How to tell whether a motor is an AC or DC motor?

I watched a youtube video http://www.youtube.com/watch?v=hT3jJRGVdW4 The man says : by spinning the motor to see whether the output is AC or DC, then it can tell whether the motor is generating AC or DC voltage. It also tell whether the motor is AC or DC

==== What I think is below===

What I think is that "For a DC brushless motor, there is a permanent magnet rotor. If I spin the rotor, the current at the stator should be AC."

Now, if we talk about normal DC brushless motor spinning, we need to provide DC power (as power source) to the motor. That means the motor itself should have a circuit to convert the DC voltage to AC voltage to change the magnetic field at the stator to spin the rotor.

If there is a circuit to convert the DC power input to AC power to really spin the rotor, why the youtube video shows the circuit could reverse AC voltage (generated by the guy moving the rotor) to DC voltage for voltmeter measurement

===========

Then it goes to two questions

Do the built-in circuit in the motor allow backward voltage convention? What is that circuit?

Here is a transcript of the relevant part of the video:

The first thing that you need to know is that only DC motors can be used as a generator. So that makes this a really easy test. So we're going to connect our pins up to our multimeter, and then we're just going to lightly spin the shaft. Doesn't really matter which way we connect it. We've got the multimeter set to 2 volts. I'm going to spin and you see I'm spinning it clockwise and we get a positive voltage about 1.5 volts if I spin it really fast. If I spin it in the opposite direction I get a negative voltage at about the same amount. I can actually spin it really hard and get past 2 volts. This means this is a DC motor. So now we can move on to figuring out how to drive this motor and whether or not we can use it for some cool project.

I don't know the operating voltage of the motor, but I'm going to assume with a motor this size that it's either 9 or 12 volts. Most DC motors have a faily safe operating range. I've seen 5 volt motors be listed as 3 volts to 6; sometimes to 3 volts to 9. So even if this is a 9 volt motor, I'm going to, in the next video, connect it to a 12 volt power supply and we're going to see if we can drive it. And you know what, if we smoke the motor, it doesn't matter.

• You can't spin a single phase induction motor and tell what it is. Neither can you spin a lot of DC motors with separate field windings that need to be energized. He's talking rubbish if you've quoted him correctly. – Andy aka Mar 3 '14 at 20:47
• Dude, If you've quoted him correctly then what's the deal. Sorry I'm not going to watch the vid because my comment and your quote accuracy should be enough. – Andy aka Mar 3 '14 at 22:46
• Going by the transcript supplied above, it appears the demonstration is of a simple permanent magnet DC motor. This type of DC motor has a permanent magnet stator and a wound rotor utilizing commutating brushes to energize the rotor. This type of DC motor can be used as a DC generator as is being described in the transcript. – FiddyOhm Mar 4 '14 at 1:23
• I think Fiddy is correct. A permanent magnet brushless DC motor can also act as a generator if you spin it, but you'll see (typically) a 3 phase AC waveform at the terminals. An induction motor CAN be used as a generator assuming it is connected to a source of excitation (like the power grid). If you spin it with no excitation you won't get anything out of it. (Or maybe just a very tiny signal due to some residual magnetism.) – John D Mar 4 '14 at 1:26
• A simple cue is the presence or absence of a commutator. If a motor has no commutator, it can't be a DC motor (except for impractical curiosities such as homopolar motors). The reverse isn't true; universal motors do have a commutator but can be driven with AC as well as DC. Brushless DC motors don't have a commutator, but they are in fact AC motors driven by an inverter. – jms Oct 16 '17 at 11:07

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:

1. Permanent magnet DC brushed. DC back emf.
2. 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).
3. Permanent magnet AC synchronous (three phases). Three phase AC back emf.
4. 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.
5. 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.
6. 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.
7. 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).
8. 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.

• The field winding on a "Coiled rotor DC brushed" AKA universal motor is in the stator (not on the rotor as "rotor coil as a separate winding" would imply), since the armature (the commutated windings) must be on the rotor – jms Feb 8 '16 at 12:51
• Stepper motors are divided into two general flavours. Permanent magnet and variable reluctance. The PM types make sort of OK polyphase generators, the VR types will present like an AC polyphase induction motor and not generate well. – KalleMP Oct 13 '17 at 7:55
• Very small single phase AC squirrel cage motors are often of the shaded pole type (found in disco ball rotators, fans, record players etc.) and will not generate anything. – KalleMP Oct 13 '17 at 7:58

I try to keep it simple (might not be exactly accurate description, but maybe easier to understand): In a brushed DC motor there is no electronic circuit, it could be called mechanically controlled commutator, which ensures that the magnetic field inside the stator rotates (the excitation can either be from magnets as you said or done by another coil which has to be powered by a DC current). In return, when the rotor is forced to rotate this mechanic commutator serves as some kind of rectifier and allows you to measure DC voltage at the terminals.

In brushless motors (also called synchronous drives) you need some external electronics and sensors in the motor to generate that rotating magnetic field by switching the three phases on and off at the right time. If you remove that electronic circuitry (often called ESC or frequency inverter) and spin the rotor by hand and measure the voltage at one of the terminals with an oscilloscope, you can observe an AC waveform. By rectifying this AC voltage you could generate a DC voltage as well. So those brushless motors are in fact three phase AC motors, the brushless DC (BLDC) term, which is often used for them is strictly speaking not correct. Usually generators in power plants work like that.

There are some other types out there like (single phase) asynchronous AC motors or stepper motors, but I think you were more confused about the two major types of "DC" motors.

But they all have something in commen: Every electric motor is also a generator.

• The most common type of brush-less AC motor, the three phase motor is most often connected directly to the grid with NO electronics. A high quality variable reluctance stepper motor or a shaded pole induction motor will not generate anything by design, residual magnetism may generate a weak signal but no usable power. – KalleMP Oct 13 '17 at 8:02