Osborne Transformer Blog

Alpha Version of Spec Tool

2007-01-05T12:57:07Z

Have you tried the spec tool yet? It´s pretty fun. Check it out.

Our programmers are calling this the alpha version of the spec tool. We think it´s good enough to share and we know there are some bugs. You´re invited to help direct the process of improving the tool. Please share any fixes or features that you´d like to see. We´ll continue to rely on your opinions to guide future tool upgrades. Thanks for sharing your thoughts.

Concerns Using Reduced Duty Cycle Transformer

2007-01-05T05:08:02Z

Designing transformers to take advantage of low duty cycle applications can save on physical size and save on original investment cost. But if the user does not appreciate what duty cycle means, it can quickly mean the loss of the original investment.

]]> When sizing a Reduced Duty Cycle transformer, there are some important facts to consider:

1.) The On-Off cycle must be considerably shorter than the thermal time constant of the transformer. Thermal time constants for transformers are usually expressed in hours. A 50 KVA, 100% duty cycle transformer has a thermal time constant of approximately 6-8 hours. With an On-Off cycle of less than 60 seconds, you can consider taking advantage such a design.

2.) The Duty Cycle is calculated as follows:
Duty Cycle (As a Percent) = [Cycle On Time / (Cycle On Time + Cycle Off Time)] x 100
Example: On-Time = 6 seconds, Off-Time = 14 seconds, Duty Cycle = [6 / (6 + 14)] x 100 = 30 Percent

3.) The relative KVA size is calculated by multiplying the 100% Duty Cycle KVA size by the square root of the duty cycle.
Example (Single Phase Application):
If the Duty Cycle = 30.0 Percent and the load during the On-Time = 50 KVA,
the Design KVA = 50 KVA x SQRT(0.300) = 50 x 0.548 = 27.4 KVA Design.
For easy calculation, assume the On-Time load is 100 Volts and 500 Amps (50 KVA).
The "Continuous" (100% DC) Design Current is 27.4 KVA / 100 Volts = 274.0 Amps.
Summarizing, a transformer designed to deliver a continuous current of 274.0 Amps
can perform well in providing 500 Amps, operating at a 30.0 percent duty cycle.

If a user is not aware of using a reduced duty cycle transformer, they may inadvertently increase the duty cycle beyond the capability of the transformer. We recently had an automotive parts customer who had been using a 30 percent duty cycle transformer successfully for many years on an induction heating machine. In designing an additional production line, the user desired to reduce the cycle-time and increase the thru-put of the new line. By increasing the duty cycle to 50 percent, they catastrophically destroyed two transformers before determining the root cause for the failure.

Calculations showing the impact of the application are:
"Design" Duty Cycle: 30 percent, On-time = 6 seconds, Off-time = 14 seconds
"Actual" Duty Cycle: 50 percent, On-time = 6 seconds, Off-time = 6 seconds
Design Equivalent Continuous Current (30% Duty) = 500 x SQRT(0.30) = 274.0 Amps
Actual Equivalent Continuous Current (50% Duty) = 500 x SQRT(0.50) = 353.5 Amps

Since copper losses vary with the square of the current, running a transformer at 353.5 amps rather than 288.5 amps results in increased copper losses by (353.5 / 274.0)^2 = 1.66 times.

By increasing the power dissipated in the transformer windings by 1.66 times (66% increase), the result is typically an significantly overheated transformer. If the design temperature rise is 115 degrees Celsius, and the actual rise is 1.66 x 115 = 191 degrees Celsius, the life of the insulation system is seriously compromised.

When properly applied, a reduced duty cycle transformer does save size and money. But when misapplied, the results can damage your equipment. Don´t let inappropriate transformer designs waste your time and money.

New And Improved Website

2006-12-30T20:42:21Z

Welcome to the new and improved Osborne Transformer website. We're very excited about the new look and feel, and in particular we'd like you to check out our new spec tool, which offers you the ability to define characteristics that you're looking for in 17 types of custom transformers and inductors.

Using a Transformer as a Switch

2006-12-30T17:59:17Z

An electrical transformer is an impedance matching device. This is a well-known fact. Here is an interesting application taking advantage of that fact. If you load the secondary of a transformer with zero (0) ohms (short circuit), the impedance looking into the primary is also zero (0) ohms. Conversely, if the load on the secondary of the transformer is infinite ohms (open circuit), the impedance looking into the primary is also infinite ohms.
Now, if you insert the transformer primary in series with any load across a voltage source, you can switch the voltage to the load on and off. If you short the secondary, the voltage source is applied directly to the load since the transformer primary impedance is zero (0) ohms and all the voltage is fully dropped across the load. And, if you open-circuit the secondary, the source voltage is now fully dropped across the infinite primary impedance and zero (0) volts reaches the load.

Unfortunately electrical transformers are not perfect switches because of inefficiencies. The primary and secondary windings have some impedance which is present even when the secondary is short-circuited. Further, for a transformer to function, there is an excitation impedance present which prevents the primary impedance from reaching infinity, even when the secondary is open-circuited.

But, when a designer knows the load impedance, and the actual open-circuit and short-circuit impedance's of the transformer, they can very often use this application to perform as effective switch. A good application for such a switch would be where the actual switching is done in a high-voltage circuit but the recognition of the switching action is required in a low-voltage control circuit. The transformer insulation system will isolate the high-voltage switch from the low-voltage control.

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Interesting Research at MIT

2006-12-29T07:55:55Z

This from a press release type post at Renewableenergyaccess.com:

"Work at MIT's Laboratory for Electromagnetic and Electronic Systems (LEES) holds out the promise of the first technologically significant and economically viable alternative to conventional batteries in more than 200 years."

A Saturable Reactor is a Magnetic Amplifier

2006-12-29T05:56:35Z

Electrical engineers are not often taught in undergraduate school why a saturable reactor is often called a magnetic amplifier. So, here is a post to explain why. A saturable reactor is a magnetic amplifier since it can control the transfer of a significant amount of AC volt-amperes with a relatively small amount of DC volt-amps. The amplification factor is the ratio of the output volt-amperes divided by the DC control volt-amperes.

]]> Saturable reactors are used to transfer a system voltage to a load through a typical series impedance division circuit. The saturable reactor is in series with the load impedance and the combination is connected across the system voltage. The magnitude of the voltage reaching the load is determined by the ratio of the load impedance to the total series impedance (load + reactor).

By reducing the impedance of the saturable reactor, the load impedance becomes a higher percentage of the total impedance and thus more of the input voltage is dropped (transferred) across the load. Since both the saturable reactor and the load may consist of resistive and reactive components, each device has an impedance that is the vector sum of its resistance and reactance.

The method used to change the impedance of the saturable reactor is to change its inductance, thus its inductive reactance and therefore its impedance. The inductance is changed by changing the permeability of the magnetic core.

With no DC control current present, the permeability of the core is at its highest value and the inductance is therefore at its highest value. An series analysis will show how much of the system voltage reaches the load under this condition. With the application of a DC control current significant enough to saturate the inductance, the impedance of the reactor is reduced to nearly the value of only its resistance. Under this excitation, the greatest value of the system voltage is dropped (transferred) to the load. For the full system voltage to reach the load, the load impedance must be significantly higher than the resistive component of the reactor.

A typical amplification factor for a 10 KVA saturable reactor, requiring 150 Watts of DC control for full output, is 10,000 / 150 = 67. This is why such a device is also known as a magnetic amplifier.

Advantages for Repairing a Transformer

2006-12-29T04:15:04Z

The process of repairing transformers can be a dirty job. But tearing down a transformer can be very educational and technically rewarding. I recently had the opportunity of disassembling a defective transformer for a local utility. This transformer is part of a 1950's vintage transmitter used to locate "system faults" on a 4,800 volt distribution system.

Closely evaluating designs during their disassembly provides a technical education of design aspects that have been successful, in some cases, for many years.

Carefully monitoring the disassembly can often provide knowledge of why the design failed, thus providing an opportunity to improve the device when re-manufacturing.

Researching the techniques used by the original transformer manufacturer.

Satisfy a customer. Supporting a customer in maintaining equipment of older technology is often a valuable asset.

Repairing a transformer provides an opportunity to recycle reusable material rather than adding to landfill.

In conclusion, disassembling and reverse engineering a failed transformer can be a dirty job and the process is very educational.

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Awareness in Choosing Inductors for Simulating Automotive Loads

2006-12-28T22:14:02Z

Passive inductors are often used in test equipment for Design Verification (DV) and Functional Testing (FT) of automotive and other vehicle loads. Test equipment manufacturers should be aware of the characteristics that determine the physical sizes of these components.

]]> Inductors are often used with series resistors to either simulate the shape of the initial current transient by the L/R ratio or to simulate the energy storage and resulting transient voltage generated by the L*di/dt at circuit turn-off.
The size of the inductor is determined by the current flowing thru the inductor, the duty cycle of that current flow, the inductance required and the DC resistance of the inductor.
To maintain a stable current throughout the duration of the testing process, it is necessary to keep the DC resistance of the inductor relatively low. The inductor is most likely wound with copper wire. If the inductor wire size is not properly picked, it will heat up during the testing process, resulting in lower test currents in the devices under test (DUT) after a period of testing. This then results in lower test currents, lower stored energy and lower transient voltages being applied to the DUT.
Actual loads on a vehicle are allowed to heat up and may have lower operating currents over time. But when providing short-term tests, it is desirable to keep the test uniform for all devices being tested.
Designing for relatively low DC resistance in the simulation inductor may make the inductor larger than the actual tested component. Further, a passive resistor is also used to maintain a stable total-circuit resistance. The resistance of the this resistor does not change much during long testing periods due to its low coefficient of resistivity. In selecting this resistor, the engineer must be aware of the heat produced and its effect on the test circuit layout.
Quite often design engineers, in trying to minimize the physical size of the overall test circuit instrumentation, neglect the importance of properly sizing the passive inductors and resistors.

Creating Circuit Diagram Illustrations

2006-06-20T17:49:03Z

This PDF from MSU's engineering school describes how to create circuit schematic diagrams (PDF) using Microsoft Visio 2003 and it's electrical engineering tool set.