BLDC and its analogy to 3 phase induction motor
BLDC and its analogy to 3 phase induction motor
(OP)
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
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





RE: BLDC and its analogy to 3 phase induction motor
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.
RE: BLDC and its analogy to 3 phase induction motor
what's the difference then between a stepper motor and a bldc then?
RE: BLDC and its analogy to 3 phase induction motor
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
RE: BLDC and its analogy to 3 phase induction motor
RE: BLDC and its analogy to 3 phase induction motor
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
RE: BLDC and its analogy to 3 phase induction motor
RE: BLDC and its analogy to 3 phase induction motor
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
RE: BLDC and its analogy to 3 phase induction motor
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