GB2247999A

GB2247999A - Brushless DC motor monitor

Info

Publication number: GB2247999A
Application number: GB9116178A
Authority: GB
Inventors: Andrew David Knatt
Original assignee: PAPST PLC
Current assignee: PAPST PLC
Priority date: 1990-07-27
Filing date: 1991-07-26
Publication date: 1992-03-18
Anticipated expiration: 2011-07-26
Also published as: GB9016508D0; IE912637A1; GB2247999B; GB9116178D0

Abstract

The frequency of commutation current pulses sensed at the brushless DC fan motor M terminals is monitored. The pulses are shaped by AMP, D1, RF, C so that the pulse period controls the peak voltage at node 0. As the motor slows, the period of the shaped pulses increases, C charges to higher levels, and comparator COMP outputs an abnormality signal when a reference R3, R4 is exceeded. <IMAGE>

Description

A METHOD AND CIRCUIT FOR MONITORING THE OPERATION OF A BRUSHLESS DC MOTOR The present invention relates to a method of monitoring the operation of a brushless dc motor and to a monitoring circuit for a brushless dc motor.

Brushless dc motors are used, for example, to power fans provided in electrical equipment which needs to be cooled, for example in microprocessing units, printers and the like. Such equipment is likely to be damaged if high temperatures arise, and it is therefore important to be alerted if the fan fails.

Brushless dc motors are commonly used to power such fans because they can be made very compact. With such brushless dc motors, a dc current is supplied to the stator winding. The commutation of the current is provided by a Hall IC which senses the angular position of the rotor and generates switching pulses for controlling the dc supply and thereby providing current commutation pulses to the stator winding which are matched to the speed of rotation.

It has long been known to be desirable to provide early warning of fan failure, and it is recognised that the square wave output of a Hall IC is very good for this purpose as it provides an accurate measure of the rotation of the rotor and hence of the fan. However, if the motor manufacturer has not made appropriate provision, anyone wishing to install the motor and to provide a fault detection circuit, has to make a connection to the electronic circuit including the Hall IC. This, of course, can be troublesome.

It is an object of the present invention to provide a simple means of monitoring the operation of a brushless dc motor which does not require an electrical connection to be made internally of the motor.

According to the present invention there is provided a method of monitoring the operation of a brushless dc motor comprising the steps of monitoring the electrical supply to the stator winding, determining if the electrical supply meets predetermined criteria, and generating an output signal if the electrical supply does not meet said predetermined criteria.

Because the method of the invention monitors the electrical supply to the stator winding, it is not necessary to make any connections internally of the motor, a connection to receive the same supply as the stator winding being all that is necessary.

The supply to the stator winding is controlled by commutation current pulses, and accordingly, it is these pulses which are effectively monitored. It will be appreciated that if the rotational speed of the rotor decreases, for example, showing a fault in the fan, the amplitude of the commutation current pulses will fall, the duration of the pulses will become greater, and their frequency will decrease. Any or all of these criteria can be monitored and be used to provide an output signal where they differ from predetermined values.

The method preferably further comprises the step of shaping signal pulses derived from said commutation current pulses before determining if the predetermined criteria are met.

The pulses are preferably shaped to enhance the performance of subsequent means.

In an embodiment, said shaped signal pulses are applied to comparator means arranged to compare said signal pulses with a reference and thereby determine if the commutation current pulses meet the predetermined criteria.

For example, the comparator means may be arranged to provide said output signal when the period of said commutation current pulses is greater than a predetermined value.

The output signal generated when the electrical supply to the brushless dc motor does not meet the predetermined criteria may be utilised in a number of ways. For example, the output signal may be applied to an LED or other warning device whereby a warning of imminent failure is given.

Additionally and/or alternatively, the output signal may be applied to logic means arranged to control other circuits, for example, to cut off the electrical supply to elements at risk of overheating.

The present invention also extends to a monitoring circuit for a brushless dc motor, the monitoring circuit comprising input means for coupling to the electrical supply to the dc motor, and monitoring means for determining if the electrical supply meets predetermined criteria, and for generating an output signal if the electrical supply does not meet said predetermined criteria.

The input means may be coupled indirectly to the input to the stator windings. For example, a capacitive or inductive coupling may be utilised to provide signal pulses representative of the commutation pulses, which signal pulses are used to generate an output signal when appropriate. However, in a preferred embodiment, the commutation current pulses themselves are applied by the input means to the monitoring circuit. In this respect, said input means is connected to input supply terminals of the stator of said dc motor to receive commutation current pulses.

Preferably, the monitoring circuit further comprises shaping means for receiving commutation current pulses from said input means and for deriving shaped signal pulses from said commutation current pulses.

In a preferred embodiment comparison means are arranged to receive a reference signal and said signal pulses and to generate said output signal when said signal pulses exceed said reference.

The present invention also extends to a brushless dc motor connected to a monitoring circuit as defined above.

Embodiments of the present invention will hereinafter be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a graph of voltage against time of the stator winding current commutation pulses of a dc brushless motor during operation, Figure 2 is a graph showing a rectangular wave form produced from the current commutation pulses of Figure 1, Figure 3 is a graph of a ramp voltage produced when the charging of a capacitor is controlled by the wave form of Figure 2, and Figure 4 is a block diagram of one embodiment of a monitoring circuit of the invention.

Figure 1 shows the current commutation pulses applied to the stator winding of a brushless dc motor. The brushless dc motor might be utilised, for example, for driving a fan. The electrical supply to the dc motor is a dc current which is switched or commutated in dependence upon the angular rotational position of the rotor. This position is determined by a Hall IC and, in known manner, the output of the Hall effect circuit controls the application of the dc supply to the stator winding resulting in the current pulses shown in Figure 1.

Figure 4 shows a monitoring circuit which is to be connected to receive the current commutation pulses of Figure 1. In this respect, Figure 4 shows a brushless dc motor M and the dc supply terminals P2 and P3 which are connected to provide a dc supply to motor M. The monitoring circuit also has supply terminals Fl and P4 by way of which power is supplied thereto. In the illustrated embodiment, the monitoring circuit is arranged to be connected to a 12V dc supply. Of course, a dc supply having a different power rating may be used and/or the circuit may be adapted for connection to an ac supply. The monitoring circuit has a signal input S which, in the embodiment shown, is directly connected to one input P3 to the motor M so that the commutation pulses are fed directly to the terminal S.Of course, an indirect coupling between the terminal S and the input to the motor M may alternatively be provided.

The commutation pulses at input terminal S are fed to the non-inverting input 3 of an operational amplifier AMP.

A reference level is fed to the inverting input 2 of the operational amplifier AMP. It will be seen that the reference level is obtained from a resistors R1 and R2 which are connected between the supply rails of the monitoring circuit. The inverting input 2 of the amplifier AMP is connected to the series connection of the two resistors Rl and R2. Where the fan has two speeds, for example, an additional resistor (not shown) may be provided in parallel with the resistor R1. Switch means (not shown) may control which of the resistors Rl and the additional resistor are connected into the circuit.

The output 1 of the amplifier AMP is connected by way of a diode D1 to an output 0. A feedback resistor RF connects the output 0 to the inverting input 2 of the amplifier AMP by way of the resistor R1. The output 0 is also connected to a capacitor C which provides the load for the amplifier AMP. The voltage across the capacitor C is fed to the non-inverting input 5 of a comparator COMP. The inverting input 6 of the comparator COMP is supplied from the positive rail by way of a resistor R3 which, together with a series connected resistor R4, sets a reference level for the comparator COMP. The output 7 of the comparator COMP is connected by way of a reverse biassed diode D2 and an output resistor RO to output terminals T1 and T2. An output capacitor C2 is connected across the output terminals T1 and T2.

It will be appreciated that in operation the commutation pulses fed to the non-inverting input 3 of the amplifier AMP will be amplified by the amplifier and shaped to produce at output 1 the substantially square wave output shown in Figure 2. These rectangular output pulses are fed to the output 0 where they cause charging of the capacitor C. In this respect, Figure 3 shows clearly how the change of an output pulse of the amplifier AMP from low to high level causes charging of the capacitor C at a speed dependent upon the size of the capacitor and the value of the resistance RF. Of course, if the duration of the pulses of Figure 2 is increased, the capacitor C will be charged to a higher level before it is discharged again by a change in the output pulse.The output on output 0 which is fed to the non-inverting input 5 of the comparator COMP, therefore approximates to a sawtooth waveform.

The comparator COMP compares the voltage level on its non-inverting input 5 with a reference level on its inverting input 6 which is determined by the value of the resistors R3 and R4. If the level on input 5 exceeds that on input 6, the output at 7 goes high putting a high level output across the terminals T1 and T2 and this high level output can be used to drive a warning device. In the circuit illustrated, a LED 10 is indicated. The diode D2 and capacitor C2 connected to the output 7 of the comparator COMP provide a delay in the occurrence of a high level output to avoid the production of false warning signals for transient conditions.

It will be appreciated that as the speed of rotation of the motor M decreases, for example, indicating that there are problems, the duration of the commutation pulses will increase allowing the voltage to which the capacitor C is charged to increase. This voltage will exceed the reference level applied to the input 6 of the comparator COMP thereby putting a high level output on the comparator COMP which is used to produce a warning signal.

The particular circuit illustrated utilises an amplifier and diode combination to provide shaping and amplification and then uses a comparator to generate the warning signal. Of course, other circuit elements providing for amplification, shaping and level detection as necessary may be used.

The circuit shown particularly determines whether or not a warning signal is to be output in dependence upon the duration of the commutation pulses. It would additionally and/or alternatively be possible to use the frequency of these pulses, or their amplitude, to generate the warning signal.

The warning signal may be provided by a warning device such as the LED 10. It may additionally or alternatively be used to provide a logic output controlling other means.

In this respect, a further output resistor (not shown) may be connected between the input end of the resistor RO and an additional terminal (not shown) to provide output means for a 24 volt input.

It will be appreciated that other variations and modifications may be made to the invention as described above within the scope of the following Claims.

Claims

1. A method of monitoring the operation of a brushless dc motor comprising the steps of monitoring the electrical supply to the stator winding, determining if the electrical supply meets predetermined criteria, and generating an output signal if the electrical supply does not meet said predetermined criteria.

2. A method as claimed in Claim 1, wherein the commutation current pulses applied to the stator winding are monitored, and wherein an output signal is generated if the amplitude of said pulses is below a predetermined level, and/or if the period of said pulses is greater than a predetermined value, and/or if the frequency of said pulses is less than a predetermined value.

3. A method as claimed in Claim 2, further comprising the step of shaping signal pulses derived from said commutation current pulses before determining if the predetermined criteria are met.

4. A method as claimed in Claim 3, wherein said shaped signal pulses are applied to comparator means arranged to compare said signal pulses with a reference and thereby determine if the commutation current pulses meet the predetermined criteria.

5. A method as claimed in Claim 4, wherein said comparator means is arranged to provide said output signal when the period of said commutation current pulses is greater than a predetermined value.

6. A method as claimed in any preceding Claim, further comprising the step of providing a warning in response to the generation of an output signal.

7. A monitoring circuit for a brushless dc motor, the monitoring circuit comprising input means for coupling to. the electrical supply to the dc motor, and monitoring means for determining if the electrical supply meets predetermined criteria, and for generating an output signal if the electrical supply does not meet said predetermined criteria.

8. A monitoring circuit as claimed in Claim 7, said input means is connected to supply input terminals of the stator of said dc motor to receive commutation current pulses.

9. A monitoring circuit as claimed in Claim 8, further comprising shaping means for receiving commutation current pulses from said input means and for deriving shaped signal pulses from said commutation current pulses.

10. A monitoring circuit as claimed in Claim 9, further comprising comparator means arranged to receive a reference signal and said signal pulses and to generate said output signal when said signal pulses exceed said reference.

11. A method of monitoring the operation of a brushless dc motor substantially as herein before described in the reference to the accompanying drawings.

12. A monitoring circuit for a brushless dc motor substantially as hereinbefore described in the reference to the accompanying drawings.