BLDC and its analogy to 3 phase induction motor 1

[hyboxis79]

At times BLDC are refered to as 3 phased since they carry a few poles. Creating similarity in the manner they are connected. Other than that the BLDC utilizes direct current - is it that by the switching pulses a rotating magnetic flux ensues? that doen't make so much sense since in the 3 phase motor what makes it possible is the alternating current

On that matter do we also have to go into considerations of slippage(i.e lag of rotating rotor from the rotating flux) in a BLDC
appreciate anyone enlightening me

 

[sreid]

The stators of induction motors and BLDC motors are very much alike.

Traditionally, an Induction motor starts and runs off AC line power. The rotor must be "Fluxed" from the stator. Load is picked up through change in the stator-rotor magnetic angle.

In a BLDC motor, the rotor field is fixed by permanent magnets. The motor must be controlled through some kind of elecrtonics. Usually the stator and rotor magnetic fields are kept at 90 degrees.

 

[hyboxis79]

if I understand correctly in the BLDC current is only present in one pair of poles each time while in an induction motor current is present all the time in all the poles but it's alternating right? that's how a similar effect is acheived right ?
what's the difference then between a stepper motor and a bldc then?

 

[cswilson]

It is important to realize that a "brushless DC" motor is an AC motor -- that is, it requires alternating-current input to maintain motion in one direction. ("Brushless DC" is a marketing term, not a technical term, originated to emphasize that you could replace a brush DC motor and drive with this type of motor and drive.)

A "brushless DC" motor is best regarded as a synchronous AC motor with a permanent-magnet rotor field, whereas an induction motor is an asynchronous AC motor with a rotor field created electromagnetically.

Ironically, what distinguishes a motor marketed as a "brushless DC" motor or "brushless servo" motor from one marketed as an "AC synchronous" motor is that the AC synchronous motor has a secondary asynchronous torque generation mechanism that permits it to accelerate up to synchronous speed running directly off the line. Brushless DC motors are intended to run off some sort of inverter, and usually with feedback, so they do not add this asynchronous torque-generation mechanism.

But if you had a brushless DC motor already running at 1800 rpm (4-pole), it could run fine off a 60 Hz line. Being a synchronous motor, it would run exactly at 1800 rpm, whereas an ayschronous induction motor would "slip" and run at somewhat less speed, typically 1740 rpm at rated load.

It is entirely possible to use the same stator with an induction motor rotor and a brushless DC rotor. I've seen teaching systems that literally allow you to mix and match rotor and stator types this way.

Curt Wilson
Delta Tau Data Systems

 

[sreid]

The term BLDC Motor" has come to mean a motor that is driven using "Six Step" commutation. With six step commutation, every 60 electrical degrees, one phase has current flowing into it and one phase has current flowing out of it. The third phase has zero current flowing (open circuit).

 

[MotorExpert]

I agree with Curt that brushless DC and synchronous AC motors are primarily the same. The difference is the winding-configuration, a synchronous AC motor has a sinusoidale winding (the same as an asynchronous motor) and a brushless DC has a trapezoidal winding. The stator may be the same, its just the winding that is different (some brands offer motors with the same stator but different windingconfigurations).

Important is that the commutation has to match the winding to be sure you do not get a torque-variation (cogging caused by false drive-motor-combi). Six-step-commutation is intended to be used by a trapezoidale winding.

If you would apply a six-step drive to a synchronous motor you get a 13% torque ripple (because mismatch drive-motor). Dependant on the speed your motor operates you may notice this as acoustical noise.

But back to your question, the difference between synchronous and asynchronous is a difference between day and night. Synchronous motors use permanent magnets and has no slippage, asynchronous uses the Lorentz-principle (electromagnets) and has slippage.

I know there are some applications for asynchronous motors in servo-applications but I have no experience with that (usually I go for DC motors or synchronous motors, good lineair characteristics).

Edward Hage

http://www.specamotor.com

 

[Clyde38]

Brushless DC motors are not limited to "3 phase". Nor do all of them have to have alternating current (unipolar). There is also no limit to the number of poles for the Brushless DC. The first time that I was introduced to the term "Brushless DC" (mid 80’s) it was with the understanding that the performance more closely matched the permanent magnet DC motor than any other motor. That is to say that if you look at the speed vs. torque profile, the Brushless DC should look very similar to the permanent magnet DC motor. This (performance) is quite different than any AC motor, step motor, reluctance motor etc. I've seen brushless DC motors with either sinusoidal phase distribution or trapezoidal distribution driven from both sine drives and trap drives. They may be driven by AC inverters and directly by the AC mains. Some schemes apply a step function for a period of time and then an AC signal. There are advantages to matching drives schemes to the motor, but it is not a requirement.

 

[hyboxis79]

Thanks for all your replies.
Some more questions have risen:

1. I wanted to ask seried regarding what you said about always having a pole where the current comes out and another pole where the current goes in. Physically a pole is a pair of wound wire on a metal core, that is one core on one side and one core on the other side? or is it just one of the wound cores. I hope that what i refer to as core is clear

2. How does a trapezoidal winding look like and how does a sinusoidal one look like ?


thanks

 

[MotorExpert]

Regarding your question:
2. How does a trapezoidal winding look like and how does a sinusoidal one look like ?

See page 25 until 29 of the book "Electrical drives for precision engineering designs" from prof.Compter at

http://www.specamotor.com/en/freebook.php

The book is free of charge.

Also the 6-step drive for a trapezoidale winding is described and the 3-phase sinusoidale drive for a synchronous motor.

Edward Hage

http://www.specamotor.com