High voltage motor control circuit

This disclosure relates to a circuit for controlling energization of a start winding of a single-phase motor connectable by power lines to an AC power supply. The circuit includes at least two triacs connected in back-to-back series relation and connected in series with the start winding. The circuit further comprises a reed switch including reed contacts and a reed coil. The gates of the two triacs are connected across the reed contacts, and the reed coil is connected in one of the power lines and receives the line current during energization of the motor.

DETAILED DESCRIPTION 
This invention relates to a circuit for controlling the application of 
electric power to a load, particularly an inductive load such as a winding 
of a single phase electric motor. A single phase motor includes a main or 
run winding and a start winding, and a control circuit is provided to 
energize the start winding only at motor start up and at low motor speed 
conditions. In recent years many control circuits of this nature have 
included a solid state gateable or triggerable device such as a triac or 
SCRs. 
U.S. Pat. Nos. 4,307,327 and 3,573,579 disclose control circuits of this 
character. In FIG. 1 of U.S. Pat. No. 4,307,327, the circuit includes a 
triac connected in series with a start winding, and a reed switch trigger 
is connected to the gate of the triac. When a motor including such a 
control circuit also includes a start capacitor connected in series with 
the start winding and with the triac and is connected to a 230 volt power 
supply, it has been found that an excessively high voltage may exist 
across the triac in certain operating conditions. For example, when the 
triac is switched off as the motor approaches running speed, the potential 
remaining on the start capacitor will equal the instantaneous voltage 
present at the time the triac is switched off. The voltage across the 
triac then equals the capacitor voltage plus the sinusoidally varying line 
voltage plus the voltage induced in the start winding, as will be 
discussed hereinafter in connection with FIG. 3. In a 230 volt, 
capacitor-start induction motor, the voltage across the triac can exceed 
850 volts, and in these circumstances a triac having a rated voltage of at 
least 1,000 volts should be used. At the present state of this art, such 
triacs are not available in production quantities, and any that are 
available are very expensive. 
It has been known that the voltage across a triac may be reduced by 
connecting a second triac in series with it. In a circuit including two 
series connected triacs as shown in FIG. 2 herein and as shown in FIG. 3 
of U.S. Pat. No. 3,573,579, the voltage is divided between the two triacs, 
thereby effectively reducing by one-half the voltage requirement of each 
triac. As will be discussed in more detail in connection with FIG. 2, a 
disadvantage of such an arrangement is that a separate external trigger 
voltage source must be provided for the triacs. This requirement adds 
expense to the control circuit, and in an application where the control 
circuit must be installed within the housing of an AC motor, the space 
requirement for a separate trigger source is a distinct disadvantage. 
It is a general object of the present invention to provide a triac control 
circuit for an inductive load, which is capable of withstanding the high 
voltages occurring in such a circuit and which does not require a separate 
trigger source. 
A control circuit in accordance with the present invention is particularly 
useful with a single-phase, capacitor-start induction motor including a 
main winding and a start winding connected in parallel to AC power lines, 
and a start capacitor connected in series with the start winding. The 
control circuit includes at least two triacs connected in series with the 
start winding. The triacs are connected in back-to-back relation, and the 
gates of the two triacs are connected to the opposite contacts of a reed 
switch. The coil of the reed switch is connected in one of the power lines 
leading to the two windings and receives the line current. The 
normally-open reed contacts isolate the gates and thereby prevent the 
triacs from being continuously triggered. The series connection of the 
triacs reduces the voltage across each triac, and the inductive impedence 
of the start winding prevents the current through the gates from exceeding 
the rated gate current before the triacs are triggered on.

The circuit shown in FIG. 1 is essentially the same as the single-phase 
motor circuit shown in FIG. 1 of U.S. Pat. No. 4,307,327, with the 
addition of a start capacitor. The circuit shown in the present FIG. 1 
includes a main winding 10 and a start winding 11 which are connected in 
parallel to two power lines 12 and 13. The lines 12 and 13 are adapted to 
be connected to a single-phase AC power supply (not shown). Connected in 
series with the start winding 11 are a start capacitor 14 and a triac 16. 
The triac 16 includes a T1 terminal 17, a T2 terminal 18, and a gate 19. 
The terminal 18 and the gate 19 are connected across the two contacts of a 
reed switch 21. The switch 21 further includes a reed coil 22 that is 
connected in the power line 13. 
As described in detail in U.S. Pat. No. 4,307,327, during operation of the 
motor shown in FIG. 1, the line current flows through the reed coil 22, 
and the magnetic field of the coil closes the contacts of the reed switch 
21. When the contacts are closed, the triac 16 is triggered into 
conduction. As described in the patent, the triac 16 is triggered in each 
half cycle of the AC supply voltage until the changing phase angle of the 
start winding current relative to the line current results in the triac 
failing to be triggered. This phase angle changes as the motor increases 
in speed. 
With reference to FIGS. 1 and 3, at the time that the triac 16 stops 
conducting, the voltage across the start capacitor 14 will be equal to the 
instantaneous voltage that occurred at the time conduction ceased. In FIG. 
3, this DC voltage is indicated by the voltage level 26. The sine wave 27 
represents the varying AC wave that continues to appear across the triac 
16 and it is equal to the sum of the line voltage plus the voltage induced 
in the start winding 11. This sine wave voltage 27 is superimposed on the 
DC capacitor voltage 26. In an instance where the power supply connected 
to the power lines 12 and 13 is 230 volts RMS, the capacitor voltage may 
equal approximately 300 volts (the peak voltage of the AC supply). 
Consequently the peak of the resultant voltage shown by the curve 27 may 
exceed 850 volts, and it appears across the power terminals 17 and 18 of 
the triac 16. The reed contacts are, of course, open at this time. 
To reduce the magnitude of the voltage across the triac, a pair of triacs 
31 and 32 may be connected in series relation as shown in FIG. 2 and as 
shown in FIG. 3 of U.S. Pat. No. 3,573,579. An inductive load 33 and a 
capacitor 34 are connected in series with the two triacs 31 and 32. To 
trigger the two triacs, a separate trigger voltage source 36 is provided, 
and in the example shown in the patent, the source 36 comprises a 
transformer including a primary winding 37 and two secondary windings 38 
and 39. The windings 38 and 39 are respectively connected to the gates of 
the two triacs 31 and 32, and when a current pulse appears in the primary 
winding 37, voltages are induced in the gate circuits of the two triacs 
which trigger them into conduction. The two triacs, of course, fail to be 
triggered in the absence of a varying current in the primary winding 37. 
Even if the resultant voltage 27 appears across the triacs it is divided 
between the two triacs, and this prevents the voltage across the power 
terminals of each triac from being excessive. The circuit shown in FIG. 2, 
however, has a distinct disadvantage in that a separate trigger source 
must be provided for the triacs. In addition to being a substantial 
expense, the transformer requires considerable space and in an instance 
where such a control circuit is used in an AC induction motor, it would be 
difficult to mount it within the motor housing where limited space is 
available. 
U.S. Pat. No. 3,573,579 also suggests, in lines 51-56, column 10, that a 
single secondary winding may be used in place of the separate secondary 
windings. Nevertheless, such an arrangement requires a separate source for 
the triggering voltage, and it isn't clear from the patent how such an 
arrangement would operate. 
The circuit in accordance with this invention, shown in FIG. 4, possesses 
the advantages of the series connected triacs without the disadvantage of 
requiring a separate trigger voltage supply. The single-phase motor 
circuit shown in FIG. 4 includes a main winding 41, a start winding 42 and 
a start capacitor 43 connected across power lines 44 and 45. Two 
triggerable or gateable devices 47 and 48, preferably triacs, are 
connected in back-to-back fashion in series with the start winding 42 and 
the capacitor 43. The phrase "back-to-back" as used herein means that the 
T2 power terminals of the two triacs are connected together. The gates 49 
and 50 are connected across the contacts 52 of a reed switch 53. The 
switch 53 further includes a reed coil 54 connected in the power line 44, 
and it thereby receives the line current. 
During startup of the motor shown in FIG. 4, line current flows through the 
lines 44 and 45 and through the main winding 41 and the field of the coil 
54 closes the contacts 52. Triac trigger current then flows between the 
line 44, the T1 terminal and the gate 49 of the triac 47, through the 
closed reed contacts 52, through the gate 50 and T1 terminal of the triac 
48, through the winding 42 and the capacitor 43, and the power line 45. 
Thus, a portion of the line current flowing through the power lines 44 and 
45 is utilized as the trigger current for the two triacs. This current 
triggers the two triacs into conduction and current then flows through the 
main terminals of the triacs and through the start winding 42 and the 
start capacitor 43. The circuit then continues in operation similar to the 
circuits described in U.S. Pat. No. 4,307,327. When the triacs 47 and 48 
are turned off, the resultant voltage is divided substantially equally 
between the two triacs. If a closer equalization of the voltages across 
the triacs is desired, a conventional RC network may be connected in 
parallel with each triac. 
It should be noted that the start winding current initially flows through 
the gate circuits of the two triacs 47 and 48 and through the reed 
contacts 52 immediately after the contacts 52 are closed and before the 
triacs are triggered into conduction. Those skilled in this art would 
expect that the circuit would be inoperative because this start winding 
current would be expected to exceed the rated gate voltages of the triacs 
and damage them. It is an important feature of the present invention that 
this does not occur because the trigger current flows through the start 
winding 42, and the inductance of the start winding limits the rate of 
rise of the trigger current during the time interval required for the 
triac to be triggered. The current builds up relatively slowly in the gate 
circuits which thereby enables the two triacs to be triggered into 
conduction before the gate current becomes excessive. When the triacs are 
triggered on, they form a low resistance path which shunts the gate 
circuits and thereby maintains a low gate current level. At the running 
speed of the motor, the phase angle is such that the contacts 52 are not 
closed and the triacs are not triggered, and the open contacts 52 isolate 
and protect the gates of the triacs. 
The circuit shown in FIG. 4 does not require an additional trigger voltage 
supply because it utilizes the load current for triggering the triacs, and 
this trigger current is prevented from exceeding the rated gate circuit 
currents because of the inductance of the start winding 42. The safe 
operating region of the triacs is a function of the start winding 42 
inductance, the line voltage, the turn-on time of the triacs, and the 
maximum permissible gate current. The start winding 42 inductance L should 
be at least approximately 
EQU L=V.times.(T/I) 
where V is the applied line voltage, T is the turn-on time of the triacs, 
and I is the maximum allowable gate current of the triacs. 
It should be apparent that a novel and useful control circuit for an 
inductive load has been provided. The control circuit is relatively 
inexpensive because it does not require triacs having a high voltage 
rating, and it does not require a separate trigger source.