ECN Forum Forums Technical Discussion Electrical Theory and Applications shaded pole motors

I often take amp draws of shaded pole motors used in furnaces. I have noticed that if the amp clamp is near the motor the ampdraw will register higher than the fla but if the amp clamp is farther away from the motor then the reading is usually within spec. Anyone have an idea why being near the motor causes the ammeter to give a false reading? Thanks

Just a guess:

The "body" of the shaded pole motor is made of laminated steel designed to "conduct" the magnetic flux right where its needed to turn the rotor. Othe types of electric motors have the magnetic parts inside a metal shell.

I'd guess that when you're near the motor with your meter, that magnetic the flux is also going through the clamp jaws that are trying to use to measure the magnetic field surrounding the conductors.

A related matter with ammeter probes is that the silicon-steel pole faces must be clean and close tightly and squarely for accurate measurements. Try parting the jaws during a measurement and notice if there is any variation.

ds247,
I'd agree that shaded pole motors have a habit for stray Magnetic fields.
Whereas, a standard induction motor has a steel body around the stator, to provide somewhat of a sheilding effect, the S.P motor has no such thing.
I'd also venture to say that most of the stray field comes from the shading rings themselves, not the laminated stator core.

what size is this SP motor? and what sort of readings did u get?

Another thought, following on Bjarney's observations. . .an experiment to try.

The field in open air around the motor will be fixed in orientation. Although the field is alternating, that is, expanding, collapsing, reversing, expanding, collapsing, etc., the orientation of the field, at a point in space, will be along the a line that doesn't change direction.

Imagine the iron filing - bar magnet experiment that uses a piece of stiff paper. The magnet is placed under the paper, the filings are poured on the paper, and then the paper is tapped. The filings line up along the lines of force. The shape of the lines of force is the same regardless of field intensity. The shape of the magnet will alter the shape of the field. In the case of the furnace motor, the shape of the motor doesn't change, so, the shape of the lines of force around the motor doesn't change.

Here's the experiment: Have the motor running and do a current measurement close to the motor, the way you have when you've noted past high readings. For this experiment, lets say that the direction of the wire that is in the clampon's jaw is the X axis of an X, Y, Z three dimensional axis for side to side, front to back, and up and down. We're going to bend the wire, but the direction of X, Y and Z won't change from the first way the wire was. While keeping the wire in the clampon, turn the pick up coil of the clampon ammeter 360º along the X-Y axis and then 360º along the X-Z axis. Watch the reading for a dip, or decline. This will occur as the pickup coil of the clampon approaches a 90º orientation to the open air magnetic field from the motor. The motor field will be at some unique angle that most likely will not be along the X, Y, or Z axis of the wire. Placing the pickup coil of the clampon at a right angle to the open air motor field results in the motor field being "invisible" to the pickup coil.