basic transformers

Misc. Topics on power transformers

by Lewis Loflin
  

  

A general overview of transformers and their properties.


autotransformer
Autotransformer

An autotransformer (sometimes called autoformer)is an electrical transformer with only one winding. The "auto" prefix refers to the single coil rather than any automatic mechanism. In an autotransformer a portion of the same winding acts as part of both the primary and secondary winding.

An autotransformer has only a single winding with two end terminals, plus a third at an intermediate tap point. The primary voltage is applied across two of the terminals, and the secondary voltage taken from one of these and the third terminal. The primary and secondary circuits therefore have a number of windings turns in common.

Since the volts-per-turn is the same in both windings, each develops a voltage in proportion to its number of turns. An adjustable autotransformer is made by exposing part of the winding coils and making the secondary connection through a sliding brush, giving a variable turns ratio. Such a device is often referred to as a variac. Ref. Wiki



Magnetic reluctance

Magnetic reluctance, or magnetic resistance, is a concept used in the analysis of magnetic circuits. It is analogous to resistance in an electrical circuit, but rather than dissipating magnetic energy it stores magnetic energy. In likeness to the way an electric field causes an electric current to follow the path of least resistance, a magnetic field causes magnetic flux to follow the path of least magnetic reluctance. It is a scalar, extensive quantity, akin to electrical resistance.

Magnetic flux always forms a closed loop, but the path of the loop depends on the reluctance of the surrounding materials. It is concentrated around the path of least reluctance. Air and vacuum have high reluctance, while easily magnetized materials such as soft iron have low reluctance. The concentration of flux in low-reluctance materials forms strong temporary magnetic poles and causes mechanical forces that tend to move the materials towards regions of higher flux so it is always an attractive force(pull).

The inverse of reluctance is called permeance, measure of the ability of a magnetic circuit to conduct magnetic flux.



Electromagnetic Induction

electromagnetic induction

Electromagnetic induction is the complementary phenomenon to electromagnetism. Instead of producing a magnetic field from electricity, we produce electricity from a magnetic field. There is one important difference, though: whereas electromagnetism produces a steady magnetic field from a steady electric current, electromagnetic induction requires motion between the magnet and the coil to produce a voltage.

Connect the multimeter to the coil, and set it to the most sensitive DC voltage range available. Move the magnet slowly to and from one end of the coil, noting the polarity and magnitude of the induced voltage. Experiment with moving the magnet, and discover for yourself what factor(s) determine the amount of voltage induced. Try the other end of the coil and compare results. (Polarity should reverse.) Try the other end of the permanent magnet and compare.

If using an analog multimeter, be sure to use long jumper wires and locate the meter far away from the coil, as the magnetic field from the permanent magnet may affect the meter's operation and produce false readings. Digital meters are unaffected by magnetic fields.

Ref. http://www.allaboutcircuits.com/vol_6/chpt_2/10.html





Wire Gauge versus Amperage

AWG Dia Inch Cir Mil Dia cm Area Inch2 lb/kft ohms
/kft
Ohms
/km
CU Max
free-air
Amps
CU Max
enclosed
Amps
32 0.008 63.2 0.020 4.964E-05 0.19 164.1 538.4 .53 0.32
30 0.010 100.5 0.025 7.894E-05 0.30 103.2 338.6 .86 0.52
28 0.013 159.8 0.032 1.255E-04 0.48 64.9 212.9 1.4 0.83
26 0.016 254.1 0.040 1.996E-04 0.77 40.81 133.9 2.2 1.3
24 0.020 404.0 0.051 3.173E-04 1.22 25.67 84.22 3.5 2.1
22 0.025 642.4 0.064 5.046E-04 1.94 16.14 52.95 7.0 5.0
20 0.032 1,021.5 0.081 8.023E-04 3.09 10.15 33.30 11.0 7.5
18 0.040 1,624.3 0.102 1.276E-03 4.92 6.385 20.95 16 10
16 0.051 2,582.7 0.129 2.028E-03 7.82 4.016 13.18 22 13
14 0.064 4,106.7 0.163 3.225E-03 12.43 2.525 8.284 32 17
12 0.081 6,529.9 0.205 5.129E-03 19.77 1.588 5.210 41 23
10 0.102 10,383.0 0.259 8.155E-03 31.43 1.215 3.985 55 33
8 0.128 16,509.7 0.326 1.297E-02 49.98 0.998 3.274 73 46
6 0.162 26,251.4 0.412 2.062E-02 79.46 0.395 1.296 101 60
4 0.204 41,741.3 0.519 3.278E-02 126.35 0.248 0.8136 135 80
2 0.258 66,371.3 0.654 5.213E-02 200.91 0.156 0.5118 181 100
1 0.289 83,692.7 0.735 6.573E-02 253.34 0.123 0.4035 211 125
0 0.325 105,534.5 0.825 8.289E-02 319.46 0.0983 0.3225 245 150
00 0.365 133,076.5 0.927 1.045E-01 402.83 0.0779 0.2556 283 175
000 0.410 167,806.4 1.040 1.318E-01 507.96 0.0618 0.2028 328 200
0000 0.460 211,600.0 1.168 1.662E-01 640.53 0.04901 0.1608 380 225

Ref. http://wiki.xtronics.com/index.php/Wire-Gauge_Ampacity



Temperature versus Resistance

You might have noticed on the table for specific resistances that all figures were specified at a temperature of 20 degrees Celsius. If you suspected that this meant specific resistance of a material may change with temperature, you were right!

The "alpha" (a) constant is known as the temperature coefficient of resistance, and symbolizes the resistance change factor per degree of temperature change. Just as all materials have a certain specific resistance (at 20 degrees C), they also change resistance according to temperature by certain amounts.

For pure metals, this coefficient is a positive number, meaning that resistance increases with increasing temperature. For the elements carbon, silicon, and germanium, this coefficient is a negative number, meaning that resistance decreases with increasing temperature.

For some metal alloys, the temperature coefficient of resistance is very close to zero, meaning that the resistance hardly changes at all with variations in temperature (a good property if you want to build a precision resistor out of metal wire!). The following table gives the temperature coefficients of resistance for several common metals, both pure and alloy:



 
Material     Element/Alloy    "alpha" per degree Celsius
==========================================================       
Nickel -------- Element --------------- 0.005866         
Iron ---------- Element --------------- 0.005671          
Molybdenum ---- Element --------------- 0.004579         
Tungsten ------ Element --------------- 0.004403           
Aluminum ------ Element --------------- 0.004308          
Copper -------- Element --------------- 0.004041           
Silver -------- Element --------------- 0.003819          
Platinum ------ Element --------------- 0.003729           
Gold ---------- Element --------------- 0.003715          
Zinc ---------- Element --------------- 0.003847         
Steel* --------- Alloy ---------------- 0.003             
Nichrome ------- Alloy ---------------- 0.00017           
Nichrome V ----- Alloy ---------------- 0.00013            
Manganin ------- Alloy ------------ +/- 0.000015          
Constantan ----- Alloy --------------- -0.000074           

REVIEW:

Most conductive materials change specific resistance with changes in temperature. This is why figures of specific resistance are always specified at a standard temperature (usually 20 degrees or 25 degrees Celsius).

The resistance-change factor per degree Celsius of temperature change is called the temperature coefficient of resistance. This factor is represented by the Greek lower-case letter "alpha" (a).

A positive coefficient for a material means that its resistance increases with an increase in temperature. Pure metals typically have positive temperature coefficients of resistance. Coefficients approaching zero can be obtained by alloying certain metals.

A negative coefficient for a material means that its resistance decreases with an increase in temperature. Semiconductor materials (carbon, silicon, germanium) typically have negative temperature coefficients of resistance.

Ref. http://www.allaboutcircuits.com/vol_1/chpt_12/6.html



Question - Would Temperature Effect the Amount of Current Traveling Through A Copper Wire?

Yes, very much. There is even a name for a condition that when the copper wire (or any material) is so cold that there is no resistance to electrical flow. That condition is called superconductivity. The temperature that copper doesn't have any resistance to flow is around -410 F (very, very cold).

The general rule is "the higher in temperature the wire is, the higher the resistance to electrical flow. It would be nice to have no resistance to electrical flow, but we can't always have copper at very cold temperatures.

That's why researchers are working on superconductors that have no resistance to current flow at higher temperatures. Currently most superconductors are ceramics that act as superconducting at around -320 F.

That's still cold, but that is also the temperature of liquid nitrogen, so if you keep the superconductor in a bath of liquid nitrogen, it will stay superconducting.




Skin effect

Skin effect is the tendency of an alternating electric current (AC) to distribute itself within a conductor so that the current density near the surface of the conductor is greater than that at its core. That is, the electric current tends to flow at the "skin" of the conductor, at an average depth called the skin depth.

The skin effect causes the effective resistance of the conductor to increase with the frequency of the current because much of the conductor carries little current. Skin effect is due to eddy currents set up by the AC current. At 60 Hz in copper, skin depth is about 8.5 mm. At high frequencies skin depth is much smaller.

Methods to minimize skin effect include using specially woven wire and using hollow pipe-shaped conductors. Another method is the use of wide thin conductors (ribbon) instead of round wire at high currents.





Radio Shack Transformers

 
12.6VCT @ 3A Transformer  (273-1511A)  Specifications  Faxback Doc. # 16365
 
ELECTRICAL DATA
 
Rating
   Primary:.......................................................120V/60Hz
   Secondary:.....................................12.5VAC @ 3 Amp 14.05 VAC
Tolerance:...........................................................+/- 5%
Primary Current
   No Load (Io):.................................................140 mA MAX
   Loading (IL):..................................................600mA MAX
DC Resistance
   Primary:.............................................14.702 Ohms +/- 10%
   Secondary:............................................0.286 Ohms +/- 10%
Core Loss:....................................................4.0 Watts Max
Temperature Rising:........................75C MAX at Primary Coil measured
                                                 with hot resistance method
Insulation Resistance:..............................500 VDC 100 M Ohms min.
Hi-pot Test:......................1080 VAC for 1 minute between primary and
                                      secondary and core without breakdown.
 
MATERIAL
 
Bobbin:............................................................Nylon 66
Lead Wire
   AWG #:.............................................................22/18
   Insulation:.................................................UL-1015 VW-1
Core:.............................................Hilite silicon steel core
   Size:..........................................................EI-60/0.5
Impregnation:.......................................................Varnish
 
 
(ALL-08/07/95)

 
500 Watt Step Down Transformer
(273-1416)                 Specifications             Faxback Doc. # 16640
 
Model No.: ....................................................... TC-500D
 
Voltage Conversion: ........................... 220/240 VAC to 110/120 VAC
 
Dimensions: ............................................. 5 x 3 x 4 Inches
 
Weight: ........................................................... 8 lbs.
 
Fuse: .............................................................. 5 Amp
 
Specifications are typical; individual units might vary.  Specifications
are subject to change and improvement without notice.
 
(/be 7/13/98)

 
24VAC CT UL Transformer  (273-1366A)   Specifications  Faxback Doc. # 9881
 
ELECTRICAL DATA
 
Rating:
   Winding:
      Primary:................................................120VAC 60 Hz
      Secondary:
         Loading:.......................................25.2VAC CT @ 0.45A
         No Load:...............................................28.5VAC CT
 
Tolerance:..........................................................+/- 5%
 
Primary Current:
   No Load (Io):.....................55 mA TRMS MAX at input 120 VAC 60 Hz
   Loading (IL):...........................................150 mA TRMS MAX
DC Resistance:
   Primary:.......................65 Ohms +/- 15% measured at 25 degrees C
   Secondary:.............................................3.0 Ohms +/- 10%
Core Loss:..............................2 Watts MAX. AT INPUT 120VAC 60 Hz
Temperature Rising:...............50 degrees C MAX AT 25 degrees C ambient
Insulation Resistance:..............:...................500 VDC 100 M min.
Hi-pot Test:.....................1240 VAC for 1 minute between primary and
                                               secondary, primary and core
                           600 VAC For 1 minute between secondary and core
 
MATERIAL
 
Bobbin:..............................................................Nylon
Lead Wire:.........................................................AWG #22
   Insulation:............................................Ul1015 1007 FR-1
Core:......................................................Size:  EI-48/.5
                                                 Hilite silicon steel core
Impregnation:......................................................Varnish
 
(ALL-01/25/95)

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