
Basic Technical Knowledge / Basic Brushless motor
If current is caused to flow in the armature conductors, torque is produced. There is an application of a law of physics which is expressed as: F = KBli Where: F = force  K = a constant  B = air gap flux density  l = length of the conductor  i = current in a conductor If
more than one conductor is carrying the same current (multiple turns
per coil), than F = KBliz Where Z = number of conductors is series. In
a motor the conductors rotate abput a central shaft (see figure 1).
Than torque, T = FR, where R = radius at the air gap. So, T = KRBliz Figure 1
shows the coil in the zero torque position. The maximum torque position
is 90 electrical degrees from the position shown. As the conductors
ratate from the maximum torque position, torque drops off in a
sinusoidal fashion and becomes zero when the coil has moved 90 degrees. A
brush type motor has more than one coil. Each coil is angularly
displaced from one another so that when the torque from one coil has
dropped off, current is automatically switched to another coil wich is
properly located to produce maximum torque. The switching is
accomplished mechanically with brushes and a commutator as shown in Figure 2 Two, three and four phase motor design are
common. This configuration optimizes performance even though it requires
more electronic components. Three types of three phase windings are
available: Delta bipolar, wye unipolar and wye bipolar. These three
winding configurations and their transistor orientation are shown in Figure 3 Figure 4
illustrates the sequential steps in the commutation of a three phase,
bipolar system. Closing the transistors 1 and 4 will enable current to
flow through phase A and B. The permanent magnet rotor will then align
itself in a zero torque,preferred position. If 1 is opened and 5
closed, current will flow through phases B and C, the rotor will move
120 electrical degrees. (Note that the current through phase A is now
flowing in the direction opposite the one at the start of this example.)
Obviously, there must be some logic in the order and rate the
transistor are switched. Hall Effect sensors are typically used in the
logic scheme. Graph 1
may help illustrate how this works.5. For istance, if one were to
energize individual phases of a three phase brushless motor one would
generate, as a function of electrical degrees of rotation, a torque
curve as shown in Graph 1. Each phase would be 120 electrical
degrees apart. (It should be noted that elelctrical degrees is simply
mechanical degrees multiplied by the number of pole pairs of the
motor). Now, imagine the rotor in Figure 4 resting in its zero torque position (i.e. the 180 electrical degree point of the Graph 1),
with current flowing trough winding A. If the rotor is physically moved
back from its rest position, torque will build up roughly sinusoidally
and become peak at 90 electrical degrees. Since the objective is to have
the motor run at its peak operating point, the position still another
30 degrees back from the peack torque point, or 60 degrees, is the point
at which the winding must be switched on. A
sensor is located to trigger from a rotor magnet at this specific
event. If the roor is allowed to turn back towards its original rest, or
zero torque point, but the current is switched from winding A to
winding B at 180 electrical degrees, the motor will operate on a new
sine wave, or torque vs. angle, resulting in another point of peak
performance. Again, a sensor is located in such a manner to mark this
event. Similarly, the third sensor is set to trigger at 300 electrical
degrees. These Hall Effect sensor setting, 120 electrical degrees apart
from sensor to sensor, automatically sequence the switching of currents
from one phase to another, at the appropriate time. Another important
point to note from Graph 1 is the sign of the torque generated as
a function of rotor position. If the currents in individual phases
were switched at the proper electrical position, positive torque could
always be generated, as illustrated in Graph 2. With the proper
selection of phase energization (i.e. the proper commutation scheme) the
resultant torque output of the motor is as illustrated in Graph 3.
The successful commutation of the brushless motor is knowing the rotor
position in electrical degrees and having the proper commutation scheme. The
nature of the application under consideration dictates what information
is required to properly select a motor candidate. For example,
operating at a fixed speed will have a different demand than operation
under servo conditions. In general, three parameters will determine
motor selection: 1 Peak torque requirement 2 RMS torque requirement 3
Speed of operation PEAK TORQUE REQUIREMENT Peak
torque Tp is the sum of the torque due to acceleration of inertia, Tj,
load, Ti, and friction Tf: Tp=Tj+Ti+Tf Looking at the separate
components, the torque due to inertia is the product of the load
(including motor rotor) inertia and the load acceleration: Tj=J*a (a =
acceleration) The torque due to the load is defined by the configuration
of the mechanical system coupled to the motor. The system also
determines the amount of torque required to overcome the friction. RMS TORQUE REQUIREMENT RootMeanSquare
or RMS torque is a value used to approximate the average continuous
torque requirement. It is a statistical approximation described by the
following equation Where t1 is the acceleration time, t2 is the run
time, t3 is the deceleration time, and t4 is the time in a move SPEED OF OPERATION Speed
of operation is also dictated bythe configuration of the mechanical
system that is coupled to the motor shaft, and by the type of move that
is to be effected. For example, a single speed application would require
a motor with rated speed equal to the average move speed. A point to
point positioning application would require a motor with a rated speed
higher than the average move speed. (The higher speed would account for
acceleration , deceleration and run times of the motion profile). Figure
8A and 8B relate rated operating speed to average move speed for point
to point positioning move profiles. 