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Brushless DC Motor Fault Detection Circuit

IP.com Disclosure Number: IPCOM000114363D
Original Publication Date: 1994-Dec-01
Included in the Prior Art Database: 2005-Mar-28
Document File: 2 page(s) / 94K

Publishing Venue

IBM

Related People

Ramage, JG: AUTHOR

Abstract

Disclosed is an improved method of speed sensing of brushless DC motors. It solves the problem of accurate detection of full or partial failure of a cooling fan in electronic enclosures. Novel circuit features are: 1. use of diodes, not resistors, in series with the motor current. 2. uses motor current zero crossings to trigger speed detection circuit. 3. a bi-directional switch commutates each motor winding. 4. motor current zero crossings provides a measure of partial drive failures to be actioned.

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Brushless DC Motor Fault Detection Circuit

      Disclosed is an improved method of speed sensing of brushless
DC motors.  It solves the problem of accurate detection of full or
partial failure of a cooling fan in electronic enclosures.  Novel
circuit features are:
  1.  use of diodes, not resistors, in series with the motor current.
  2.  uses motor current zero crossings to trigger speed detection
       circuit.
  3.  a bi-directional switch commutates each motor winding.
  4.  motor current zero crossings provides a measure of partial
drive
       failures to be actioned.

      Fig. 1 shows a drive circuit for a brushless DC motor as used
in a DC Fan or Disc File.  The motor has two windings, w1 and w2
which are wound round all poles of the stator.  Current is steered
through either winding by electronic switches, sw1 and sw2, from a
positive voltage source derived in the speed control block.  Sw1 and
sw2  will be either transistors or MOSFETS.  The polarity of the
windings is such that if current flows through w1 the pole pieces
will be magnetized in opposite directions to those if w2 is
energized.  The currents in the motor windings are commutated via sw1
and sw2 by position sensing pulses derived from the rotor position.
The rotor has permanent magnet poles and its position is sensed by a
Hall effect device.

      A speed control block such as a linear voltage regulator or a
further electronic switch is connected in series with the positive
feed to the motor windings as shown in Fig. 1.  By adjusting the
output of the linear regulator or by varying the duty cycle of the
switch, the effective voltage across the windings can be changed.

      Motor speed is measured by a circuit using a frequency to
voltage converter followed by a comparator as shown in Fig. 1.  The
frequency to voltage converter is typically triggered from the supply
current.  The dotted line in Fig. 2 shows a typical supply current
waveform for such a motor.

      The triggering method that is the subject of this disclosure
uses the zero-crossings of the supply current waveform to trigger the
fault detector (speed sensor) circuit.  This method requires
bi-directional switches to implement sw1 and 2.  This is easily
achieved by using transistors with shunt reverse diodes or standard
power MOSFETS that have built-in reverse diodes.  Referring to Fig.
1, at each commutation of the motor windings reverse...