I haven't even bothered watching after "only DC motors can be used as a generator".
As far as I am aware, a motor can be of the following families:
- Permanent magnet DC brushed. DC back emf.
- Coiled stator DC brushed (as a separate winding, or internally wound as series or parallel). DC back emf IF the stator is powered. Single phase universal motors are a subset of series connection types for which, regardless of the polarity of the voltage, torque is always generated (needs moveable brushes or a different wiring to change direction though).
- Permanent magnet AC synchronous (three phases). Three phase AC back emf.
- Coiled rotor AC synchronous. I think those generally are not brushed but rather rectify the current induced by the stator. If brushed, no back emf unless the rotor is powered.
- DC brushless. This one is basically a permanent magnet AC synchronous with hall sensors built in, to be able to electronically switch the phases. The back emf is however square or trapezoidal to maximise flux linkage.
- Stepper motor (2, 3, 5 phases). Close to the PM AC synchronous in its construction, except that the motor is made to maximise the number of stable equilibrium positions of the rotor (many alternating magnetic poles at the rotor or variable reluctance). Back emf depends on how it's driven.
- AC asynchronous (3 phases). The rotor is a closed loop (a coil, or a squirrel cage made of bars) which creates its field from currents induced by the stator. Can only be used as a generator beyond the synchronous rpm (+voltage at stator). AC back emf (TBC).
- AC asynchronous (single phase). The motor cannot be self-started unless an out-of-phase auxiliary supply is created via a reacting capacitor, and fed to windings 90° from the main windings. Can only be used as a generator beyond the synchronous rpm (+voltage at stator). AC back emf (TBC).
There are many more (e.g. hybrids), but I think those represent 95% of the production. I'm sure I've missed a few important ones, please feel free to comment and I'll update the list.
The biggest clue to the type of a motor is the number of wires, but as you can see this is not enough. Some motors cannot generate power without an excitation, some not at all, and even if they do, the back emf is funny sometimes (trapezoidal for example) depending on its construction.
You could plan to try the various types of supplies on the motor, ramping up the voltage, and see if it does anything, but what's your "OK that's not it, better cut the power before I smoke it" point? If you don't know what type of motor it is, I assume you don't know anything about it. Including the voltage and current ratings, Max rpm. You could get that from eyeballing it, but there is no guarantee then.
For your specific problem though, if you are certain your motor is a DC bruhless but you don't know if the inverter+control circuit are integrated, look at the number of wires. Generally the motor does not have a circuit built in, and an ESC must be connected to it. You will have to identify which wires are the hall sensors.
ESC might or might not be used for current generation, it depends on how they are made. I don't think there can be any harm in hooking up a resistive load compatible with its current range at the input and test it.
Firstly, a point about the title of your question.
You initially ask why it is that a magnetic field is "transmitted" with alternating current. In terms of transmission, electromagnetic radiation only occurs where a circuit has (or becomes) an antenna. Otherwise, only an electromagnetic field is generated around the conductors. This is called a "near-field" and it is not 'transmitted'.
With regard to magnetic fields, a magnetic field is generated when a current flows through a wire. This current can be D.C. too - the phenomena is not limited to A.C. current. The direction of the magnetic field depends upon the direction of current flow. Therefore, if you connect a D.C. source a magnetic field is generated with a constant polarity. If you connect an A.C. source the magnetic field alternates in polarity.
It is interesting to note that a magnetic field is only measured or observed when a charge moves relative to you as an observer. It is a relative property! If you were able to move down the wire with the charge (or alongside the wire at the same speed as the charge), so that the charge is no longer moving relative to you, you would see that the magnetic field disappears completely!
The electric and magnetic fields are related in a peculiar way, similar in some sense as to space and time in that they are relative properties and depend entirely upon the observer.
Here's a really good book which I think you would enjoy:
[http://books.google.co.uk/books/about/Electromagnetics_Explained.html?id=MLzPNpJQz9UC&redir_esc=y][1]
Best Answer
This answer was given prior to the op changing the question. Originally, the question referred to the motor as being a squirrel cage type. Subsequently, after several comments, the op changed it to a brushless motor. You just can’t get the staff any more!
When you stall a 3 phase motor it becomes a transformer and, because the rotor is the secondary AND is shorted out (by necessity for it to operate as a motor), you end up with a shorted transformer.
Ask yourself why a transformer burns when the secondary is shorted.