Step down transformer define,design,calculation

Posted by : admin Thursday 20 November 2014

Step down transformer: is one whose secondary voltage is less than its primary voltage. It is designed to reduce the voltage from the primary winding to the secondary winding. This kind of transformer “steps down” the voltage applied to it.
As a step-down unit, the transformer converts high-voltage, low-current power into low-voltage, high-current power. The larger-gauge wire used in the secondary winding is necessary due to the increase in current. The primary winding, which doesn’t have to conduct as much current, may be made of smaller-gauge wire.

Step-down transformers are commonly used to convert the 220 volt electricity found in most parts of the world to the 110 volts required by North American equipment.

How to Wire a Step Down Transformer

Observe and identify the schematic and rating of the step down transformer to be installed. Remove the terminal connection box cover placed at the lower side of the transformer. Only the high amperage types will have this enclosure, while lower powered transformers will have an exposed screw terminal.
Know termination identification follows for all step down transformers: H1, H2, H3 and H4 signify the high voltage side or power feed end of the transformer. This holds true regardless of the size of the transformer. Interconnection of the transformer will vary depending on the manufacturer and voltage used for feeding the transformer.
Terminate the feed power wires first by cutting the wires to length. If you are using large wire lugs be sure to take into consideration the length of the lug and the amount of wire that can be inserted into the female crimp area.
Strip back the outer insulating of the wires with the pocketknife or wire strippers. Insert the eye ring or wire lug over the bare copper wire and crimp the connection device, using the appropriate-size crimper, permanently to the wire.
Terminate the high side, high voltage of the step down transformer. If the high side terminals are bolts, be sure to follow any torque requirements that are listed by the manufacturer.
Terminate the low side, low voltage of the transformer. Note these terminals will be identified by X1, X2, X3 and X4. Again follow the manufacturer’s individual schematics for that particular type of transformer. Note that on small control transformers there will only be an X1 and X2. X1 is the power or “hot” side and X2 is generally the grounding and neutral portion of the low voltage.
Terminate the small control transformer for X1 and X2. X1 will go directly to the control circuit after passing through a small fuse that is rated for the circuit. X2 will be terminated not only to the neutral side of the control circuit, but the grounding safety as well. In other words, the X2 side of the small control transformer must be tied to the grounding system of the electrical circuit.
Replace all covers on the transformer and any enclosures that protect you from electricity. Apply the high voltage to the transformer by switching on the feeder power circuit. Turn on the low side safety circuit control.
Use a volt meter to test for proper voltage on the step down side of the transformer. It should be the same that is listed on the specs tag provided by the manufacturer.

How to Check a Step Down Transformer

Remove all wires from the transformer terminals using the screwdriver. Identify the wires if they are not already identified. Use a clear tape and pen. Write the terminal that the wires are attached to and place the identified tape on the wire’s end.
Turn the volt ohmmeter to the “Ohms” position and place the red lead into the connector identified as “Ohms.” Touch the black lead to the metal frame of the transformer.
Touch the red lead to the transformer’s terminals in the following order: H1, H2, X1 and then X2. The meter should read infinite ohms or wide open. Infinite ohms on a digital meter will be identified as a blank screen or a wide open will have the word “Open” displayed. If the meter registers any form of resistance, there is an internal problem with the windings. The copper coils may be shorted to the metal frame of the transformer. The transformer will have to be replaced.
Check the continuity of each separate coil using the ohmmeter. Touch the black lead to H1 and the red lead to H2. The meter should give a resistance reading. Generally, it should read in the range of 3 to 100 ohms, depending on the style and type of transformer. Perform the same test to the X1 and X2 terminals. You should receive the same results. If the meter reads infinite ohms or a wide open when checking between the terminals of the same coil, the wires are broken. Replace the transformer.
Use the ohmmeter to conduct the transformers isolation circuit. Touch the red lead to H1 and the black lead to X1. The meter should read infinite ohms or a wide-open circuit. Perform the same test, but to H2 and X2 respectively. If any resistance at all is read on the meter other than a wide-open circuit, the isolation of the transformer has been compromised and must be replaced.

The problem with surge protectors

Usually, having 220 volts between neutral and ground in an appliance designed for 110 volts is not a problem the insulation has a large safety margin. However, if you connect a surge protector (or a piece of equipment with built-in surge protection) on the 110 volt side, bad things can happen.
Surge protectors contain varistors components which protect against surges by effectively shorting out any excess voltage. Some surge protectors contain only a single varistor connected between hot and neutral; those will work fine with a step-down transformer. However, many surge protectors have additional varistors connected between hot and ground and between neutral and ground. When a surge protector of this kind is used with a step-down tranformer, one of these varistors can be subjected to the full 220 volts. This is enough to trigger the varistor into its conducting mode, effectively treating the 220 volts as a surge.
Varistors are designed to absorbed short-lived surges, but they can't handle a persistent overvoltage. A varistor subjected to twice its rated voltage will quickly be destroyed, usually causing a short circuit and a blown fuse.

step down transformer working

The function of any transformer is to change one AC voltage value to another AC voltage value. A step down transformer will transform a higher AC voltage to a lower AC voltage. A step up transformer will transform a lower AC voltage to a higher AC voltage. The transmission of electrical power uses both of these types of transformers. From the generation station the voltage is stepped up to a very high transmission voltage and at the end of the transmission line it is stepped down to a voltage that consumers can utilize.

step down transformer calculation

EMF Equation of transformer can be established in a very easy way. Actually in electrical power transformer one alternating electrical source is applied to the primary winding and due to this, magnetizing current flowing through the primary winding which produces alternating flux in the core of transformer. This flux links with both primary and secondary windings. As this flux is alternating in nature, there must be a rate of change of flux. According to Faraday’s law of electromagnetic induction if any coil or conductor links with any changing flux, there must be an induced emf in it. As the current source to primary is sinusoidal, the flux induced by it will be also sinusoidal. Hence, the function of flux may be considered as a sine function. Mathematically, derivative of that function will give a function for rate of change of flux linkage with respect to time. This later function will be a cosine function since d(sinθ)/dt = cosθ. So, if we derive the expression for rms value of this cosine wave and multiply it with number of turns of the winding, we will easily get the expression for rms value of induced emf of that winding. In this way, we can easily derive the emf equation of transformer

Instructions

1.Obtain the windings ratio of the transformer. This number is often printed on the transformer case, and will take the form of "primary:secondary." For example, a "2:1" transformer will have twice as many windings in the primary as it has in the secondary.

2.Divide the secondary number by the primary number. For a 2:1 transformer, this number is 1/2, or 0.5.

3.Multiply the input voltage you intend to apply to the transformer by the number you calculated in the previous step. For example, if you apply 12 volts to a 2:1 transformer's primary, you will obtain 6 volts across the transformer's secondary.