DC and Brushless motors

[KansaiRobot]

Hello everybody and thanks for the help.

I have a new project in which I am working and would like to know the difference between DC and Brushless (brushless DC??) motors. Specially concerning how should I control them from a microcontroller.

In few words what is different about them?


Any push in the right direction would be very appreciated

Kansai

 

[dknguyen]

A brushless DC motor is just a type of DC motor. Just like a brushed motor is a type of DC motor. THat's all.

To control them from a microcontroller you need at minimum the following things:
-a three-phase interver (three half-bridge circuits, each connected to one of the three phases of the DC motor)
-a way to know the rotor position of the motor so you can properly commutate the current through the windings using the three-phase inverter (can be a directly sensed using something like a hall-sensor, or indirectly sensed by measuring back EMF. Without this you lack the sequencing information to get the motor to spin and it will lock up rather than spin.

Intro:
https://www.electro-tech-online.com/custompdfs/2010/05/00885a.pdf

Sample Projects:
https://www.rcgroups.com/forums/showthread.php?t=140454

 

[MikeMl]

I take issue with the statement that a DC motor is the same as a BLDC (brush less dc) motor:

A DC motor usually has two external wires. You hook DC Voltage to the two wires (sometimes even AC, if it is a "universal" motor) , and the motor runs.

A BLDC motor has at least seven wires (three for the stators and four for the Hall Effect sensors). It takes a very complicated algorithm to get the motor to run efficiently. It takes a dedicated IC chip controller or a microcontroller with three external half H-bridges to run the motor.

 

[dknguyen]

No one said they're the same. But I admit it does get murky because there are AC synchronous motors and DC brushless motors which are pretty much the same thing, but meant for different purposes and are driven differently.

Then you get people debating whether it should be classified as a AC or DC motor because the motor actually does require AC waveforms (whether sinusoidal or trapezoidal) to run, but the motor driver generates these from a battery which is DC. Technically calling them just brushless motors alone is vague because no AC motors of any kind has brushes, but in common usage brushless motor tends to mean it was meant to be powered from a battery or DC source (despite a motor driver being needed to turn the DC into an AC waveform). Technically they should be called synchronous motors, but in common usage that usually implies that they were meant to be powered by an AC source (either directly or via a variable speed drive).

Incidentally, a common way of controlling the speed of a synchronous motor being powered from an AC source is to actually convert the AC into DC, then convert that DC into the waveform you want. THis is because the frequency of the AC waveform determines the speed of the motor and it's tricky to convert a AC waveform of one frequency (50 or 60Hz from the wall) directly into an AC waveform of another frequency. It is possible though, but expensive and tricky and is only used in really realy high power applications. In essence a "DC brushless motor" just doesn't have the step of turning into AC to DC since it can get DC straight from a battery.

 

[schmitt trigger]

I've read that there are discussions about adding another category to the traditional two (AC and DC) motor categories.

It would be called electronically commutated (EC) motor, which would describe brushless DC, stepper, and other motors which cannot function unless there are electronic devices to drive them.
A three-phase induction motor driven from a variable frequency drive would NOT be classified as an EC, because if connected directly to an AC source it will still run. On the other hand, a BLDC cannot run if connected directly to a battery..

Unfortunately, I cannot find the original article...the author made a very compelling argument.

What do you think?

 

[KansaiRobot]

How can Brushless motors be controlled? with PWM? H-bridge?

 

[indulis]

If your DC brushless motor already has the controller in it (2 wire), then you will be unable to “brake” the motor. It will just “coast” as it slows. With a DC motor you can forcibly slow it by changing the duty cycle… think along the lines of regenerative braking in electric golf carts.

Regenerative braking

 

[Ubergeek63]

If your DC brushless motor already has the controller in it (2 wire), then you will be unable to “brake” the motor. It will just “coast” as it slows. With a DC motor you can forcibly slow it by changing the duty cycle… think along the lines of regenerative braking in electric golf carts.

Regenerative braking

it is not changing the duty cycle though... it is shorting the low side of the motor to ground and PWMing the other side to ground creating a boost regulator using the armature inductance and motor speed generated voltage.

 

[gabeNC]

it is not changing the duty cycle though... it is shorting the low side of the motor to ground and PWMing the other side to ground creating a boost regulator using the armature inductance and motor speed generated voltage.

I was following you up until this part...

boost regulator using the armature inductance

 

[indulis]

Yes... IF you were to reduce the duty cycle (effective input voltage), it will in turn reduce the speed of the motor.

 

[KansaiRobot]

I am thinking about using the Toshiba TA7257P.

http://www.datasheetcatalog.org/datasheet/toshiba/4018.pdf

Any opinions on this?

 

[indulis]

You haven't said what the end application is...

 

[dknguyen]

I've read that there are discussions about adding another category to the traditional two (AC and DC) motor categories.

It would be called electronically commutated (EC) motor, which would describe brushless DC, stepper, and other motors which cannot function unless there are electronic devices to drive them.
A three-phase induction motor driven from a variable frequency drive would NOT be classified as an EC, because if connected directly to an AC source it will still run. On the other hand, a BLDC cannot run if connected directly to a battery..

Unfortunately, I cannot find the original article...the author made a very compelling argument.

What do you think?

Intriguing. Except...synchronous motors still run though when connected to AC...that is...if you can get them started. I was told some have induction motor components built into them and function like an induction motor to get started until they become synchronous. WHat would that be classified as?

 

[schmitt trigger]

Synchronous motors, hmmmm.....those are borderline classification.
Technically they are powered from AC, and the literature classifies them as AC motors. But they do require additional circuit to start them.
The first step is to short the field winding to act as an induction motor. As the rotor reaches synchronicity, another circuit determines the exact phase angle when the DC field will be applied. And lastly, one that the motor is running syncronously, the field is adjusted to provide the proper power factor. All of those require electronic circuits.

 

[KansaiRobot]

You haven't said what the end application is...

Well, basically I am going to use a small motor that has a pump attached to it to generate some discharge pressure. The motor I am thinking on using is a small DC motor for pumping. 25mmx 31 mm with a drive voltage of 6~12V.

Now, I have understood more or less DC motors but not brushless. I have the option between these two but since I dont know how to control brushless I suppose I am going for the DC.

Do brushless even use H-bridge???

 

[schmitt trigger]

DC motors are very simple to control; if you don't require rotation reversal a single transistor will do.

 

[Ubergeek63]

I was following you up until this part...

look at the motor as a battery whose voltage is motor RPM times some constant in series with the armature inductance and resistance.

that battery voltage is ALWAYS less than the supply voltage.

in regenerative braking you are reversing the armature current by shorting the motor and then when you remove the short the motor inductance "boosts" the energy back up into the battery