Basic Components of a Transformer

A transformer can be used to increase or decrease voltage and current levels. This article examines common transformer faults, ratings, and testing procedures.

Welcome to Part 3 of a four-part series of excerpts from the Refrigeration Service Engineers Society publication, “Electricity for HVACR Technicians.” (You can find Part 1 online, at Part 2 can be found at This publication won a Contracting Mechanical Systems WEEK Product Showcase Award in 2009, in the category of Education. NOTE: this series is not intended to serve as a replacement for concentrated, formal classroom and/or field training by a qualified electrical professional.

A transformer is a device that can be used to increase (“step up”) voltage and current levels. The principal parts of a transformer are two coils of wire, called the primary winding and the secondary winding, wound on some type of core material. An enclosure protects the internal components from dirt, moisture, and mechanical damage. Figure 3-1 shows a schematic diagram of a simple transformer.

Common Defects

There are at least five common faults that you may encounter in working with transformers:

Open primary. This occurs when the coil of wire in the primary breaks or separates from the leads or terminals of the transformer.

Open secondary. This occurs when the coil of wire in the secondary breaks or separates from the leads or terminals of the transformer. This condition also results from a blown internal fuse.

Winding-to-winding short. This occurs when the insulation on the coil of wire in the primary or secondary breaks down, and current can pass from one winding to the other.

Winding-to-case short. This occurs when the insulation on the coil of wire in the primary or secondary breaks down, and current can pass directly to the case or ground.

Overheated transformer. This occurs when the load or demand on the transformer is too great. This condition can be detected by discoloration of the winding area of the transformer.

Transformer Ratings

A transformer is rated according to its primary voltage, secondary voltage, and power-handling capacity. The power-handling capacity of a transformer is measured either in watts or volt-amperes (VA). You can determine the maximum current that a transformer can safely handle by using a “rule of thumb” for converting volt-amperes to amperes. Simply divide the volt-ampere rating of the transformer by the secondary voltage.

Example: For a 40-VA transformer with a 120-V primary and a 24-V secondary, calculate the current draw as follows:

A = 40VA/24V = 1.66A

The VA rating of the controlled devices must not exceed the VA rating of the transformer. If the total VA draw is greater than the transformer’s rating, the secondary voltage can drop drastically, and the transformer will overheat. If you must replace a transformer, make sure that you select a transformer with a volt-ampere rating equal to or greater than that of the transformer being replaced.

Residential Transformers

Figure 3-2 shows a typical low-voltage transformer with wire leads. This type of transformer is normally found in residential equipment. The primary voltage of such a transformer will vary according to whether the transformer is placed in a condensing unit or a furnace. If the transformer is in a condensing unit, the primary voltage is 230 V. If it is in a furnace, the primary voltage is 120V. The secondary voltage will range from 24 to 28V. Most residential transformers have VA ratings of 20 or 40VA. Some may go as high as 75VA for very large systems.

Figure 3-3 shows a low-voltage transformer with screw terminals and a dual-voltage or tapped secondary. The schematic diagram for this transformer is shown in Figure 3-4. This type of transformer is also found in residential equipment. In the past, it was frequently used where a lower voltage was required for some form of glow coil ignitor in a heating system.

Transformer Fusing

Transformers with VA ratings of 40VA and above usually have an internal fuse built into the secondary windings. This makes it imperative for the service technician not to short across the secondary with a screwdriver (to see if it “sparks.”) If the secondary is shorted for any reason, the fuse will open and the transformer will have to be replaced. This can be a costly mistake. The schematic diagram for a transformer with an internal fuse is shown in Figure 3-5.

A number of newer transformers, especially the general replacement types, have an external fuse. Some local codes require external fusing, and some manufacturers make transformers with external fuses in order to meet UL approval. An external fuse may be an in-line type or a chassis-mounted type. Regardless of the type of fuse, remember that if you find an open fuse, something must have caused the excessive current. Look for the trouble elsewhere in the circuit.

Testing Residential Transformers

To test a transformer, first turn off the power supply. Remove all the controlling circuits, and place a voltmeter across the terminals on the secondary side of the transformer, as shown in Figure 3-6. Turn the power back on. The voltage read across the secondary, with no load, is called the open circuit voltage (OCV). The OCV is usually about 5 to 10% higher than the nameplate voltage. If no voltage is read, turn off the power, disconnect the transformer, and do a continuity test of the transformer’s primary and secondary.

If there is a short in the secondary, usually the fuse will open, or the winding itself may be found open. If there is a short to the case or ground of the transformer, usually a circuit breaker will open or a line fuse will have blown. This is because there is a potential difference between the primary and secondary windings of the transformer. As a result, a current is allowed to pass from the grounded case to the neutral wire of the transformer’s primary winding. This is illustrated in Figure 3-7.

To learn more about Electricity for HVACR Technicians and other educational offerings from RSES, visit Click the “eLearning” link for online versions of this course.


Relays: Components & Testing

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