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Transformer Design Guide Our design guide takes you step by step through the process of designing a t oroidal transformer.If you have any questions about your design, our engineering and design team is available at 800-836-5920 to help you. You can fill out our Design Form or e-mail us your specifications. At Tortran, we offer:
For most custom transformer designs, there's no need for special costly tools, such as stamping dies required for custom lamination forms. Unlike E-I cores which require special tooling. There are no engineering design charges for our standard designs. Step 1. Choose your Primary Windings On the Toroidal Transformer design sheet, enter the voltages that you would like or specify a custom voltage. |
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Step 2. Choose your Operating Frequency
The standard line operating frequency in North
America is 60 Hz. The rest of the world operates at 50Hz. A
transformer designed for 50Hz will also work at 60Hz but a transformer
designed for 60Hz will not work at 50Hz.
A 50Hz transformer is 20% larger than a 60 Hz design so allow space for a 50Hz design if you plan to export your product at a future date.
400 Hz is used for weight sensitive airborne and seaborne applications as it allows the transformer to be about 1/3 smaller.
Step 3. Specify your Power
Requirements
Our toroidal
power transformers are available from 7VA to 30KVA (Single Phase) or 90KVA
(Three-Phase).
You can specify the output load in one of two ways:
Method 1: Specify the AC (RMS) voltage and current or power (VA) and duty cycle for each secondary output. Please specify whether the secondary voltage are open or under load.
Method 2: Specify the DC load parameters. Please tell us the: DC voltage, current, rectifier type (full wave, full wave bridge, etc.) and specify the capacitor type and value, regulator type and specifications, any special load characteristics, including duty cycle.
If possible, please supply a schematic so that we can determine the optimum secondary DC specifications for each output.
Due to its construction with the magnet wire evenly distributed over the circumference of the core, our toroidal transformers have no hot spots like the laminated type.
If the transformer is not operating continuously under full load, please specify the duty cycle so that we can calculate a smaller power rating saving you space and cost.
Step 4. Specify the Insulation
Class
The standard
insulation is Class A (105șC). Class B (130șC) or Class F (155șC) are
available as options.
Step 5. Specify the Primary Windings (Input Voltages)
For 60Hz applications single phase:
117V, 208V 220V or 277V. 380V for three-phase configuration.
For 50Hz applications single phase : 230/240V and 440V for three-phase configuration.
For 50-60Hz worldwide applications single phase: 100V, 117/120V, 220V and 234/240V voltages. You need 100V/50Hz for the Japanese market.
If your overseas market and especially the Japanese market is very small, you would save money by having a dedicated 50Hz design as the price of a 60Hz design is lower.
Specify the Secondary Windings (Output Voltages)
You can specify the output loading in one of two ways:
Method 1: Specify the AC (RMS) voltage and current or power (VA) and duty cycle for each secondary output. Please specify whether the secondary voltage are open or under load.
Method 2: Specify the DC load parameters. Please tell us the: DC voltage, current, rectifier type (full wave, full wave bridge, etc.) and specify the capacitor type and value, regulator type and specifications, any special load characteristics, including duty cycle.
If possible, please supply a schematic so that we can determine the optimum secondary DC specifications for each output.
Step 6. Choose the Mounting Method
A metal washer and rubber pads provide the most economical way of mounting
the transformer horizontally and is adequate for transformers under 600 VA,
provided the transformer is not installed in a product that receives rough
handling. Above 600 VA, epoxy potted center is standard mounting method.
Prices include mounting hardware, either a steel washer and two rubber pads
or as shown for larger sizes, an epoxy potted center with one or two drilled
mounting holes.
Optional Potted Center
For a nominal cost you can order smaller transformers with epoxy potting, either with drilled hole or threaded inserts. The potting has a recessed top to accommodate screw head. Flat bottom is a standard feature.
The L-Bracket
An L-shaped steel bracket is an economical solution for vertical mounting. A metal washer and two rubber pads are furnished with each L-bracket. Larger transformers will have an epoxy potted center.
Standard Sizes of Steel Washers Dimensions in Inches Size A B C T D 1 2.00 .75 .188 .04 .18 2 2.38 .88 .188 .05 .22 3 2.75 1.13 .25 .06 .26 4 3.50 1.25 .25 .06 .28 5 4.38 1.38 .31 .08 .33 6 5.70 1.75 .31 .08 .50 
Step 7. Choose the Connections and Leads
Our standard design is to provide toroidal transformers with multi-stranded
leads.
Self leads and Connector assembly are also available as an option.
Step 8. Choose the Thermal Protection
When a fuse or thermal cut-out is desirable, we use a cut-out
that opens at 110C unless otherwise specified. You can choose between
auto-resettable or non-resettable thermal fuse. 95% of our designs have the
thermal fuse installed in-line on the primary leads.
Step 9. Choose the Static Shielding
An Electrostatic shielding (a.k.a. Faraday shield) may be
needed to minimize capacitive coupling between primary and secondary
windings if transformer operates in an extremely noisy environment.
Step 10.
Is your design sensitive to electromagnetic strayfields?
Our standard design greatly reduces stray fields compared to a laminated
transformer. For sensitive electronic applications, our optional Ultra-Low
Stray Field Design achieved through a proprietary process further reduces
the emissions. In applications such as audio equipment, high-resolution CRT
displays, a magnetic shield around the circumference of the transformer
achieves even lower stray field levels.
Step 11.
Factors that influence Physical Size and
Weight
Experience has shown that for the conventional transformer using stacked
lamination cores with air gaps, the working flux densities of 12 to 14
kilogauss are the practical limits, i.e. before the transformer goes into
saturation. Toroidal transformers can be designed with flux density of up
to 16 kilogauss. With this higher working flux density our toroidal
transformers require fewer turns and/or a smaller cross sectional core area.
We can significantly reduce transformer size and weight where the
transformer is loaded intermittently. In such cases, the load is energized
for a small time duration, which is much shorter than the overall thermal
time constant of the transformer. The same is true for transformers mostly
running at half the load. To know the actual duty cycle will therefore be
very helpful in designing the adequate power rating.
Step 12.
Power Ratings / Size and Weight
information
Typical sizes and weights
listed in the chart below serve as a basic guideline to determine size and
weight based on power (VA) rating. Height and diameter can be varied, as
long as the core cross section holds constant. The most economical height
to diameter ratio is approx. 1:2. You may specify the diameter and height
you would like or simply tell us the maximum physical envelope available.
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Quick Reference Table for Approximate Physical Sizes | ||||
| Rating | Dimensions OD x HT | Weight | ||
| VA | inches | mm | lbs | kg |
| 25 | 2.6 x 1.4 | 66 x 36 | 1.1 | 0.5 |
| 50 | 3.2 x 1.5 | 81 x 38 | 1.7 | 0.8 |
| 100 | 3.9 x 1.6 | 99 x 41 | 2.4 | 1.1 |
| 150 | 4.3 x 1.7 | 109 x 43 | 3.4 | 1.6 |
| 200 | 4.5 x 1.9 | 114 x 48 | 4.4 | 2.0 |
| 250 | 4.7 x 2.1 | 119 x 53 | 5.4 | 2.5 |
| 300 | 5.0 x 2.3 | 127 x 59 | 6.2 | 2.8 |
| 400 | 5.2 x 2.5 | 132 x 64 | 7.4 | 3.4 |
| 500 | 5.4 x 2.8 | 137 x 71 | 8.8 | 4.0 |
| 600 | 5.7 x 2.9 | 145 x 74 | 10.3 | 4.7 |
| 750 | 6.3 x 2.9 | 160 x 74 | 12.7 | 5.8 |
| 1000 | 6.6 x 3.0 | 168 x 76 | 15.4 | 7.0 |
| 1500 | 7.8 x 3.5 | 198 x 89 | 26.0 | 11.8 |
| 2000 | 8.4 x 3.9 | 213 x 99 | 34.0 | 15.5 |
| Power rating (VA) is determined by secondary RMS data. Physical size may vary from above data depending on number of primary and secondary windings and whether duty cycle is 100%. | ||||
Step 13. Choosing the best Rectifier Circuit for your application
When using a toroidal power transformer, some rectifier circuit designs are more efficient than others. Four typical circuits are illustrated here with recommendations. Consult us for further information and assistance.
Dual Center Tap Rectifier
Full Wave Bridge
Full Wave Center Tap
Half Wave Rectifier
This is a very efficient use of toroidal transformers, and the best choice for two balanced outputs with a common return. The output windings are wound for precisely matched series resistance, coupling and capacitance
Full wave bridge is the most efficient use of toroid technology and secondaries; best for high voltage outputs.
The full wave does not make full use of secondaries. However, it is more
efficient than the half wave. It is good for high current, low voltage
applications.
Avoid half wave rectifier circuits, as they are an inefficient use of toroidal transformers. They cause the core to become polarized and saturate in one direction.
Output voltage regulation varies with the size of the toroidal electrical transformer. Regulation can be improved by selecting a transformer with a higher VA rating than actually required.
Step 15. Avoiding Shorted Turn Condition
A completed path by any
conductor passing through the center hole of the toroid constitutes a
shorted turn. A through-the-center screw making contact to the chassis at
both ends can inadvertently establish a shorted turn. As with any short
circuit, this condition will result in high circulating currents and high
local heat. Such mounting must be avoided.
Step 16. Avoiding the Nuisance Inrush Current Phenomeon
Because toroidal
electrical transformers have excellent magnetic properties and no air gaps,
the inrush current when power is turned on is sometimes higher than with
stacked transformers. Inrush current can be as high as 15 times the peak
steady state rated current. It typically lasts less than 10 ms and rarely
lasts over a half cycle.
Transformer Rating
Suggested Protection Up to 300VA
None 300VA to 1,000VA
Use slow-blow fuse in primary circuit 1,000VA to 2,000VA
Add a small value resistor in series with primary circuit. 2,000VA and up
Add a by-pass relay that will momentarily short out the resistor after 100-200 milliseconds. An NTC thermistor may be sufficient for some applications.
Step 17. Aspects of Efficiency
The graph illustrates the effect of increasing load on the efficiency for
various nominal ratings.
Using an
oversized core will increase the efficiency.
Temperature rise varies with the actual output power (P-out) in relation to nominal power (P-nom) for a given core size.
Our basic design guideline is not to exceed 55 șC to comply with Class A (105 C) and Class B (130C) requirements for room temperature applications with a comfortable safety margin. . Actual temperature increase will depend on how and where the transformer is mounted and how well it is cooled. When higher temperature ratings are needed, we offer transformers built to Class F (155C).
Please specify the temperature rise or tell us the operating ambient temperature.
Using a larger core size will reduce the temperature rise. The small core losses will cause the temperature rise to drop drastically when reducing the output power. At half the load, the temperature rise will only be about 25% of the rise at full load.
Total losses for the transformer, including winding loss and core loss per pound of silicon steel at a given flux level, may be calculated from design data and data furnished by steel suppliers. The graph illustrates the rise in transformer temperature as the actual power approaches the toroidal transformer's nominal power rating.
