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MMD > Archives > May 1998 > 1998.05.13 > 07Prev  Next


Switch Arcing in Player Piano
By John Wale

Hello to John Tuttle and the MMD group!  Switch arcing usually occurs
when you try to disrupt the flow of current in an inductor (i.e., by
switching a motor off) and the stored inductive energy creates a spark
at the switch contacts.  To suppress the arcing you need a device that
can absorb this energy safely.

Traditionally, this is done with a voltage dependent resistor (or
'varistor') which is connected across the motor terminals.  Varistors
have little effect at ordinary mains voltages, but when the voltage
rises at switch-off, they go low impedance and turn the energy into
heat.  Philips have a comprehensive set of data sheets at their web
site -- www.passives.comp.philips.com -- do a search for 'varistor' in
their product finder.  It's worth noting that varistors have limited
life if they absorb a lot of transients.

An alternative to the varistor is the 'snubber' network that consists
of a resistor and capacitor in series across the switch contacts.  The
component values need to be chosen correctly to suit the motor
characteristics, but this circuit doesn't age like a varistor.

There could also be an amount of arcing due to the inrush current of
the motor when it starts.  If the problem is due to brief interruptions
in the power when the motor is running, though, the motor speed won't
have changed much, so the inrush current should be small.  Either way,
the suppressors mentioned above should do the job.

I hope this helps.  If I can help further, send me an e-mail.

Regards,

John Wale.
jw Electronics,
Coventry, UK.


(Message sent Wed 13 May 1998, 09:44:13 GMT, from time zone GMT+0100.)

Key Words in Subject:  Arcing, Piano, Player, Switch

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