Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!pt.cs.cmu.edu!sei!rsd
From: rsd@sei.cmu.edu (Richard S D'Ippolito)
Newsgroups: sci.electronics
Subject: Re: Fluorescent light ballasts
Message-ID: <5144@ae.sei.cmu.edu>
Date: 30 Nov 89 14:36:06 GMT
References: <7835@ttidca.TTI.COM> <5068@ae.sei.cmu.edu> <2956@psivax.UUCP>
Reply-To: rsd@sei.cmu.edu (Richard S D'Ippolito)
Organization: Software Engineering Institute, Pittsburgh, PA
Lines: 49

In article <2956@psivax.UUCP> Torkil Hammer writes:

>In article <5068@ae.sei.cmu.edu> Richard S D'Ippolito writes:
>#
>#with two 40w tubes, fixture, cord and ballast.  The ballast nameplate reads
>#"120V 60Hz .85A", UL, minimun starting temperature 50 deg. F."
>#
>#More enterprising folks might be able to calculate the power factor.
>
>We can't do so without knowing the efficiency factor, aka cosine of phase
>angle, which is measured as the ratio between watts and voltamperes.

Sure you can -- make some assumptions about the limits:

	If the ballast is purely resistive, the power factor is 1.0
	and the ballast dissipates 120V*0.85A - (2*40W) or 22 watts.

	If purely reactive, it dissipates zero watts and the power
	factor of the circuit is (2*40W)/102VA or 0.78.

This should put an end to the erroneous claims for both outrageous ballast
losses and low power factors.  Note that worst case for ballast losses is
best case for power factor, and v.v.  So, if a typical ballast dissipates 15
watts, the power factor is on the order of 0.93.

>Questions:
>1. Why is there a minimum starting temp for the ballast?

It has nothing to do with the ballast.  The lamp must be above a certain
temperature in order to get enough vaporized mercury to initiate the arc.

>2. Do today's ballasts come with a capacitor or other phase compensation
>to optimize the irregular, but predictable, current pattern of flourescent
>tubes?

The current pattern is very close to the voltage waveform; it is the
phosphor characteristic that minimizes flicker.  Some ballasts are lead-lag,
so that the two tubes have their maximum brightness at slightly different
times to reduce flicker.  Also, good commercial/industrial installations
will run the fixtures from alternate phases to reduce the strobe effect,
especially where there is rotating machinery.


Rich
-- 
When you can measure what you are speaking about, and express it in numbers,
you know something about it.
Lord Kelvin						      rsd@sei.cmu.edu
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