Control Transformer sizing for contactor actuation:
Introduction
A contactor is an electromagnetic device consisting of a coil and magnet system along with fixed and moving contacts. When the coil is energized, it produces a magnetic field thereby attracting the moving magnet. This causes the fixed and moving contacts to connect and the contactor is said to be actuated. The energization of contactor coil is usually done through a control transformer.
This is mainly done because voltage requirements vary with control systems and with an intermediary control transformer the desired voltage can be obtained.
When a contactor coil is energized, it draws in a high inrush current momentarily. Apart from contactor coils, relays and solenoids are some other devices which draw inrush current when energized. The control transformer selected must be able to accommodate this momentary high inrush current for a satisfactory operation.
Selection of a control transformer
For a proper selection of control transformer, three parameters of the load circuit must be determined in addition to the minimum voltage required to operate the circuit. These are Hold on VA, Pick-Up VA, and Inrush load power factor.
Hold-On VA: Hold-On VA is the product of load voltage (V) multiplied by the current that is required to operate the circuit after initial start up or under normal operating conditions. It is calculated by adding the hold-on VA requirements of all the electrical devices of the circuit that will be energized at any given time. Hold-On VA is also sometimes referred as steady state VA.
Pick-Up VA: Pick-Up VA is the product of load voltage (V) multiplied by the current (A) that is required during start up. It is calculated by adding the pick-up VA requirements of all devices (contactors, timers, relays, solenoids, etc) which will be energized together. Energization of electromagnetic devices takes 20-50 milliseconds. During this inrush period, the electromagnetic devices draw 3 to 10 times the normal current.
Inrush Load power factor: Inrush load power factor is difficult to determine without a detailed vector analysis of all the load components. Generally such analysis is not feasible; hence a safe assumption would be 40% power factor. Until recently 20% power factor was commonly used for transformer calculations; however tests conducted on major brands of control devices indicate that 40% power factor is a same assumption.
It is recommended that a control transformer be sized at 40% power factor. Some electromagnetic devices typically operate at that level due to their inherently low power factor. Selecting a control transformer at 40% power factor will be more than the adequate size for all the various loads in the circuit.
Besides the above parameters there are two parameters of primary and secondary voltage. Primary voltage is the voltage available from electrical distribution system which is connected to the transformer supply terminals. Secondary voltage is the voltage required for load operation which is connected to the transformer load voltage terminals.
Steps for selection of control transformer
1) Determine the supply and load voltages as per requirement. The supply voltage is the voltage available to control transformer and load voltage is the operating voltage of all the devices connected to the transformer output.
2) Determine the hold-on and pick-up VA of each coil in the control circuit. This data is provided by the product manufacturer in the datasheet.
3) Calculate the hold-on VA by adding the VA requirements of all the equipment that will be energized together (timers, contactors, relays, solenoids, pilot lamps etc).
4) Calculate the Pick Up VA of all the coils that will be energized together. Be sure to include the hold-on VA of components that don’t have inrush (lamps, timers) as they present load to the transformer during maximum inrush.
5) Calculate the application Inrush VA by using the following industry accepted formula,
Application Inrush VA= Sq.root (Sq.(pickup VA)+Sq.(Hold on VA))
Let us further understand this with the help of an example,
Consider MNX 110 contactor,
Pick Up VA = 550VA
Hold On VA = 36VA
Application Inrush = √ (5502 + 362) = 552 VA