GB2161334A - Safety device for alternating current motors - Google Patents

Abstract

A safety device (1) for an alternating current motor (2) is interposed between the motor and a phase-shifting controller or motor current switching power regulator (3) incorporating a triac (4) and comprises a fuse network containing a diode (D1) or (D2) in series with a fuse (Si1) or (Si2) in each of two parallel branches (7, 8) of a parallel shunt arrangement (6), the diodes (D1, D2) being disposed in antiparallel. The device will respond to half-wave energisation of the motor due to partial failure of the triac. <IMAGE>

Description

SPECIFICATION Safety device for alternating current motors The invention relates to a safety device for alternating current motors, having the features set out in the preamble to Claim 1.

In the field of heating and ventilation technology it is usual nowadays to regulate the rotary speed of the fan of a room heating appliance on the basis of comfort. Control is effected as a function of the room temperature. When the appliance is first switched on in an unheated room, the fan will start up under full power in order to achieve the fastest possible transfer of heat to the room from the heater.

In other fields too, the power of electric motors is regulated by control devices. The conventional power controller nowadays generally includes a triac for phase shift control of the alternating current half-waves, the triac obtaining its gate potential from a control voltage with a predetermined input value, using amplifiers and the like.

While the use of phase shift-controlling triac regulators on exclusively ohmic devices presents no problem whatsoever, the combination of a triac, a control device and an alternating current motor, particularly a split-pole motor, gives rise to the problems set out hereinafter by virtue of the effective inductive resistance inherent in the motor.

In the case of a defective triac, it is necessary to distinguish its switching condition.

(a) If the triac is permanently fired, then both mains half- waves are passed to the motor, which is thus supplied with the rated current. The motor thus runs under acceptable conditions, but regulation of the motor output is no longer possible.

(b) If on the other hand the triac is not fired, then either the positive or the negative mains half-wave will be passed to the motor winding. Because of the one-sided half-wave operation caused by the defect, the inductive portion of the effective resistance will fall markedly due to the direct current component inherent in the pulsating alternating current and with it the shift of pre- magnetisation of the motor into the saturation range. Because of the one-sided half-wave situation, the motor will draw many times the current it needs, will overheat and eventually burn out.

Attempts have already been made to counteract the risk of burning out of triac regulated electric motors by providing them with thermal sensors in the form of bimetal switches which were capable of detecting elevated motor temperatures and of preventing further supply of current thereto. Such control systems are expensive and operate only after the motor has already become overheated. Furthermore, they require modification of the motor which makes it more complicated and expensive.

The invention is concerned with the problem of providing a safety device for a triac-regulated alternating current motor which is simple and inexpensive to produce, can be added to existing triac- controlled electric motors and which, in the event of a triac becoming defective, responds before the motor becomes overheated.

This problem is solved, according to the invention, by the features set out in the characterising portion of claim 1.

The invention utilises the phenomenon observed in alternating current motors whereby the inductive portion of the effective resistance falls markedly when an unacceptable direct current component is fed to the alternating current motor, resulting in the magnetic components of the motor being run in the saturation magnetisation range, but without reversal of magnetisation actually taking place due to the absence of the other mains half- wave. The effective motor resistance changes from high-ohm inductive to low-ohm ohmic which is why the result is an increase in current through the motor.

In order to be able to react to this special instance of resistance drop in the motor (triggered by the triac defect) before the motor becomes unacceptably heated, a parallel branching or doubling of the current carrying conductor is provided between the triac output and the motor, and a diode and a fuse in series are incorporated in each of the branches, the two diodes being oppositely oriented. This means that one half-wave (for example the positive) flows through one parallel branch while the other half-wave (for example the negative) flows through the other parallel branch. The total nominal current fed to the motor is thus evenly distributed over the two parallel branches, so that the response current of each of the two fuses must be 1,,,/2.

In the case of a triac defect in accordance with (b) above (case (a) is harmless), the current in one of the two parallel branches becomes nil while the current in the other parallel branch becomes a multiple of 1,,,/2, which is attributable to the drop in the inductive proportion of the resistance. The fuse in the branch through which the current is flowing reacts to counteract the risk of the motor catching fire.

If the triac is fired again afterwards, then once again full mains voltage is applied to the safety device and thus also to the motor. However, since the fuse in one branch has already responded only a pulsating direct current can flow through the still intact branch. Since this pulsating direct current is likewise very much greater than half the rated motor current, in accordance with which the fuse is dimensioned, then the second fuse also responds. The motor is then switched off completely.

Advantageously the response value of the fuses in each of the parallel branches should correspond substantially to 0.6 to 1.5 times the total current flowing through the safety device. Such dmensioning ensures that on the one hand, negligible increases in current above the mean rated current will not bring about an accidental response of the fuses, and on the other, that in the event of a triac defect, one of the fuses will in any case be triggered since the observed increased in current leads to a multiplication of the current in the corresponding parallel branch by virtue of the fall in inductive resistance.

It is ensured, according to Claim 3, that on the one hand brief start-up current pulses will not lead to a response of the fuses and on the other hand, that the entire safety device can be produced at an extremely favourable cost. On the other hand, if, according to claim 4, each of the fuses takes the form of a fused-on- load switch, then after the triac defect has been rectified, the fuse can be restored to the connected state.

If, according to claim 5, the entire safety device consisting of the two fuses, the diodes and the required wiring, is integrally moulded within a common housing of insulating material which is provided with two wire terminals for connection purposes, then it can be easily inserted between the two parts of the motor lead without having recourse to any further conversion.

The invention will be described in greater details hereinafter with reference to one embodiment thereof shown in the accompanying drawings, in which: Figure 1 is a wiring diagram showing the safety device connected between a power regulator and a motor; Figure 2 is a circuit diagram corresponding to Figure 1 in which a first type of triac defect is simulated; Figures 3 and 4 are circuit diagrams in which a second type of triac defect is simulated; Figure 5 is a circuit diagram corresponding to Figure 1, showing the situation when a fuse has already responded; and Figure 6 is a circuit diagram corresponding to Figure 1 in which is shown additional switching elements to direct individual currents to the safety network.

A safety device 1 for an alternating current motor 2 is connected to the output 4 of a power regulator 3 containing a triac 5. The safety device 1 comprises a shunt 6, of which the parallel branches 7, 8 each contain a diode D, or D2 in series with a fuse Si1 or Si2. The fuses and diodes forming the parallel branches are disposed in an insulating housing 9 which is provided with two external connections in the form of screwed terminals 10, 11 connected to the output 4 and a connection 12 on the motor 2 respectively.

During normal mains operation, the positive half-wave flows through D1, Si, and the alternating current motor 2 while the negative half-wave flows through the diode D2, the fuse Si2 and the motor. The alternating current motor 2 operates as an inductance with an effective resistance.

If the triac 5 becomes defective (e.g. the positive mains half- wave is permanently switched off), then one of the following occurs: 1. If the triac 5 is permanently fired, both mains half-waves pass through the two parallel branches 7, 8 of the safety device 1 and through the alternating current motor 2. The rated motor current flows so there is no danger to the motor.

2.lf the triac 5 is not fired, due to a defect, the positive mains half-wave is fed through the safety device and the alternating current motor 2. A pulsating direct current flows through the branch 7, i.e. through the diode D, and the fuse Si1 and also the alternating current motor 2, of which the inductive resistance is greatly reduced by the direct field premagnetisation. In consequence, the current flowing through the branch 7 increases quite considerably and causes the fuse Si, to respond. In the case of a triac defect in which the negative half-wave is fed through the safety device, the fuse Si, responds (Figure 4). If subsequently the triac 5 is again fired, then full mains voltage is applied to the safety device and to the alternating current motor.However, since one of the fuses (e.g.Si,) has already responded, then only a direct current can flow through the diode D2 and the fuse Si2 (Figure 5). Since this direct current is likewise very much greater than the half of the rated motor current which normally flows through the parallel branch, then the fuse Si2 also responds. The alternating current motor 2 is then switched off completely.

The measured data ascertained from a test structure will be set out hereinafter with reference to Figure 6.

The measuring instruments used were digital multimeters HP 3466A. The object being measured was a type OLK 45/2400-2518 fan motor manufactured by ITT.

Triacfired; S, and S open: U(V) I,(mA) 12(my) l2(mA) noted 220 82 81 159 braked 20 80 79 151 idling To ascertain the half-wave curents 1, and 12 alone, Si, was bridged and Si2 removed or Si, removed and Si2 bridged and in each case the triac was briefly fired; Si, and Si2 are open.

Inom I" (positive half-wave) = 347 mA=4 2 Inam 12 (negative half-wave) = 353mA=4.

2 Pw fan = 40W unacceptably high U mains Pw = II or 12 2 Measurement at phase shift U = 130 V 12 = 49.6 mA Triac defect simulated: S, closed, S2 open I, = 320mAoSi, responds 12 = 14.5 mA S, open, S2 closed I1 = 14.9 mA 12 = 320 mA~Si2 responds In the event of the triac operating only as thyristor, i.e. the negative half-wave is not let through, then with falling room temperature the voltage U rises through Si1, D1 and the motor. At some time, the rated fuse current of 100 mA will be exceeded. The fuse Si, can respond with a delay. However, since the effective power Pw at the motor is only 25 W (admissible) even at twice the rated fuse current of 200 mA, then there is no risk to the fan in this case either.

The same applies to a situation where the positive half-wave is blocked and the negative half-wave is regulated.

Claims (6)

1. A safety device for an alternating current motor, in particular a split-pole motore such as a fan motor which is operated through a phase-shift controller or motor current switch power regulator incorporating a triac, characterised by the provision, between the triac output and the motor, of a fuse network containing at least two diodes, at least two fuses, and a parallel shunt arrangement each branch of which contains at least one of said diodes in series with one of said fuses, the diodes in the parallel branches being oppositely oriented.

2. A safety device according to claim 1, characterised in that the response value of the fuses is substantially 0.6 to 1.5 times the total current flowing through the fuse network.

3. A safety device according to claim 1 or 2, characterised in that the fuses are slow-acting fuses.

4. A safety device according to claim 1 or 2, characterised in that the fuses are reconnectable fused on-load switches.

5. A safety device according to any preceding claim, characterised in that the fuses and diodes are incorporated in a common insulating housing having two external connections.

6. A safety device for an alternating current motor, substantially as hereinbefore described with reference to and as shown in the accompanying drawings.