Combining different types of Power Conditioner

Generator and Constant Voltage Transformer

Sophisticated computer systems sometimes use diesel engine driven generators for backup in case of mains failure. Where our CVTs are used for power conditioning in the normal mains mode it is desirable to take advantage of the CVT performance when using the generator. These notes provide some guidance on the potential problems which can be met together with solutions.


Some generators do not refer the low side of the output to earth. This MUST be tied down to avoid damage to any of our larger catalogue units which have double primary shields.

Care must also be taken that the generator neutral is not connected to the CVT output low.


All ferroresonant devices are frequency sensitive. The generator must run close to 50 Hz for the unit to operate correctly. Unfortunately the speed/output voltage curve for the generator goes the same way as the CVT so speed should be adjusted at actual running load.

We publish a curve for the effect of frequency variation in our faq001.

Short term off-frequency operation will not damage the CVT.


The output from the CVT will be out of phase with the input in cases where the generator supplies other equipment directly some care is needed if a phase sensitive triac firing circuit is installed.


Unless phasing circuits are fitted all circuits should use 'break before make contactors and enforced supply separation. Some thought needs to be given to the Regulations regarding out of phase supplies in the same area.

Constant Voltage Transformers and UPS


Modern on-line UPS give excellent performance when applied as expected by the designer.

Sometimes superior noise immunity or voltage variation handling is required. This is usually accommodated by adding a power conditioning product to the UPS. Such installations are attempting to provide a much higher protection than provided by the simple RF filters in the UPS which are designed to stop internal noise leaving the UPS.

Potential pitfalls

The UPS was probably not designed with the expectation that it would be driven from something other than an ordinary mains supply. Some UPS have output circuitry which does not manage difficult loads very well. Once the user decides to add the two products together it becomes apparent that there are several different ways to connect up the critical load. These different approaches give rise to various problems which can cause problems to the unwary.

Alternate scenarios

The UPS will normally have an automatic bypass if it is of the on-line type. We recommend that all such units should also be fitted with a maintenance bypass (either manual or automatic) for properly planned installations. The main problem is to assess whether or not the power conditioner should be put in front or behind the UPS. There also needs to be some consideration about how the bypass operates and whether or not power conditioning is available in either bypass mode.


It is not often realised that a UPS supplied in today's modern market place may have characteristics which are not properly described in the sales literature.

For example the UPS output power rating is usually quoted with a 'power factor'. The implication of this is that the unit may be rated at 1000 VA but is quite incapable of supplying 1000 watts.

An on-line UPS has to support the full rated load whilst the system is recharging it's batteries. This usually means that the unit requires considerably more input power than expected from the output rating.

The UPS may not have facilities to allow the bypass line to be connected to a different supply from the normal incoming mains.

The two major transformer based power conditioning technologies are 'ferro-resonant' or 'low impedance' types. Both also have internal inefficiencies and will require more power at the input when fully loaded than is available at the output.

The ferro-resonant types often based on CVT technology also have power factor considerations at both input and output.

The CVT is also less efficient than conventional transformers. However the CVT represents one of the most effective ways to solve noise and voltage problems when applying UPS products in tough working environments.

Working solutions

UPS + low impedance conditioner (AIT)

AIT powering UPS

In this case the conditioner may be used in front of the UPS to protect it against mains borne spikes and common mode noise. Clearly if it protects the UPS then it will protect the load against the same mains borne problems. The AIT must be rated to provide enough power for the worst case input requirements of the UPS. These must be considered for an exhausted battery full load and worst case temperature conditions. In the absence of proper data use a rule that the AIT should have a rating which is at least 50% bigger than the UPS.

UPS powering AIT

It is also possible to use the AIT after the UPS to provide galvanic isolation and/or protect the critical load against common mode noise generated by the UPS. The AIT will NOT provide any protection to the UPS input circuitry. With this connection attention must be given to the capacity of the UPS to drive the AIT. Typically the AIT wattage rating should be no larger than about 50% of the rated VA of the UPS.

UPS + CVT power conditioner

CVT powering UPS

There are huge benefits to feeding a UPS from a CVT based power conditioner. These include lightning protection better battery recharge times and a benign mains feed for the UPS. The mains becomes essentially clean and complements the battery back up features of the UPS. In this case however more problems are apparent. It is important to remember that the output of the CVT is completely Galvanically Isolated and that most UPS need an earthed neutral to operate correctly, when connecting a UPS to the output of the CVT ensure the CVT is output low connected to Earth.

Further attention should be given to the fact that the CVT may provide a clean sinewave into a resistive load but it may not provide the high crest factor current required by some UPS chargers. This can cause the UPS to run it's batteries flat.

UPS powering CVT

The CVT is a resonant device which has unusual input characteristics. (See faq001) Consideration must given to the fact that the input power factor of the CVT is affected by line voltage and load value. Some UPS units will not drive inductive loads at all. Others may struggle to cope with the varying power factor. Either way the UPS load rated at 1000VA with a 0.6 PF rating on the output can only drive a CVT which is considerably smaller than one designed to deliver 1000 watts. A useful rule is that the CVT should be considered to be 80% efficient and therefore the rating is:

UPS load rating VA * PF * 0.8

So a normal 1000VA UPS may only be adequate to drive a 480 watt CVT! For CVTs driving the input of the UPS the issue is entirely one of the worst case UPS needs. The rule above for the AIT can be used.


The combination of power conditioners requires serious planning BEFORE buying equipment. If the two units have similar ratings they will probably not operate satisfactorily in any combination.

If one item is already installed it probably means that the only combination which will work is either a larger unit in front or a smaller unit after the existing one. This usually means the second unit is too small to drive the critical load!

Our engineers would be delighted to assist with any problems relating to a specific installation.

Generators driving UPS

Sophisticated ICT systems sometimes use local engine driven generators for backup in case of mains failure

Some modern small UPS boxes are designed to operate primarily from traditional mains supplies and some care should be taken when operating them from a small generator

These notes provide some guidance on the potential problems which can be met together with suggested practice

Size of generator

The average modern UPS need a much larger generator than one might expect. Allowance must be made for re-charging the batteries as well as providing full load and the effects of UPS efficiency.

Typically Advance recommends a generator capable of delivering at least TWICE the output power of the UPS however some generator manufacturers quote unrealistic performance specifications and it is best to check with our engineering team.

In addition some UPS input circuits have high crest factors and generator control circuits cannot handle them.

The Master Elite range of UPS has a specially developed input filter for use with generators and other sensitive sources.


Some generators do not refer the low side of the output to earth. This MUST be `tied down' to avoid possible damage to the UPS

Care must also be taken that the generator neutral is connected to the UPS input `low'

Conventional mains phasing is expected by the UPS and the generator should be wired through to the UPS input in the normal way


The generator must run close to 50 Hz for the unit to operate correctly if the frequency goes outside the UPS operating window it will operate from the UPS battery which will not be re-charged

Electrical noise

Some medium sized generators may be connected to other loads. In this case care must be taken to ensure that the other loads do not create excessive spikes or other electrical noise

The generator exhibits a much higher impedance than a conventional mains supply. Noise from triac controlled motors etc. may affect the UPS rather than just disappear in the distribution system

Clearly any generator which is not brushless may also cause a similar problem

A generator providing a clean waveform with a reasonably stable frequency and voltage correctly connected only to the UPS is the most attractive arrangement


Some thought needs to be given to the Regulations regarding out of phase supplies in the same work area