Glossary of Terms

Transformers designed for 50 Hz and 60 Hz operate at different frequencies, influencing core losses and temperature. Small transformers can often function at both frequencies with minor design adjustments. For larger transformers, the core must accommodate higher no-load losses for 50 Hz operation. Transformers rated for 50/60 Hz typically cost 5–10% more than 60 Hz models due to these design considerations.

Aluminum and copper are commonly used materials for transformer windings, each with distinct advantages and trade-offs. Aluminum weighs significantly less than copper and costs less on a per-pound basis, making it a cost-effective option for constant-load applications. However, aluminum has only about 60% of copper’s conductivity, requiring larger windings to achieve comparable performance. Aluminum windings typically result in larger transformers with shorter lifespans, often about one-fourth that of copper-wound transformers. Copper, by contrast, offers superior conductivity, durability, and efficiency, making it the preferred choice for high-performance applications, especially those with varying or intermittent loads. While both materials can be suitable for different use cases, not all transformer types and designs are compatible with aluminum windings.

An auto transformer electrically connects its primary and secondary windings, allowing energy to flow directly between them. This design reduces cost and size but lacks the electrical isolation provided by standard transformers. Auto transformers are ideal for applications where only a small voltage adjustment is needed. They are available in single-phase or three-phase configurations but are not suitable where circuit isolation is required.

Transformers of 2 windings can be reconnected as an auto transformer depending on the relative connection of the secondary with respect to the primary. One can obtain either the sum of the input and output or the difference between the input and output voltages. An example would be 120 volt primary, 12 volt secondary. By connecting the secondary winding to one end of the primary, the secondary may be 132 volts, or by reversing the secondary leads, the output may be 120 volts minus 12 volts or 108 volts. The boosting transformer connection is often used where consistently low voltage prevails. 
For example, you may have 105 volts, so you use a transformer reconnected to boost this voltage, as outlined in the above procedure. The output voltage would be 10% greater than the input of 105 volts or approximately 116 volts. This type of transformer connection is often used to boost 208 volts to 230 volts.

A choke, or inductor, is a magnetic device with a single winding used to buffer power surges and control current flow. It reduces voltage without significant power loss and is commonly used in SCR drives to smooth current and eliminate stray harmonics. Control Transformer Chokes are specifically designed and tested for linearity, particularly for high-load systems, and charts can be produced for your engineering files if needed.

A combination transformer combines the functionality of two or more transformers or power supplies into a single device. It can range from simple single-phase configurations to complex three-phase setups with multiple outputs, including isolation and auto-transformer options.

A DC power supply is a device that will deliver DC voltages and currents to your system. Most DC power supplies used in the industrial environment will convert the AC power generated by the utilities to DC power. This is achieved through various rectifier configurations.

DC power supplies come in many sizes and configurations, but will generally fall into one of four categories: battery, switch-mode, linear, or regulating. Each of these devices has their own unique application benefits and pitfalls.

To determine which type of DC power supply is suitable for your application, please contact Control Transformer's Engineering Department for application service. 
 

A delta connection is a term that describes the connections used on 3-phase transformers or three single-phase transformers when connected to a 3-phase supply (sometimes called closed delta connections). The connection is similar to an equilateral triangle or the symbol delta. A side of the delta or equilateral triangle represents each single-phase transformer. The three transformers are connected together at the corners of the delta. The supply lines or load lines are connected at the same junctions from each corner. This results in a 3-wire, 3-phase system.

The term Open Delta Connections describes the connection of two single-phase transformers for use in a 3-phase power supply for transforming a 3-phase voltage. The open delta is the same as the closed delta described above, except that one transformer is removed from the closed delta circuit. Open delta is less expensive, but it presents certain limitations compared to normal 3-phase connections.

In many applications, a transformer will not be called upon to continuously provide the maximum amount of current required by a system. Just like motors rated by a duty cycle calculation, a transformer rated for continuous duty will be bigger and more expensive than one rated for partial duty. For example, a duty cycle of 50% is assumed in a typical machine tool application. By calculating the percentage of full-load use required, you can properly size the transformer and motor in a system. The square root of the duty cycle times the full-load KVA requirement gives the effective KVA rating required for the transformer. For example, take a 50% duty cycle for which the square root is .71. Thus, the transformer must be 71% of the full-load rating.

Transformers are among the most efficient electrical apparatus commercially manufactured. Most transformers of sizes from 1 KVA through 1000 KVA will have a full-load efficiency of from 95% to 98.5%. The items which lower the efficiency of a transformer are losses, which appear in the iron and those which appear in the windings. The iron losses are explained in the above question. The losses in the windings are due to the resistance of the wire. When current flows through a wire, losses appear in the form of heat, just the same as when an ordinary electric hot plate is turned on. The wire used in quality transformers is generally made of copper. Aluminum is sometimes used in low-quality transformers where life expectancy or transformers with a constant load are unimportant. These metals are among the best commercial metals for conducting electricity. They offer the least amount of resistance. Therefore, little energy is lost in the form of heat through the windings. These losses run from 1% to 2% of the full-load rating. They vary inversely as the square of the load.

When used in connection with transformers, exciting current is the current or amperes required to get the transformer to the point where it will operate. A certain amount of energy is required to overcome the internal resistance of the steel core. The exciting current on most transformers varies from approximately 10% on small sizes of about 1 KVA and smaller to approximately .5% to 4% on larger sizes up to 1000 KVA. The exciting current comprises two components, one of which is a real component and is in the form of losses referred to as no-load watts; the other is in the form of reactive power and is referred to as KVAR.

Impedance is the combined resistance, inductance, and capacitance in an AC circuit, affecting voltage regulation and fault current levels. Lower impedance improves voltage regulation but can increase short-circuit risks. Most transformers have impedance levels between 3% and 6%.

This transformer electrically separates primary and secondary windings to provide circuit isolation, enhancing safety and reducing noise interference. Isolation transformers are critical for sensitive equipment and systems requiring minimal electrical interference.

A line reactor is a special form of inductor that is typically used between the line and the load to smooth current inrush, reduce harmonics and noise, and buffer the systems connected to it. Specifically, it is an inductor that adds inductive impedance to a circuit.

These devices are available in single-phase or three-phase configurations, with three-phase units being the most common, and are connected in series to the load they are protecting. Line reactors are typically specified as a percent impedance at a certain voltage and current level.

For example, a 480 volt, 5%, 25 amp line reactor is a typical specification. This specification means that with a 25 amp load current, the line reactor will have a 5% inductance impedance voltage drop on a 480 volt system. If the load is an inductive impedance load, the voltage to the load will be 5% lower than the voltage to the line reactor. If the load is a capacitance impedance load, the voltage to the load will be 5% greater than the line voltage input to the line reactor. If the load is a resistance impedance load, the load voltage will be less than 1% lower than the voltage input to the line reactor. Often, line reactors are used in circuits to replace considerably larger and more expensive drive isolation transformers.

Paralleling transformers allows them to share loads, provided their voltages and impedance match closely. Voltage mismatches can lead to circulating currents and potential transformer damage. Proper impedance matching ensures balanced load sharing.

In most commercial installations, a tolerance of 10% impedance is permissible when transformers are paralleled. Three-phase transformers must have similar angular displacements, meaning the phasing must be the same on each transformer.

The Scott T connection is a transformer configuration that converts three-phase power to two-phase power or vice versa using two transformers. Developed by its namesake, this method provides an economical solution for phase conversion in specialized applications. The setup involves a "main" transformer with a center tap and a "teaser" transformer with an 86.6% tap. The teaser is connected to the center tap of the main, with their windings joined to the three wires of the three-phase supply. The secondary windings of the main and teaser transformers generate voltages that are 90 electrical degrees apart, creating the two-phase output. This configuration is commonly used due to its efficiency and cost-effectiveness and can be implemented in both auto-transformer and isolating transformer designs.

Taps allow for voltage adjustments on a transformer’s windings, enabling fine-tuning of input or output voltage. They improve flexibility, efficiency, and compatibility with varying supply voltages.

A transformer is an electrical device that, by electromagnetic induction, transforms electric energy from one or more circuits to one or more other circuits at the same frequency. By varying magnetic relationships or values of the input versus the output, a transformer produces changed voltage and current values. A transformer works by having the input windings made around a core of special steel that conveys the pulses of AC current to output windings around a core of special steel that conveys the pulses of AC Current to output windings around the same or connected portions of the steel core. 

An auto transformer has its primary and secondary connected to each other electrically. A portion of the energy in an autotransformer comes from this connection while the balance comes directly from the supply. Building inspectors often object to auto transformers because they do not isolate one circuit from the other. One ground may be at a considerably higher voltage than the ground in another section of the same circuit. Local inspectors and utility companies should be consulted before installing autotransformers. Where the use of autotransformers is not objectionable, they do represent a considerable saving in price over that of a regular separate winding transformer. This saving varies as the ratio of windings changes. After the ratio of windings reaches approximately 4:1 or 5:1, there is very little economy in using an autotransformer. Autotransformers are most practical where a small percentage of voltage raising or lowering is required and isolation between the two circuits is not required. Autotransformers can be single phase or three phase. In neither case is any isolation provided.

An isolation transformer insulates the input (primary) from the output (secondary) winding. This can be done by several means, including separating the windings and for more isolation and shielding. 

A step-down transformer is one in which the input (primary) windings exceed in number the output (secondary) windings, thus the ratio of voltage is from the high voltage winding to a low voltage winding or windings. The offset is current, which is higher in the secondary than it is in the primary.

A step-up transformer is one in which the voltage change is from the low voltage winding to a high voltage winding or windings. High current is used in the primary to produce a relatively lower current in the secondary.

A 2-winding transformer has the primary winding and secondary winding electrically insulated from each other. Therefore, all of the power flowing through the transformer is done by transformer action, not by partial conduction, as explained in the auto transformer section. A brief description of how a 2-winding transformer operates is as follows: The electrical energy flows through the primary windings and is converted to magnetic energy, which is conducted by the lamination or iron in the center of the coil. This magnetic energy is called lines of force or flux and varies according to the supply frequency. When these lines of force vary or move, they affect the winding of the secondary, generating a voltage in the secondary. This voltage may be larger, smaller, or equal to the input of primary voltage and is determined by the ratio of turns between primary and secondary. For example, the primary of a transformer has 100 turns and is connected to a 100-volt supply, the secondary has 50 turns, therefore, the secondary voltage will be 50 volts. 

A 3-winding transformer is identical to a 2-winding transformer except that the secondary has two sections and is quite often suitable for series or parallel. A typical example would be a 480-volt primary with a secondary of 120/240 volts. "Windings" can be effected by putting taps at various places on one large winding or by making separate coils (either primary or secondary).

A 4-winding transformer is the same as a 2-winding transformer, except the primary and secondary are generally split up into two sections usually suited for series-parallel connection. An example would be a transformer with a primary of 240/480 volts and a secondary of 120/240 volts. The voltages indicated are such that the primary can be connected in series for a 480-volt supply or paralleled for a 240-volt supply. The secondary can be in series for 240 volts paralleled for 120 volts or connected in series with the mid tap brought out for 120/240 volts, 3-wire operation. This is the most common connection found on industrial lighting panels and in most residences.

Voltage regulation in a transformer refers to the difference between its no-load voltage and full-load voltage, expressed as a percentage. It indicates how well a transformer maintains a stable output voltage under varying load conditions. For example, a transformer delivering 100 volts at no-load and 95 volts at full-load has a voltage regulation of 5%. Lower regulation percentages signify better performance, which is particularly important for sensitive equipment such as instruments or computers. Most power and lighting transformers have a voltage regulation range of 2% to 4%, depending on their size and application.

A WYE connection is a three-phase transformer configuration where the ends of the windings (primaries or secondaries) are connected to a central neutral point, forming a "Y" shape. The other ends of the windings are connected to the three line leads of the three-phase supply. If a fourth wire is present, it connects to the neutral point, enabling both three-phase and single-phase operations. Transformers in a WYE configuration operate at higher phase-to-phase voltages than their single-phase ratings. For instance, a transformer with 120-volt single-phase windings can produce 208 volts in a three-phase WYE configuration. This setup is commonly used in industrial power systems for its flexibility and efficiency.

A zigzag transformer is used in 3-phase systems and is made up of 6 coils connected in a "Y" manner. Each leg of the "Y" is made up of a coil on a different phase leg of the transformer. The neutral formed by the zigzag connection is very stable. Therefore, this type of transformer lends itself well to establishing a neutral for an ungrounded 3-phase system.

This type of transformer will carry a fairly large rating, yet physically be relatively small. This particularly applies in connection with grounding applications. The reason for this small size in relation to the nameplate KVA rating is due to the fact that many types of grounding auto transformers are rated for 2 seconds. This is based on the time to operate an overcurrent protection device such as a breaker. Zigzag transformers used to be employed to enable size reductions in drive motor systems due to the stable wave form they present. Other means are now more common, such as 6 phase star.