I have confusion about back emf in motors. I mean I know that winding of a motor is actually inductor and when magnetic field changes on it, it creates a back emf . But also it is said when motor rotates there is back emf induced on motor windings.
What I want to ask if these both are the same thing ? (Is it like two different back emf , one is due to inductance of winding and the other is due to motor rotating ?
I tried to explain with my english (not good). sorry for it.

 

[edit] comments removed by self.

 

ALL motor armatures that spin their coils in a magnetic field, create an Counter AC Voltage called Back-EMF. according to Lenz's Law.
It does not matter if the motor armature has a Slip Ring attached or a Commutator attached.
The Back-EMF is caused by the motion of the armature coils moving through the N-S magnetic poles of the Field.

Now if the motor shaft does have a Commutator, then the brushes periodically switch from coil-to-coil on the armature.
So, as the brushes change from coil-to-coil on the armature,
there is a quick large spike in the armature coil's Back-EMF Voltage,
as the magnetism in that armature coil "inductor" collapses, to maintain the current flow.
This can be seen as a spark between the Commutator and the Brushes.
This additional "spike in Back-EMF voltage" does not occur in motors with Slip Rings on the armature

 

[edit] comments removed by self.

 

I remember taking a very very very big 12 volt transformer and touching the 12 volt coil to a large 12 volt battery. When breaking the contact there would be a huge spark and loud snap. THAT is what I mostly think of as BEMF. But that's just me.

Were the 12V leads the primary or secondary?

 

AC motors with slip rings (slip rings are contacts that maintain steady contact with electrical power throughout the rotation of the motor) do not produce BEMF. If you have an AC motor with permanent magnets you actually have an alternator.

You are wrong in a few ways. this should be edited out of your response.

Only one type of AC motor has slip rings, one with a wound field rotor. And those are so very few and far between that most will never see one in their life time., other than in pictures.

Once you add permanent magnets, you no longer have an AC motor, you then have an alternator. This is one of the ways some people use to make a wind mill power unit.

 

There are still a few wound rotor AC motors around used in traveling gantry cranes etc, the generated armature voltage is fed into a resistor bank for speed control, when shorted, they run at maximum RPM and are the same as a normal AC induction motor.
Another is the synchronous motor, it is ran up as a normal AC induction motor, when slip frequency is very close, just a few cycles away, DC is injected into the rotor via slip rings and the motor comes up to synchronism.
All BLDC motors are also generators, the same as their DC brush commutator counterparts.
PDF shows the 3ph sine back EMF when back rotated.
Max.

 

[edit] comments removed by self.

 

Hello,

Back EMF in a motor is usually considered to be a different thing than back EMF in say a relay coil or inductor. That is because with the latter it is usually caused by a collapsing magnetic field, but in the motor it is caused by rotational speed.
The so called "back EMF" of the motor is considered proportional to the speed of the shaft, and because it opposes the voltage applied to the motor, the voltage that actually gets to power the armature is the difference between the applied voltage and the back emf, and since the back emf is proportional to the speed, the voltage that actually powers the armature is:
Varm=Vapplied-Vbackemf
which is:
Varm=Vapplied-Speed*Kb
where Kb is the back emf constant of the motor.

By measuring this motor back emf you can measure the speed of the shaft, and thus use that to regulate the speed to keep the speed constant under load and various disturbance torques and friction.

Just to note, the armature torque is Varm*Km/Ra during steady state where Km is the motor torque constant.

 

Also, were it not for BEMF that opposes the applied voltage, the current in a DC motor for example, would be extremely high when based on the actual resistance of the rotor winding.
Max.

 

I have seen slip ring motors. The alternator in my vehicles have slip rings, at least they did last time I opened one up.

But then again it is an alternator, not a motor. Most AC motors that aren't "universal motors AC or DC, with brushes", or shaded pole motors are "squirrel cage induction motors". No slip rings involved.

 

Here is the type still used in some of the crane yards around here for travel and hoist.
Some have been converted to VFD, but some of the originals are still in operation after many decades of service.
https://www.motioncontroltips.com/what-is-a-wound-rotor-motor/
Max.

 

[edit] comments removed by self.

 

Just guessing here, but I would venture to say they don't have BEMF.

I will repeat what I wrote way back in message #3 ...

ALL motor armatures that spin their coils in a magnetic field, create an Counter AC Voltage called Back-EMF. according to Lenz's Law.

 

Just guessing here, but I would venture to say they don't have BEMF.

You guessed wrong.
Virtually all motors generate a BEMF when the rotor is rotating.
An induction motor has current induced in its rotor by the AC field current, which generates a rotor magnetic field.
When the rotor rotates, this generates a field that induces a BEMF in the stator (transformer action).
When there's no rotation of the rotor (such as at startup) there is no BEMF and the stator current is limited only by the winding inductance and resistance, which is very low, causing the normal high current surge when a motor starts.

You need to divorce your thinking about the inductance of a motor (which is generally incidental and parasitic to the motor operation) and the back emf produced by rotor rotation, which is the primary voltage that counters the input voltage.
An operating AC motor with a load, primarily looks like a resistance to the line, not an inductance.

 

In a AC induction motor the rotor field and BEMF almost reaches zero as the slip frequency approaches synchronism, but never reaches by around 5 cycles.
Post #12 illustrates the generation which is used to control rpm.
Max.

 

@alexfrey @mvas @djsfantasi @shortbus @MaxHeadRoom @MrAl @crutschow I've removed my comments. Consensus is clear, my comments were in error and it's obvious I need to learn more about this subject. Until I know more I shall endeavor to refrain from contributing to such threads.

 

@alexfrey @mvas @djsfantasi @shortbus @MaxHeadRoom @MrAl @crutschow I've removed my comments. Consensus is clear, my comments were in error and it's obvious I need to learn more about this subject. Until I know more I shall endeavor to refrain from contributing to such threads.

Hi,

I wouldnt go that far, it's always interesting to hear other people's opinions and ideas.

 

@MrAl I pride myself on knowing when I am wrong and admitting such. Too often pride prevents people from admitting their mistakes and learning from them. The last thing I want to do is to give wrong or bad advice or information. I thought I knew more about this particular subject than I did. It's an opportunity to learn something more correctly. By now it should be obvious I lack knowledge in BEMF. I thought I knew; but I don't. So I'm willing to admit my error and allow others the opportunity to correct my statements. Also, @shortbus asked me to remove my erroneous statement, and for good reason. So I did. Again, I have no problem doing that.

@alexfrey The people here are knowledgeable, and I would trust their advice. Especially when there is such an overwhelming lean in one direction. Not only do you have the opportunity to learn here, so do I. This is a good website to gain insight, knowledge and help on many levels. It's why I like visiting this site nearly daily.

To all; have a good day.

 

@Tony1084, if I came across as being a know it all in my first post and telling you you HAD to delete, that wasn't my intention. Making statements about things we (I do it quite often) think we know, is how we (at least I) learn. That is what is great about the internet, you can make a statement and learn from where what your thinking is, is wrong.

When you don't have a one on one teacher or mentor, it is the fastest way to gain new knowledge. At least for me. The problem comes when a person doesn't expand their knowledge and goes on thinking the rest of the world is wrong, not them.