Body
OleI have been experimenting with dc motord the last few days.I have a rather large 240 volt motor which I have a flywheel on, driving a 40 volt with a geared up vee belt arrangement using a Variac and bridge rectifier power source.Using almost 500 ma, at about 40 volts turning the smaller motor at a much higher speed, I am getting substantial voltage above the running voltage as well as much more amperage.I thought, well why not try it? You know?
With a growl, it churns down and dies when the power is removed.Hmmm?So then I got out an identical 240 volt big dc motor and hooked up the leads from the smaller motor.At the same input from the variac to the first motor and generator combo, that big motor on the output from the little motor belted to a big motor turns the same speed as the first big motor with such torque, I can't stop it.What is going on here? What do I have to do to make up that invisible difference?To measurements, it seems like higher energy, yet it just bogs down and stops when I try to drive itself.Yet the power produced is sufficient to strongly drive that third motor which is identical to the first.I am thinking that it may be back emf or what you describe as the flyback effect, shorting against itself, in a braking manner.It seems remarkably close, yet there is something stealing that energy when recirculated.Cheers Warren
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On Sun, Apr 8, 2018 at 8:54 PM, [email protected] [EVGRAY]<[email protected]> wrote:
The energy isn't OU but the voltage is being transformed.
The high voltage is the flyback voltage (inductive kick back) generated when trying to break the current through a coil. A buzzer is actually a relay wired to power itself through a normally closed contact.
Source of animation: https://wiki2.org/en/Electric_bell
The inside of a buzzer:
Source of image: http://www.electrachime.net/resonator-door-chimes/confused-buzzer/
The next image shows the inductive flyback voltage formed when disrupting the current through an inductor:
Inductive kickback: (a) Switch open. (b) Switch closed, electron current flows from battery through coil which has polarity matching battery. Magnetic field stores energy. (c) Switch open, Current still flows in coil due to collapsing magnetic field. Note polarity change on coil. (d) Coil voltage vs time.
Source of the above image and text: https://www.allaboutcircuits.com/textbook/semiconductors/chpt-3/inductor-commutating-circuits/
When putting a low impedance like a diode across the inductor the flyback voltage is limited to the forward conduction voltage of the diode (voltage level d to the right). If putting a resistor in parallel to the coil the induced voltage is limited to the resistance multiplied by the current at cut off which is the voltage level (e)..
When opening the switch without any snubber circuit the voltage is limited only by the breakdown voltage of the switch and the parasitic capacitance between the turns of the coil. This makes a door bell or abuzzer or a simple relay kick back.
Because the inductive spike happens fast a wire hasn't got low enough impedance to short the spike. A wire connected to earth has too high inductance to short the spike which just follows the wire. EMC rules applies to dampen such spikes. This requires guiding the spike to some resistance that can dissipate the energy as heat or store the energy in a capacitor. For best result the snubber has to be placed close to the EMP source. This would be close to the coil by using short wires. If the snubber is placed across the switch the wires between the switch and the coil will radiate the spike through space because of the loop area formed. Any loops are to be avoided as they form coils of one turn. Remove loops by twisting the wires.
A coil carrying current acts like a current source when the circuit is opened. The current always declines from the value it has during breaking the circuit. The voltage will adjust according to the impedance of the load and of course also to the parasitic capacitance of the coil.
Regards
Ole
---In [email protected], <nwootan@...> wrote :
-------- Forwarded Message --------
| Subject: | Re: [EVGRAY] buzzer OU |
| Date: | Sat, 7 Apr 2018 16:40:53 -0500 |
| From: | Norman Wootan <nwootan@...> |
| To: | [email protected] |
Back in the day (70s) W.W. Grainger sold these Lungren mousetrap buzzers in 6 or 12 VDC! Measured over 1,000 violts fromthese guys from frame ground side. Knock the crap out of you ifyou were earth grounded and held the buzzer while ir was poweredup with 6VDC input!
On 4/7/2018 2:54 PM, Mick mkjekyll@...[EVGRAY] wrote:
Norm,
I found an old buzzer from the 70's in a box of junk. What did you used
to do with these to study neutral spikes?
Were you ever able to get one to ring continuously withthe right
capacitor and no other input besides momentary one?
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