Download Multiphase Brushless DC Motor

MULTIPHASE
BRUSHLESS DC
MOTOR
Introduction

A BLDC motor accomplishes commutation electronically
using rotor position feedback to determine when to switch
the current unlike brushed dc motor which uses carbon
brush to provide unidirectional current.

STATOR= Stator windings of a BLDC motor are connected
to a control circuit (an integrated switching circuit).It is
wound with a specific number of poles.

ROTOR=Permanent magnets are mounted on the rotor of
a BLDC motor.The permanent-magnet rotor is typically
made either of rare earth magnets (Sm-Co) or of ceramic
magnets (ferrites).
continued..

There are two types of stator windings: trapezoidal and sinusoidal,
which refers to the shape of the back electromotive force (BEMF)
signal.

The shape of the BEMF is determined by different coil
interconnections.

A sinusoidal motor produces smoother electromagnetic torque than
a trapezoidal motor.
Operational Theory

Motor operation is based on the attraction or repulsion between
magnetic poles.

Using the three-phase motor shown in figure , the process starts
when current flows through one of the three stator windings and
generates a magnetic pole that attracts the closest permanent
magnet of the opposite pole.

Sequentially charging each winding will cause the rotor to follow
in a rotating field
continued..
Brushless DC Motor Control

The essence of BDCL lies in controlling the polarity of the stator
which in turns control the rotation of the rotor.

Brushless DC motors use electric switches(connected in Hbridge structure) to realize current commutation, and thus
continuously rotate the motor.
Single Phase

BLDC commutation relies on feedback on the rotor position to
decide when to energize the corresponding switches to
generate the biggest torque.

Hall sensor is used to detect the correct position of the rotor
when the change in polarity of stator is required.

A Hall position sensor (“a”) is mounted to the outside stator,
which induces an output voltage proportional to the magnetic
intensity.

the sensor goes HIGH when the rotor’s North Pole passes by,
and goes LOW when the rotor’s South Pole passes by.
3-Phase

A three-phase BLDC motor requires three Hall sensors to detect
the rotor’s position.

Three Hall sensors are mounted on the stator at 120° intervals.

For every 60° rotation, one of the Hall sensors changes its state; it
takes six steps to complete a whole electrical cycle
Advantages of
Brushless DC Motor
continued..
•
Minimized torque ripple
•
Reduced Noise
•
Increased Power density
•
Improve fault- tolerance w.r.t open circuit lags

High Efficiency

High Torque to Weight Ratio

Longer Life Time

Compact and Light weight

Improve fault-tolerance w.r.t open circuit
Speed Control
Speed Control Method

One can control speed of motor using two methods:
1.
No. of stator poles per phase.
2.
Frequency.
Speed Control Method

We can find the speed of motor by formula:
S= 120 x f
P
S= Speed of motor in rpm
f= Frequency
P= No. of Stator Poles per phase
Torque
Comparison
Torque Comparison
•
•
2 wire motor is not smoothly started.
Multi Phase motor is smoothly started and rotated with
high torque at low speed and high power at high speed.
Torque Comparison
The BLDC is controlled by two types of waveforms produced by
inverters:

Trapezoid

Sinusoidal
Torque Comparison
Trapezoid Waveform:

Motors powered by trapezoid waveform produced
10% more torque than sinusoidal.

But motors are not smooth and have ripples in
torque.
Torque Comparison
Sinusoidal Waveform:

Used for low speed but it is smooth and have less
ripple in torque.
Torque Comparison

With the increase in phases ,the torque becomes
less rippled because of smoother rotation of rotor
Torque Comparison

Torque also depends upon the configuration of stator
windings.
1.
For Y-configuration, motor gives high torque at low speed
but not as high as at top speed.
2.
For Delta-configuration, motor gives low torque at low speed
but can give high torque at top speed.
Torque Comparison

Furthermore, Torque can be increased by increasing
no. of rotor poles.
Applications
Applications

Multiphase Brushless DC motors are used in:
•
In Electric Vehicles (Evs)
•
PC fans , DVD players
•
Aero modeling
•
Heating and Vantilation
•
Radio Controlled Cars
BLDC in Electric Cars
Windings on stator Permanent magnets are on the rotor
Side.
BLDC used in Evs due to following advantages :

Low maintenance

80-90% efficiency

High operating speeds

No brush sparking

Compact size
continued..

Quick response

Zero tailpipe emissions

Charge itself on applying brakes

Cheap to drive
continued..

Pc fans and DVDs for more efficient and less noisy
…

In Heating , Ventillation and Air conditioning
industries (HVAC)

Used in aero modeling like aircrafts and helicpters …

Radio controlled cars having less input power and
more torque