What happens / why do motors stall?

[maclaren]

When I was reading this thread I was doing great till somone mentioned "back EMF" and then I got confused. So I decided to go out and look for more info on the back EMF phenomena. So I went to google and found very useful and very CONCISE which, in my opinion, all the replies to this thread singly lacked. This is what I found:

-EMF stands for Electro Motive Force or Voltage (being it's more common name).
-Back EMF occurs when a magnetic field is collapsed.
-This collapse of magnetic field cuases a spark (easily visible in AC motors).
-The voltage of the power source to the motor creates a positive potential which is then neutralized by the negative potential of the spark or back EMF when the brushes lose contact with the commutator contacts.
-The flow of electrons that cause the motor in this case to move is from positive to negative the spark that occurs is in the opposite direction from negative to postivie hence the back part of back EMF.

Useful Website

I don't know If this clarified anything for others but it sure did for me. Thanks for your time.

[Mike Betts]

Mclaren,

A few of your statements are either misleading or you have reached the wrong conclusions.

Four Basic Principles describe how magnetic fields are used in electromechanical devices:

1. A current-carrying wire produces a magnetic field in the area around it.
2. A time-changing magnetic field induces a voltage in a coil of wire if it passes through that coil (basis of transformer action).
3. A current-carrying wire in the presence of a magnetic field has a force induced on it (basis of motor action).
4. A moving wire in the presence of a magnetic field has a voltage induced in it (basis of generator action).

Items (3) and (4) are particularly important to DC motors and item (4) defines “back-EMF”.

The magnetic field between the rotor and stator does the work. Any spark on the commutation brushes is lost power (voltage times current), does not contribute to the magnetic field and does no useful work.

In fact, the brush losses are the main motivation for the “DC Brushless Motor” that has permanent magnets in the rotor and electrically commutated windings in the stator. The trade off is cost versus efficiency. Since the expense of most of our FIRST motors is of top concern, we usually use permanent magnet DC brushed motors.

Last item: Almost all AC motors have no brushes. The rotor currents are induced by a rotating magnetic field created by the stator windings.

Now that I have been critical, let me praise you and all of the students who are posting here for getting out, doing research and piecing together "The Puzzle”.

[Al Skierkiewicz]

The last two posts have some valuable information, if you still are having a hard time try this explanation...
1. Whenever a current passes through a wire, a magnetic field occurs around the wire. In a motor that wire is in turn wound around a metal structure that concentrates the magnetic field produced by the current flowing through the wire. A coil of wire resists the current flowing through it during the time that the magnetic field is changing to a steady state. In DC coils, this occurs when current is first applied and then reduces to the wire resistance. Input current is low when first applied and then builds to Ohm's Law values after some length of time. Likewise when the source is removed, the collapsing magnetic field will try to keep the current flowing. Coils that have many windings and strong magnetic fields will self generate kilovolt outputs while the field is collapsing. The ignition coil in cars operate on this principle. If the collapsing field can generate sparks in your car, the field in a motor can generate sparks across the brush assembly.
2. A loaded motor draws more average current than one with little or no load. (Remember the brush is in contact with a motor winding for a longer period of time.) Since the strength of the magnetic field in a DC coil is a function of the number of turns and the current in the wire, it follows that a loaded motor will have a much higher field generated by the winding. Since there is several windings in close proximity inside the motor all of them are within the field generated by the winding that has current flowing in it.
3. A coil of wire either moving inside a steady magnetic field or non-moving inside a changing magnetic field will have current induced in it. In most of the motors we use on the robot, the wire is moving inside a stationary magnetic field, so as the motor turns current is induced in the wire if the circuit is a closed loop.(coil is connected through brushes to the speed controller.)
4. The collapsing field causes a current to flow in the opposite direction of the current that created the field in all windings that are in the presence of the magnetic field. (i.e. all motor windings)
5. This phenomena was first studied in the early stages of motor development. Those engineers used the term EMF (Electro Motive Force) to describe the power supply (battery) in their experiments. Since the current induced by the collapsing field is opposite of the current that created it and in observation it appeared that another voltage source was opposing the applied power, the engineers called the phenomena "back EMF".
The changing magnetic fields in a motor creating high voltage arcing are not peculiar to DC motors as anyone with an AC drill can attest. Simply, any wire in a changing magnetic field will have current induced in it. The stronger the field, the more wire in the field, and the time it takes for the field to change all affect the level of voltage induced in the winding.
Commonly AC drill motors are "series wound", i.e. the field and armature windings are wired in series through the brushes. Other types of AC motors have no brushes and use the sine wave of the AC line to create the changing magnetic field. Brushless DC motors use drive circuitry to change the DC current into AC current and then operate as an AC motor.
This is a difficult principle to discuss in this forum. If anyone has any questions, please contact me. askierkiewicz@wttw.com

Good Luck All

[Mike Betts]

Al,

Semantics... "AC drill motors" are usually referred to as "universal motors" because they can function with either AC or DC as an input.

Anyone who wants to learn more, can go to:

http://www.engin.umich.edu/labs/csdl.../ac/universal/

Other than that, we are in violent agreement. Thanks

[Al Skierkiewicz]

Mike,
"we are in violent agreement" brought all kinds of frightening images to mind. How about if we just smile and shake hands when we next meet like always?
Smiling and writing....