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Transformers are electromagnetic devices that transfer electrical energy from one source to another by mutual induction. In the example shown below, an AC generator provides the power to the 'primary' side of the transformer. The magnetic field created by the primary induces voltage into the secondary coil...which provides power to a load. (In this case a motor)

So you ask, why is this useful? The above transformer is producing the same voltage, current and power as the generator supplied it. How is that helpful? For instance, the type of transformer you are viewing above is an isolation transformer. The main advantage to this type of transformer is the reduction of voltage spikes. Some equipment can become damaged with fluctuations in power. This is an economical way of reducing these spikes that are created on the supply side.

Now that we have a basic understanding of a transformer, we can move on to the different types of common ones you'll see:

-primary and secondary are one continuous winding
-not isolated from each other
-they reduce inrush that is caused by starting large motors
-buck-boost transformers (small changes in voltage)
-variable output (variac- variations in voltages)

Instrument Transformer
-used for small instruments that have small voltage (so it steps down voltage)
-isolate metering equipment from high voltage
-PTs (Potential Transformer)
- CTs (Current Transformer)

Distribution Transformer
-poletop or padmount transformers (near all buildings with electricity)
-step down transformers (to provide electricity to consumers)
-could be indoor dry, or outdoor oil filled
-usually rated from 1.5 to 500 kVA

Power Transformer
-very large transformers
-step up transformers at generating stations
-step down transformers at sub station for local distribution
-rated from 500kVA to over 100,000kVA

Signal Transformer
-small transformers
-step down from 120V to 8-24V
-used to supply control voltage to fire alarms, door bells, HVACR

Control Transformer
-usually under 1kVA
-step down transformer (480V to 120V for a motor control)
-used to supply signal or control circuits

Ratio for Transformers

Ratio is something that will really help you when figuring out calculations for transformers. Notice in the diagrams below that the ratio matches every portion of the transformers? The voltage, the current and the turns are all matching the ratio.

So, for an example...if ratio is known and you know the primary has 1800 turns, then you can easily deduce that the secondary has 900 turns because it is half the value. This can be used for any other value as well. If all else fails, you can work through ratio and ohm's law. It always works.(Be careful that the higher the voltage-the lower the current, the lower the voltage - the higher the current)

Formulas for Transformers

The following formulas are the most common for transformers. Everybody has a different way of figuring out things, and for some the easiest way is ratio. For other people, it's learning six other formulas. So here they are:

To find voltage...

To find current...

To find the number of coil turns...

ES = secondary voltage
EP = primary voltage
IS = secondary current
IP = primary current
NS = turns in the secondary coil
NP = turns in the primary coil

Isn't that useful? Now let's actually use it in a situation...

If a transformer has 300 turns on the low winding, and a voltage ratio of 208V-240V. How many turns are required on the high voltage winding?

What is known:

Using your

= 240Volts x 300 Turns on the Primary
208 Volts

= 346 turns on the high voltage winding.

Or do you understand Ratio better?

voltage ratio is 208V-240V, (so divide 240 by 208, which equals 1.15)
So now you know the ratio is 1: 1.15
Take the 300 (known value) and multiply by 1.15
It equals 345

(which is the turns on the high voltage winding) Another decimal point and it would have matched the above calculation.

Either are right.

Markings on Transformers

The markings on a transformer are as follows:
High Voltage = H1, H2, H3
Low Voltage = X1, X2, X3
or you can remember visually from the following graphic

Terminal markings are critical for doing such things as:
1) connecting multi-coil transformers
2) connecting three phase transformer banks
3) connecting metering and protective relaying

Additive and Subtractive Polarity

If X1 is adjacent to the H1 terminal = subtractive
If X1 is diagonal to the H1 terminal = additive

This is a designation of the relative instantaneous polarity of the transformer leads.
Generally 200kVA and smaller (HV 8660V or less) = Additive Polarity
200kVA and above (HV 8660V and above) = Subtractive Polarity

Temperature Rise

This indicates the insulation temperature rise of the transformer windings. The maximum temperature rise above 40 degrees with which the insulation may be subjected to without shortening it`s life.
Example...A transformer has a temperature rise of 115 degrees Celsius. Take the 115 degrees then add the 40 degrees, which equals 155 degrees it can be subjected to.

Cooling Methods

- usually air and oil are used for cooling
- additional cooling can increase the kVA capacity
-since heat is the result of losses, this has a negative effect on the life of the insulation


The total kVA rating refers to the total kVA that the whole transformer can transform. One winding handles the whole kVA in and the other handles the total kVA out. kVA is divided equally between the windings. For instance, if there were two windings for the primary and the total was 100kVA...Each winding would handle 50kVA.
That is why the kVA is divided by the lower voltage to get the full load current. Like a primary having two windings of 120V/240. kVA is divided by 120V to get the full load current.

Transformer Impedance

The percent impedance (%Z). This is determined by the physical construction of the transformer such as core construction, wire size, number of turns in the windings, and the magnetic coupling in the wiring. It is very important to know the transformer impedance to determine:
-Fault Current Calculations (See Below)
-To determine voltage drop from no load to full load
-To connect transformers in parallel. Different impedance percentages cannot be paralleled.

Fault Current

- of the secondary

Three Phase Transformers

Three phase transformers are used to transform three phase power. They may be made up of a single transformer, or a bank of single phase transformers.

Single Transformer: This three phase transformer has a slightly higher efficiency than a bank of transformers. It is obviously lighter and smaller than a bank of transformers. It also is less complex, and costs less than a bank.

Bank of Transformers: This bank of three single phase transformers can be operated at a lower kVA capacity if one needs to be removed for repair. Its also less expensive to carry parts or repair.

Certain conditions MUST be met in order to bank transformers
1) voltage ratings must be the same
2) impedance must be the same
3) frequency ratings must be the same
4) tap settings must be the same

Lets look at the six choices we have for three phase transformer connections:
1) Delta - Delta
2) Wye - Delta
3) Delta - Wye
4) Wye - Wye
5) Open Delta (Emergency Use)
6) T- Connected (Better phase balance than open delta)

Here are some three phase calculations that show the primary transformer, the secondary transformer, and the load. The first example is a wye-delta transformer, and the second example is a delta-delta transformer.

Transformers in Parallel

1) KVA is the sum of the individual kVA ratings
2) Fault current is the sum of the available fault currents
3) The primary current rating is the kVA rating of the bank/primary voltage rating
4) The secondary current rating is the kVA rating of the bank/secondary voltage rating

Here is a complete calculation of a 167.5 kVA and a 37.5 kVA transformer paralleled together to supply a 120V bus.

Transformer Tap Changers and Multi-Tap Transformers

1) Used to adjust the output voltage of a transformer by changing the number of turns in the primary ( Change the turns ratio)
2) Used to compensate for changes in the secondary voltage caused by changes in the load (transformer % regulation)
3) For any fixed primary voltage
-any increase in the number of turns in the primary, reduces voltage per turn lowering secondary.

-any decrease in the number of turns in the primary, reduces voltage per turn increasing secondary.

Sizing Conductors for Transformers

1) Do calculations and come up with the total current for the conductor
2) CEC 26-258 (not less than 125%)
3) Take ampacity and multiply by 1.25
4) Go to appropriate Table 1, 2, 3, 4
5) Check for size conductor needed for ampacity calculated

Breaker or Fuse Sizing for Transformers

1) CEC 26-252 or 26-254 or 26-256
2) Otherwise Table 50 shows values that are not more than rated.