What is a power transformer?
Products in the power transformers family include those designed for transformation of AC utility power at nominal frequencies of 400 Hz or less that contain galvanically isolated primary and secondary windings. They are not packaged or equipped with the common connectors used for AC utility powered products, making them unserviceable as an end-use product in an isolation transformer capacity.
An engineer’s perspective on transformer selection
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Power transformers are always incorporated into larger systems. This requires us to think about specifications in terms of the AC mains (feeder) and in terms of the load. There are also dynamic considerations, as the load on the transformer’s secondary will often fluctuate. A prime example is a transformer used in an audio amplifier. There is a wide variation between a quiet passage and a thundering bass line. Selection of a high-efficiency transformer suitable for both situations can be challenging.
The complexity of the transformer selection process increases significantly when the transformer is incorporated into a power supply with rectification and capacitor filtering. In this common application, the steady-state AC specifications serve as guidelines as the current waveform is pulsed as opposed to sinusoidal.
Must-know facts about power transformers
There are a few must-know facts about power transformers:
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Rectification: Power transformers are often used as part of a DC power supply. In this application, a step-down transformer is followed by a half- or full-bridge rectifier. The rectifier is then followed by a large energy-storing capacitor. This forms a low-pass filter removing the AC power line fluctuations. This is a non-linear system. The current waveform consists of a series of spikes associated with the peaks of the AC waveform. This significantly complicates power transformer selection as the pulse current is not directly related to the sinusoidal assumptions as described later in the specifications section. As a mitigation factor, select a larger transformer than the power would otherwise indicate. This should minimize overheating as the lower resistance windings are better able to handle the current spikes.
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Dual primary windings: Many transformers feature dual primary windings. This is a convenient feature that allows the transformer to be used with different feeder voltages. The typical American step-down transformer features two 120 VAC windings. Both windings must be used to obtain the transformer’s rated power. The windings are connected in parallel for use on a 120 VAC system. They are connected in series when installed in a 240 VAC system. In both cases, proper configuration prevents excessive I2R losses in the windings.
Tech Tip: Observe coil polarity (schematic phase dots) when connecting coils in parallel. Crossed connections will appear as a short circuit leading to transformer destruction.
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Size and Weight: Power transformers are designed to operate on main power. In North America this is a 60 Hz system while other parts of the world use 50 Hz. The 400 Hz specification mentioned in the opening paragraph is for aviation systems. As a rule, transformer size and weight are inversely related to frequency. That is why aviation generators, transformers, and AC motors are designed for 400 Hz as size and weight are critical factors for flight. On a related note, many modern power supplies such as your laptop include high frequency transformers. A 100 W high-frequency transformer is tiny in comparison to its 60 Hz equivalent.
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Overvoltage: Recognize that electromechanical devices are a study of efficiency. Designers take every opportunity to make the devices as physically small and low cost by using the least amount of materials in terms of silicon steel and copper. The result is that electromechanical devices operate very close to the knee of magnetic saturation. Recall that an induced magnetic field is the product of amps and turns. Given the optimized transformer design, overvoltage will cause the magnetic core to saturate. Consequently, applied voltage is a hard specification that applies to each winding. For example, a 10 to 1 stepdown transformer designed for 120 VAC operation provides 12 VAC. It cannot be reversed to step 120 up to 1200 VAC, as the core will saturate, resulting in a spectacular failure with melted windings and insulation breakdown.
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Volt-Amps vs Watts: Transformer power is expressed in terms of Volt-Amperes (VA) instead of Watts (W). This concept is mysterious for people who have yet to complete mid-level introduction to electronics class. We can start with a recognition that the VA and W specifications are the same when the transformer is driving a purely resistive load. The specifications depart when capacitive and inductive loads are included. In this case, the real power as expressed in W will be less than the apparent power as expressed in VA. As previously mentioned, things are even more complex when the transformer is used to drive a rectifier. Once again, as a rule of thumb, select a larger transformer than the power rating would otherwise indicate.
Looking forward to continuing the conversation on this forum.
Best wishes,
Aaron Dahlen
LCDR USCG (Ret.), MSEE
DigiKey Applications Engineer
Introduction to DigiKey’s Product Selection Guide
This page is one of many in DigiKey’s Product Selection Guide (PSG). This particular page is focused on power transformers. It provides a description of the individual parameters used to characterize the part. This information will allow you to better understand and navigate DigiKey’s parametric search engine. Click here for a case study showing how to use the search tools.
How are power transformers categorized?
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Type: Refers to the specific construction or design configuration of the transformer’s core and winding arrangement.
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Voltage - Primary: Indicates nominal AC input voltages with which a device is designed for use: multiple values indicate different connection options, allowing production of the listed secondary voltages from any of the listed primary voltages.
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Voltage - Secondary (Full Load): Nominal AC RMS output voltage appearing at the device output windings when the rated output current is being drawn and the nominal primary voltage is applied.
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Current - Output (Max): The maximum amount of current that can be output by the device.
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Primary Winding(s): Describes the configuration of the transformer’s primary winding(s), which are those typically connected to a power source.
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Secondary Winding(s): Indicates the number of separate windings present on the side of the transformer that is typically used to deliver power to a load.
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Center Tap: Indicates whether or not a user-accessible connection to the midpoint of the secondary winding is present.
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Power - Max: Typically this represents the maximum power rating to which a device will meet its given specifications. Exceeding this power rating may damage the device and other system components.
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Mounting Type: Indicates how the device is attached.
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Termination Style: Selection of termination style used to connect the device to a system, such as PC pin, solder pad, or wire leads.
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Size / Dimension: The physical dimensions of the device, typically listed in length and width or diameter.
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Height - Seated (Max): Indicates how far the device will extend above the surface to which it has been attached.
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Voltage - Isolation: Indicates the level of electrical isolation provided between the primary and secondary windings of the transformer.
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Weight: Indicates the overall weight of the transformer, typically listed in pounds or grams.
Examples of power transformers
- The Triad Magnetics brand VPM240-4170 is a 1000 VA toroidal transformer designed for the demanding requirements of medical electrical equipment. Datasheet
Schematic symbol
The schematic symbol for a transformer is shown below.
Schematic symbols courtesy of Scheme-it.
Related media links
Videos
DigiKey Tech Forum articles
- Power Transformer - inherently and non-inherently limited - Power / Transformers
- Transformers vs. Power Supplies
- Transformer Rated Power Capacity
- Triad Magnetics Transformers “B” Suffix
- Transformer Primary / Secondary Impedance
Follow up
If you have any general questions on the specifications for selecting power transformers, please reply using the button below. If you have questions on a specific transformer, please post your questions in the forum’s transformers category.
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