Pulse Transformer – An Overview


The transformer which handles voltages and currents in the form of pulse are called Pulse Transformer. Pulse Transformer is mostly used in Power Electronic circuits as an Isolating Transformer to isolate source and load. It is also used in television, radar, digital computers etc. The main functions of the Pulse Transformer are as below:

1)      For changing the amplitude of voltage pulse
2)      For inverting the polarity of the pulse
3)      For coupling different stages of pulse amplifier
4)      As an Isolation Transformer
It shall be noted here that the turn ratio of an Isolation Transformer is 1:1. The Pulse Transformer core is made of Ferrite.


The input voltage to a Pulse Transformer is of discontinued type as shown in figure below. The most and basic requirement of such transformer is that it should reproduce input voltage at its primary to secondary as accurately as possible.



Figure above shows the square input voltage pulse at the input terminals. The pulse with varies from fraction of microsecond to 25 microseconds. Normally enough time elapses before next pulse arrive at the input terminals. The waveform of output voltage can be determined by using the equivalent circuit of the pulse transformer. A typical output voltage waveform is shown in figure below. In the figure below, the pulse parameters are also shown and it must be noted.



Rise time here is the time interval required for the output to rise from 0.1 to 0.9 of its final value. The transformer response for to the flat top portion of the input voltage pulse can be determined using low frequency equivalent circuit. The output voltage seems to have downward tilt or drop off during pulse duration time. The output voltage cannot be flat as this would mean that DC is flowing through Transformer which is not possible. The drop off of the output voltage pulse can be minimized by using high magnetizing inductance for the transformer.

Again, when the input pulse is zero, the output pulse do not reduces to zero instantaneously because of magnetic energy stored in the inductance of the transformer rather there is some definite fall or decay time to reach the output voltage pulse to zero value.

You may like to read Concept of Transformer Action

Pulse Transformers are quite small in size. To minimize the leakage inductance, there are few truns of primary and secondary but to have high magnetizing reactance, its core is either made of Ferrite or high permeability alloys like permalloy.

Comments

Post a Comment

Popular posts from this blog

Concept of Subtransient, Transient & Steady State

Image
The concept of Subtransient, Transient and Steady State arises in case of fault in an Alternator. Let us assume a sudden short circuit in three phase of alternator. The fault current will flow in all the three phases of alternator and its waveform will be as shown in figure below. When the alternator is short-circuited, the currents in all the three-phases rise rapidly to a high value of about 10 to 18 times of full load current, during the first quarter cycle. The flux crossing the air gap is large during a first couple of cycles. The reactance during these first two or three cycle is least and the short circuit current is high. This reactance is called subtransient reactance and is denoted by X". The first few cycles come under sub-transient state. After a first few cycles, the decrement in the r.m.s. value of short circuit current is less rapid than the decrements during the first few cycles. This state is called the Transient  State and the reactance in th...
Read more

Open Circuit and Short Circuit Characteristics of Synchronous Machine

Image
Open Circuit Test and Short Circuit Test are performed on a Synchronous Machine to find out the parameters of Synchronous Machine and hence to have an idea of their performance.  Open Circuit Test of Synchronous Machine is also called No Load, Saturation or Magnetizing Characteristics for the reason which will be clear after going through the post. For getting the Open Circuit Characteristics of Synchronous Machine, the alternator is first driven at its rated speed and the open terminal voltage i.e. voltage across the armature terminal is noted by varying the field current. Thus Open Circuit Characteristic or OCC is basically the plot between the armature terminal voltage E f versus field current I f while keeping the speed of rotor at rated value. It shall be noted that for OCC, the final value of E f shall be 125% of the rated voltage. Figure below shows the connection diagram for performing the Open Circuit Test of Alternator. As clear from the figure a...
Read more

Transformer Cooling Classes

Image
Losses in the transformer are of the order of 1% of its full load kW rating. There are mainly two types of losses in a Transformer i.e. Core Loss and Ohmic Loss. Because of these losses the winding of transformer as well as Core gets heat up and we need to provide sufficient cooling to remove heat and maintain a constant winding and Core temperature. Various types of cooling methodologies are used for Transformer but before going to each methodology, we will have a look at the General Method of designating a particular cooling method i.e. class. In the nameplate of Transformer, four letter represents the cooling method of the Transformer. The above table is self explanatory, therefore if you see ONAN on the Name Plate of Transformer then it means that Oil with flash point less than 300° is used as cooling media and Natural convection is used for removing heat. In secondary side of cooling mechanism, Air with Natural convection is used by employing Radiator. ...
Read more