How to Select Transformers
Image credit: ABB Low Voltage Systems | Grainger | RS Components, Ltd.
Transformers are electrical devices that are designed to transfer energy from one circuit to another by electromagnetic induction.
A basic transformer consists of two coils of wire wrapped around a core, known as the primary coil and secondary coil; the coils are separated by a medium such as air, water, or metal. The primary coil is typically connected to an alternating current (AC) power supply. When voltage is applied to the primary coil, it creates an electromagnetic field and induces a voltage in the second coil.
If both coils contain the same amount of wound wire, the induced voltage will be equal. Frequently, the secondary coil has fewer windings than the primary coil, and therefore the induced voltage will be reduced. For this reason, transformers are often used to channel high voltages into a lower-voltage circuit. Conversely, if voltage is applied to the secondary coil, it will be increased to the voltage of the primary coil by induction.
A transformer used to reduce 120 VAC to 24 VAC. Image credit: Automation Notebook
Transformers are extremely common devices used in many applications, primarily:
Widespread power distribution over a power grid
Power distribution and voltage matching for power distribution to buildings
Providing low voltages for machine control
While transformers are frequently used for power distribution, they can also be used in audio and radio frequency (RF) applications for impedance matching.
The video below provides an excellent visual overview of transformer basics and wiring.
Video credit: Afrotechmods
Principle of Induction
A transformer's ability to step voltage up or down is achieved and determined by the principle of induction, which describes the relationship between coil windings and voltage. The principle of induction is as follows:
VS / VP = NS / NP
VS = secondary coil voltage
VP = primary coil voltage
NP = number of primary coil turns
NS = number of secondary coil turns
Examining this proportion can be used to determine a coil's voltage and to determine the type or function of a transformer. For example, if NS / NP is less than one, the secondary voltage is less than the primary voltage and the device would be known as a step down transformer. Conversely, if this ratio is greater than one, the secondary voltage is larger than the primary, rendering the product a step up transformer.
Power transformers typically use relatively high voltages and may be as large as a substation transformer or as small as an electronic component. For information about specific types of power transformers, please read the Power Transformer Selection Guide.
RF / Signal Transformers
RF and signal transformers transfer information from one circuit to another. These devices generally use low voltages. Specific types include:
Audio transformersare typically used for voice and data transmission, and operate on frequencies between 300 Hz and 20 kHz. Digital transformers are used in audio applications for signal isolation and impedance matching.
Current sense transformers detect and measure current. Some of these products are designed to precisely measure current in instrumentation applications, while others are used in switch-mode power converter applications.
Radio frequency (RF) transformers are used for impedance matching on the radio frequency / microwave spectrum.
Telecom transformers are used for signal isolation in telecommunication applications, which typically require high bandwidth and fast switching.
Rectifier transformers produce a DC current or voltage and are used to drive diodes or thyristors.
A current sense (left) and RF transformer. Image credit: Octopart | Exax
Power rating is an important specification when selecting a transformer, and is measured in volt-amps (VA). Power rating is calculated by multiplying the total current load of a circuit by the circuit's secondary voltage, such that:
Volt-amps (VA) = total current (A) x voltage (V)
For example, a particular 24 V circuit may have the following components and current load:
6 heating elements at 500mA (or .5 A) each
4 indicators at 100 mA (or .1 A) each
1 lamp at 1 A
This circuit's power rating is calculated as:
[(6 x 0.5) + (4 x 0.1) + 1] x 24 = 105.6 VA
When precisely calculated, power rating represents an appropriate threshold when dealing with resistive loads. However, when selecting a transformer for use with a solenoid or large motor, inrush loads, or high momentary loads generated upon startup, must be considered. For solenoid valves, the calculated power rating may be 16 VA, while the inrush current may boost that level to 27 VA. A typical control transformer is designed to effectively handle these high currents, either by modifying the duty cycle or by using "slow-blow" fuses.
Dry vs. Fluid-filled
Because transformers naturally generate heat, they must be cooled by either air or a kind of fluid. Dry transformers feature the two coils facing each other and separated by air; this type does not have a core. The transformer's case or enclosure is ventilated to allow airflow, which cools the windings. Encapsulated transformers are special types of dry transformers that utilize a solid resin to encase and seal the core and coils.
Fluid-filled transformers immerse the coils and core in oil or water to keep the transformer cool.
Transformers may be constructed using a variety of core materials.
Laminated transformers, also known as E-I transformers, contain laminated steel cores insulated with varnish or some other nonconducting material.
Split core transformers use a hinge and locking snap to allow attachment without interrupting a current-carrying wire. They are typically low-cost devices.
Toroidal transformers consist of copper wire wrapped around a cylindrical core, preventing magnetic flux that occurs within the coil from leaking.
(left to right) A laminated, split core, and toroidal transformer.
Image credit: made-in-china.com | Rapidonline | Altran
Standards and Certifications
RoHS / WEEE
The Restriction on Hazardous Substances directive, also known as RoHS, and the Waste Electrical and Electronic Equipment (WEEE) directive are two closely linked pieces of legislation adopted by the European Union in 2003. RoHS and WEEE were both passed to limit high-tech waste and to improve the safety of existing waste.
Although RoHS is often referred to as a "lead-free" directive, it actually restricts the use of lead and five other hazardous chemicals: mercury, cadmium, hexavalent chromium, polybrominated biphenyls, and polybrominated diphenyl ether. While there is no standard RoHS logo or mark, a typical imprint is shown below.
The WEEE directive sets targets for the recycling and recovery of all electrical and electronic goods in order to reduce waste. The directive relies on equipment manufacturers to provide reasonable opportunities for recycling of their goods. The standard WEEE mark is a trash receptacle crossed with an "X" above a thick black line, as shown below.
Image credit: Tecartex | Wikipedia
Transformers may be designed and manufactured according to standards published by various agencies and standards bodies. Common transformer standards include:
- Audio Transformer
- Balun Transformer
- Buck-Boost Transformer
- Chassis Mount
- Chip Transformer
- Constant Current Transformer
- Current Sense Transformer
- Digital Transformer
- Dish / Disk Mount
- Distribution Transformer
- Dry-type / Air Cooled
- Electronics Transformer
- Encapsulated Transformers
- Flyback Transformer (FBT)
- Harmonic Mitigating Transformer
- High Frequency Transformer
- High-Voltage Transformer
- Impedance Matching Transformer
- Industrial Control Transformer
- Interface Transformer
- Inverter Transformer
- Isolation Transformer
- Lighting Transformer
- Medical Transformer
- Modular Jack
- Multi-Ratio Transformer
- Oil Filled
- One to One
- Transformer Category:Other
- Mounting / Form Factor:Other
- PC / PCB Mount
- Planar Transformer
- Power / Power System / Distribution Transformer
- Power Transformer
- Pulse Transformer
- RF Transformer
- RoHS Compliant
- Signal / RF Transformer
- Single Phase Transformer
- Solar Power Transformer
- Step-Down Transformer
- Step-Up Transformer
- Substation Transformer
- Switch Mode Transformer
- Telecom / LAN Transformer
- Three Phase / Polyphase Transformer
- Trigger / Gate Drive Transformer
- Variac / Variable Autotransformer
- WEEE Compliant