TECHNICAL ADVISORS’ COLUMN: Alan Ross
The step-down system of power grids is what makes transformers so critical from generation to distribution, to power industrial and commercial equipment, and ultimately to light the lights that the world relies upon. In many cases we lose site of the seemingly insignificant control transformer as we pay attention to the big brother of the transformer family, both fluid filled and dry type.
But control transformers have their own important part to play. Control transformers are transformers that are used to step down the voltage to power the control devices of a circuit or machine, considering that the reduced voltage provides a much safer environment for technicians working on or around the equipment. They are also referred to as industrial control transformers, control power transformers, machine tool transformers and isolation transformers, providing much needed voltage regulation where they are designed to produce a high level of secondary voltage stability during brief periods of overload condition, typically known as inrush current.
Additionally, control transformers are used to reduce or eliminate harmonics or transients – power quality issues that can have a significantly negative affect on solid state production equipment, an ever-increasing evolution within the move to “everything digital”.
Growing Demand – Growing Criticality
A recent marketing report indicated that “the industrial control transformer market is expected to grow at a CAGR of around 3.5% over the next decade. Factors such as the increasing number of industries (such as chemical, and metal and mining), and the demand to reduce equipment failure on account of frequent voltage fluctuations are likely to drive the market, especially in the emerging economies. However, due to the outbreak of COVID-19, slow down, or delay of different industrial projects are likely to hinder the market growth during the forecast period.”
Now that the effects of Covid seem to be waning, will we see a rebounding effect, where the actual growth rate for the next few years will rebound beyond 3.5%? It is more likely to happen than not to happen.
Applications
Control transformers are most often used in an electronic circuit that requires constant voltage or constant current with a low power or volt-amp rating when power quality issues impact the circuit whether due to harmonics or transients. Capacitors are also used to minimize the variations in the output.
Where electromagnetic devices like solenoid switches or relays are used, especially in industrial applications, the control transformer maximizes inrush capability and output voltage regulation when electromagnetic devices are initially energized.
Control transformers utilize high-quality insulating materials to insulate turn-to-turn windings; layer-to-layer windings, primary-to-secondary windings; and for grounding. Control transformers are vacuum impregnated with VT polyester resin and oven-cured, which seals the surface and eliminates moisture. The resin creates a strong mechanical bond and offers protection from the environment.
Specification
For proper control transformer specification, three characteristics of the load circuit must be determined in addition to the minimum voltage required to operate the circuit.
These are total steady-state (sealed) VA, total inrush VA, and inrush load power factor as follows:
-
Total steady-state (sealed) VA is the volt-amperes that the transformer must deliver to the load circuit for an extended period — the amount of current required to hold the contact in the circuit.
-
Total inrush VA is the volt amperes that the transformer must deliver upon initial energization of the control circuit. Energization of electromagnetic devices takes 30-50 milliseconds. During this inrush period, the electromagnetic control devices draw many times normal current — 3-10 times normal is typical.
-
Inrush load power factor is difficult to determine without detailed vector analysis of all the load components. Since such an analysis is typically not feasible, a safe assumption is 40% power factor.