RILTON ELECTRONICS LTD

VARIABLE TRANSFORMER SPECIFICATIONS
ISO 9001

Outline

All the Variable Transformers are constructed of a single layer of copper wire wound on a high permeability electrical steel core. This winding is both primary and secondary and is tapped for the various input voltages detailed in the data sheets. The insulated copper wire is bared and in some models silver plated for the
smooth contact of the brushgear. These brushes, made of layered long life carbon, provide an infinitely
variable output voltage from zero to line voltage or above. The resolution is only limited by the volts per turn which is typically < 0.5%.

Input Voltage

This is as stated in the data sheets and may be exceeded continuously by 10%.

Output Voltage

Infinitely variable from zero to line voltage or 15% above line voltage.

Nominal output current

This figure is the current that can be taken continuously from the transformer at all brush positions on the track.

Maximum output current

Due to the distribution of currents in the winding it is possible to exceed the nominal current by an increasing amount as the brushgear moves away from the centre position. See Figure 1.

Figure 1

Brush Life

The brushes are guaranteed for a minimum of 200,000 sweeps of the track but the expectation under normal use and load is 500,000 sweeps of the track

Transient Overloads

Because of the low contact resistance between the brushgear and the track, transient overloads up to 1000% can be tolerated. The period and extent of these overloads are shown in Figure 2.

Figure 2

Inrush Current and protection

Due to the high permeability of the core there can be a high inrush current when connected to the supply. The level of the inrush current depends where on the cycle of the input supply that switch-on occurs. At worst the inrush current could be 20 times the nominal current of the transformer. It is recommended that a "slow blow" or "motor start" fusing is used on the input side.

Fusing on the output of the variable transformer is essential because by the nature of a variable ratio device a two times overload on the output when the brush position is at 50% will not be seen as an overload by the input fuse. The output fuse does not "see" the inrush current and can be of a normal type.

Voltage Drop

There is a volt drop caused by copper and brush losses on full load which is given in the data sheets and which varies according to the type of load and the brush position. Figure 3 shows the top curve for Inductive and lower curve for Resistive loads

Figure 3

Derating for High Ambient Temperature

The nominal Current can be taken at all ambient temperatures up to 40 Degrees C. Above that the variable transformer should be derated according to the curve in Figure 4

Figure 4

Full details of these and additional design parameters are available in the Data Sheets.