Transformers

• A transformer is a device that transfers electrical energy from one circuit to another by electromagnet induction
  
  
• Electrical energy is transferred with;
• no change in frequency
• no change in voltage, or
• an increase in voltage, or
• a decrease in voltage

• A transformer will only work an input source voltage that varies in amplitude
  
  
• Basic components of a transformer
• primary winding
• secondary winding
• core

Electromagnetic Induction

• The primary winding connected to an AC source
  
  
• As power is applied to the primary a magnetic field builds and collapses around the primary winding, in sync with the AC sine wave
  
  
• The expanding and collapsing magnetic field around the primary winding cuts the secondary winding inducing as AC voltage into the secondary winding which opposite in polarity
  
  
• The Voltage causes AC to flow to the load
  
  

Core Material Characteristics

Used when source has a low frequency (below 20 kHz)

Used when heat dissipation is important for efficient power transfer

• The most efficient transformer core is one that offers the best path for most lines of flux, with the least of power
  
  

Winding Characteristics

• The wire is coated with an insulating varnish so that each turn is insulated from the other turns, sheets of paper may also be used as insulating material between the layers of windings
  
  
• The primary winding is wound first then an insulating layer of paper and the secondary winding is wound on top
  
  

Transformer Formula for Voltage and Turns

Es = Ns
Ep    Np

Es = voltage induced in the secondary

Ep = voltage applied in the primary

Ns = number of turns in the secondary

Np = number of turns in the primary

Relationship of Turns, Voltage, and Current in Transformers

• The turns ratio is deadly proportional to the number of turns
  
  
• The Voltage ratio is directly proportional to the turns ratio
  
  
• The current ratio is inversely proportional to the turns ratio

Power loss in Transformers

• Transformers are not 1005 efficient.  There are three main reasons for transformer power loss
  
  
• Resistance in the winding – looses due to heat I2 R1
  
  
• Hysteresis – losses (heat) due to friction between molecular caused by changing magnetic fields
  
  
• Eddy Currents – losses due to current flow in the core material.  Solid cores have high eddy current losses.  Losses due to eddy currents are reduced by using a laminated core
  
  

Transformer Phase Relationship

• Electromagnetic induction causes a phase shift
  
  
• Transformer secondary winding may be wound to be either in phase or out of phase
  
  
• Some equipment requires that transformers be in phase
  
  

Transformer in phase
            Subtractive polarity

Transformer out of phase
            Additive polarity

Control Transformer Ratings

1. Voltage Primary & Secondary
   
   
2. Volt ampere VA

Example:

A tramsformer rated at 220V primary, 24V secondary, and a VA rating of 48

This means that Maximum of 2 amps can be drawn from the transformer secondary.      

2 amps x 24V = 48VA

VA = Volts of the secondary x the current of the secondary

• Primary
• Secondary
• VA rating
• Phase (in & out)