| Author: IMS |
| The Difference between Transformers and Variable Transformers |
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What is a transformer?
A transformer is a device that transfers energy from one circuit to another
through electromagnetic induction. This term can be explained as an
electrical current in one circuit causes a time-varying flux (a changing
magnetic field) in the air. This flux generates a current in the second
circuit. Knowing this definition makes the transformer symbol used on any
drawing quite clear: |
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| V1 / V2
= N1 / N2 |
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| If you apply a voltage, V1, to the circuit
on the left, then a current I1, induces flux (the magnetic field
that is changing with respect to time). The flux induces the voltage, V2,
in the right circuit and hence the current I2. The voltage in each
circuit is proportional to the number of windings in each circuit. V1 / V2
= N1 / N2 This is Faraday’s Law and is the
fundamental theory of how a transformer works. |
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 So,
what is a variable transformer?
It is a transformer where the induced voltage (V2 in the
illustration above) can be adjusted. In the case of our models, this adjustment
is made by turning the dial on the top of the unit. The pointer on the dial
tells you the output voltage. (Model #3PN1010B shown at right). |
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Can the dial be adjusted to any voltage?
The L501B (IMS #104247), 120VAC input, can be set to 140 VAC.
The 3PN1010B (IMS #109303), 120 VAC input, can be set up to 140 VAC.)
The 3PN1020B (IMS #104295), 120/240 VAC or input, can be set up to 280 VAC.)
The 3PN1520B (IMS #117522), 120/240 VAC or input, can be set up to 280 VAC.
The 1520CT (IMS #104210), 120/240 VAC or input, can be set up to 280 VAC.
The 5021CT (IMS #104289), 120/240 VAC or input, can be set up to 280 VAC. |
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| Getting these voltages is dependent upon how the input
(supply) is wired. There are three types of connections: |
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| Line Connection: The output voltage can be no higher
than the input (supply voltage). |
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| Boost Connection: The maximum output voltage is 17%
higher than the input voltage. |
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| Step-Up Connection: An output voltage of up to 280
VAC can be obtained from a 120 VAC input (supply) voltage. |
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If the output voltage is higher than the input
(supply) voltage, then where does the extra voltage come from?
Physically, the extra voltage can be obtained from the extra turns in the coil
which are illustrated in the previous wiring sketches as being above the
highest input wire. In the actual transformers, the coil is not straight as
shown in the picture, but is round. The turns of wire are wrapped around the
core to form a toroid, a donut-shaped coil. |
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| Any output voltage that is higher than the input
voltage does not appear from thin air and is not “generated” by the
transformer. The amount of energy that comes out of the transformer is equal to
(for an ideal device) or less than (for a real device) the amount of energy
which is put into the transformer.(The Law of Conservation of Energy). |
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| This real device gives up some of the input power in
the form of heat and other losses. Power which is left over and available for
output is constant such that increasing the output voltage beyond the input
voltage, overvoltage mode, will lower the current amp rating. Here’s why: |
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| For single phase power: Power = Volts X Amps |
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| Holding this Power constant at the maximum level
(input power minus losses) requires the current (Amps) to decrease as the
Voltage increases. THIS IS ONLY TRUE WHEN THE TRANSFORMER IS OPERATED IN
OVERVOLTAGE MODE AND THE LOAD, THE DEVICE WIRED TO THE OUTPUT, IS DRAWING THE
MAXIMUM AMOUNT OF POWER AVAILABLE (EQUIVALENT TO THE INPUT POWER MINUS LOSSES
SUCH AS HEAT). |
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| Here’s an example: |
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| Suppose the input (supply) voltage to the transformer
is 120 VAC and is coming from a 20 Amp circuit breaker. The maximum power
available through the transformer is 120 x 20 = 2400 Watts. If we lose 400
Watts to heat and other losses, then there is, at most, 2000 Watts available
for the load. If a heater band is wired to the transformer and requires all
2000 Watts, then at 120 Volts, the current from the transformer is 16.7 Amps
(2000 divided by 120). At 140 Volts (overvoltage mode using a Boost
Connection), the current would be 14.3 Amps (2000 divided by 140). Because the
current that could be handled by the transformer dropped from 16.7 to 14.3
Amps, the maximum current rating is decreased. |
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What if I need a higher voltage?
For voltages other than 120 VAC and 240 VAC single phase, multiple decks
(toroids) are connected together by the manufacturer and are in the same
enclosure. The variable transformer looks no different, except for enclosure
size, than the single deck transformers. |
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| Products related to this article: |
Voltage Controllers
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