Controls & Transformers

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Questions and Answers

Duty cycle is a measurement of the % of the time that the transformer is conducting current during one minute. This value is used to insure that the electrical components are not operating above their thermal capability. Resistance welding transformers are rated at a 50% duty cycle. Each application may operate at different duty cycle up to and including 100% continuous.

THE FORMULA TO CALCULATE DUTY CYCLE FOR AC is:

Duty Cycle AC

THE FORMULA TO CALCULATE DUTY CYCLE FOR MFDC is:

Duty Cycle MFDC

Reference:  RWMA Manual Chapter 19

Resistance welding transformers can have very high demand for a very short period of time. Installation of correctly sized primary conductors is imperative for the machine to run properly. The proper procedure to size these conductors for AC equipment is:

Transformer Labeled Sketch

When the duty cycle of the machine is known
1. Calculate the duty cycle
2. Determine the weld current
3. Divide the weld current by the turns ratio of the weld transformer. This is the Primary Demand Current.
4. Multiply the Primary Demand Current by the square root of the duty cycle. This is the Primary Effective Continuous Thermal Current (ECTC).
5. Refer to the Ampacity Chart of the cable type to be used. Select the size based on the Primary Effective Continuous Thermal Current (ECTC).

If the Duty Cycle or Weld Current is not known or this is a general purpose machine:
1. Divide the transformer nameplate KVA by the primary voltage.
2. Multiply that result by the square root of 0.5 (RWMA transformers normally are rated at 50% Duty Cycle)
3. The result is the Primary Effective Continuous Thermal Current (ECTC).
4. Refer to the Ampacity Chart of the cable type to be used. Select the size based on the Primary Effective Continuous Thermal Current (ECTC).

Ampacity charts are based on the National Electric Code (NEC). There may be other state, local or company standards that must be followed.

Reference: National Electric Code
                   RWMA Manual Chapter -21.7

MFDC stands for MID FREQUENCY DIRECT CURRENT. There are two components needed to convert standard alternating current (AC) to MFDC. They are the weld control and the transformer. The weld control is made up of two major areas, the first being the Bridge Rectifier where the three phase 60 Hz AC input is rectified into a DC output. The second section is the inverter, or H=Bridge, which is a polarity reversing switch. This switch also operates at the desired increased frequency of 1000 Hz. In the H-Bridge the rectified DC input is converted back into a single phase AC square wave output at 1000 Hz frequency, as shown below.
This mid frequency AC is the input to the primary of the transformer. The diodes in the transformer then converts this AC back to DC (MFDC) for welding in the secondary circuit.

In many facilities it is beneficial to insure that the multiple welders in a station not fire at the same time to prevent a sudden large draw on the plant power supply. To spread this out cascade controls can be employed where one control will sequence through several contactors in sequence. The sequence and separation between the welds can be controlled to give an appropriate modest draw on the power supply.

In North America the power grid is 60 hertz. All schedules and operations are based upon that power.  In other parts of the world power is generated at other power levels. Many use 50 hertz. If one has a good weld schedule in a 60 hertz area and a new operation were set up in a 50 hertz region a conversion would be needed for weld time. The formula is:

Weld Time @60 Hz X (50/60) = Weld Time @ 50 Hz

                            OR

Weld Time @50 Hz X (60/50) = Weld Time @60 Hz

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