What is a "TRU" (Transformer Rectifier Unit)?

[cupoftea]

Hi,

Can you advise what exactly is a “Transformer Rectifier Unit”? (TRU) Google doesn’t help much.

1...Does it always involve an iron transformer rather than ferrite?
2...Does it usually operate at around 50-60Hz? (ie not 50khz plus)?
3...Is the “rectifier” usually a thyristor based rectifier? If so is it leading or trailing edge fired?
4...What is the power factor of a TRU like?
5...What sort of loads do they supply?
6...Is it usually used in 3 phase systems? (is it only used in 3 phase systems?)
7...Is it for use where ferrite based SMPS’s simply cant handle the power levels?
8...Is the switching frequency of the “rectifiers” always less than 1khz? (and most often around 100-120Hz?)
9...Presumably the supply for the rails of the London Underground trains come from a TRU?
10...Do any EV chargers use TRU’s?
11...What power level is it where a TRU must be used?
12...Most SMPS (eg full bridge) have a “transformer and a rectifier”, so why are they not called a “TRU”?
13...Do they usually use big capacitor banks to smooth the output?...or is the output usually inductive?
14...Does a TRU ever use IGBTs?

 

[scopeprobe]

Hi,

Can you advise what exactly is a “Transformer Rectifier Unit”? (TRU) Google doesn’t help much.

1...Does it always involve an iron transformer rather than ferrite?
2...Does it usually operate at around 50-60Hz? (ie not 50khz plus)?
3...Is the “rectifier” usually a thyristor based rectifier? If so is it leading or trailing edge fired?
4...What is the power factor of a TRU like?
5...What sort of loads do they supply?
6...Is it usually used in 3 phase systems? (is it only used in 3 phase systems?)
7...Is it for use where ferrite based SMPS’s simply cant handle the power levels?
8...Is the switching frequency of the “rectifiers” always less than 1khz? (and most often around 100-120Hz?)
9...Presumably the supply for the rails of the London Underground trains come from a TRU?
10...Do any EV chargers use TRU’s?
11...What power level is it where a TRU must be used?
12...Most SMPS (eg full bridge) have a “transformer and a rectifier”, so why are they not called a “TRU”?
13...Do they usually use big capacitor banks to smooth the output?...or is the output usually inductive?
14...Does a TRU ever use IGBTs?

My view of a TRU is it takes a AC Voltage (Single or 3Phase) and converts it to a DC voltage. To do this it uses a Low Frequency Transformer (50/60Hz or 400Hz in aircraft) and a Bridge Rectifier hence the name.

1...Does it always involve an iron transformer rather than ferrite? Iron Core

2...Does it usually operate at around 50-60Hz? (ie not 50khz plus)? It operates at mains frequency the transformer just takes the AC current and transfers to a secondary winding at a higher/lower voltage which is then rectified.

3...Is the “rectifier” usually a thyristor based rectifier? If so is it leading or trailing edge fired? At Low power It can be standard rectifiers but higher powers it requires Active rectification with Thyristors to reduce losses. The diode needs to be on through the entire half cycle so switching would need to be around the zero crossing point.

4...What is the power factor of a TRU like? Given its a transformer power factor can be corrected using line capacitors power factor can be high > 0.98

5...What sort of loads do they supply? They Provide DC so it can be anything where you have to get Unregulated DC from an AC Line, Often the DC loads connected to a TRU can have their own power supplies to regulate locally.

6...Is it usually used in 3 phase systems? (is it only used in 3 phase systems?) They can be Single or 3 phase it really depends on the power of the system.

7...Is it for use where ferrite based SMPS’s simply cant handle the power levels? No they can be used as required, TRUs are inherently low noise as they operate from low frequency mains so can be favourable where low noise, unregulated DC power is desirable. They don't normally regulate. The beauty with the transformer is they can simply stepup or step down line voltage. A TRU would normally form the front end of a Linear Power Supply. The only real difference would be the addition of regulation and current limiting features for it to be considered a power supply. They are also inherently reliable due to low component count.

8...Is the switching frequency of the “rectifiers” always less than 1khz? (and most often around 100-120Hz?) The rectification is typically a Bridge Formation so 2 diode will forward bias on each half cycle of the mains which as you state is (100Hz for a 50Hz system and 120Hz on 60Hz). Aircraft and Miltary often use 400Hz Mains which means a 800Hz Zero Crossing point.

9...Presumably the supply for the rails of the London Underground trains come from a TRU? Outside of my knowledge base but If its a DC system then i would think so, if its a DC system i would presume a high DC voltage level to keep current on the track to a minimum over long distance. I would have thought in this case they would use multiple TRU at feeder points accross the network.

10...Do any EV chargers use TRU’s? Don't know sorry

11...What power level is it where a TRU must be used? You can use it where its practical.

12...Most SMPS (eg full bridge) have a “transformer and a rectifier”, so why are they not called a “TRU”? A PSU has regulation a TRU doesn't.

13...Do they usually use big capacitor banks to smooth the output?...or is the output usually inductive? They normally specifiy the output voltage ripple.

14...Does a TRU ever use IGBTs? Designers perogitive

 

[cupoftea]

Thanks, Can you make them regulate their output voltage by having a big output capacitor bank, and varying the firing angle of the thristors?

 

[scopeprobe]

Thanks, Can you make them regulate their output voltage by having a big output capacitor bank, and varying the firing angle of the thristors?

It will adjust the DC Level as thats effectively a switched capacitor circuit but they are typically inefficient at higher powers which is why SMPS use L's and C'. Also i'm not sure how well the tranformer would respond and for sure the power factor and harmonic content would be ugly. Thats why HF SMPS use PFC front ends.

 

[cupoftea]

Thanks, i think above 1kW (for industrial users) , there is no legal need for PFC.....?...but companies who use over 1kW equipment have to pay for their reactive power?

I think most SMPS above 1kW have PFC anyway, but its to reduce the rms input current? ..rather than legal requirement? (unless for domestic non-industrial use, in which case, PFC is needed >1kW)

 

[FvM]

Seriously, it makes no sense to read the discussed details into a simple and unspecific technical term.

As far as I'm aware of, TRU refers to devices that are comprised of a transformer and a rectifier as main components.

 

[cupoftea]

As far as I'm aware of, TRU refers to devices that are comprised of a transformer and a rectifier as main components.

Thanks, and i think its more than that.....
It refers to a power level where ferrite based SMPS cannot be used.
Also, the rectifiers are "controlled rectifiers", which are switched at most at say 500Hz....but usually 100Hz.
They are virtually always three phase, otherwise output capacitance requirement is ridiculously high.
There is no PFC requirement, but the co's using them pay for their reactive power.

A "TRU" is the only way to get a regulated DC vout at extremely high power levels.

A "TRU" is by definition, a massive big chunk of iron and copper, with huge semiconductor switches ( controlled rectifiers). A 100W mains transformer followed by a full wave bridge would not be termed a "TRU" (even though it is one)


Would you agree?

 

[KlausST]

Hi,

What power levels are you talking about?

Klaus

 

[FvM]

Thanks, and i think its more than that.....
It refers to a power level where ferrite based SMPS cannot be used.

Empirically wrong. I see e.g. a reference to a 84 W supply https://www.electrocube.com/pages/transformer-rectifier-unit-tru-data-sheet

This supports my assumption that "TRU" is a rather unspecific technical term, used in different industries with partly different meaning. Respectively it's useless to read-in an exact specification. Or to hold one usage against another in a different industry.

One application range is aviation power supply, here "TRU" apparently refers to transformer/rectifier combination with 28 VDC output and several ten W to kW rating, as far as I understand typically without regulation. If you have other applications in mind, you should quote references.

 

[cupoftea]

Thanks, i see, you are correct, we have to remove the spec of power in "TRU" definition......but it does appear that we can say a "TRU" is definetely not containing a ferrite transformer.
It would be interesting to know what is the max frequency of operation of controlled rectifiers in a "TRU"?....Also, whether they ever feature closer vout regulation by control of the controlled rectifiers?
Also, whether they are always three phase input, or where single phase, they feature single phase to 3 phase transformer?