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BACKGROUND OF THE INVENTION
This invention relates to starting circuits for electric motors having a
main winding and a start winding and, more particularly, to a circuit
which continuously monitors an average value of the magnitude of current
in the main winding for switching electric power to the start winding.
A common form of electric motor is the induction motor which is
manufactured in fractional horsepower capacities typically in the range of
from one-quarter horsepower to one horsepower. Such motors are provided
with two windings, one winding being the main winding and the other
winding being a start winding, also known as a phase or auxilliary
winding. The main winding continuously draws alternating current from a
source of electric power throughout the operation of the motor. The start
winding is required in addition to the main winding for generating a
rotating torque field in order to initiate rotation of the motor shaft
after which the start winding is disconnected from the source of power. In
some motors, a capacitor may be included in series with the start winding,
such motor being known as a split-phase motor or capacitor-start motor. In
either case, the start winding must be energized for activation of the
motor in the event of a stall or when initiating operation of the motor.
The start winding is not to be energized during continuous operation of
the motor to avoid burnout of the start winding. Also, there is no need
for the start winding during continuous operation of the motor become
rotation of the rotor in cooperation with the magnetic fields of the main
winding induces the necessary rotating torque field for maintaining
rotation of the rotor under load.
In order to disconnect the start winding from the source of power after
rotation of the motor has commenced, various forms of circuitry have been
employed. Some of these circuits take advantage of the relationship
between main winding current and motor speed. The main winding current has
a maximum value when the rotor is stationary, this value of current being
referred to sometimes as a locked-rotor current. As the rotor begins to
rotate, the current in the main winding begins to drop such that, at 75%
of full speed of rotor rotation, the main winding current has dropped
approximately 20% in magnitude. At full speed, the main winding current
drops to a magnitude in the range of one-sixth to one-quarter of the
locked-rotor current. One form of motor control circuit makes use of this
relationship and operates with a threshold based on a testing of a
specific model and size of motor. A relay responsive to the motor current
is set to operate at the threshold to disconnect the start winding.
Alternatively, an electronic switch set to the predetermined threshold has
also been employed to disconnect the start winding. Methods of measuring
the main winding current include the use of a sense resistor in series
with the winding, as well as computerized circuitry which calculates peak
value of the current based on measurements of the slope of the current
waveform.
A problem arises in that the use of a fixed value of threshold is
disadvantageous in that such threshold must be preset for each size and
type of motor. In addition, such setting of the threshold presupposes that
the operating voltage provided by a power supply will have its stated
value. However, as is well known, the voltages of electric power sources,
such as an incoming residential power line, may vary considerably
depending on the loading of the line. Also, the characteristics of the
motor itself may change in the event that the motor becomes heated. Also,
computerized circuitry tends to be excessively costly.
A further disadvantage of the foregoing control circuitry lies in the fact
that some of these circuits are designed to operate with initial values of
current established when the motor is first activated. In the event of a
stall due to a momentary overload, an operator must activate a switch to
restart the motor. This is also true of starting circuits employing a
timer. It is also noted that the circuitry employing a fixed value of
threshold suffers from the disadvantage that it is not interchangeable
among the various sizes and types of motors because of different
requirements for specific settings of th values of threshold for the
different motors.
SUMMARY OF THE INVENTION
The foregoing problems are overcome and other advantages are provided by a
starting circuit for an electric motor having a main winding and a start
winding. The starting circuit is constructed in accordance with the
invention and includes means for sensing the magnitude of main winding
current. Also included within the starting circuit is an electronic switch
in series with the start winding for disconnecting and reconnecting the
start winding to the source of power which drives the main winding. A
circuit is provided for the generation of a reference level, or threshold,
based on measurements of the winding current. A comparator compares the
reference level to the present value of main winding current as sensed by
the sensing means for operating the electronic switch. The switch can be
activated at any time during the operation of the motor by the comparator
whenever the threshold is crossed. The switch disconnects the start
winding when the main winding current drops below the threshold, and
reconnects the start winding whenever the main winding current rises above
the threshold.
In accordance with a feature of the invention, the reference generation
circuit comprises a resistor-capacitor circuit which is charged to a
voltage proportional to a moving average of the main winding current. In
addition, the starting circuit includes its own internal power supply
which is activated simultaneously with energization of the main winding.
The internal power supply experiences a delay in producing its output
voltage and current for powering the starting circuit, the delay
commencing with initial activation of the motor. The delay excludes the
first few cycles of the AC (alternating current) voltage applied to the
motor from entering into the generation of the threshold, thereby reducing
the influence of current switching transients on the starting circuit.
The starting circuit is particularly advantageous in that the threshold is
set at a predetermined fraction of a measured value of the main-winding
current. After motor turn-on, the threshold rises rapidly towards the
desired value so as to enable the comparator to activate the electronic
switch. Thereafter, threshold value responds to any downward drift in main
winding current as averaged over an interval of time in excess of
approximately one-half minute. Future operation of the switch, as might
occur during a temporary stall condition, is based on the average value of
the main winding current. The foregoing form of threshold generation
circuit is particularly useful because it automatically adapts to any
value of current in the main winding of split phase motors and capacitor
start motors independently of the size of such motors. Thereby, a single
circuit may be used for a quarter-horsepower motor as well as for a one
horsepower motor without any need for presetting a specific value of
threshold. Circuit parameter values are selected readily for adaptation of
the starting circuit to various motor supply voltages.
BRIEF DESCRIPTION OF THE DRAWING
The aforementioned aspects and other features of the invention are
explained in the following description, taken in connection with the
accompanying drawing wherein:
FIG. 1 is a block diagram of the motor starting circuit of the invention;
FIG. 2 is a graph of motor current in the main winding versus motor speed;
FIG. 3 is a schematic diagram of the motor starting circuit of the
invention;
FIG. 4 is a timing diagram showing the operation of various portions of the
starting circuit; and
FIG. 5 shows signal waveforms at various locations within the starting
circuit.
DETAILED DESCRIPTION
With reference to FIG. 1, there is shown a start circuit 10 incorporating
the invention for use in starting an electric motor 12. The circuit 10
receives input electric power, such as 110 volts AC at terminals A and B.
The input power may be of a voltage conventionally employed in residential
electric circuits, or other electrical voltages suitable for driving a
motor. The motor 12 is of a well-known induction construction having the
form of either a split phase or capacitor start. The motor 12 comprises a
main winding 14 and a start winding 16, the main winding also being known
as a run winding and the start winding also being known as a phase or
auxilliary winding. Terminal 18 of the main winding and terminal 20 of the
start winding are connected together directly in the split phase form of
motor (shown in FIG. 3), and are connected via a capacitor 22 (FIG. 1) in
the capacitor-start form of motor. The capacitor 22 is shown in phantom
since it may or may not be present depending on the form of motor. The
start circuit 10 is operable with either form of the motor 12.
The motor 12 is connected in series with the start circuit 10 by means of a
terminal 24 of the main winding 14 connected to terminal C of the circuit
10, and a terminal 26 of the start winding 16 connected to terminal D of
the circuit 10. The input power is provided to the circuit 10 by a pair of
lines 28 and 30 connecting respectively with terminals A and B of the
circuit 10, and to the motor 12 via connection of the line 28 with
terminal 18 of the main winding 14 and terminal 20 of the start winding
16. Line 30 may be either neutral or grounded depending on external
considerations in the site of motor installation. By way of example in the
ensuing description, line 30 is understood to be grounded at terminal B,
the following description of operation of the invention applying equally
well to the case wherein line 30 may be electrically neutral. It is to be
understood, however, that the circuit 10 is operable with various power
connections. For example, the circuit 10 can be operated with an AC
voltage of 60 hertz, 50 hertz, or other frequency which may be employed.
In the case of input power provided by a center-tapped transformer,
wherein the center tap is grounded or at a neutral voltage and the
transformer provides 220 volts, terminals A and B may be connected across
the transformer terminals for receiving the 220 volts AC, or terminal B
may be connected to the center tap for application of 110 volts AC between
the terminals A and B.
The start circuit 10 comprises an internal power supply 32 which coverts
the alternating current from the power lines 28 and 30 to direct current,
at terminal P, for distribution to other components of the circuit 10 at a
voltage suitable for their operation. The circuit 10 further comprises a
level detector 34, a reference generator 36, and a comparator 38 which are
powered by the supply 32. Also included within the circuit 10 are a
current sensor 40 for measuring current in the main winding 14, and an
electronic switch 42 operated by the comparator 38 for switching current
on and off in the start winding 16.
The reference generator 36 comprises three sections, a capacitive storage
unit 43 which stores a reference voltage based initially on the peak value
of the main-winding current, a compensation unit 44 for retaining the
value of the reference voltage during running of the motor 12 subsequent
to the starting phase of the motor 12, and a disable unit 45 which
disables the compensation unit 44 during activation of the start winding
16. Included within the disable unit 45 is a delay function 46 which
retards activation of the compensation unit 44 for a preset delay after
deactivation of the start winding 16 to desensitive the compensation unit
from starting-current transients in the motor 12.
With reference also to FIG. 2, a graph shows a relationship between current
in the main winding 14 and speed of rotation of a rotor 48 of the motor
12. A characteristic of the induction motor is the magnitude of current
flowing in the main winding as a function of rotation speed. Maximum
current flows when the rotor 48 is stationary, such a condition being
referred to as a locked rotor. Upon rotation of the rotor, the
main-winding current drops in magnitude, and reaches a value of 80% of the
maximum current value when the rotor 48 has attained a speed of
approximately 75% of full motor speed. The current then drops more rapidly
with increasing speed of rotation, the main winding current reaching a
value of approximately one-quarter to one-third of maximum value at full
speed of rotation.
The foregoing relationship of current versus speed is employed by the start
circuit 10 to determine when the motor 12 has reached the correct speed
for switching current in the start winding 16. Initially, before rotation
of the motor 48, the switch 42 is operated to apply current to the start
winding 16. Later, after the rotational speed has reached a value of
approximately 75% of full speed, the switch 42 is operated to terminate
the start-winding current. Should the motor 12 stall during use, then the
switch 42 is again operated to initiate current in the start winding 16,
thereby to restart the motor 12. Such stalling can occur in a situation,
by way of example, wherein the motor 12 is driving a variable load such as
a grinding wheel wherein a person operating the motor may exert excessive
force on the grinding wheel to stall the motor.
In accordance with a feature of the invention, all operating levels in the
circuit 10 for determining when current is to be initiated and terminated
in the start winding 16 are based on measurements of main-winding current.
No preset levels are employed. Thereby, the circuit 10 may be employed
with a motor drawing a relatively small amount of current or a relatively
large amount of current. In addition, the characteristics of the motor may
change during operation, such as a reduction in current due to increased
resistance associated with increased running temperature, and the circuit
10 automatically adjusts the levels to correspond to the changed values of
current. The level detector 34 responds rapidly to changes in current, as
sensed by the sensor 40, to provide an output voltage to the comparator
38, which voltage tracks the present value of main-winding current. The
level detector 34 also provides signals for operation of the reference
generator 36 which responds more slowly to changes in main winding
current. The magnitude of the reference provided by the generator 36 is
stored, and is allowed to change slowly so as to follow any drifting in
the value of main-winding current, such as that associated with the
aforementioned change in electrical resistance. With respect to a downward
drift in the main-winding current, a reference voltage outputted by the
generator 36 follows a moving average value of the main-winding current.
The comparator 38 operates in response to a difference in voltages
outputted by the detector 34 and the generator 36. This operation of the
circuit 10 will be explained now in further detail with reference to FIGS.
3, 4, and 5.
As shown in FIG. 3, the power supply 32 of the start circuit 10 comprises a
diode 50, a resistor 52, a capacitor 54, and a zener diode 56. Alternating
current from terminal A is applied via the diode 50 which rectifies the
current. The diode 50 is serially connected by the resistor 52 to the
capacitor 54 for charging the capacitor 54 with the rectified current. The
resistor 52 in combination with the capacitor 54 provides a filter
function for smoothing output voltage appearing at terminal P of the
supply 32. The capacitor 54 and the zeneor diode 56 are connected in
parallel between terminal P and ground, the zener diode 56 serving to
regulate the output voltage to the preset zener voltage. The output
voltage at terminal P is applied to a corresponding terminal P appearing
in each of the units 34, 36, and 38 for supplying power thereto.
In the connection of an induction motor with a start circuit, it is common
practice to identify the respective wires by use of color. Four wires are
involved, and are colored red, blue, yellow and white. The white wire
connects input power to the junction of the main winding 18 and the start
winding 16 and also to terminal A of the start circuit 10. The yellow
wires applies input power to terminal B of the start circuit 10. The blue
wire interconnects terminal 24 of the main winding to terminal C of the
start circuit 10. The red winding connects terminal 26 of the start
winding to terminal D of the start circuit 10. This facilitates
interconnections among the start circuit 10, the motor 12, and the source
of input power. A switch 57 may be inserted in the white wire between the
motor 12 and the input power line 28 to energize and to deenergize the
motor 12 and the start circuit 10.
The level detector 34 includes a rectifier 58 and a filter 60. The
rectrifier 58 comprises an operational amplifier 61, two resistors 62 and
63, and a capacitor 64. In operation, the resistor 63 receives an input
signal voltage from the current sensor 40, and inputs the signal voltage
to the negative input terminal of the amplifier 61 The positive input
terminal of the amplifier 61 is grounded. The resistor 62 and the
capacitor 64 provides a feedback path between the negative input terminal
and the output terminal of the amplifier 61. The power supply terminals of
the amplifier 58 are connected between terminal P of the supply 32 and
ground and, therefore, output a unipolar voltage in the form of a
half-wave rectified sine wave. The capacitor 64 introduces an integration
function to the rectifier 58 which tends to smooth pulsations in the
rectified sine wave. The unsmoothed and smoothed pulsation of the
rectified since wave are shown, respectively, in the solid and dashed
traces in the second graph of FIG. 5.
The filter 60 comprises an operational amplifier 66, two resistors 67 and
68, and two capacitors 69 and 70. The resistors 67 and 68 are serially
connected between an output terminal of the amplifier 61 and the positive
input terminal of the amplifier 66. The capacitor 70 is coupled between an
output terminal of the amplifier 66 and a junction point of the resistors
67 and 68. The capacitor 69 is coupled from the junction of the resistor
68 and the positive input terminal of the amplifier 66 to ground. The
output terminal of the amplifier 66 is directly connected to the negative
input terminal thereof. The combination of the resistor 68 and the
capacitor 69 provide an integration function to the filter 60. The
combination of the resistor 67 and the capacitor 70 provide a further
integration function to the filter 60. The operation of the filter 60 has
the advantage of an adequately fast response to pulsations in the smoothed
rectified signal outputted by the rectifier 58, and sufficient integration
capacity to produce on line 72 an essentially smooth indication of the
magnitude of the main-winding current of the motor 12. Such smooth
indication is most desireable to ensure against any chatterring in the
operation of the switch 42. An output signal of the filter 60 is provided
by the output terminal of the amplifier 66 and applied via line 72 to
input terminals of the storage unit 43 and the compensation unit 44 of the
reference generator 36, as well as to the comparator 38.
In the reference generator 36, the storage unit 43 and the compensation
unit 44 output voltages which are summed together at line 74. The storage
unit 43 comprises three resistors 76, 78, and 80, a diode 82 and a
capacitor 84. In operation, the resistors 76 and 78 are connected in
series between the output terminal of the amplifier 66 and ground to serve
as a voltage divider, the junction of the resistors 76 and 78 serving as
an output terminal of the voltage divider and being connected to an input
terminal of the diode 82. The divider outputs a voltage equal to 80% of
the voltage on line 72, this corresponding to the reference value of 80%
of the locked-rotor current disclosed hereinabove with reference to FIG.
2.
The capacitor 84 and the resistor 80 are connected together in parallel
between an output terminal of the diode 82 and ground. The filtered
rectified voltage of the amplifier 66 is applied via the divider and the
diode 82 to charge the capacitor 84. Charging of the capacitor 84 is
accomplished via a time constant equal to the product of the resistance of
the resistor 76 and the capacitance of the capacitor 84. The capacitor 84
discharges through the resistor 80 with a time constant equal to the
product of the capacitance of the capacitor 84 and the resistance of the
resistor 80. The charging time of the capacitor 84 is substantially less
than the duration of a cycle of the AC input power, typically a 60 Hertz
signal, on the input power lines 28 and 30 so as to follow any increase in
current amplitude. The discharge time is sufficiently long, approximately
one half minute, for storage of a voltage representative of the amplitude
of main winding current in the motor 12.
The compensation unit 44 of the reference generator 36 comprises an
operational amplifier 86, three resistors 88, 90, and 92, and a diode 96.
The resistor 92 and diode 96 couple an output signal of the unit 44 for
summation with the output signal of the unit 43. The output signal of the
unit 43 appears across the capacitor 84, and the output signal of the unit
44 appears at the output terminal of the amplifier 86.
In operation, the signal outputted on line 72 by the level detector 34 is
applied to the positive input terminal of the amplifier 86. The negative
input terminal of the amplifier 86 is connected via the resistor 90 to the
output terminal of the amplifier 86, and is also connected via the
resistor 88 to ground. The resistor 90 serves as a feedback resistor. The
output terminal of the amplifier 86 is coupled via the resistor 92 to
circuitry of the disable unit 45. The gain of the amplifier 86 is
established by the ratio of the values of resistance of the resistor 90
and the resistor 88. The resistance ratio is selected to provide a gain of
approximately 2.9 in the preferred embodiment of the invention, this gain
being equal to the ratio of the reference signal on line 74 to the voltage
outputted by the sensor 40 during a normal running current of the motor
12. The ratio of 2.9 has been established experimentally by observation of
motors which are available commercially. It is to be understood that, by
way of example, special purpose motors may have other values of running
current resulting in some other value of the ratio of reference voltage to
voltage outputted by the sensor 40, in which case the resistance ratio of
the resistors 90 and 88 would be altered to produce a corresponsing value
of the gain of the amplifier 86.
It is noted that the capacitor 84 of the storage unit 43 can be charged by
two charging paths, either via the resistor 76 and diode 82, or the
resistor 92 and diode 96. During the start phase in the operation of the
motor 12, the voltage applied to the diode 82 exceeds the voltage applied
to the diode 96 so that charging of the capacitor 84 is via the diode 82.
During the run phase in the operation of the motor 12, the voltage applied
to the diode 96 exceeds the voltage applied the diode 82 so that charging
of the capacitior 84 is accmplished via the diode 96.
The foregoing factor of 2.9 applied to the charging of the capacitor 94 has
been selected to compensate for a discharging of the capacitor 84, as will
be explained subsequently with reference to FIG. 4, so as to provide for a
substantially constant output voltage on line 74 as the main-winding
current of the motor 12 assumes its full-speed value.
The comparator 38 comprises two resistors 98 and 99, and an operational
amplifier 100 having a positive input terminal and a negative input
terminal. The positive input terminal of amplifier 100 receives the output
voltage from the level detector 34 via line 72 and resistor 98. The
negative input terminal receives the reference voltage signal on line 74
from the reference generator 36. Both the resistors 98 and 99 are
connected to the positive input terminal of amplifier 100, the resistor 99
being connected between the positive input terminal and the output
terminal of the amplifier 100 to provide a feedback function. The
connection of the two resistors 98 and 99 serve to stabilize the gain of
the amplifier 100 in its use as a comparator. In view of the connections
of electric power to the amplifier 100 between terminal P and ground, the
output voltage of the amplifier 100, in its operation as a comparator, has
either one of the voltage states, wherein one voltage state has a value of
approximately zero volts, and the other state has a value approximately
equal to the voltage at terminal P. The amplifier 100 outputs a signal, a
relatively high voltage, for activating the switch 42 when the voltage of
the level detector 34 exceeds the reference level on line 74. When the
output voltage of the level detector 34 drops below the reference voltage
on line 74, the output signal of the amplifier 100 of the comparator 38
shifts to an alternate value, near zero, which deactivates the switch 42.
The switch 42 comprises two triacs 102 and 104, two resistors 106 and 108,
and a capacitor 110. The resistor 106 couples current of the output signal
of the comparator 38 to a control electrode of the triac 102, the latter
outputting a signal to the control electrode of the triac 104. The triac
104 is connected in series between terminal D and ground so as to conduct
current through the start winding 16 of the motor 12. Upon activation of
the switch 42 by the comparator 38, the triac 104 is placed in a state of
conduction for conducting current through the start winding 16. Upon
deactivation of the switch 42, the triac 104 is placed in a state of
nonconduction for terminating the flow of current in the start winding 16.
The first triac 102 provides sufficient power for operation of the second
triac 104 for initiating and terminating start-winding current. The
resistor 108 and the capacitor 110 are serially connected between input
and output terminals of the second triac 104 to act as a snubber circuit
for attenuating any voltage spikes which might otherwise develop as
switching transients during a termination of start-winding current.
The current sensor 40 comprises a resistor 112 which is serially connected
between terminal C and ground, so as to conduct current through the main
winding 14 of the motor 12. The resistance of the resistor 112 is
sufficiently small so as to provide no more than a negligible voltage drop
in series with the voltage drop across the main winding. Thereby, the
motor 12 is able to function without any significant effect of the series
connection of the resistor 112. The voltage appearing across the resistor
112 is proportional to the main-winding current, and is amplified by the
amplifier 61 of the level detector 34. The voltage gain of the amplifier
61 is determined by the ratio of the feedback resistor 62 to the input
resistor 63, the latter being connected to the junction of the resistor
112 with the main winding of the motor 12. Typically, a gain of 40 is
provided by the amplifier 61.
The disable unit 45 comprises a transistor 114, a resistor 115, a capacitor
116, and a diode 117. The transistor 114 includes a collector terminal
connected to the junction of the resistor 92 and the diode 96, a base
terminal connected to the resistor 115, and an emitter terminal connected
to ground. The resistor 115 and the diode 17 are serially connected
between the base terminal of the transistor 114 and the junction of the
output terminal of the amplifier 110 and the resistor 106. The capacitor
116 is coupled between ground and the junction of the resistor 115 and the
diode 117. In operation, the output voltage of the comparator 38 is
applied across the base-emitter junction of the transistor 114. Thus, when
the comparator output voltage is relatively high, base current is supplied
via the resistor 115 to place the transitor 114 in a state of conduction.
When the comparator output voltage is approximately zero volts, the
transistor 114 is in a state of nonconduction. The diode 117 is poled to
prevent any negative switching transients which may be present at the
output of the comparator 38 from reaching the transistor 114, thereby to
protect the transistor 114. During a state of conduction, the transistor
114 conducts current from the output terminal of the amplifier 86 via the
resistor 92 to ground. Thereby, during a state of conduction, the
transistor prevents the current outputted by the amplifier 86 from
charging the capacitor 84. During a state of nonconduction of the
transistor 114, current outputted by the amplifier 86 can pass via the
resistor 92 and the diode 96 to charge the capacitor 84 without
interference by the disable unit 45.
The delay function (identified by the numeral 46 in FIG. 2) is accomplished
by operation of the capacitor 116. During a transistion in the state of
the output voltage of the comparator 38 from a low voltage to a high
voltage, current flows via the diode 117 into the capacitor 116.
Sufficient base current for activating the transistor 114 is not provided
by the resistor 115 until after a period of time, or delay, in which the
capacitor 116 charges with current provided by the comparator 138. The
amount of delay is preset to a desired value by selection of a suitbale
value of capacitance for the capacitor 116. Also, during at transistion in
the state of the output voltage of the comparator 38 from a high voltage
to a low voltage, the diode 117 is initially back-biased as the capacitor
116 discharges by providing base current through the resistor 115 to the
transistor 114. Thus, there is also a delay in the termination of the
state of conduction in the transistor 114.
The connection of both the disable unit 45 and the switch 42 to the output
terminal of the comparator 38 provides for operation of the disable unit
45 in concert with operation of the switch 42. Upon activation of the
switch 42 to provide a start current to the motor 12, the disable unit 45
essentially grounds the junction of the resistor 92 and the diode 96,
thereby back-baising the diode 96 to ensure that no electrical transients
associated with operation of the start winding 16 are applied via the
amplifier 86 to the capacitor 84. This provides insurance, in addition to
that of the filter 60, against any undue influence of start motor
transients upon a value of reference voltage stored within the capacitor
84. After deactivation of the switch 42, the delay imparted by the
capacitor 116 prevents flow of current form the compensation unit 44 to
the capacitor 84 until after all switching transients essentially have
died out.
With reference also to the timing diagram of FIG. 4, the operation of the
circuit of FIG. 3 is explained further. The timing diagram is composed of
three graphs in time registration with each other. Near the left end of
the top graph, the input AC power is turned on by a closure of the switch
57. Thereafter, current flows in the main winding of the motor 12, but
there is no rotation of the motor because the start winding has not yet
been energized. Also, upon closure of the switch 57 the capacitor 54 of
the power supply 32 begins to charge, and the output voltage at terminal P
begins to rise to provide DC (direct current) power as indicated at 118.
When the output voltage at terminal P reaches a value suitable for
operation of the start circuit 10, the amplifiers 61 and 86 become
operative and the capacitor 84 begins to charge. The voltage on the
capacitor 84 rises to a value which corresponds to 80% of the locked-rotor
current by action of the voltage divider of the resistors 76 and 78. The
rise in voltage of the capacitor 84 results in activation of the
comparator 38 and operation of the switch 42. Therefore, as shown in the
first graph of FIG. 4, the start winding of the motor 12 is energized
momentarily after the circuit 10 begins operation. Thereupon, the rotor of
the motor 12 begins to rotate, as indicated by a dashed line in the first
graph of FIG. 4.
As the motor speed increases, the current in the main winding drops. At
approximately 75% of full speed, the start winding is deactivated, as is
indicated at 120. Thereafter, the motor speed increases until full speed
is reached. With reference to the foregoing example of a person employing
the motor to drive a grinding wheel, in the event that the person
overloads the motor, the rotation slows down the motor stalls, as
indicated at 122. The main-winding current rises upon the occurrence of
the stall. The increased current is detected by the level detector 34
which, in turn, activates the comparator 38 to operate the switch 42 to
energize the start winding of the motor 12. Thereupon, the rotor of the
motor 12 beings to rotate as has been described above for the case of
initial power turn-on. Upon increase of motor speed to approximately 75%
of full speed, the start winding is again deenergized by operation of the
switch 42.
The operation of the switch 42 for turning on current and turning off
current in the start winding is further explained in the second graph of
FIG. 4. The second graph shows the main-winding current level as detected
by the level detector 34. The trace shown in the second graph is the
output voltage of the detector 34. At the initial stage of operation of
the start circuit 10, the voltage outputted by the detector 34 begins to
rise and, as noted above, activates the comparator 38. The detector
voltage is proportional to the magnitude of the mean-winding current.
Thereafter, the detector voltage begins to drop as the magnitude of the
main-winding current drops. As has been explained in FIG. 2, the
main-winding current drops upon increase in rotation speed.
As the detector voltage drops, the state of charge of the capacitor 84 in
the generator 36 is retained, except for a slow discharge via resistor 80
as indicated at 123 in FIG. 4, so that no corresponding drop of voltage
occurs at the output signal of the generator 36 on line 74. The reference
voltage level on line 74 is shown in the second graph at 124, and also at
126 and 128. As the output voltage of the detector 34 drops below the
reference level at 124, the aforementioned action of the switch 42 takes
place to terminate the start current. Later, at the time of the foregoing
stall, the output voltage of the detector 34 rises in correspondence with
the rising magnitude of the main-winding current. As the output voltage of
the detector 34 passes the reference level at 126, the switch 42 is
operated to initiate the start-winding current. Later, as the output
voltage of the detector 34 drops below the reference level at 128, the
foregoing operation of the switch 42 occurs to terminate the start-winding
current.
The development of the reference level on line 74 is explained further with
reference to the third graph of FIG. 4. There, the reference level on line
74 is shown by a solid line. There is a slight deviation from the
reference level to due to the slow discharge of the capacitor 84 as the
main-winding current drops below the locked-rotor value. The slow
discharge is indicated by a dashed line at 123, shown somewhat exaggerated
to facilitate explanation of the operation. It is noted that the voltage
outputted by the amplifier 86 of the compensation unit 44 is greater than
the voltage putted via line 72 to the storage unit 43 for charging the
capacitor 84. Accordingly, after operation of the comparator 38 to
deactivate the switch 42, and after the further delay produced by the
capacitor 116 in the disable unit 45, current outputted by the amplifier
86 is applied via the resistor 92 and diode 96 to restore charge in the
capacitor 84. This additional source of charging current compensates for
the falling voltage on line 72. The compensation is indicated in the
dotted line at 130 in the third graph of FIG. 4. The aforementioned
multiplicative factor of 2.9 imparted by the gain of the amplifier 86 is
generally sufficient to restore the value of the reference signal on line
74 to its desired value based on the measured magnitude of the normal
running current of the motor 12. Accordingly, the reference level
portrayed in the third graph of FIG. 4 is shown as a straight line. The
staight line is an adequate approximation to the operation of the
reference generator 36 for explanation of the operation of the start
circuit 10. As has been noted hereinabove, the values of resistors in the
start circuit 10 can be altered to accommodate other motor voltages, and
other relationship between locked-rotor current and normal running
current.
With reference also to FIG. 5, there are shown waveforms of voltages
appearing at various points in the circuit of FIG. 3. The first graph
shows the alternating voltage appearing across the sense resistor 112 in
response to the flow of main-winding current in the motor 12. The peak
voltage has initially a relatively high amplitude which decreases later is
the speed of rotation increases. The dashed trace of the second graph
shows the output of the amplifier 61 which amplifies, rectifies and
smoothes the voltage across the sense resistor 112. The solid trace shows
what the output of the amplifier 61 would be in the absence of capacitor
64, so as to demonstrate the smoothing feature of the capacitor 64. All of
the graphs of FIG. 5 are in time registration with each other to permit a
comparison of the waveforms of the re | | |
