Welding transformers are a vital piece of hardware used to reduce the voltage from the electrical source. The device changes the alternating current (AC) from a power line to a high amperage and low-voltage current suitable for welding.
On a welding transformer, the primary and secondary taps are used for macro adjustment of the welding current and voltage. A deep understanding of the operating principle of winding transformers is important to know the function of taps.
So, let’s dig deeper to know what are taps on a winding transformer.
What is the Operating Principle of a Welding Transformer?
Welding transformers are used to adjust the voltage of the power source to a voltage required to form a welding arc. The time it takes for the voltage to rise from zero to the desired voltage required for welding an arc is known as the arc recovery time.
The arc recovery time should be kept to low to ensure that the arc is steady. This is important otherwise the cathode may become cold that will prevent the generation of sufficient ions and electrons to create and maintain an arc.
One method to reduce the time is to increase the circuit voltage of the power source. The arc recovery time is significantly lower for lower voltage peak value. The welding circuit should have an inductance that results in a phase difference between current transients and voltage ranging between 0 to 35 and 0 to 45.
Taps help in adjusting the voltage to generate the desired arc. The taps help increase the current when the voltage is low. Low voltage prevents the creation of the desired arc due to loss of heat by the cathode.
An arc can easily be formed when the current is up to 250 amperes. This requires a voltage of about 60 volts. In case the current is lower than 70 amperes, the voltage can be increased to 80 volts. But an increase in the voltage poses a safety risk and also impairs the arc to open-circuit voltage ration of the welding transformer. Taps prove invaluable in this situation as they help in keeping the voltage within the constraints thereby preventing any damage.
Construction of Taps on Winding Transformers
Taps are present in the secondary windings and connected to a high-current switch or plug receptacles. They are used to reduce the voltage commonly between 15 and 45 volts. One side of the secondary winding is connected to an electrode while the other end is joined to the welded pieces.
Taps on the winding transformers help reduce the voltage thereby preventing heat-up problems. Points on the secondary coil can be attached to modify the welding current. Taps are connected in some welding transformers to the secondary coil windings to provide the correct voltage. These taps provide a full output at the terminals.
Most large welding transformers have multiple phase inputs while smaller ones have single-phase inputs. A lot of heat is generated in case of a high current due to resistance between the welded pieces and electrode in the secondary windings.
The impedance is generally higher in welding transformers as compared to the general transformers. The high impedance results in the establishment of an arc. The current remains sinusoidal and the voltage is distorted in the welding current.
Location of Taps
Taps can be located at a different position on the secondary coil. The main method of providing taps is to join on the last layer of the coil. The layer of the coil is situated away from the finish line. In most cases, there are breaks between the taps number four and five on the tap section. Besides, a jumper is provided for a coil that helps in selecting the desired voltage at the specific nameplate.
Another tap configuration is to join the loop at the end of the secondary coil with no break in between the taps. The last tap that is usually tap number seven is located near the finish line. In this configuration, the wye or delta connection on the input side is closed at the tap depending on the nameplate. This configuration of taps is called as ‘the line taps or end of the coil taps.
The tap jumpers join two tap terminals of a coil in the welding transformer. In another configuration, the tap jumpers connect only one terminal to the relevant phase terminal. The location of the tap on the phase terminal corresponds to the desired voltage.
Welding Transformer Taps Design
Welding transformer designs vary with different tap configurations. But the lowest tap always connects to the maximum turn cycle in the transformer. This results in the generation of the lowest voltage for the transfer. The low voltage results in lower current and power generated for the welding device. Other tap configurations produce higher voltage and current for the resistance welding.
Terminals of the taps can be either welded brazed or loop type. The coil conductor for the loop traps is generally stripped of the insulation to form a loop suitable for the hardware size. Terminal taps are brazed to the coil at the appropriate location.
How Taps Help in Adjusting the Voltage?
Taps are used in the welding transformer to adjust the voltage and the current. They are connected to the secondary coil in different configurations. These taps allow the welder to adjust the normal voltage to one required by the device. The lead of the taps is connected to a terminal or multiple terminals.
Taps are invaluable when the lines voltages are lower or higher than the voltage required for creating the arc. They provide higher or lower secondary voltage source depending on the line voltage. The voltage ratio of the transformer changes when the taps are used.
On large power transformers, taps help offset variations in the voltages. The connections of the taps are normally set at the default for the line voltage. The welder can change the taps to get the desired voltage.
For instance, a single winding transformer rated at 480V-120V with 456 V input line voltage will have a secondary voltage rated at 114 V. The secondary voltage figure is arrived at by dividing the higher voltage figure with the lower one. Here the secondary voltage ratio is 4 that is arrived at by dividing 480 by 420. As a result, the secondary voltage for a 456 V input transfer is 114 V or 456 divided by 4. The same transfer having a 400 V will have the secondary voltage of 100 V.
Large transformers allow only a few turns. In this situation, the taps cannot be placed at the exact precise voltage. A large transformer generally has 5 V per turn due to which a 2 ½ tap on a winding with 480 V voltage allows 2.4 turns. As taps are only possible for whole turns, generally the tap is located at 2 turns for 10 V.
What are Tap Switches and Turns?
Most weld transformers have a tap switch. The switch allows a welder to change the turns ratio in the transformer. The welder can increase or decrease the output voltage by turning the tap.
As the voltage output changes, the ability to increase the current availability to different parts also changes. Higher voltage results in higher current delivered to different parts.
A whole number of turns is important between taps. This is essential otherwise the tap won’t be turned to set the desired voltage. For instance, tapping of the winding is not possible at 7 ¼ and 12 ¾ turns. The turns should be 7 or 12 for proper operation.
Let’s suppose that we have a 480 V transformer with 960 turns. The tap will be 24 turns in case of 2 turns for volt. In contrast, at 2 ½ percent, a low tap will result in the reduction of the voltage by about 12 V or 0.025 times the normal 480 V tap.
The number of turns per volt can be changed. Most tap voltages are marked to ensure precise voltage adjustment. The tap voltages are normally marked on the nameplates of the transformer.
How to Determine the Best Setting for the Tap?
AC welding generally generates a lot of heat. That is why the off time should be minimized during each cycle for optimum results. The weld schedule should be optimized for a particular application. In the case of a very high or low tap setting, the current flow will be insufficient due to which the required heat is not produced.
For optimum tap setting, the general rule of thumb is to use the lowest transformer tap at the highest percent current for a minimum duration. This will result in the generation of the desired current required for the constant and quality weld. The tap setting will result in the best thermal and mechanical weld.
The optimum tap settings will result in the heat that is ideal for welding. Too large or small weld current can damage the welding device. The low current time can result in insufficient heat while a high current can result in too much heat. Following the general rule of thumb will result in the desired current and heat for creating an optimum weld.