Using Constant Voltage Transformers outside published specification

All Advance catalogue CVTs are supplied and guaranteed to a published specification. In general the CVT is specified to industry recognised norms and can be operated well outside its described performance specification. This note describes where liberties may be taken and should be read in conjunction with our GT generic specification which gives curves for many situations. This very robust product is particularly suitable for the unpredictable electrical mains supply in third world countries. Our CVTs are still the most reliable and effective mains protection for such applications.

High input voltage

if the CVT is operated with the correct input fuse or circuit breaker it should work OK until the protection opens at about 150% of the nominal input voltage rating.

The output voltage will rise with increasing input at about 20% of the change - i.e. if the input goes up 5% the output will go up about 1%.

Low input voltage

the output voltage will sag as the input voltage falls.

To operate under expected very low input voltage select a larger unit than normally required. Under loading the unit will provide significantly improved results. Most units will provide usable power down to 30% of the rated input voltage.

Non-sinusoidal input voltage

If it is the correct frequency and alternating the CVT will operate.

A THD up to 25% or even a square wave is NO problem for short term durations.


The CVT may deliver up to 50% more power than specified this is very dependant on actual input voltage.

After this the unit will self protect by reducing the output voltage progressively until it reaches nearly zero. The unit can be operated into a short circuit indefinitely. Electric motors take large currents at switch on. If the CVT will start the motor it is big enough.

Power factor loads

Inductive loads depress output voltages and can usually be corrected by adding capacitors.

Capacitive loads have the opposite effect. If you can tell us about the load we can usually advise how to drive it.

Switching loads

Ordinary switched mode power supplies are particularly suitable for use with our CVTs. Care must be taken with units with self adjusting input voltage arrangements. Some dimmer circuits or phase controlled circuits can cause problems.

Wrong frequency

1 or 2 Hz off the correct frequency will produce low output volts for low frequency and vice versa.

50 Hz units will function at 52 Hz but will eventually fail if operated at 60 Hz.

Low temperature

Down to -25 deg C is usually no problem after that the capacitor bank becomes the limiting factor.

High temperature

For short term excursions of ambient temperature up to 70 deg C the only damage is to the life of the capacitor bank.

For every 5 deg C above 40 deg C expect the life to be halved from the calculated 200 000 hours MTBF.

High humidity

If the unit is stored at 100% RH it will probably require drying before starting up.

The unit will operate at 99% RH without problems.

Failed capacitor

If the unit has several capacitors and one fails the unit may still provide reduced power.

Shorted capacitors will stop operation but open circuit failures can be tolerated. Problems will occur at switch on if the unit is operated at high input voltage and light loads when a capacitor has failed. If the unit makes a'humping' or'motor-boating' noise it should be turned off and on again. Failed capacitors should be replaced as soon as possible.

Damaged casework

We recommend that a thorough visual examination be made by a competent person prior to switch on.

Don't connect a critical load without prior testing. Light bulbs and fan heaters make excellent test gear in remote locations. So long as all the wiring and insulation seems intact and the situation demands it we suggest you try it.

Other problems

Please ask for technical assistance via our sales office.