Sensing a magnet in motion

[atferrari]

I expected to sense a magnet in motion passing close to this coil (from a discarded relay), with the scope in the lowest scale (2mV/div) but no joy.



In case you ask, continuity is OK and magnets were moved in any position and in any sense you could imagine wrt the coil.

I tested all the magnets in my drawers. While I know that no one is made of Neodymium and the diameter of the coil is really small, I expected a minimal output at least.

What could I be doing wrong?

 

[JimB]

Maybe the coil is short circuit, rather than countinuous.

Maybe the scope was not set correctly,
may be you set the sensitivity on channel A and the probe was connected to channel B,
maybe the probe was not making good contact with the connector pins on the coil.

JimB

 

[atferrari]

Maybe the coil is short circuit, rather than countinuous - around 270 ohms measured.

Maybe the scope was not set correctly - 2 mV - DC

may be you set the sensitivity on channel A and the probe was connected to channel B - yellow probe in the yellow channel triggered by the yellow channel.

maybe the probe was not making good contact with the connector pins on the coil. -

JimB

Gracias for replying

 

[atferrari]

Solved Jim.

Found a magnet that is giving up to 10 mV and 20 mV with a core inserted (take from another relay).

Gracias again.

 

[JimB]

So, just not enough magnetic flux to give enough induced volts to see on the scope.

JimB

 

[atferrari]

Definitely something I took for granted but never tested or thought about in detail.

 

[Mikebits]

Wouldn't a Hall effect device be better for detecting a magnetic field?

 

[atferrari]

Wouldn't a Hall effect device be better for detecting a magnetic field?

Sure. Easier to implement!

 

[Reloadron]

Just out of curiosity are you using DC or AC coupling on the vertical input to your scope? I would think DC coupling would show the best response depending on how quickly the mgney is passing the coil. Also try a few different angles.

Ron

 

[atferrari]

DC, Ron.

But after thinking of all what I did today, I suspect that I am not moving the magnet fast enough. Tomorrow I will do a more consistent testing, even with the not so powerful magnets.

 

[chemelec]

Most Important was Putting in that Core.
It (Amplifies) Concentrates the Lines of Flux.

 

[Reloadron]

This is actually a pretty cool little experiment. I used such circuits for many, many years. We wrapped a coil on a nylon form with a steel bolt running through the core, no laminations, just a steel bolt. This part we secured to the outside of the stator windings on a very large and very unusual AC type motor. These motors ran at a very low AC frequency, typically sub 1 Hz. The coil was part of what we called a "pole slip detector". We could watch the stator magnetic field during normal run and monitor for any pole slips during running. Generally a pole slip or pole slipping is associated with a generator but in the case of some very special and unusual motors there is a need to detect pole slips.

If a special motor is driving a lead screw and the screw bottoms out or reaches a bottom or top stop the motor will begin to pole slip. The normal waveform picked up by the coil will show spikes in amplitude. So my signal from my coil can be amplified and passed along to a level detector (comparator) and when the spikes start to happen (pole slips) I can count them. I know I am at a top or bottom stop of the lead screw I am driving. I could for example allow 3 (or any number) of pole slips to occur then reverse two phases to my 3 phase motor and reverse the direction of my motor. Something else I could do is while my motor is running I could suddenly change the frequency to my motor which should result in a change to my motor's speed. A good motor should not have any pole slips happen when the speed is changed. A test of the motors ability to respond to speed changes while the motor is under a load.

So again a cool little experiment that has some very real world applications. While I did over all those years try many times to improve this test method using all sorts of hall effect sensors I could never seem to beat the method of the coil of wire and steel bolt. Simple and very, very effective.

Keep everyone up on how it goes and your observations.

Ron

 

[atferrari]

One of my test yesterday was with the core. Then I realized that if I use it for the passage of a pendulum, the core-magnet interaction will affect the pendulum period. That is why I never see coils with core used with pendulums.

 

[alec_t]

the core-magnet interaction will affect the pendulum period

I would expect that. But provided the interaction is consistent does that matter?

 

[atferrari]

I would expect that. But provided the interaction is consistent does that matter?

Hola alec

Clever way to kick the ball back to my field.

Honestly I do not know what to say... but I think I would avoid it anyway. For a sensible pendulum it would add an additional factor that could alter the natural period.

 

[alec_t]

Even without a core in the coil there will be an affect (albeit small) on the pendulum period, since energy needed to induce the coil voltage/current will be extracted from the pendulum's kinetic energy.

 

[Reloadron]

Even without a core in the coil there will be an affect (albeit small) on the pendulum period, since energy needed to induce the coil voltage/current will be extracted from the pendulum's kinetic energy.

Yes, so the only way to overcome this would be if the pendulum or force driving the magnet was great enough to exceed the energy being removed each time the magnet passed the coil, with or without an iron core in the coil. I guess you are converting a form of mechanical energy to electrical energy aren't you? We can't get something for nothing.

Ron

 

[MrAl]

Hi,

Sensing a magnetic field is all about speed, distance, field strength, and orientation.

The faster the magnet moves the higher the output voltage from the coil (not the same with a Hall Effect Sensor).
The closer the magnet is to the coil the higher the output voltage.
The stronger the field strength the higher the output voltage.
The better the orientation of the magnet to the coil the higher the output voltage.