Quote:
Originally Posted by winston_1
Bell motors are largely inductive and adding series resistance does little.
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Actually a bell-movement is nearly as resistive as it is reactive, typically 1776 + j2085. This gives an impedance of 2729 Ohms Z. The capacitor tunes out some of the inductance and, on a single line, results in an impedance presented to the ringing circuit of (typically) 1776 - j1451 and a capacitive impedance of 2293 Ohms Z, resulting in an increase of current through the bell movement.
The telephone bell that has served us for long has been extremely well designed. It is robust and simple and almost indestructable. It can be overloaded without damage. Fifty, sixty, seventy and eighty-year-old movements will still work as well as on the day they were manufactured. I don’t know about the design specifications, but I do know that an electromagnet will dissipate most power when the resistance is the same as the inductive reactance, i.e: the phase angle is 45° and the modulus is at its maximum.
But yes... Adding a resistor just causes even less current to flow, causing nearly half the ringing voltage to be dropped across it. I tend to leave them out. I believe REN is referenced to a more modern 59D 4000 Ohm d.c. bell, typically 7186 + j8275.
To quote from the European Telecom Standard ETS 300 001:
'Instead, these parameters are defined in terms of the concept of the "ringer
equivalent number" or REN of the TE; this is a measure of the numbers of items of equipment that can be connected in parallel, the reference for a REN = 1 being a simulated ringing detector circuit consisting of an inductor of 55 H in series with a resistor of 7 kohms.'
Which fits in nicely with the spec of the 59D bell-movement.
