AGT series Constant Voltage Transformer (CVT) MTBF with ambient

Advance's AGT series power conditioners have an ambient temperature rating of 40C.

The family can be used in higher ambients without derating but MTBF is reduced.

Units up to 2900VA need not be derated. For large units please consult the sales office.

Ambient Air degC MTBF hrs
40 200 000
45 100 000
50 50 000
55 25 000
60 12 500
65 6 250

MTBF calculated to MIL 217C ground benign.

In addition Advance can design tailor made versions for a specified Temperature.

CVT Failure Modes

The CVT is basically a saturated transformer with an output resonant circuit maintained by a capacitor.

All transformers have the failure mode of 'shorted turn' where winding damage causes a part of the winding to short (either to earth or to another layer ). This leads to large current flow in a small part of the winding typically blowing an input fuse or breaker.

In addition windings can go open circuit leading to either no output from an energised transformer or no energisation.

This can happen only on the INPUT of a CVT, because the OUTPUT winding is current limited, in the case of a shorted turn on the CVT output, it is unlikely the current draw will be sufficient to blow an input fuse before the CVT output 'folds' to zero volts. In fact it is possible to directly short the entire output winding of a CVT and then remove the short without damage to the CVT and restoring the output as soon as the short is removed.

In addition to standard transformer failure modes the CVT capacitor can be considered a weak point. Advance uses only GE Capacitors that are specially designed for the rigorous duty required in a CVT. These capacitors are oil filled and some 'ageing' of this oil occurs causing a recommended replacement after 8 years, based on a 10 year expected life span. The capacitor has an integral fuse, which can cause an open circuit on failure, since this leads to a loss of saturation in the output the symptoms of capacitor failure are low output voltage.

Some CVTs are fitted with protective earth screens on windings to improve attenuation of high frequency noise. These screens can short to each other but the only effect is to reduce the attenuation at high frequencies.