DC BRUSHLESS FANS
APPLICATION NOTE DC BRUSHLESS FANS
Eric Adamson April 12, 1998
The term "brushless" refers to the means of commutation employed by a fan or motor.
(For the remainder of this document, motor and fan may be used interchangeably,
the latter being merely a motor with blades attached to its shaft.)
Commutation is a necessary feature of DC motors - in such motors, sustained rotation is impossible,
unless the direction of current flowing through the motor windings is reversed every 180° of rotation.
Figure 1 (above) depicts a simplified DC motor,
consisting of a single-turn winding positioned between two permanent magnets of opposing polarity.
As current passes through the winding, an electromagnetic field is established.
Magnetic forces compel the winding to turn until its poles align with those of the permanent magnets.
Commutation guarantees that the poles never actually align - every 180°,
the winding current reverses direction, causing the cycle to repeat indefinitely.
In brushless fans, commutation is not achieved through mechanical means,
but instead, through the use of an electronic control circuit. By using a fixed coil,
and placing permanent magnets on the rotating portion of the motor, this becomes possible.
Figure 2 shows a disassembled brushless fan - the Comair/Rotron Flight 80, which is currently used in the PPBS:
-Primary components in this design are labeled for clarity:
N-Channel JFET (Junction Field Effect Transistor)
PTC (Positive Temperature Coefficient) Thermistor
Hall Effect Sensor (see text below)
Rotor Magnet (arrow pointing at N-S pole transition)
The fan commutation circuitry consists of two N-channel JFET's, one PTC thermistor, three diodes,
and one Hall Effect IC, which is mounted directly beneath the stator armature.
(Duplicate components have not been labeled in Figure 2, to keep the diagram uncluttered.)
BRUSHLESS DC FAN OPERATION
Fan operation is relatively simple. A Hall Effect sensor (see Figure 3, below) is the primary control component.
The fan rotor rests in the stator armature such that a circular permanent magnet,
mounted inside the rotor, is in proximity with the Hall Effect sensor.
When the N-S pole transition (see Figure 2) of the rotor magnet passes the Hall Effect sensor,
two 50% duty cycle pulses are generated, 180° out of phase.
During normal operation, these pulses are issued repeatedly, forming two complementary pulse trains.
These pulses are used to drive the two JFET's - each of which switches drive current to a separate pair of stator coils.
Because the pulse trains are 180° out of phase, when one JFET is on, the other is off.
Thus at any given instant, one pair of coils is energized.
Alternate firing of these coil pairs in succession results in steady rotation of the fan rotor.
In case of high temperature due to fault or fan stall, the PTC thermistor's resistance increases,
thereby throttling coil current until fan temperature returns to a reasonable level.
Two reverse-biased diodes protect the JFET's from inductive back-emf during switching,
and a third protects the control circuitry, should the fan be connected with reversed polarity.
◆ ADVANTAGES AND LIMITATIONS OF DC BRUSHLESS MOTORS
Brushless motors provide a number of advantages, particularly the following:
Improved Efficiency Conventional DC motors have high inertia,
due to the presence of bulky rotor coils.
Because permanent magnets can be made relatively lightweight,
brushless motors may have significantly lighter rotors.
These rotors are capable of faster changes in speed, and deliver torque more efficiently,
because less energy is required to turn the rotor.
Consequently, more energy is available for transfer to the load.
Low EMI Brushes in conventional DC motors cause sparking,
and broadband electromagnetic interference.
Brushless motors do not exhibit this effect.
In fact, brushless motors are highly suitable for use in hazardous atmospheres,
and with proper sealing, they are suitable for underwater applications.
High Reliability Conventional DC motors are subject to greater physical wear.
Particularly, the commutator brushes must eventually be replaced.
The operation of brushless motors involves less mechanical activity, leading to superior reliability.
Brushless motors, as a consequence of their low inertia, are more prone to stalling than conventional DC motors. Because their electronic control circuitry is not tolerant of high temperatures,
thermistors are typically included in brushless designs as a means of safeguarding against these conditions.
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