Theatre Design & Technology - Oct 1966 - 18

An inductor which has been designed so that the current
flow does not cause saturation will have a different leading edge of its waveform.

LINE REGULATION AT FULL LOAD
B

120

A
6

115

Figure 7 shows the shape of the non-saturating inductor
type of filter. Figure 8 shows the result for a saturating
design. Note that the slow initial rate of rise gives way
to a faster rate as saturation begins.

3
5

110
4

105

Figure 9 tabulates results of the oscillographs of rising
waveforms. The first column of the table gives the rise
time of the most rapid part of the waveform. (The rise
time is defined as the interval between the 10% and
90% points of the waveform.)

B
A

100

~ 95

~ 90

> 85

One point to be noted is that the saturating filters have
rise times in the saturated region which vary only in
2: 1 range, but the same filters with unsaturated region
have rise times with a range of better than 23: 1. Thus
even though the filters are quite different while active,
after saturation all four designs perform in about the same
way.

6
4

I-

::::> 80

I-

::::> 75
0

5
2

70
65
60

10,-----------------------------,

55
50

45
-15

-10

Fig. 6:

Line Regulation at Full Load

regulation is easiest to establish. As the output voltage
increases the regulation becomes harder to establish. The
ninety volt level shows a wider spread of control. At
full output all of the designs lost regulation, with the output voltage following the input voltage changes.

Since all of the dimmers used all of the available waveform at full-on, it was not possible to compensate for
falling line voltage by an increase of the firing angle.
In the process of lighting a scene, the output level of
each dimmer is arbitrary. It would seem desirable that
a dimmer should have enough regulation to avoid significant changes in output for the expected range of line
voltage changes and at all output voltage settings.
The rapid turn-on characteristic of the SCR produces a
burst of high frequency energy. All of the designs tested
employed a series inductor to slow down the rate of
turn-on and thus reduce radio frequency interference.
The rapid turn-on of the SCR also creates a noise, hum
or buzz in some types of lamp filaments. This noise is
produced by the magnostrictive contraction of the lamp
filament and the vibration of the filament supports. A
second purpose of the series inductor is to slow down
the rise of current pulse and thus reduce the noise at
the lamp.
Four of the five designs tested used a saturating iron core
inductor in the filter. The saturation of the iron by the
magnetic flux results in greatly reduced inductance. The
rate of rise of the waveform is directly related to the
effective inductance value at that instant of time. The
large reduction in inductance upon saturation allows a
faster rate of rise. The result of saturation is thus a
waveform with two parts; a slow rate of rise before saturation, and a faster rate thereafter.

-ho
Fig. 7:

Leading Edge Wave form for a Non-Saturating Inductance

10,-----------------,-----==-------,

I
/;
08

OlL..L-_,.-

-'-.,--

~
Fig.

I USITT I

Leading

Edge

-1

T2 - - T1

Waveform for a Saturating Inductance

THEATRE DESIGN AND TECHNOLOGY

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