66_114_97_100 said...
Looks like I will have to dig up my old text books because my recollection of Back EMF is that it is produced by any coil, (solenoids, transformers, etc.) not just permanent magnet motors. It is dependent on inductance and a varying current.
I apologize Ugha for taking your post off on a tangent
Thanks guys
A brief tangent, and I apologize too.
Back EMF will be generated when the armature is rotating in a magnetic field.· With a permanent-magnet motor there's always a magnetic field.· With a motor whose field comes from current in a field winding, then cutting off the power cuts off the field current.· No field current, no magnetic field, and -- poof! -- no back EMF.
But while it's running with applied power, you're quite right, there's always back EMF.
Looks like I will have to dig up my old text books because my recollection of Back EMF is that it is produced by any coil, (solenoids, transformers, etc.) not just permanent magnet motors. It is dependent on inductance and a varying current.
I think he's referring to the current spike generated by the collapsing magnetic field of an inductive device.· I to thought·this·was considered back EMF.
So wouldn't non-permanent magnet motors also generate this?· Also wouldn't this effect the method discussed for measuring motor speed, or lack of, like having to wait a short period of time for the signal to stabilize?
I wasn't going to continue this discussion on this thread but since Ugha gave permission, I will. [noparse]:)[/noparse]
Exactly my thoughts agfa, Back EMF is generated by any inductive load. It is the reason you need to use protection diodes on inductive loads. Using the motor as a generator would be totally different thing.
No, the terminology is very specific. Back EMF is the EMF that is generated by the motion of the armature of an electric machine in the magnetic field of that machine and that opposes the applied external power. It's "back" because it tends to drive through the terminals of the machine a current in the OPPOSITE direction from the current driven by the applied power (thus reducing the total current through the armature).
The inductive kick created by interrupting the current through an inductor is not included in the meaning of the term "back EMF". That's good, because technical terms ought to be as specific as possible. It's also reasonable, because the inductive kick isn't "back" at all. It's forward. It drives current through the inductor in the SAME direction it was driven by the applied EMF.
Carl Hayes said... ·It's also reasonable, because the inductive kick isn't "back" at all. It's forward. It drives current through the inductor in the SAME direction it was driven by the applied EMF.
This cannot be true. It would render protection diodes useless and more important would violate the law of conservation of energy. We would end up with more energy out than we put into the machine.
It certainly is true.· You misunderstand how the kick diode works. Consider the circuit shown in my first image, BEFORE.· The arrows show the direction of current in each branch of the circuit.· Note that there is no current in the diode, because it is back-biased.
Now disconnect the battery, and you get the circuit of the second image, AFTER.· Current is still flowing in the same direction·in the inductance, which resists change in current.· The kick diode gives this current a place to go, preventing the inductance from developing a large inductive voltage in its attempt to maintain the same current flow.· Without the kick diode, that current must flow through the surrounding countryside, including whatever switch disconnected the battery.· If that switch is a transistor in your Propeller or Stamp, then when it tries to shut off the current the inductor will develop whatever voltage is necessary to keep the same current flowing.· Goodbye transistor.
Energy is conserved, as in all physical processes.· The energy the inductor delivers in forcing a continued current was originally delivered by the battery and stored in the inductor's magnetic field.
You're not alone. When I first encountered it I misunderstood it too. I burned up a number of 2n3053 transistors, driving printer solenoids, before the light dawned. That was in a blood analyer that printed a ticket for each blood sample, with values for (if I remember) seven different measurements. Not my finest hour as a design engineer.
Comments
Back EMF will be generated when the armature is rotating in a magnetic field.· With a permanent-magnet motor there's always a magnetic field.· With a motor whose field comes from current in a field winding, then cutting off the power cuts off the field current.· No field current, no magnetic field, and -- poof! -- no back EMF.
But while it's running with applied power, you're quite right, there's always back EMF.
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· -- Carl, nn5i@arrl.net
I only wish I understood half of what your talking about... time to check wikipedia!
Keep at it guys [noparse]:)[/noparse]
So wouldn't non-permanent magnet motors also generate this?· Also wouldn't this effect the method discussed for measuring motor speed, or lack of, like having to wait a short period of time for the signal to stabilize?
Post Edited (agfa) : 12/20/2008 1:50:42 PM GMT
Exactly my thoughts agfa, Back EMF is generated by any inductive load. It is the reason you need to use protection diodes on inductive loads. Using the motor as a generator would be totally different thing.
The inductive kick created by interrupting the current through an inductor is not included in the meaning of the term "back EMF". That's good, because technical terms ought to be as specific as possible. It's also reasonable, because the inductive kick isn't "back" at all. It's forward. It drives current through the inductor in the SAME direction it was driven by the applied EMF.
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
· -- Carl, nn5i@arrl.net
Now disconnect the battery, and you get the circuit of the second image, AFTER.· Current is still flowing in the same direction·in the inductance, which resists change in current.· The kick diode gives this current a place to go, preventing the inductance from developing a large inductive voltage in its attempt to maintain the same current flow.· Without the kick diode, that current must flow through the surrounding countryside, including whatever switch disconnected the battery.· If that switch is a transistor in your Propeller or Stamp, then when it tries to shut off the current the inductor will develop whatever voltage is necessary to keep the same current flowing.· Goodbye transistor.
Energy is conserved, as in all physical processes.· The energy the inductor delivers in forcing a continued current was originally delivered by the battery and stored in the inductor's magnetic field.
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
· -- Carl, nn5i@arrl.net
Post Edited (Carl Hayes) : 12/20/2008 8:37:05 PM GMT
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· -- Carl, nn5i@arrl.net