GB2251522A

GB2251522A - An electric motor

Info

Publication number: GB2251522A
Application number: GB9026671A
Authority: GB
Inventors: Michael Edward Dennis; Paul Harden; Derek Lewis Bayley
Original assignee: IST Laboratories Ltd
Current assignee: IST Laboratories Ltd
Priority date: 1990-12-07
Filing date: 1990-12-07
Publication date: 1992-07-08
Also published as: GB9026671D0

Abstract

An electric motor (1) of the brush less type includes circuit means (15), connected to the stator coils, comprising a commutation circuit including a thick film circuit 11 formed on a metal substrate (3) comprising a portion of the housing. Magnetic field sensors 12-14 are also surface mounted on the thick film circuit for sensing the rotor position together with a silicon controlled rectifier circuit supplying DC to the motor. External connection to a current switching device is avoided and radio frequency interference is reduced. <IMAGE>

Description

"AN ELECTRIC MOTOR" This invention relates to an electric motor of the brushless motor type and in particular but not exclusively to a brushless DC electric motor.

It is known to provide electric motors in which commutation of supply current to motor coils is carried out by means of electronic switching devices without the need for electro-mechanical commutators.

Such brushless electric motors may be arranged to supply coils with AC current as in the case of a switch reluctance motor or may be arranged to supply DC current as in the case of a brushless DC motor.

It is known to provide in a brushless DC motor a commutation circuit which switches current from a DC supply to sequentially energise stator coils, the motor having a magnetic rotor provided with an angular position sensor which provides a sensor output signal used to control the timing of current switching by means of a control circuit.

It is also known to control the speed of the rotor by controlling the average current conducted by the commutation circuit using pulse width modulation at a frequency of the order of 10 kHz.

In most practical applications of such motors the DC supply is derived from an AC supply using a regulated DC supply circuit. These circuits are connected externally to the motor and are provided with eat sinks.

A problem associated with each of the above motors is that of electromagnetic interference generated by current switching and radiated by current carrying conductors connected to the motor.

According to the present invention there is disclosed an electric motor of the brushless type comprising a housing, a plurality of stator coils fixed to the housing, a rotor rotatable in the housing and circuit means connected to the stator coils and circuit means comprising a commutation circuit operable to selectively supply current to stator coils selected. according to the angular position of the rotor relative to the stator coils wherein the commutation circuit comprises a thick film circuit formed on a metal substrate and wherein the substrate comprises a portion of the housing.

An advantage of such an arrangement is that the disposition of the metal substrate in close proximity with the components of the commutation circuit ensures effective electromagnetic screening and provides an integral heat sink. The need for a separate printed circuit board for the communication circuit externally of the motor housing is thereby obviated. Conductors carrying switched current between the commutation circuit and the motor are no longer located externally of the motor housing thereby avoiding a possible source of electromagnetic interference.

In a preferred embodiment, a brushless DC electric motor has a rotor comprising a plurality of permanent magnetic poles and further comprises sensor means connected to the circuit means and operable to produce a sensor output signal representative of the relative angular position of the rotor and the stator coils, the commutation circuit being operable to supply direct current to the coils.

Preferably the circuit means comprises a DC supply circuit connected in use to a source of alternating current and operable to supply direct current to the commutation circuit wherein the DC supply circuit constitutes a part of the thick film circuit.

The need for connection between the motor and an external DC supply circuit is thereby obviated.

The length of current path between the DC supply circuit and the commutation circuit is also significantly reduced and improved screening is ensured by the close proximity of the metal substrate.

Preferably the DC supply circuit comprises a switching amplifier operable to modulate each half wave portion of the AC supply to provide a corresponding pulse of length which is variable in response to a control signal from a control means to thereby vary the average direct current supplied to the commutation circuit.

Conveniently the control means comprises a microprocessor connected to the sensor means and operable to vary the control signal for the DC supply circuit so as to regulate the rotor speed in a programmable manner.

The switching amplifier of the DC supply circuit may conveniently comprise a silicon controlled rectifier.

The thick film circuit comprises an insulating layer formed on the substrate and conductive tracks formed on the insulating layer, the DC supply circuit and the commutation circuit respectively comprising semi-conductor devices surface mounted on the insulating layer and interconnected by the conductive tracks.

Preferably the portion of the housing constituting the substrate comprises an end cap of the housing and the thick film circuit may be formed on an inner face of the end cap.

The housing and the cap may thereby form a metallic enclosure within which the thick film circuit is contained so that the thick film circuit is electromagnetically screened.

The thick film circuit may comprise an array of magnetic field sensing devices cooperating with a magnetic actuating means of the rotor to constitute the sensor means.

A specific embodiment of the present invention will now be described by way of example only and with reference to the accompanying drawings of which: Figure 1 is a part sectioned elevation of a motor; Figure 2 is a sectional elevation taken at II-II in Figure 1 and showing a thick film circuit; Figure 3 is a schematic circuit diagram of a circuit means for the motor of Figures 1 and 2; Figure 4 is a waveform illustrating the current output of a switching amplifier of the circuit of Figure 3; and Figure 5 is a waveform showing the modulated and rectified direct current supplied to the commutation circuit portion of the circuit of Figure 3.

In Figure 1 a motor 1 comprises a cylindrical metal housing 2 having an end cap 3 at a first end 4 of the housing 2. A shaft 5 projects from a second end 6 of the housing and is journalled in the housing 2 so as to be axially rotatable with a magnetic rotor 7.

The rotor 7 comprises a multipole magnetic structure of the type normally found in such brushless DC motors and in the present example comprises three segments (not shown) having circumferentially spaced north and south poles.

The motor 1 includes three circumferentially distributed stator coils 8.

The end cap 3 comprises a stainless steel circular sheet 9 having an inner face 10 on which is formed a thick film circuit 11. In Figure 2 some, but not all, of the components surface mounted on the thick film circuit are illustrated schematically including Hall effect sensors 12, 13 and 14 and six current switching devices 15.

A magnetic actuator 16 is mounted on the rotor 7 in proximity with the sensors 12, 13 and 14 and is arranged to induce signals in the sensors during rotation of the rotor such that the angular position of the rotor relative to the housing 2 can be determined.

A power supply cable 17 carrying single-phase alternating current is connected to the thick film circuit 11 and extends through a grommet (not shown) provided in the sheet 9. A signal output cable 18 and a signal input cable 19 are also connected to the thick film circuit 11 in like manner.

Figure 3 shows schematically the circuitry associated with the motor 1 and those components within the broken line 20 form part of the thick film circuit 11. As shown in Figure 3 a microprocessor 21 (not shown in Figure 1) is connected to both signal output cable 18 and signal input cable 19 in order to receive sensor signals and in order to transmit a control signal to the thick film circuit 11.

The position of rotor 7 is sensed by means of the sensors 12, 13 and 14 and the output of these sensors is combined in a combining circuit 22 to produce a combined signal representative of the rotor speed which is input to the microprocessor 21.

The outputs from the sensors 12, 13 and 14 are also connected to control logic circuits 23a and 23b which produce switching signals to switch on and off sequentially the current switching devices 15a to 15c and 15d to 15f respectively such that at a given time only two of the six current switching devices are turned on. The sequence in which pairs of devices are turned on to complete a single rotation of the rotor 7 is as follows: 15a 15e 15a 15f 15b 15f 15b 15d 15c 15d 15c 15e The coils 8a, 8b and 8c are thereby sequentially energised to drive the rotor 7.

Alternating current received at terminals 24 and 25 is gated by means of a silicon controlled rectifier (SCR) switching circuit 26 in which the current is switched on at a variable firing angle controlled by the input signal from the microprocessor 21. Current is switched off at the zero crossing point of the AC waveform according to the normal operator of SCR circuits. The output of the switching circuit 26 at terminal 27 is shown in Figure 4. The modulated alternating current from the switching circuit 26 is rectified by means of a bridge circuit 28 to supply modulated direct current across terminals 29 and 30 having a waveform shown in Figure 5.

Current is drawn from terminals 29 and 30 by the current switching devices 15 via a current sensing circuit 31. The control logic circuit 23a receives signals from the current sensing circuit 31 and controls the amplitude of current pulses conducted by the current switching devices 15d, 15e and 15f so as to limit the instantaneous current delivered to the coils 8 to a maximum level. This current limiting control function is necessary to protect the magnetic circuit of the rotor 7 against demagnetising effects of the coil current.

The control circuit 23a limits the coil current amplitude by pulse width modulation at a fixed frequency of 25 kHz, the current being switched at this frequency by the current switching devices 15d, 15e and 15f which thereby perform the dual functions of commutating the coil current and limiting the coil current by pulse width modulation.

The average current drawn by the coils and hence the motor speed is determined by the pulse length of the current output from the switching circuit 26.

A typical application of the motor 1 is in a washing machine the micro-processor 21 is located behind a control panel some distance from the motor.

Motor speed is controlled by output signals from the micro-processor which are delivered to the motor via input cable 19 and are input to the switching circuit 26 to determine the average DC current supplied to the motor coils 8 via the commutation circuit comprising switching elements 15 and control circuit 23. A feedback signal representative of the speed of rotor 7 is provided to the micro-processor 21 via a signal output cable 18 enabling the micro-processor to control the motor speed to any required level.

The thick film circuit is formed by initially firing a stainless steel substrate in an oven to form a chromium oxide surface layer, the firing process being carried out at a temperature of 8500C to 10000C. A first dielectric adhesion layer is then adhered to the oxidised steel substrate, the adhesion layer being selected to have a coefficient of thermal expansion approximately equal to that of the steel.

One or more further separate coatings are then separately applied such that the final coating has a coefficient of thermal expansion approximately equal to a thick film ink. A thick film circuit layout is then applied by silk-screen printing in which conductive tracks are printed so as to interconnect solder pads which are subsequently connected to surface mounted components. The circuit including the substrate and insulating layer is formed into the end cap by suitable machining which may include bending and the surface mounted components are then added and external connections made.

Alternative embodiments of the present invention are envisaged in which for example the Hall effect senses are located at the opposite end of the housing on a circuit board having an aperture through which the shaft extends. The sensor may alternatively be an optical sensor or any other suitable form of rotation or position sensor.

The motor may be of any polyphase configuration other than the three-phase motor described above.

Other forms of current limiting control may be incorporated in the commutation circuit. For example the current switching devices may operate in a pulse frequency modulation mode or the switching devices may be transistors operating as linear amplifiers to control the coil current.

The control means for the DC supply circuit may be other than a microprocessor and may for example be a logic circuit dedicated to a specific application of the motor.

The substrate may be formed of metals other than stainless steel and may for example be aluminium.

Claims

CLAIMS:

1. An electric motor of the brushless type comprising a housing a plurality of stator coils fixed to the housing, a rotor rotatable in the housing and circuit means connected to the stator coils, the circuit means comprising a commutation circuit operable to selectively supply current to stator coils selected according to the angular position of the rotor relative to the stator coils wherein the commutation circuit comprises a thick film circuit formed on a metal substrate and wherein the substrate comprises a portion of the housing.

2. An electric motor as claimed in claim 1 wherein the rotor comprises a plurality of permanent magnetic poles, the motor further comprising sensor means connected to the circuit means and operable to produce a sensor output signal representative of the relative angular position of the rotor and the stator coils and wherein the commutation circuit is operable to supply direct current to the coils.

3. An electric motor as claimed in claim 2 wherein the circuit means comprises a DC supply circuit connected in use to a source of alternating current and operable to supply direct current to the commutation circuit wherein the DC supply circuit constitutes a part of the thick film circuit.

4. An electric motor as claimed in claim 3 wherein the DC supply circuit comprises a switching amplifier operable to modulate each half wave portion of the AC supply to provide a corresponding pulse of length which is variable in response to a control signal from a control means to thereby vary the average direct current supplied to the commutation circuit.

5. An electric motor as claimed in claim 4 wherein the control means comprises a microprocessor connected to the sensor means and operable to vary the control signal for the DC supply circuit so as to regulate the rotor speed in a programmable manner.

6. An electric motor as claimed in any of claims 4 and 5 wherein the switching amplifier of the DC supply circuit comprises a silicon controlled rectifier.

7. An electric motor as claimed in any of claims 4, 5 and 6 wherein the thick film circuit comprises an insulating layer formed on the substrate and conductive tracks formed on the insulating layer, the DC supply circuit and the commutation circuit respectively comprising semi-conductor devices surface mounted on the insulating layer and interconnected by the conductive tracks.

8. An electric motor as claimed in any preceding claim wherein the portion of the housing constituting the substrate comprises an end cap of the housing.

9. An electric motor as claimed in claim 7 wherein the thick film circuit is formed on an inner face of the end cap.

10. An electric motor as claimed in any of claims 2 to 9 wherein the thick film circuit comprises an array of magnetic field sensing devices cooperating with a magnetic actuating means of the rotor to constitute the sensor means.

11. An electric motor substantially as hereinbefore described with reference to and as shown in the accompanying drawings.