**From the ICMEESA archive**

**Loads from resistance welding machines often cause trouble, especially where large machines not fitted with power factor correction are involved.**

The load taken by a resistance welding machine is invariably intermittent and it draws a large single phase current of low power factor (0,3 to 0,5 lagging). With modern “tube” control, the load is often on only for a few cycles during which the kVA demand is high. There follows a much longer rest period.

The half-hour kVA demand tariff as charged by supply authorities is low for this type of load. However, the brief peak kVA demand determines the size of cables and transformers to avoid undue drop in voltage. For this reason, the load from these machines is not welcomed by supply authorities or by the user.

**Effects of this type of load**

The load from a welding machine causes dips in the voltage to occur in the works. These dips sometimes affect other users, producing some or all the following side effects:

- Lamp flicker.
- Interference with other electrical machinery or apparatus, or with other welding machines.
- Adjustment of the welding machine is difficult due to interference by other machines, so causing reduction in the uniform quality of welds.
- It is not possible to do heavy welds.
- A special individual feeder to each welding machine is often necessary.
- Restrictions are placed on the use of welding machines and their size by supply authorities or by the management of the works.

**Benefits of power factor correction of welders**

Some or all of these effects can be overcome, or the bad effects thereof reduced by correctly applied series connected power factor correction capacitors tailored to the welding machine. The kVA demand of the works can be reduced, which saves on the power bill.

**How power factor correction is achieved**

The application of a series connected capacitor to a new or existing welding machine is not straightforward as is the case with shunt connected capacitors.

Fig. 1 illustrates a typical simplified circuit of a welding machine. If a shunt capacitor is connected permanently as shown dotted at A, it would be energised all the time the isolator is closed and a heavy leading current would flow when the machine was not actually welding. This is undesirable.

If a shunt capacitor is connected permanently as shown dotted at B, then it would only be energised when the welding operation was taking place. However, after every welding cycle, the capacitor would discharge via the primary winding and, when re-energised, the charging current of the capacitor would flow and this could last for the whole, or the greater part of, the weld time, which is often only a few cycles. This upsets the adjustment of the welding machine and affects the quality of the weld.

It is not possible to switch a bank of capacitors in and out of circuit fast enough by a contactor or similar device due to the brief time of welding. The only possible way of correcting the load is using series capacitors. Fig. 2 shows a typical circuit for a new welding machine where the welding transformer windings can be arranged correctly during manufacture.

Fig. 3 shows how the circuit of an existing welding machine can be rearranged to apply a series capacitor.

Fig. 4 is the simplified vector diagram for the machine with the series connected capacitor of Fig. 2.

*VW*: Voltage across welding machine.

*VS*: Voltage across supply.

*VC:* Voltage across capacitor.

*Ø*: Uncorrected machine power factor angle.

*Ø*_{1}: Corrected machine power factor angle.

*I*: Current vector.

Fig. 5 is a vector diagram of a typical shunt connected capacitor application. In the shunt application, the capacitor supplies part of or all the reactive current of the load and the current drawn from the supply is the vector difference of the load current and the capacitor current, i.e. OC = OA – AC on Fig. 5. In the series connected application, the current through the capacitor and the primary winding of the welding machine transformer is the same and the capacitor generates a voltage across it so that the voltage across the supply is the vector difference of the primary voltage and the voltage across the capacitor.

*IW*: Welding machine current.

*IS*: Supply current.

*IC*: Capacitor current.

*V*: Voltage vector.

*Ø*: Uncorrected machine power factor angle.

*Ø _{1}*: Corrected machine power factor angle.

Depending upon the load on the machine therefore, various voltages will be generated across the capacitor. The capacitor bank chosen must be suitable for a range of voltages. The procedure adopted for design is as follows:

- The maximum kVA during welding and the power factor at this kVA must be known, as well as the supply voltage.
- Considering the application shown in Fig. 2, the vector diagram Fig. 4 enables the voltage across the capacitor and the primary winding voltage of the machine to be computed, and it is usual to correct the power factor to unity.
- As it is uneconomic to make capacitors for every small increment of voltage, a capacitor is chosen that covers a range of voltages. The duty cycle of the welding machine should be known if it is desired to use a short time rated capacitor to reduce costs. For example, a capacitor with a continuous voltage rating of 100 units can be used for a “during weld” voltage of 150 units.
- The number of capacitors required is now determined as follows: Knowing the welder kVA and the voltage across the welding machine primary winding, the current can be found:

I = kVA x 1000/VW.

Knowing the voltage across the capacitor and the current, the total capacitive reactance required can be found: XC = VC/I.

The standard capacitor unit available to suit the voltage VC has a certain capacitive reactance, say XC_{1}.

The number of such units to be used is therefore XC_{1}/XC.

- A check has now to be carried out to see if the selected capacitor bank will be overloaded under the worst condition, that is when the two welding electrodes are shorted. It is necessary to know the kVA and the power factor with the electrodes shorted. By a similar process as detailed previously, the voltage now generated across the capacitor must be calculated. The manufacturers of capacitors, by experience, have drawn up a set of maximum values for each voltage range of a capacitor and a check is made that this value is not exceeded for the capacitor in question.

There are two courses to follow if the value is exceeded:

The capacitor with the next highest standard voltage is chosen, its reactance per standard unit is known and the number of units is re-computed. The number will increase compared with the lower voltage units.

A voltage limiting device can be incorporated across the capacitor bank which will protect the bank from an undue rise in voltage. An arc gap type of device with certain other auxiliary equipment is used.

The first of these solutions is often the best and least expensive method for welding machine applications. The application of series capacitors to any existing welding machine where the primary winding is already fixed, necessitates either rewinding the primary or incorporating an additional transformer as shown in Fig. 3. The design procedure is like the outlined.

**Conclusion **

The correction of the power factor of the load taken by a resistance type welding machine using series capacitor is the only feasible method and it provides instantaneous automatic correction.

The correction of the power factor must be tailored to the design of the welding machine and should preferably be carried out in full consultation with the designer and manufacturer of the machine.

The resistance type of welding machine is an attractive tool and any objection to its use can often be overcome by properly applied power factor correction.

Contact Mariana Jacobs, ICMEESA, Tel 011 615-4304, icmeesa@icmeesa.org.za