GB2249441A

GB2249441A - DC motor with thermal cutout in AC supply line

Info

Publication number: GB2249441A
Application number: GB9121368A
Authority: GB
Inventors: Hitoshi Yasufuku
Original assignee: Toshiba Corp
Current assignee: Toshiba Corp
Priority date: 1990-10-09
Filing date: 1991-10-09
Publication date: 1992-05-06
Anticipated expiration: 2011-10-09
Also published as: GB9121368D0; JPH04150754A; KR920009026A; JP2877481B2; GB2249441B

Abstract

A DC brushless motor and its controller 56 are fed from an AC supply 38 through a rectifier 52 and a single thermal cutout 34. e.g. a bimetal switch or a thermal fuse. When the cutout 34 detects motor overheating it interrupts the supply to the rectifier 52 and controller 56, and thereby prevents energisation of any DC motor winding 16. DC circuit breaking is obviated. Noise filter 42 prevents motor noise propagating onto the AC line. <IMAGE>

Description

A DC MOTOR HAVING AN ELEMENT FOR PROTECTING AGAINST OVERHEATING The present invention relates to a DC motor having an element such as thermal protector for protecting against overheating.

Prior art DC motors, for example brushless DC electric motors, have a rotor having permanent magnets, stator windings coupled to a DC circuit through a driving circuit, and a thermal protector which is provided near the winding for protecting against overheating. The thermal protector interrupts an electrical circuit thereof when the temperature of the stator exceeds a predetermined temperature. The thermal protector is connected in series with the winding. Accordingly if the motor is overloaded electrically, or if the bearings, which are provided at an end of a shaft of the motor,- have no lubricating oil,causing the motor to lock, the thermal protector opens up for protecting against overheating of the motor. Thus direct current isn't supplied to the windings of the motor and the motor stops.

In the case of a known brushless motor having thrttase driving windings, even if direct current isn't supplied to the first winding, the motor doesn't stop, because direct current is supplied to the second and third windings.

Accordingly two thermal protectors connected in series with at least two windings are needed to stop such a motor or to stop the supply of current to the windings.

Mbrecver, when such a motor is locked and stopped, direct current is always supplied to the specific winding determined by the Hall element detecting a rotational position of the rotor. In the above case, an are discfiarge rmy be caused between a movable portion and a fixed portion of the thermal protector electrically insulated from each other, because direct current is always supplied between these portions. Then these portions may be melted and bonded tightly to each other, so that the operator can't shut down the circuit of the motor,even if the motor has a thermal protector.

Consequently, because a small thermal protector isn't effective for the above treasons, if the operator wants securely to prevent the thermal protector from melting and bonding, the operator needs a large mass thermal protector.

Therefore, the present invention seeks to provide a DC motor having an element such as thermal protector for protecting against overheating, wherein current supplied to windings is securely shut down without large thermal mass elements, when an accident, for example when the motor is locked, haEFens.

A motor in accordance with this invention comprises (a) a stator having windings to which direct current is supplied, and (b) a rotor having a shaft, said rotor provided in close proximity to said stator, and (c) protecting means, having two terminals adapted for connection to an AC voltage line, for interrupting the direct current to the winding when the stator temperature exceeds a predetermined value.

When a DC motor has a trouble such as locking, then in accordance with the temperature of the motor during overheating, the element such as a thermal protector is actuated. In the case of the present invention, a current running between two terminals of the element is alternating current. Accordingly a movable portion and a fixed portion of the element should not be melted, and not bonded, so that the two portions of the element are securely shut down electrically, and a current running through the stator wfixHngs iS certainly shut down also.

Preferably the present invention also seeks to provide a DC motor having an element, which when the element is actuated, interrupts noise signals to an AC p > r source.

To achieve this, the protecting means for interrupting the direct current to the winding when the stator temperature exceeds a predetermined value, is connected to an AC voltage line between a noise filter connecting to an AC power source and DC source connecting to a driving control circuit for supplying a direct current sequentially to the windings.

When the element is shut down electrically , noise is removed by the noise filter so that the noise does not affect the AC power source.

The present invention is best understood with reference to accompanying drawings in which: FIG 1 is a circuit diagram of the embodiment including a DC motor in accordance with the present invention; and FIG 2 is a sectional view of a DC motor in accordance with the present invention.

In the following description of an embodiment, the DC motor in accordance with this invention is a DC brushless motor having three-phase windings and a rotor having permanent magnets.

The present invention will now be explained by referring to Fig.1 and Fig.2 in the following description of the embodiment.

Fig.2 shows a whole structure of the DC motor in accordance with the present invention. As shown in Fig.2, a stator 10 comprises twenty four teeth 12 including stacks of laminated plates 14, and windings 16 wound around the teeth 12. The stator 10 is provided between an upper cup-shaped frame 18 and a bottom cup-shaped frame 20. The upper frame 18 and the laminated plates 14 and the bosom frame 20 are fixed together by rivet 22. A rotor 24, provided on the inner side of the stator 10, comprises a shaft 26, a iron core 28, a permanent magnet 30, and a cylinder shaped metal cover 32, having four holes 33 on the surface thereof, made of a non-ferromagnetic metal such as alminium. The shaft 26 is provided at the center of the iron core 28 comprising stacks of laminated plates.The magnet 30, comprising four magnet pieces 31 aranged circularly with respect to the iron core 28, is provided around the iron core 28 made by laminated plates, the magnet being inserted on the inner side of the cover 32. The holes 33 of the cover 32 are arranged in accordance with each of the magnet pieces 31. A thermal protector 34,for example "17 AM TYPE KLIXON" ;(TRADEMARK OF TEXAS INSTRUMENT), having a fixed portion 34a and a movable portion 34b as protecting means contacts the winding 16. Terminal 35a connected to the fixed portion 34a and an another terminal 35b connected to the movable portion 34b are drawn out through an opening 36 provided on the bottom frame 20.

The thermal protector 34, made of a bimetal, may have a normal stage and an abnormal stage alternatively.

In the normal stage, the fixed portion 34a and the movable pprtion 34b contact each other, in the abnormal stage, the portions 34a, 34b are detached from each other.

When the temperature of the winding 16 exceeds a predeterminded temperature, for example about 120 C.,the thermal protector 34 detects the condition and changes from the normal stage to the abnormal stage, and so the thermal protector 34 interrupts a current between the fixed portion 34a and the movable portion 34b. A thermal fuse to be used only one time may be used as protecting means instead of the thermal protector 34.

A Hall element 37 is provided against the inner face of the magnet 30 and on the basis of the signal fran this Hall element 37, direct current is supplied to each of the windings 16. Therefore the DC brushless motor of three-phase driving windings 16 comprises the above-mentioned elements.

As shown in Fig.1, illustrating the circuit diagram of an embodiment in accordance with the present invention, a noise filter 40 comprising coils 42 and 44 and a capacitor 46 is connected across an AC power source 38 through a current fuse 47. An AC power supply line 48 connected to the noise filter 40 is coupled to the terminal 35a of the thermal protector 34. The terminal 35b and another AC power supply line 50 connected to the noise filter 40 are connected to a voltage stabilizing circuit 51 used as a DC source, including a rectifier circuit 52 and a smoothing capacitor 54. A driving control circuit 56 is connected to the voltage stabilizing circuit 51, the circuit 56 being for supplying direct pulse width current including phase difference of 120 electrical degrees to the windings on the basisof the signal from the Hall element 37.One end of each of the three windinas 16 is connected to a carton terminal 58. The other ends of the windings 16 are connected to the driving control circuit 56.

The operation of the motor having the above arrangement will next be described.

When the motor is locked or the motor is overloaded, the temperature of the windings 16 increases. When the thermal protector 34 detects the abnormal temperature, the movable portion 34b detaches from the fixed portion 34a. Accordingly, an electrical connection between the terminal 35a connected to the fixed portion 34a and the terminal 35b connected to the movable portion 34b is cut down, and so alternating current from the AC power source 38 isn't supplied to the voltage stabilizing circuit 50, and direct current from the voltage stabilizing circuit 50 isn't supplied to the windings 16. Then the rotating of the motor is stopped.

In this case, the current passing between the fixed portion 34a and the movable portion 34b is alternating current before the motor is stopped.

Accordingly, when the movable portion 34b is detached from the fixed portion 34a, there is an instant in which the voltage between the movable portion 34b and the fixed portion 34a reaches zero and an arc does not occur between the fixed portion 34a and the movable portion 34b. Thus the portions 34a, 34b aren't melted by an arc and aren't bonded together.

According to the above mentioned embodiment, since the thermal protector 34 is provided on the winding 16 mechanically, and is connected between the AC power source 38 and the voltage stabilizing circuit 50 electrically, alternating current flows between each portion 34a, 34b of the thermal protector 34. When the motor develops problems, and the thermal protector 34 is operated, an arc between the portion 34a and 34t should not occur continuously.

Consequently, although a thermal protector of large capacity isn't used, an are shouldnot occur continuously, so that the fixed portion 34a and the movable portion 34b aren't melted by an arc. Since the thermal protector 34 is operated certainly, the thermal protector 34 inhibits the motor from smoking and igniting.

In such a motor having three-phase windings 16, even with only one thermal protector 34, and the motor has a problem, current supplied to all of the windings 16 is stopped. Thus there is a need for fewer thermal protectors as compared to aboveimentioned prior art motor. Cbfeequently, cost is lowered and it is easy to assemble the motor.

Moreover, since the thermal protector 34 is connected between the noise filter 40 and the voltage stabilizing circuit 51, noise caused by the windings 16 aid carried to the AC power supply line 48 through the thermal protector 34 is absorbed by the noise filter 40.

Therefore, since the noise isn't carried to the AC power source 38, other apparatus connected to the AC power source 38 aren't affected by the noise.

The present invention is not limited to the above embodiment but various change and modification may be made within the spirit and scope of the invention.

For example, instead of a DC brushless motor, a DC motor using direct current running through the windings may be used. The thermal protector 34 is not always touching the winding 16, but may be provided near the winding 16; for example the thermal protector 34 may be provided on the teeth 12 or the frame 18, 20, so that the thermal protector 34 may detect the abnormal temperature of the winding 16.

Claims

What is claimed is:

1. A DC electric motor comprising: a stator having windings to which direct current is supplied; a rotor having a shaft, said rotor provided in close proximity to said stator; and protecting means, having two terminals adapted for connection to an AC voltage line, for interrupting the direct current to the winding when the stator temperature exceeds a predetermined value.

2. A DC motor according to claim 1, wherein the protecting means includes a thermal protector.

3. A DC motor according to claim 1 further including a driving control circuit coupled to the windings, for supplying direct current sequentially to the windings.

4. A DC motor according to claim 3 further including a DC source, coupled to the driving control circuit, for supplying direct current to the driving control circuit.

5. A DC motor according to claim 4 further including a noise filter provided between an AC power source and the DC source, wherein the protecting means is electrically connected to the AC voltage line between the noise filter and the DC source.

6. A DC motor according to claim 1, wherein the rotor includes a permanent magnet.

7. A DC motor according to claim 1, wherein the motor is a DC brushless motor having a detecting means for detecting the rotational position of the rotor.

8. A DC motor according to claim 7, wherein the DC brushless motor has a three-phase winding each separated by an angle of 120 electrical degrees.

9. A system for protecting against overheating of a motor, comprising: a direct current motor powered by a direct current power supply, and including a rotor and a stator; and means for interrupting the direct current to the motor when the temperature of the rotor or stator exceeds a predetermined level; said means for interrupting being connected to an AC voltage line and to the direct current power supply.

10. A DC electric motor arrangement comprising: a DC electric motor, an AC voltage scarce, a rectifyng arrangement for rectifying the AC voltage from said source to provide a DC voltage supply for said DC motor, and a thermal interruptor, mounted in or on said motor, connected in series in one of the connections between the AC source and the rectifying arrangement and being such that an increase in the temperature of the interruptor above a predetermined level results in the thermal interruptor interrupting the flow of current in said one of the connections.

11. A DC electric motor or motor arrangement substantially as hereinbefore described with reference to the accompanging drawings.