THE CORRECTION OF POWER FACTOR ON RESISTANCE WELDING MACHINE LOADS

BY H. J. YELLAND (ASSOCIATE MEMBER) - The Certificated Engineer Jan 1967

ABSTRACT

Users and power supply authorities alike often experience trouble with the loads from resistance welding machines especially if large machines that are not fitted with power factor correction are involved. The nature of the welder load is explained. The only possible way of correcting the power factor is using series-connected static capacitors, the design, and application of which must be closely tailored to the design of the welding machine.

 THE NATURE OF THE WELDING MACHINE LOAD

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 kV A demand is high. There follows a much longer rest period.

The ? hour kVA demand tariff as charged by supply authorities is low for this type of load. However, the brief peak kV A demand determines the size of cables and transformers to avoid an 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 consumers, producing some or all the following side effects:

1.      Lamp flicker.

2.      Interference with other electrical machinery or apparatus or with other welding machines.

3.      Adjustment of the welding machine is difficult due to interference by other machines so causing a reduction in uniformly high quality of welds.

4.      It is not possible to do heavy welds.

5.      A special individual feeder to each welding machine is often necessary.

6.      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 the effects listed 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 to an 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 energized 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 energized when the welding operation was taking place. However, after every welding cycle the capacitor would discharge via the primary winding and when re-energized 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 the 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 by using series capacitors. Fig. 2 shows a typical circuit for a new welding machine where the welding transformer windings can be correctly arranged during manufacture.

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

VW-Voltage across the welding machine.

VS-Voltage across the supply.

VC-Voltage across the capacitor.

? -Uncorrected machine power factor angle.

?1-Corrected machine power factor angle.

I-Current vector.

Fig. 4 is the simplified vector diagram for the machine with the series-connected capacitor of Fig. 2. Fig. 5 is a vector diagram of a typical shunt connected capacitor application. In the shunt application, the capacitor supplies part 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, i.e. OC = OA-AC in Fig. 4.

IW-Welding machine current.

IS-Supply current.

IC-Capacitor current.

V-Voltage vector.

?-Uncorrected machine power factor angle.

?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:

a)      The maximum kV A during welding and the power factor at this kVA must be known as well as the supply voltage.

b)     Considering the application shown in Fig. 2, then 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.

c)       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 often used for a "during weld" voltage of 150 units.

d)     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 XC1.

The number of such units to be used is, therefore, XC1 / XC

e)     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.

If the value is exceeded, there are two courses to follow:

i.     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.

ii.     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.

f)       Solution (i) 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 outlined.

CONCLUSION

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

DISCUSSION

Mr. N. D. Kennedy-Potts (Visitor): Can the author give a comparison of the cost of converting an existing welder to improve the power factor as described and the cost of correcting the power factor using an automatic correction unit at the busbars.

I notice that in Fig. 3 the primary of the welding transformer had a supplementary set of coils in parallel with the original coils. This presumably means a re-design of the transformer, and because the bulk of flash welders, as they are called, are imported. Does this mean that the transformer must be returned to the manufacturer to be re-designed to take care of the improvement in the power factor?

We have in our factory about a dozen of these flash welding machines. An arc is held across the two bars to be joined until the material becomes plastic and then for a period of two to three cycles, these plastic bars are pressed together, during which time the power is held for a period of two or three cycles during which the power factor is low. At this stage, the current increases to about twenty times normal and then fall away to zero on switch-off. We found after carrying out experiments with a series of capacitors that better results were obtained by installing bus-bar power factor correction. How can we modify our existing machines to give us an advantage of the machine itself?

Mr. H. J. Yelland: Automatic power factor correction applied to' the bus-bars of an installation at some suitable point will not be effective in overcoming the voltage drop and related effects associated with resistance welding machines. Automatic power factor correction of this nature comprises several capacitors switched in and out of circuit by means of contactors under the control of relays which are sensitive to the kVAr requirements of the system or to the system voltage or the load current.

As outlined in the paper, resistance welding machines have an 'ON' time of a few cycles only, followed by a much longer 'OFF' period. Firstly, contractors switching capacitors IN and OUT cannot do so fast enough to match this sort of duty and even if they could, the conventional power factor control relays are intentionally designed to have a time lag to prevent too frequent a duty. Further, the charging currents associated with the switching on and off shunt connected capacitors trying to' follow this sort of load would affect the setting of the welding machines and the quality of the welds.

The automatic correction of the plant power factor by shunt capacitors will however effectively deal with the problem of reducing the power bill due to a low average power factor. This is because the conventional metering gear used has a ? hour delay characteristic and measures only the maximum demand, be it Kw or kVA, based on a maximum ? hour average.

The use of series-connected capacitors, properly designed to' suit the welding machine, be it new or existing, provides instantaneous, an automatic correction which is highly effective. On an existing welding machine, the transformer thereof would either must be re-wound to suit the changed voltage ratio required due to the addition of series capacitors, or an autotransformer would be used to feed the existing welding transformer set up as shown in Fig. 3. The comparison of costs between the two possible alternatives depends too much on the design and size of the existing gear for any values to be given. The best solution would be to leave the transformer intact and add a new auto-transformer on any existing installation. There would be no need, in either case, to send gear overseas for modification because there are firms in Johannesburg who could readily undertake this work. However, co-operation with the overseas makers of the welding machines would be a big help.