# 12 slot 4 pole 3 phase DC motor. What is the relation between the electric field and rotor motion

#### jlawley97

Joined Oct 5, 2019
25
(if there is a better location or even a different forum all together let me know)

Ive been gifted this project of simulating 12 slot 4 pole 3 phase brushless permanent magnet (in the rotor)DC motor and I have been banging my head for a while. I have a lot of questions my main one is
How many rotations should the electrical field make for each rotation of the rotor?

Second question is does anyone have any resources like books or pdfs on this topic that would be good at covering this type of thing?
there are so many types of electric motors it is driving me nuts! and again I know this is not the 100% best forum for motor discussion so any suggestions as where else to go for that.

Joined Jul 18, 2013
28,675

#### shortbus

Joined Sep 30, 2009
10,045
How many rotations should the electrical field make for each rotation of the rotor?
A BLDC motor needs two stator(not field, the magnets are the "field") electrical rotations to make one shaft rotation.

Quote - " For BLDC motors, only the knowledge of six phase-commutation instants per electrical cycle is needed; " From- https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3231115/

To learn more about motors you need to first learn the terms of their parts, then Google will give you more answers than you ever need to understand. And Google will tell you the names if you type in the kind of motor.

#### nsaspook

Joined Aug 27, 2009
13,257
The Rotor of the 2 pole motor completes one cycle for every cycle of the source, while the rotor of the 4 pole motor completes only half a cycle for every single cycle of the source.

#### shortbus

Joined Sep 30, 2009
10,045
The Rotor of the 2 pole motor completes one cycle for every cycle of the source, while the rotor of the 4 pole motor completes only half a cycle for every single cycle of the source.
Except he was asking about a BLDC motor not an AC motor.

#### nsaspook

Joined Aug 27, 2009
13,257
Except he was asking about a BLDC motor not an AC motor.
A BLDC motor is a synchronous AC motor.

Brushless DC motors were developed from conventional brushed DC motors with the availability of solid-state power semiconductors. So, why do we discuss brushless DC motors in a chapter on AC motors?

Brushless DC motors are similar to AC synchronous motors. The major difference is that synchronous motors develop a sinusoidal back EMF, as compared to a rectangular, or trapezoidal, back EMF for brushless DC motors.

Both have stator created rotating magnetic fields producing torque in a magnetic rotor.
https://www.motioncontroltips.com/faq-whats-the-difference-between-bldc-and-synchronous-ac-motors/

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Joined Jul 18, 2013
28,675
BLDC 4 pole motor. commutation.

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#### shortbus

Joined Sep 30, 2009
10,045
A BLDC motor is a synchronous AC motor.
In theory. Have you ever tried running say a hobby BLDC motor on AC? Doubting very much that it will do anything but let out the magic smoke.

#### nsaspook

Joined Aug 27, 2009
13,257
In theory. Have you ever tried running say a hobby BLDC motor on AC? Doubting very much that it will do anything but let out the magic smoke.
In theory and practice.
I design BLDC driver circuits and software for various types of AC or DC motors.

I know exactly how to handle 6-step commutation (trapezoidal or quasi-square waveform), sinusoidal commutation or FOC depending the the construction of the target motor.

https://www.motioncontroltips.com/faq-trapezoidal-back-emf/
Motor terminology can be tricky. A common way to categorize motors is by their input voltage: AC or DC. But many engineers argue that brushless DC (BLDC) motors are actually AC motors, because they are driven by DC voltage that switches from one stator coil to the next, generating an alternating (hence, AC) trapezoidal or quasi-square waveform.

Another way to classify motors is by their back EMF profile. BLDC motors are referred to as trapezoidal motors, since they produce trapezoidal back EMF, and BLAC motors (aka PMSM) are referred to as sinusoidal motors since they produce sinusoidal back EMF. But, as discussed below, the back EMF of a BLDC motor is not truly trapezoidal in shape. In reality, its shape is more sinusoidal.
This is why BLDC motors can use trapezoidal or sinusoidal commutation.

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Joined Jul 18, 2013
28,675
Wave form produced when BLDC back-fed rotated in order to align encoder commutation tracks.

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#### shortbus

Joined Sep 30, 2009
10,045
I design BLDC driver circuits and software for various types of AC or DC motors.
Yet another of your many skills and jobs. Amazing. And still you find the time to watch and post the many Youtube videos to the site.

#### nsaspook

Joined Aug 27, 2009
13,257
Yet another of your many skills and jobs. Amazing. And still you find the time to watch and post the many Youtube videos to the site.
Evil never sleeps.

Yes, it's amazing what you can do if you put your mind to it old friend. It's clearly amazing to you but it's fun and enjoyable to me to build semiconductors at the chip level, design with semiconductors, design circuits and PCB's for them and write software for semiconductors all while understanding the simple electrodynamics of motors as one of many skills.

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#### shortbus

Joined Sep 30, 2009
10,045
What ever. You seem to forget all your other claims of the things you've done. all the while earning a degree in electrical/electronic engineering.

#### Ya’akov

Joined Jan 27, 2019
9,117
In theory. Have you ever tried running say a hobby BLDC motor on AC? Doubting very much that it will do anything but let out the magic smoke.
3 phase AC would do it, since that is what the driver produces.

Joined Jul 18, 2013
28,675
The difference between a BLDC and a 3ph AC, both with a PM rotor motor is in the commutation, I have used the AC version with BLDC just by adding the 3 commutation devices or tracks on the encoder.
Stripped down, they are physically identical. And can be used either way.

#### shortbus

Joined Sep 30, 2009
10,045
3 phase AC would do it, since that is what the driver produces.
Execept what started this tangent of the thread was nsa's posting a single phase induction motor video explaining the motor speed. The original post was asking about a BLDC motor as is the topic/post headline wording. When I called him out on it he then went into the BLAC motor talk to cover his mistake. He can never be wrong.

#### Ya’akov

Joined Jan 27, 2019
9,117
Execept what started this tangent of the thread was nsa's posting a single phase induction motor video explaining the motor speed. The original post was asking about a BLDC motor as is the topic/post headline wording. When I called him out on it he then went into the BLAC motor talk to cover his mistake. He can never be wrong.
I only saw the 3 phase subject and missed anything else. I lost the context.

#### jlawley97

Joined Oct 5, 2019
25
Hi, so like does the amount of stators not matter? This is the set up I have where capital letters indicate clockwise and lower case indicate counterclockwise winding. Red is North magnets and Blue is South magnets. Would it still be 2 electrical rotations for 1 mechanical rotation?

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