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Brushless Motor Basic Theories |
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1. Naming Rule:
For Example
57 BL 01
① ② ③
① FRAME SIZE ② BRUSHLESS MOTOR ③ NO.
2. Small Brushless DC Motor Theories:
Principles of operation
The differences between a DC motor having a mechanical commutation
system and a BLDC motor are mainly found in :
- the product concept
- the commutation of phase currents.
From the user's point of view, brushless DC motors follow the same
equations as those with brushes: torque is proportional to current,
speed depends on the voltage and the load torque.
The commutation of brushless motors
In the conventional DC motor commutation takes place mechanically
through the commutator-and-brush system. In a BLDC motor,
commutation is done by electronic means. In that case the
instantaneous rotor position must be known in order to determine the
phases to be energized.
The angular rotor position can be known by:
- using a position sensor (Hall sensor, optical encoder, resolver)
- electronically analyzing the back-EMF of a non-energised winding.
This is called sensorless commutation.
Use of Hall sensors
In general, BLDC motor have three phase windings. The easiest way is
to power two of them at a time, using Hall sensors to know the rotor
position. A simple logic allows for optimal energizing of the phases
as a function of rotor position, just like the commutator and
brushes are doing in the conventional DC motor.
Use of an encoder or resolver
The rotor position may also be known by use of an encoder or
resolver. Commutation may be done very simply, similar to the
procedure with Hall sensors, or it may be more complex by modulating
sinusoidal currents in the three phases. This is called vector
control, and its advantage is to provide a torque ripple of
theoretically zero, as well as a high resolution for precise
positioning.
Use of Back-EMF analysis
A third option requiring no position sensor is the use of a
particular electronic circuit. The motor has only three hook-up
wires, the three phase windings are connected in either triangle or
star. In the latter case, resistors must be used to generate a zero
reference voltage. With this solution the motor includes no sensors
or electronic components and it is therefore highly insensitive to
hostile environments. For applications such as hand-held tools,
where the cable is constantly moved, the fact of just three wires is
another advantage.
The functioning of a sensorless motor is easy to understand. In all
motors, the relation of back-EMF and torque versus rotor position is
the same. Zero crossing of the voltage induced in the non-energised
winding corresponds to the position of maximum torque generated by
the two energized phases. This point of zero crossing therefore
allows to determine the moment when the following commutation should
take place depending on motor speed. This time interval is in fact
equivalent to the time the motor takes to move from the position of
the preceding commutation to the back-EMF zero crossing position.
Electronic circuits designed for this commutation function allow for
easy operation of sensorless motors.
Small Brushless DC Motor
As the back-EMF information is necessary to know the rotor position,
sensorless commutation doesn't work with the motor at stall. The
only way of starting is to pilot it at low speed like a stepper in
open loop.
Remember:
- for commutation, position sensors are necessary when operating in
incremental mode
- sensorless commutation is recommended only for applications
running at constant speed and load.
Operating principle of BLDC motors:
It follows the same equations as the DC motor using mechanical
commutation except that parameters like iron losses and losses in
the drive circuit are no longer negligible in applications where
efficiency is of prime importance.
Iron losses
They depend on speed and, in the torque formula, may be introduced
as viscous friction. The equation for useful motor torque becomes:
Losses in the electronics
The current and votage required by the motor and the drive circuit
to operated at the desired speed and torque depend also on the drive
circuit.
As an example, a driver bridge in bipolar technique will reduce the
voltage available at the motor terminals by about 1.7V, and the
total current must include the consumption of the circuitry. |
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Stepper Motor Basic Theories |
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Stepper Motor
1. Naming Rule:
Motors convert electrical energy into mechanical energy.A Stepper
Motor converts electrical pulses into specific rotational
movements.The movement created by each pulse is precise and
repeatable,which is why step- per motors are so effective for
positioning applications.
Permanent Magnet Stepper Motors incorporate a permanent magnet
rotor,coil windings and magnetically conductive stators.Energizing a
coil winding creates an electromagnetic field with a north and south
pole .The stator carries the magnetic field.The magnetic field can
be altered by sequentially energizing or "stepping" the stator coils
which generates rotary motion.
Figure 1 illustrates a typical step sequence for a two phase
motor.In Step 1 phase A of a two phase stator is energized.This
magnetically locks the rotor in the position shown,since unlike
poles attract,W hen phase A is turned off and phase B is turned
on,the rotor rotates 90¡ã clockwise.In step 3,phase B is turned on
but with the polarty reversed from Step 1,this causes another 90¡ã
rotation.In Step 4, phase A is turned off and phase B is turned
on,with polarity reversed from Step2.Repeating this sequence causes
the rotor to rotate clockwise in 90¡ã steps.
The stepping sequence illustrated in figure 1 is called "one phase
on" stepping.A more common method of stepping is "two phase on"
where both phases of the motor are always energized.However,only the
polarty of phase is switched at a time,as shown in figure 2. With
two phase on stepping the rotor aligns itself between the "average"
north and "average" south magnetic poles.Since both phases are
always on,this method gives 41.4% more torque than "one phase on"
stepping.
Half Stepping
The motor can also be "half stepped" by inserting an off state
between transitioning phases.This cuts a stepper's full step angle
in half.For example,a 90¡ã Stepper Motor would move 45 on each half
step,figure3.
However,half stepping typically results in a 15%-30% loss of torque
depending on step rate when compared to the two phase on stepping
sequence.Since one of the windings is not energized during each
alternating half step there is less electromagnetic force exerted on
the rotor resulting in a net loss of torque.
What is a Stepping system?
Stepping systems have been used in industrial automation for many
years to accurately position machine tools, x-y tables, feeders,
etc. Unlike typical AC variable speed drives (used for velocity
control), a stepping drive and Stepper Motor is used primarily for
position control. A typical single axis Stepping system consists of
a Stepper Motor controller/ indexer, a motor drive, a motor (with or
without gearbox), and a power supply. A stepper is typically
commanded by two digital inputs: a digital pulse train and a
direction bit. A single pulse on the pulse input moves the motor one
step increment in the direction (CW or CCW) set by the direction
bit.
Controller/ Indexer
The controller/ indexer is responsible for outputting the pulse and
direction commands to the drive. PLCs distributed by MotionKing, as
well as many other 3rd party products, can be used to control the
SureStep line. The frequency of the pulse train controls the
velocity of the motor, where the number of pulses determines the
length of the move. The direction signal determines in which
direction the motor will rotate.
Stepper Power Supply
The power supply plays a dual role: it supplies the main power to
the motors as well as the power to optically-isolated digital
inputs. The motor power is typically provided by a linear
non-regulated power supply. The SureStep power supply has a 32 VDC @
4A (when fully loaded, 41 VDC unloaded) output to supply power to
the motor via the drive. Also, the digital interface between the
drive and the controller/ indexer should be powered by the isolated
+5 VDC (500 mA max) regulated output. The motor power and the
digital interface power are typically isolated since the motor power
source is extremely "noisy" when loaded. Using the motor power for
the interface might lead to false pulse commands thereby creating
undesired system movement. One SureStep power supply can provide
both motor power and interface power to a least 2 complete SureStep
drive/ motor combinations.
Stepper Drive
The drive translates the pulse and direction commands from the
controller/indexer and converts them into actual motor movement. For
each pulse from the controller, the drive will move the motor "one
step" in the direction indicated by the direction command. The
SureStep microstepping drive provides 4 different step resolutions
for use in a wide range of applications. They range from 1/2
stepping (400 steps/rev) to 1/50 stepping (10,000 steps/rev) when
used with a 1.8¡ã (200 full steps / rev motor). The SureStep drive
can be configured for use with all SureStep brand motors, as well as
many other 3rd party 2-phase, bipolar Stepper Motors that require
0.4 to 3.5 Amps/phase to drive them. The drive also features a "test
mode" that allow the drive and motor system to be tested without
being connected to a controller/indexer. This feature enables an
on-board indexer that moves the motor 1/2 revolution back and forth
in half-step mode so the user can easily "move" the system while
troubleshooting. An idle current cutback feature can be used to
conserve power and reduce heat by cutting the power to the motor by
50% if no step pulses are received for 1 second. All the SureStep
drive settings are done through a 9-position dip switch, which means
that NO SOFTWARE or external resistors are required to configure the
drive.
Stepper Motor
The motor converts the power from the drive into rotational
movement. Unlike AC motors, Stepper Motors have 100% current
(idle-current cutback disabled) applied to them all the time
regardless of load on the motor. The motor is moved in "steps" (one
per command pulse) and will hold at its present position if no
command pulses are received. The SureStep line of motors are built
to provide high-torque for their frame size which ranges from NEMA
17 to 34. All SureStep motors have only 4 leads and are
connectorized, which greatly reduces the chance of mistakes while
wiring the motor to the drive. Many other motor brands typically
have 6 or 8 leads, which can cause confusion while wiring the
system.
NOTE: For further details or technical specifications, select the
desired product category and select the "Products" tab and click on
the desired "Item Code" link, or select the "Technical Info" tab. |
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