to 220V or vice versa.

However, the autotransformer does not provide isolation between the primary and the secondary. Hence, there may a need to connect external filtering or suppression circuits. Thus, in the event of a failure of the insulation between the turns of the winding, there are chances of the primary voltage appearing on the secondary.

Another aspect which needs to be checked is the neutral point. If the neutral point is not at ground potential in the primary, the secondary neutral wil also not be at ground potential.

The autotransformer has higher voltage stability and better overload transformers than the ordinary transformers.

A Variac is a variable autotransformer with movable taps. Thus, it provides variable output voltage for a steady input voltage. The taps can be adjusted by a movable knob.

See Also:

Difference between Autotransformer, Variac and Dimmerstat

Scopemeters - An Overview

A scope meter is a handheld instrument that is used to see electrical parameters in graphical form. It can be described as a combination of a multimeter and an oscilloscope. It can be used to take measurements of the magnitude and frequency of a signal such as current and voltage.

A scope meter is an effective instrument in troubleshooting. As it displays the parameters in graphical form, transients and other momentary variations can be identified. This enables the identifications of improper and loose connections, grounding loops, etc which may be missed by indicating instruments.

Modern Scopemeters can also be connected to a computer by means of a USB port and the data collected can be transferred into the computer for analysis. The Scopemeter usually comes with software for this purpose. They also have recording and playback facility.

Since, scopemeters are handy and battery powered, they are portable and can be taken around easily unlike oscilloscopes.

image courtesy : http://www.aikencolon.com

What is the difference between a Variac, variable autotransformer and Dimmerstat?

An Autotransformer is a transformer with only one winding. See more. A variable autotransformer is an autotransformer with a sliding scale which can be used to adjust the transformer ratio. "Variac" and "Dimmerstat" are trade names of variable autotransformers. Hence, they are the same.