Are Brushless Motors AC or DC? - Well...

Ever wondered why we call brushless motors such as the ones we use with drones, we call them DC motors?? Here's a #shorts explination of why we do that. So, brushless motors are AC or DC? Comment below!

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[dtibor]

If you drive this motor with the proper amount of AC phases, it will work nicely. In practice we use DC pulses to drive it, but that's OK, it is still an AC motor

[ZoeyR]

The current on a bldc motor is Infact AC because it is infact alternating. The 3 phase bldc controller is swapping the polarity on all the pins so it's ac. Most modern esc are now using phase inductance to produce a smooth signwave now as well dji and castle do this to drastically reduce ringing and heat from the motors.

A bldc motor controller is really just a dc to 3 phase ac inverter with variable frequency control

[terjeoseberg]

Whether it crosses 0v or not depends on where you put 0v. It’s arbitrary. You can decide to put it anywhere. Simply take the average of the 3 phases and make that 0v. Then by your definition of AC, it’s AC.

[zohan]

I see this as a perspective problem.
By taking the battery ground as the absolute ground, yes, this is all DC, but once past the ESC, we should consider the frame of reference of the motor. Now we have 3 phases and the virtual 0-point is determined with the 3 resistor star thingy, you certainly know what I'm talking about.

[anishpatil]

It is an ac motor because we are using posative and negative half cycles in order to attract and repel the magnet.

[stephensu]

brushless motor broke the line between ac motor and dc motor, but on construction standpoint it more closer to the ac motor, you can consider the esc as some kind VFD or VVVF

[gromgromeson]

this should just be wrong tho. alternating current is current that periodically changes direction by having these modulated potential waves we definitely get reversing current in the motor coils => ac.
it really doesnt matter that with regard to our defined 0V potential we only have positive potentials. The direction of the Current is what matters for AC (alternating CURRENT)
the reason we call them dc is because we can supply dc to the alternator similar to how we can just supply dc to a brushed motor. the DC is therefore just user centric nomenclature. the motor itself definitely sees alternating current.

[power-max]

0:09 Nope, *YOU'RE WRONG.* :)
2 reasons:

That is a AC voltage with a DC offset, or a DC voltage is AC signal superimposed on it (whichever you prefer). At least by MY definition of AC and DC. Defining AC as a voltage that happens to cross the zero crossing seems... arbitrary. I prefer thinking in terms of AC and DC components. Just do a Fourier series analysis on it, and seeing 2 impulses, one at 0Hz (DC) and another factor at some frequency. In this case, my answer would be both AC and DC components, in roughly equal magnitudes (actually a bit more DC than AC magnitude since it is well above the zero crossing in the time domain).

HOWEVER, the motor NEVER sees that voltage. Voltage is a measure between 2 nodes, so I ask you, what 2 nodes were measured to get that waveform? I know for *FACT* it sure is hell ain't the voltage across any 2 connections of the motor. In fact, I pretty sure that waveform is the voltage measured between one of the phases and the negative of the battery terminal. Guess what? The motor has no connection to battery negative. This is *deliberate*. Doing so would result in massive DC current flow through the motor coils, saturation of the magnetic core, exploding MOSFETs, and burnt windings. So that voltage measurement is pointless and misleading because no current will (or at least should) flow in that path. and if you put a clamp-meter to measure current in any phase, the measured current will have no DC bias, just like you show in 0:41. See? AC currents ONLY. *Alternating currents*, with NO dc offset.

So yes, the BLDC motor is in fact a type of *AC synchronous motor.* The motor will never see any DC voltage on it. The back EMF is pure AC. Each phase can be electrically modeled as a series inductance and resistance, which also suggests that any DC bias would only serve to cause ohmic heating (it's bad.) and perhaps also extreme torque ripple.

IMHO, Even DC motors are AC motors, the commutator converts DC to AC rather than electronics. From the frame of reference of any particular pole on the armature, it will never see a steady-state DC. some percentage of the time it will get a blimp of positive voltage, and 180 degrees later, get an equal blip of negative DC voltage.

Now, what's the difference between a "BLDC" motor and a "PMSM" motor? both have permanent magnets and an armature, and both operate in much the same way. One has a sinusoidal back EMF and designed to work with sinusoidal drive. Whist the other has a more trapezoidal back emf and designed to work with 6 step commutation. You can also dive into the intricacies of space vector modulation and 3rd harmonic injection if you want to get real fancy. Actually, practically speaking, BLDC and PMSM motors in real life are really blurred together due to manufacturing tolerances and stuff. Things aren't so mathematically pure in practice. Both BLDC and PMSM will generate a somewhat distorted back EMF.

0:43 *This is where things get interesting.*
Yes, the torque vs RPM of a BLDC motor without an advanced control algorithm (like Direct Torque Control or Field Oriented Control) does actually behave a _lot_ like a permanently-excited DC motor. That is to say, a linear torque and RPM relationship; which has a some maximum torque at stall condition and linearly follows a line down to the no-load maximum RPM, assuming constant voltage excitation, as a result of back EMF and parasitic phase impedance. At the midpoint of half speed and half the stall torque is the maximum power point, and maximum efficiency is achieved closer to the no-load condition (light loading.) as expected. But why does this define a BLDC to be a DC motor? What does this characteristic have _anything_ to do with it being a DC motor???

So really? Well as far as I can tell, whether it should be considered a "DC" or "AC" motor has more to do with the controller than it does with the motor. You can connect a PMSM to a BLDC motor driver and it will honestly work just fine. And ditto with a PMSM motor to a BLDC motor controller. (maybe not 'optimal' theoretically) but whatever. Practically, they are in fact interchangeable.

Actually, I do really appreciate the discussion though. and the TL;DR is... it's complicated. Really both sides make good points.

[darkobul]

0 volt is arbitrary point. Any change of current is AC. So all Motors are AC. As Motors can be generators this can be seen at output.

[daycontrols]

I think the key word in Alternating Current is CURRENT. The zero voltage reference is somewhat arbitrary. Each motor lead in this BLDC motor will have current flowing in both directions. This is what defines an AC motor. For example, we could use a large 600V battery to feed the DC buss of a VFD serving an AC induction motor. This motor would not suddenly become a "DC motor".

[uwezimmermann]

Even in your definition of "alternating" current, the current through the wires to the motor is actually changing direction, each coil wire being driven by a half bridge. It is true that even for a proper DC motor the current inside the coils changes direction, but not in the wires to the motor.
But in the end arbitrary semantics are irrelevant in science and technology. Neither does the motor itself care what you call it, nor is it relevant for the application.

[gizmoguyar]

By definition all continuous motors are ac. Maxwell's equations state that the current in the windings must integrate to zero over a full wave. Otherwise magnetic flux would accumulate, and the core would saturate. Even what we call "DC motor" is an AC motor. We call it DC because commutation is done I internally. Brushless motors do not have internal commutators. This is done electronically. Therefore you must feed it AC current.

[spaceshuttle]

Learned something new today! Thank you!

[ciprianvlad]

In each phase of the motor the current flows in both directions. So it is an AC motor a guess....

[Simon-cufi]

It is an AC motor. They call them BLDC because the torque vs speed curves of both BLDC and DC brushed motors are linear. Using a DC battery to run the motor puts the DC into an inverter which converts it to AC (DC offset doesn't make a signal DC). The signal alternates regardless of what hight it alternates.

[VlogWithme-wb]

Bldc is a DC motor that uses Electronic commutator is term as brushless DC motor ..thats Why DC motor ❤😊

[jakubapszo]

AC motors whats looks quite simillar to brushless motors(BLDC) are names PMSM(Premament Magnet Synchronous Motor).

[tvamos]

IMHO the current is alternating inside the motor coil means AC. The other hand the motor coil Voltage is also change the direction obviously. So, in my theory this motor is AC. (Yes, the brushed is also AC inside.)

[multicopter]

You are definitely wrong. Every motor has either mechanical or electronic switching. The DC motor is mechanically switched and the BLDC motor is electronically switched. Every motor is a transducer, converts electrical energy into mechanical energy and vice versa. But if you run a DC motor you will have a DC generator dynamo, now if you run a BLDC motor you will have an alternator, generating three-phase AC current, and you will need a voltage rectifier to convert to DC. Cheers from Brazil.

[kevinroberts]

You can hook it up to 3 phase AC and it will run. FYI