Transformer-Rectifier Units – An Overview

Transformer-Rectifier Units
An overview

Ralph Livingstone
US Magnetics©2015

The Transformer-Rectifier Unit (TRU) is comprised of a transformer combined with a rectifier assembly designed to convert AC mains power (typically from a three-phase source) to unregulated or semi-regulated bulk DC power. The multi-phase TRU, like an active PFC (Power Factor Correction) front end is capable of suppressing undesirable input harmonic currents – but unlike the PFC front end has very few active components. It is therefore much less susceptible to damage caused by system transients or high temperatures. The decision whether to use a TRU or an active PFC front end in a system typically involves the consideration of weight vs. circuit complexity and MTBF.

In the active PFC circuit, AC mains power is converted to a DC bus voltage which is higher than the peak rectified AC input voltage at high line. This is accomplished by the use of a pulse-width modulated boost converter – with the PWM control tracking the input sine voltage in a manner which results in the presentation of a linear load to the input mains. The circuitry used to accomplish this desired result is fairly complex – and typically includes an integrated circuit and a hundred or more support components for each phase – including power semiconductors. The result is a circuit that is light in weight but with a parts count that adversely lowers system MTBF. The TRU compares extremely favorably to the electronic PFC in terms of reliability, ruggedness and MTBF – typically at the cost of increased weight. The exception is in some high-voltage bus situations, when the use of a non-isolated ATRU (using an autotransformer) may actually result in overall weight savings compared with the PFC circuit.

The TRU typically includes only the transformer and 6 to 24 active components – the output rectifiers.
The multi-phase TRU accomplishes power-factor correction and input harmonic current suppression by forced control of the output rectifier conduction angles and durations. This is achieved by a combination of the transformer secondary winding configuration and the inclusion of planned distributed leakage reactance within the coil winding structure. Secondary winding interconnections are designed to result in overlapping rectifier conduction periods – occurring in the time domain in a manner designed to minimize input harmonic currents. Various winding configurations may be used to achieve the desired result; from a simple 6-phase diametric star (wye) connection with interphase inductor to the complex 24-pulse interconnected star connection. Commonly seen secondary connections include 6, 9, 12, 15, 18 and 24-pulse configurations with both star- and delta-based interconnect schemes.

An example of one method used to shift rectifier conduction angles is the case of the interconnected star. In this illustrated example, a 9-phase, 18-pulse secondary is desired. A star-based secondary of 25 turns to neutral is the starting point for the design example shown in FIG 1. The initial three output phases are defined in terms of phase rotation as 0°, 120° and 240°. In order to meet the requirement for a symmetrical 9-phase secondary output, six additional phases will have to be “built” at 40°, 80°, 160°, 200°, 280° and 320° rotation.

Our building blocks for the new windings are segments of the three existing secondary phases, limited in flexibility only by the necessity to use integer turns. By combining unlike segments of the three original phases, a vector-equivalent of each required new phase can be constructed. The ratio of the integer turns of the individual winding segments will determine vector phase angle and amplitude, as well as secondary current reflected back to each original primary phase during the period of rectifier conduction of the vector phase. As shown in FIG 2, a combination of 10 turns on Phase A coil connected in series opposing with 18 turns on Phase C coil results in a new combined winding. In FIG 3, using the Law of Cosines (b^2 = a^2 + c^2 -2ac COS β), the equivalent vector amplitude of the new winding can be determined. Using the Law of Sines (SINα/a = SINβ/b), the equivalent phase rotation of the new combined winding can be calculated. The new winding has the equivalent amplitude of 24.58 turns and an equivalent rotation angle of 39.36°. These results are within a few percent of the goal for a 25-turn winding at 40° and are fairly typical of the degree of accuracy acceptable in a combined winding. The remaining added phases would be constructed in a similar manner, using a balanced selection of combined phases.

Another type of TRU is the single-phase TRU. Providing unregulated or semi-regulated DC power from a single-phase AC source, this type of TRU does not provide power-factor correction or suppress input harmonic currents. This is not typically an issue on a system level however, since this type of TRU is generally employed to provide a source of unregulated DC to low-power loads such as instrument lighting, solenoids and relays etc. The level of the output load compared to total system power capacity is typically low enough so as not to adversely affect overall system performance. Some improvement in power factor as well as in load regulation can be achieved by the addition to the TRU of a swinging inductor in series with the DC storage capacitor. The varying of the output inductance with load tends to limit output voltage under lightly-loaded conditions, and to “equalize” the DC output as the load varies. This economical and effective design technique is sometimes used in aircraft instrument lighting dimmer controls.

Methods of cooling

Because of the advantages of reducing weight, free-air convection is seldom used as a method of cooling a TRU with an output capability over a few hundred Watts. Air Flow Through (AFT) cooling, the use of onboard fans, conduction cooling or Liquid Flow Through (LFT) are some of the most common methods of cooling. Each method has some advantages and disadvantages in terms of cost, reliability and weight savings.
In systems already incorporating an avionics AFT cooling system, taking advantage of the existing system is a logical choice, assuming adequate air system capacity, and that a suitable cooling port is available. Using the AFT system precludes the need to install onboard fans or a bulky baseplate, so considerable savings in both weight and cost can be achieved. Because the cooling blowers are external to the TRU, there is little added effect on overall system maintenance.
Conduction cooling by means of a baseplate has the advantages of low maintenance and high reliability because of the absence of fans. It also allows the construction of a fully sealed unit – ideal for service under harsh environmental conditions. The main drawback of the conduction-cooled TRU is the increased weight of the transformer and its mounting provisions resulting from the lack of airflow over the transformer windings. The added baseplate also adds to the overall weight of the unit.
The fan-cooled unit allows the reduction of TRU weight and size without the need for centralized cooling system airflow. Passing forced air directly over the TRU transformer windings allows the use of smaller conductor cross sections, but makes environmental protection of the TRU components more problematic. Since the fan is a moving part, it is much more prone to failure due to mechanical wear or shock and vibration than other TRU components. The fan therefore becomes a maintenance point – requiring periodic inspection or replacement if used in critical applications. Since the proper cooling of the TRU depends on the operation of the fan, monitoring of its operation through a locked-rotor alarm or an over-temperature sensor is typical.
When LFT cooling using either PAO or water is available on the system level, optimum cooling efficiency and reduction of weight can be achieved. The higher thermal transfer coefficient and thermal capacitance of liquid results in greatly improved cooling efficiency compared to air. The routing of the path of the cooling liquid close to critical components and to thermal hot spots within the TRU results in superior localized cooling. Since the liquid coolant used is incompressible, greater efficiency in pumping coolant can be achieved compared to moving compressible air. The disadvantages of using LFT cooling include the higher initial cost of hardware, and the increased labor required during the initial hardware installation.

When a TRU is selected for a new application, it is very important that the system designer discuss the application in detail with the designer of the TRU. Active system elements such as pulsed loads and reactive elements such as capacitors may require changes to the TRU to meet system harmonic current limits. Active circuits such as motor controllers may necessitate the addition of transformer shields and EMI filtering to prevent conducted EMI “blow-through” to the input source. The dynamic performance of a TRU in the fielded system may be very different than its operation in the test lab with a linear load. Typically, the TRU designer and the system engineer will work closely together throughout the system design process to forego any problems during system qualification testing.

The TRU can be an ideal choice to provide bulk DC power in systems where economy, high reliability and ruggedness are high priorities. They are particularly suited to difficult applications such as in shipboard power systems or when providing high-power DC to motor controllers, where line and load transients and regenerative feedback make the use of the PFC front end problematic. At lower power levels, the single-phase TRU can be used to supply bulk DC power to lighting devices, solenoids and relays, and to electronic circuitry that may have a fairly wide input range or onboard voltage regulation. Specifying a TRU in your next system may result in a very desirable net improvement in overall system MTBF. Contact US Magnetics to discuss how we can assist you in determining if a TRU may be your smart choice.