Starting device for a single-phase induction motor

The single-phase induction motor has a starting device which is activated during the start-up phase of the motor and exposes at least a part of the motor coil to a voltage which changes temporarily, the average value of the voltage formed during one period of oscillation not being equal to zero.

BACKGROUND OF THE INVENTION 
a) Field of the Invention 
The invention relates to a single-phase induction motor with a starting 
device. 
b) Description of the Prior Art 
Motors of this type are used today in numerous areas, for example for 
driving heating pumps and submerged pumps, for driving compressors and 
other units. They are operated using alternating current and are 
essentially intended for low capacities. In the simplest form, 
single-phase induction motors are neither self-starting nor dependent on 
direction of rotation. However, in practice both are generally required, 
which is why a starting device is provided. This determines not only the 
direction of rotation, but also produces a starting moment. In the 
simplest form, this starting device comprises an auxiliary coil provided 
in the support, to which a capacitor is connected in series. This starting 
device is connected in parallel to the support coil. 
In a refined embodiment, at least one part of the actual starting device 
can be switched off after reaching a predetermined rotational speed in 
most cases by means of a centrifugal switch. When switching off, as a rule 
the capacitor capacitance existing at the auxiliary coil is reduced. This 
technology is known, reference is made to it, for example in SIHI-HALBERG, 
Grundlagen fur die Planung von Kreiselpumpenanlagen (Fundamental 
principles for planning centrifugal pump installations), 1978, pages 186 
and 187. 
Known starting devices operate independently of whether or not they are 
switched off completely or partially when reaching the nominal rotational 
speed, so that an approximately uniform starting torque rotating with the 
rotor is produced when starting the motor. The characteristics of the 
relative magnetic flux of stator and rotor are represented as ellipses 
arranged essentially symmetrically about the zero point. In terms of 
construction, starting devices of this type can be realised according to 
the state of the art by connecting capacitors next to the auxiliary coil 
which is required anyway. Capacitors are comparatively expensive 
components and occupy an increasing amount of space with increasing 
capacitance. 
Starting from this, the object of the invention is to provide an 
alternative starting device which can be constructed more favourably using 
other electronic components. 
SUMMARY OF THE INVENTION 
This object is achieved according to the invention in that during the 
start-up phase of the motor the starting device exposes at least one part 
of the motor coil to a voltage which changes temporarily, the average 
value thereof formed during one period of oscillation (of the supply 
voltage) not being equal to zero. It is clear that according to the 
invention the motor is controlled during the start-up phase using a 
voltage, the temporary average value of which is controlled intentionally 
to not be equal to zero. The variations conventionally present due to 
inequalities within the supply network and which on precise inspection 
also lead to an average voltage value which is not equal to zero, can be 
neglected and are not adequate for this specific motor control. 
The invention can be used both for starting devices which run continuously 
and for those which are switched off completely or partially after 
reaching a predetermined rotational speed. So that the average value of 
the voltage which changes temporarily and exists at at least one part of 
the motor coil is not equal to zero, the characteristics of the relative 
flux in the stator and in the rotor are displaced away from the zero 
point, wherein the measure of the displacement is a function of the 
average value of the previously mentioned voltage. This results in the 
starting moment no longer being uniform and it may even be reversed for a 
short time. The extent of the maximum moment is thus increased to two to 
three times the conventional value. Running of at least one part of the 
motor coil at a voltage, the average value of which is not equal to zero, 
does however have the same effect as that of a large capacitance connected 
in series with the auxiliary coil as regards the starting process. 
Moreover, the starting moment may be chosen by selection, arrangement and 
dimensions of the electronic components, so that a reversal of moment not 
only takes place, but the moment returns to the zero value if need be for 
a fraction of a period of oscillation. 
The advantage of the solution according to the invention lies in the fact 
that in the simplest form the starting device only requires one further 
rectifier element next to an auxiliary coil, and this rectifier element is 
available cheaply and compactly in the form of a diode. In a preferred 
embodiment therefore, the starting device has at least one rectifier 
element which is superimposed on at least one part of the motor coil 
during the start-up phase. There are numerous possibilities for connecting 
this rectifier element, hence it may be connected, for example in parallel 
or in series to the auxiliary coil or a part of the auxiliary coil. 
Preferably, the rectifier element is deactivated after reaching a 
predetermined rotational speed or after a certain time. 
The starting device of the invention operates independently of whether or 
not the auxiliary coil remains in superimposed connection with a running 
capacitor during operation of the motor to increase the breakdown torque 
and the nominal power.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The main stator coil of the single-phase induction motor is designated 1 in 
each case in FIGS. 1 and 2. The auxiliary coil connected in parallel 
thereto--often also termed the starting coil--is designated 2. An 
auxiliary coil, to which an inductively coupled further auxiliary coil 2b 
is assigned, is termed 2a. Here the auxiliary coil 2 thus comprises two 
parallel auxiliary coils 2a and 2b. 
The single-phase induction motor shown using FIG. 1 has a main coil 1 in a 
manner known per se, to which an auxiliary coil 2 in series with a running 
capacitor 3 lies in parallel. The coil 2 and the capacitor 3 remain 
superimposed during the entire motor operation and ensure increased 
nominal capacity of the motor during operation. A diode 4, which lies in 
series with a centrifugal switch 5, is connected in parallel with the 
running capacitor 3 to start the motor. When starting the motor, the diode 
4 and the centrifugal switch 5 as well as the starting coil 2 form the 
actual starting device. If the motor has reached a predetermined 
rotational speed, for example two thirds of the nominal rotational speed, 
the centrifugal switch 5 opens into the switch position shown in FIG. 1, 
after which the starting device is deactivated. 
The capacitor 3 and the auxiliary coil 2 are indeed required in practice 
for starting the motor, namely so that the direction of rotation is 
determined, but do not belong to the actual starting device. The diode 4 
replaces the further capacitor conventionally connected for the purpose of 
starting in the state of the art. A further resistance (not shown) in 
series or further electronic components may optionally be connected to the 
diode 4 to adjust the relative flux characteristic or the moment course in 
the required manner. 
In the circuit shown using FIG. 2, the auxiliary coil 2a lies in series 
with the running capacitor 3 and in parallel with the main stator coil 1. 
A switch 6, which then connects a resistance 7, a further auxiliary coil 
2b and a diode 4 one behind another in series, in parallel to the main 
stator coil 1, is closed to start the motor. The further auxiliary coil 2b 
is inductively coupled to the first auxiliary coil 2a. The switch 6 is 
controlled by means of a bimetallic member, that is to say, after a 
certain time it operates due to its heat produced by the current flux. The 
heat produced within the motor then ensures that the switch 6 does not 
return to its original position after the switching process, rather only 
when the motor has cooled down and is to be started again. The switch 5 or 
a time switch may also be used instead of the switch 6. 
During normal motor operation, as is known from the state of the art, the 
motor is also exposed to an alternating voltage in the start-up phase, the 
frequency of which may optionally be varied by means of a frequency 
converter. The average value of the supply voltage during one period of 
oscillation is however always zero. A relative magnetic flux is then 
produced in the stator of the motor, as is shown using the characteristic 
8 in FIG. 3. Such characteristics are typically ellipses arranged 
symmetrically about the zero point 9. 
A rotor flux characteristic, which represents the relative magnetic flux of 
the rotor during one motor revolution, belongs to each stator flux 
characteristic. Such rotor flux characteristics typically lie within the 
associated stator flux characteristic and are not shown in FIGS. 3 and 4 
for reasons of clarity. They also behave in similar manner to the stator 
flux characteristics. If the voltage existing at a part of one or more 
coils is set to a temporary average value which is not equal to zero, the 
shape of the characteristic of the relative magnetic flux on the one hand 
and its position with respect to the zero point 9 changes. Two further 
characteristics 10 and 11, which show the relative magnetic stator flux 
during one motor revolution when the temporary average value of the 
existing voltage is not equal to zero, are shown by way of example in FIG. 
3. 
Whereas the characteristic 10 is essentially displaced from the coordinate 
zero point 9 by an amount X.sub.10, the shape of the characteristic 11 is 
also still clearly altered. It is displaced with respect to the zero point 
9 by the amount X.sub.11. This distance X, at which the characteristics 10 
and 11 are removed from the original characteristic 8, in particular from 
the zero point 9, is a function of the temporary average value of the 
voltage U.sub.m superimposed on the auxiliary coil 2. That is to say, the 
greater this distance X, the greater also the temporary average value of 
this voltage. 
The momentary magnetic flux within the stator and the rotor is in each case 
determined by a vector starting from the zero point 9, the peak of this 
vector lying on the stator flux characteristic or the rotor flux 
characteristic (not shown). A changing angle .alpha. lies between these 
two vectors. The torque being produced between rotor and stator is a 
function of the cross product of these two vectors or of the amounts given 
by the length of the vectors and of the angle .alpha. fixed by them. It is 
clear from this that the maximum moment of the motor also grows with 
increasing distance X. Two vectors V.sub.9 and V.sub.11 are drawn in for 
this purpose by way of example. 
Whereas for the stator flux characteristics according to numbers 10 and 11 
in FIG. 3, the moment not only changes its quantity but also its 
direction, the starting device, the effect of which is shown using FIGS. 4 
and 5, is adjusted so that the stator flux characteristic 12 still just 
includes the zero point 9, and this ensures that the temporary moment 
course of the motor is always greater or smaller (depending on the 
direction of rotation) than zero. A moment course 13 of this type is shown 
in FIG. 5. The high moment peaks can be seen clearly which not only ensure 
that one particularly high torque is available for starting at least for a 
short time, which is advantageous, for example for the rotor is stuck, but 
that this moment also still pulsates so that a certain vibrating effect is 
achieved, which makes starting possible even under unfavourable 
conditions. If after reaching a predetermined rotational speed or after a 
predetermined time, the starting device is then deactivated, a stator flux 
characteristic is produced which is arranged symmetrically to the zero 
point 9, as is conventional. 
The present invention may be embodied in other specific forms without 
departing from the spirit or essential attributes thereof and, 
accordingly, reference should be made to the appended claims rather than 
to the foregoing specification as indicating the scope of the invention.