DC shunt motors (with field) should not be run without field on. The shunt field makes flux between north and south poles and when armature current flows in the armature those perpendicular wires to that flux result in a force on the wire, torque and rotation (one of those left hand dead-guy rules). “Full field” is 36 volts and the field weakening resistors/relays lower that for increased speed as less flux allows more speed. Theoretically, with no field (you disconnect it or relay is stuck open) will allow the motor to overspeed if it can draw enough current to self-destruct. In these motors with a load and losses it will draw enough current to basically be a “short” and peg the ammeter – if it does that when you are running it usually is an open field.
So, applying 12 volts to the motor lets it run 1/3 speed and 12 volts on the field will allow for higher speed so the net result is slow speed. Not a bad bench test.
…Walt
From: RJ Kanary [mailto:rjkanary consolidated net]
Sent: Tuesday, July 26, 2011 12:21 PM
To: David Plamondon; elec-trak cosmos
Subject: Re: (ET) Testing I5 motor
It could possibly run without the field energized, but more valid results would be obtained with field voltage present.
RJ
David Plamondon wrote:
On my newly acquired I5 I would like to test the motor using the 12v "method". Can i just connect 12v to the armature, or do i need to also connect 12v to the field?
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