SYSTEM AND METHOD FOR MONITORING AND CONTROLLING
STATOR WINDING TEMPERATURE IN A DE-ENERGIZED AC MOTOR
GOVERNMENT LICENSE RIGHTS
[0001] The present invention was made at least in part with Government support
under Contract No. DE-FC36-04GO14000, awarded by the United States Department of
Energy. The Government may have certain rights in the invention.
BACKGROUND OF THE INVENTION
[0002] The present invention relates generally to alternating current (AC) motors
and, more particularly, to a system and method for determining stator winding
resistance for thermal protection of AC motors.
[0003] Thermal protection is an important aspect in the monitoring of motor
conditions, as motor failures can often be related to thermal stress on stator winding
insulation. It is commonly assumed that the motor's life is reduced by 50% for every
10° C increase in temperature above an acceptable stator winding temperature limit.
[0004] Thermal protection of AC motors is important not only to running motors, but
also to de-energized motors. In many applications, the AC motor is periodically de-
energized to allow the motor to cool down prior to the next start. Also, overload relays
may be used to trip the AC motor to protect the motor windings if the motor overheats.
If the motor is tripped by overload relays, a certain amount of time is typically required
before the motor can be restarted. Either this recovery time may be too conservative
and production time is lost, or the recovery time may be too short and the incomplete
cooling accumulates after each shutdown, potentially leading to premature damage to
the winding insulation due to overheating.
[0005] Overheat protection of the stator winding insulation of AC motors is only one
aspect of thermal protection. When electric machines are shut down, the stator winding
temperature may fall below the ambient temperature, causing moisture condensation on
the stator windings, brushes, and other compartments. This condensation can be
detrimental to the life of the motor in certain applications. To avoid the moisture
condensation or accumulation, motor winding pre-heating can be desirable to maintain
the stator winding temperature above the ambient temperature.
[0006] Various methods and mechanisms for determining the stator winding
temperature are currently employed for thermal protection purposes. Aside from the
direct stator winding temperature measurement, thermal model-based and motor
parameter-based temperature estimation methods are two techniques for thermal
protection. The thermal model-based methods estimate the stator winding temperature
using motor thermal models. However, due to the thermal parameter variation and the
difficulty of thermal parameter identification, the accuracy of these methods may fall
outside acceptable ranges. Besides, due to possible changes in cooling conditions, the
thermal parameters are not always constant, and may need to be identified for each
motor under each specific cooling condition.
[0007] Also, even if a thermal model or temperature measurement is determined for
a given motor, existing stator winding heating devices heat the motor using two phases
of the stator windings, allowing a single current flow path in the stator winding. This,
however, leaves one phase unheated, or reliant on inductive heat. Also, because the
stator resistance is relatively small, a large voltage and current input is typically needed
to heat the motor. This large voltage and current input may reduce the life of the stator
winding.
[0008] Because an AC motor may sustain damage if the stator winding temperature
is outside an acceptable range or if the stator windings are heated at too high of a
voltage and current input, accurate monitoring and controlling of the stator winding
temperature in a de-energized AC motor is beneficial for motor protection purposes.
[0009] It would therefore be desirable to design an accurate, non-intrusive method
for monitoring and controlling stator winding temperature in a de-energized AC motor,
in an efficient manner and without adding further resistance to the motor.
BRIEF DESCRIPTION OF THE INVENTION
[0010] The present invention provides a system and method for remote stator
winding resistance estimation and stator winding heating for thermal protection of
induction motors in an idle or shutdown condition. The triggering of a series of
switches in a motor control device can generate a DC signal in an output of the motor
control device. This DC signal is analyzed to determine a stator winding resistance.
The temperature of the stator windings can then be determined based on the stator
winding resistance. The switches can be controlled to heat the stator windings to a
desired temperature.
[0011] Therefore, in accordance with one aspect of the present invention, a motor
control device includes a circuit having an input connectable to an AC source and an
output connectable to an input terminal of a multi-phase AC motor. The circuit further
includes a plurality of switching devices to control current flow and terminal voltages in
the multi-phase AC motor and a controller connected to the circuit. The controller is
configured to activate the plurality of switching devices to create a DC signal in an
output of the motor control device corresponding to an input to the multi-phase AC
motor, determine a stator winding resistance of the multi-phase AC motor based on the
DC signal, and estimate a stator temperature from the stator winding resistance.
[0012] In accordance with another aspect of the invention, a method for monitoring
and controlling a multi-phase AC motor includes the step of configuring a motor control
device with a plurality of switching devices to condition voltage and current to the
multi-phase AC motor. The method also includes the steps of disposing the motor
control device in series between an AC power source and the multi-phase AC motor and
selectively operating the motor control device in a temperature estimation mode.
Operating the motor controller in the temperature estimation mode also includes
transmitting a gate drive signal having a firing angle therein to a switching device on
each of two phases of the multi-phase AC motor, thereby triggering the switching
devices to introduce a DC signal into a current path formed by the two phases of the
multi-phase AC motor, measuring the DC signal introduced to the multi-phase AC
motor, determining the resistance of the stator winding based on the measured DC

signal, and calculating a temperature of the stator winding based on the determined
resistance.
[0013] In accordance with yet another aspect of the invention, a soft-starter to
control transmission of voltage and current from an AC power source to an induction
motor having a stator winding includes a plurality of supply lines, each supply line
corresponding to a phase in the induction motor. The soft-starter also includes at least
one solid-state switch located on each of the plurality of supply lines to condition a
motor line voltage and a phase current to the induction motor and a processor. The
processor is programmed to simultaneously trigger one of the at least one solid-state
switches on each of two of the plurality of supply lines to inject a DC current and
determine a resistance of the stator winding based on the DC current.
[0014] In accordance with yet another aspect of the invention, a motor control
device is electrically connected to a multi-phase AC motor. The motor control device
includes at least one solid-state switch corresponding to each phase of the multi-phase
AC motor to control current flow and terminal voltages. The motor control device also
includes a controller configured to trigger a solid-state switch on each of two phases of
the multi-phase AC motor to inject a DC signal into a current path formed by the two
phases. The controller is also configured to selectively trigger additional solid-state
switches such that the solid-state switch triggered on each of two phases of the multi-
phase AC motor is alternated between phases of the multi-phase AC motor thereby
injecting a DC signal having a substantially equal magnitude into each phase of the
multi-phase AC motor.
[0015] Various other features and advantages of the present invention will be made
apparent from the following detailed description and the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] The drawings illustrate one preferred embodiment presently contemplated for
carrying out the invention.
[0017]In the drawings:
[0018] FIG. 1 is a perspective view of a motor control device according to the
present invention.
[0019] FIG. 2 is a schematic view of an AC motor system incorporating a motor
control device for DC injection according to an embodiment of the invention.
[0020] FIG. 3 is a graphical depiction of the triggering of two of the switches as
shown in FIG. 2 to inject a DC signal according to an embodiment of the invention.
[0021] FIG. 4 is a graphical depiction of the triggering of a plurality of the switches
as shown in FIG. 2 to inject a DC signal and heat the stator windings according to an
embodiment of the invention.
[0022] FIG. 5 is a schematic view of a DC equivalent circuit of a motor control
device for DC injection according to an embodiment of the invention.
[0023] FIG. 6 is another schematic view of a DC equivalent circuit of a motor
control device for DC injection according to an embodiment of the invention.
[0024] FIG. 7 is another schematic view of a DC equivalent circuit of a motor
control device for DC injection according to an embodiment of the invention.
[0025] FIG. 8 is a technique for measuring and controlling the temperature of an AC
motor according to an embodiment of the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0026] The embodiments of the invention set forth herein relate to a system and
method for remote and sensorless stator winding resistance estimation for monitoring
the temperature of and providing heat to alternating current (AC) motors. In one
embodiment, the invention is implemented in a motor control device or a soft-starter,
for example. A soft-starter 10 is shown in FIG. 1 and includes a cover assembly 12
having air inlets 14 on a motor connection end, or load end 16. Similar air outlets 18
are located on a power source end, or line end 20 of the soft-starter 10. The cover
assembly 12 also houses an electronic controller 22. Soft-starter 10 also includes a base
assembly 24 to house each of switch assemblies 26, 28, 30. Each of the switch
assemblies 26-30 is identical in construction for a given soft-starter 10 and corresponds
to a phase of a multi-phase input to a multi-phase AC motor.
[0027] According to one aspect of the invention, the activation of a pair of switches
in the switch assemblies of the soft-starter is controlled to generate a DC signal that is
measurable in each phase of the AC motor. This DC signal is analyzed to determine a
stator winding resistance, which is used to determine a temperature of the stator
windings. Based on the measured temperature of the stator windings, the activation of a
pair of switches in the soft-starter may be further controlled to provide heat to the AC
motor. While described below with respect to a three-phase, AC motor having
windings connected in a wye-arrangement, it is also recognized that embodiments of the
invention also include other multi-phase motors having winding arrangements in
various patterns (e.g., delta arrangement). The activation of switches in each of these
various types of AC motors can be modified to inject a DC signal into each phase of the
motor.
[0028]Referring to FIG. 2, a three-phase, AC motor is schematically shown
according to an embodiment of the invention, and is generally designated by the
reference numeral 40. As is conventional, AC motor 40 is represented as three
windings 42, 44, 46. In this case, AC motor is shown connected in a wye-arrangement.
It can be appreciated that AC motor may alternatively, and according to the equivalents
of the invention, be connected in a delta arrangement without deviating from the scope

of the invention. Stator windings 42-46 of AC motor 40 are operatively connected to an
AC source 48 through corresponding multi-phase supply lines 50, 52, 54, at motor
terminals 56, 58, 60.
[0029] As shown in FIG. 2, a motor control device 62 is connected between AC
source 48 and AC motor 40. As set forth above with respect to FIG. 1, in an exemplary
embodiment of the invention, motor control device 62 comprises a soft-starter
configured to limit the transient voltages and current to AC motor 40 during start-up,
resulting in a "soft" motor starting. The basic structure of soft-starter 62 is shown in
FIG. 2 (i.e., circuitry of the soft-starter) as including a contactor 64, 66, 68
corresponding to each supply line 50-54 or each phase of the supply power. Soft-starter
62 also includes a switching device 70, 72, 74 on each supply line 50-54. In an
exemplary embodiment each switching device 70-74 is formed of a pair of anti-parallel
switches, such as solid-state switches in the form of thyristors, to control the current
flow and, in turn, the terminal voltages of the motor 40. That is, thyristor pair 70
includes thyristors 76, 78, which are opposite in polarity and are connected in parallel
for supply line 50. Likewise, thyristor pair 72 includes thyristors 80, 82, which are
opposite in polarity and are connected in parallel for supply line 52. Finally, thyristor
pair 74 includes thyristors 84, 86, which are opposite in polarity and are connected in
parallel for supply line 54. In a preferred embodiment, thyristors 76, 80, 84 are forward
conducting and thyristors 78, 82, 86 are backward conducting. It is recognized that, for
a specified supply line, a thyristor could be arranged in parallel with a diode rather than
another thyristor given the end use of the device. In any of the embodiments described
above, a thyristor on each of two supply lines can be controlled to inject a DC voltage
and DC current in the AC motor 40, as described in detail below.
(0030) Also included in soft-starter 62 is a controller or processor 88 configured to
control operation of thyristors 76-86 via the transmission of gate drive signals thereto,
as well as to control opening and closing of contactors 64-68. During start-up of AC
motor 40, soft-starter 62 operates in a "start-up" mode, during which controller 88
causes one or more of contactors 64-68 corresponding to supply lines 50-54 to open
such that the power from AC source 48 passes through thyristors pairs 70-74, thus

controlling the current flow (and therefore the voltage) applied to the AC motor 40.
Upon start-up of AC motor 40, the soft-starter 62 enters a bypass mode in which
controller 88 causes the contactor 64-68 on each supply line 50-54 to close, so as to
minimize power dissipation. The bypass mode thus is considered the "normal" mode of
operation for the soft-starter 62 and AC motor 40.
[0031] According to one embodiment of the invention, controller 88 is further
programmed to operate soft-starter 62 in a gate drive control mode (i.e., a "temperature
estimation mode") to inject a DC signal into each terminal 56-60 of the AC motor 40
when the AC motor 40 is in a standby condition or turned off. As shown in FIG. 2,
during the temperature estimation mode, controller 88 operates to open all contactors
64-68. One phase of the motor 40, for example phase a, is kept open by not activating
or triggering thyristors 76, 78, while one thyristor in each of the other two phases, for
example thyristors 76, 78, are activated or triggered to create a current path for the DC
signal. Operation of thyristors 76, 78 in this manner injects a DC signal into the AC
motor 40. This DC signal may be measured and determined for each DC injection
period by voltage and current sensors 90, 92, 94 included in soft-starter 62 that are
within the current path.
(0032]Based on the measured DC signals, stator resistance may be calculated as:
(Eqn. 1),
where vdc and i^c represent the DC signal in the phase-to-phase (i.e., line-to-line) voltage
of two lines of the AC motor 40, v, and the phase current, /', respectively, and A: is a
constant that is determined by the configuration of the AC motor 40. That is, for a
three-phase AC motor having winding resistances that are balanced and equal, k=M2.
However, for other multi-phase motors, or for windings in an alternative arrangement, k
may have a different value. Additionally, it is also envisioned that phase-to-phase
voltage and phase current may be measured between any two phases, or all three phases
if desired. That is, the injected DC signals may be induced and measured in all phases
of the AC motor 40.

[0033] Based on the estimated Rs from DC signal injection, the stator winding
temperature Ts may be monitored. The Rs variation is linearly proportional to the Ts
variation, as:
(Eqn. 2),
where Tso and Rso represent Ts and Rs at room temperature; fs and j?s are the estimated
Ts and Rs from DC injection; and /u is the temperature coefficient of resistivity.
[0034]Once the DC signal in the voltage and current, vdc and idc, are determined, the
stator resistance Rs can be estimated according to Eqn. 1 and the stator winding
temperature rs may then be monitored based on the determined Rs according to Eqn. 2
in real-time. In one embodiment of the invention, controller 88 (FIG. 2) is configured to
generate an alert (e.g., audible or visual alert) if the stator winding temperature falls
below a pre-determined threshold value. This alert allows an operator to take a desired
action, such as starting up the AC motor 40. Alternatively, the alert may automatically
trigger controller 88 to enter a heating mode, as discussed in detail below.
[0035] Controller 88 also asymmetrically controls the gate drive signal sent to
thyristor pairs 70-74 to regulate a switching time thereof. In one embodiment,
controller 88 operates to simultaneously trigger one forward conducting thyristor 76, 80,
84 and one backward conducting thyristor 78, 82, 86 at a specified firing angle, for
example a, before the falling zero-crossing of the phase-to-phase voltage. Since the
phase-to-phase voltage is positive when the selected thyristors are triggered on, current
will start flowing. At a specified angle, for example a, after the zero-crossing of the
phase-to-phase voltage, the current will drop to zero and the two triggered thyristors
automatically turn off. As such, current flows for approximately 2a and a DC signal
may be injected in two phases. The magnitude of the injected DC current may be
controlled by adjusting the firing angle a. An example of the waveforms of the motor
line voltage 96 (vjc) and phase current 98 (if,) when thyristors 80, 86 are triggered at a
firing angle 100 of a (e.g., a < 30°) is shown in FIG. 3.

[0036] FIG. 4 shows the phase-to-phase voltage 102 waveforms for all phases and
the current 104 waveforms for phase a when controller 88 (FIG. 2) is used to heat the
stator windings of AC motor 40. In one embodiment, controller 88 periodically triggers
all thyristors 76-86 and regulates the firing angle a 100 such that all three phases of the
stator winding are injected with the same magnitude of DC current. By regulating the
firing angle a, the magnitude of the DC current, and therefore the temperature of the
stator windings, may be controlled. In a preferred embodiment, each thyristor 76-86 is
turned on once every cycle to provide even heating for each phase of the stator winding.
As shown in FIG. 4, a current path is created each time two thyristors are triggered at
firing angle a 100. The firing angle a 100 may be controlled within a desired range, for
example, between 0 degrees and 60 degrees, to maintain a connection to the power
supply 48 in no more than two phases at a time. Because no more than two phases are
connected to the power supply 48 at a given time, the AC motor 40 will not start.
[0037] Referring now to FIG. 4 in conjunction with FIGS. 5-7, the control of the
thyristors 76-86 while the motor system is operating in heater mode is illustrated. As
shown in FIG. 4, for each injection, the contactors 64-68 are held open and two
thyristors are triggered at firing angle a 100, before the falling zero-crossing of the
corresponding phase-to-phase voltage of the power supply 48. For instance, thyristors
76, 80 are triggered at angle a 100, before the falling zero-crossing of vab, to inject a DC
voltage, vabjc and a DC current, iajc, to phases a and b. For ease of understanding, the
DC equivalent circuit representing this control of thyristors 76, 82 is shown in FIG. 5.
Phase c is open and DC voltage vabjc and DC current iajc are injected in the loop formed
by supply lines 50, 52.
[0038] Referring back to FIG. 4, thyristors 80, 86 are triggered next at angle a 100,
before the falling zero-crossing of VbC, to inject a DC voltage, vhc,dc, and a DC current,
ib.dc, to phases b and c. The DC equivalent circuit representing this control of thyristors
80, 86 is shown in FIG. 6. Phase a is open and DC voltage Vbc.dc and DC current ib.dc are
injected in the loop formed by supply lines 52, 54.
[0039]Referring again back to FIG. 4, thyristors 78, 84 are triggered last in the
triggering sequence at angle a 100 before the falling zero-crossing of vca to inject a DC

voltage, vca.dc, and a DC current, icjc, to phases c and a. The DC equivalent circuit
representing this control of thyristors 78, 84 is shown in FIG. 7. Phase b is held open
and DC voltage vccltdc and DC current iCitjc are injected in the loop formed by supply lines
54, 50.
[0040] As shown in FIG. 4, all six thyristors 76-86 are triggered once in each cycle
and each phase of the motor stator winding is injected with current twice in each cycle
(once with positive current and once with negative current). Due to the periodic
operation of all six thyristors 76-86, the DC signal injected to the motor 40
approximates a periodic AC signal.
[0041] According to one aspect of the invention, the temperature of the stator
winding when the AC motor is in a standstill condition, or turned off, may be monitored
and regulated according to a technique 106 as illustrated in FIG. 8. Technique 106
monitors the temperature of the stator winding and maintains the stator winding above a
target temperature, for example several degrees above the ambient temperature or
condensation temperature, by controlling the frequency and magnitude of the injected
DC signal. According to an exemplary embodiment of the invention, the stator winding
temperature may be monitored and controlled by alternating between a temperature
estimation mode and a heating mode.
[0042] Technique 106 begins at step 108 and determines whether the motor is
running. If the motor is running 110, technique 106 may enter into an optional motor
running subroutine 112. In subroutine 112, technique 106 enters a motor running
temperature estimation mode 114 in which technique 106 determines the motor
temperature. At step 116, the motor temperature is analyzed. If the motor temperature
is lower than a maximum motor operating temperature 118, technique 106 proceeds to a
first optional time delay 120 before returning to step 108. If the motor temperature is
higher than the maximum motor operating temperature 122, technique 106 shuts down
the motor at step 124 and proceeds to a motor idle estimation mode 126, which is
described in greater detail below.
[0043] If the motor is turned off or in standby mode 128 following step 124 or step
108, a motor idle temperature estimation mode 126 is entered to determine the
temperature of the stator windings. At step 130, a DC signal is injected to the stator
windings. The magnitude of the DC signal is calculated next 132. At step 134, the
winding resistance is calculated. Technique 106 next calculates the winding
temperature at step 136. In an embodiment of the invention, Eqn. 1 may be used to
calculate the winding resistance in step 134 and Eqn. 2 may be used to calculate the
winding temperature in step 136. Next, technique 106 reports the calculated winding
resistance and temperature to a temperature database 138.
[0044] At step 140, technique 106 determines whether the winding temperature is
above a target temperature. In a preferred embodiment, the target temperature may be
the ambient temperature or a condensation temperature of the stator windings. If the
winding temperature is greater than the target temperature 142, technique 106 enters a
second optional time delay 144 before returning to step 108. Even if the motor
temperature is lower than the target temperature, technique 106 may enter optional step
146 to determine whether the motor winding temperature is projected to fall below the
target temperature within a predetermined time period. If the winding temperature is
not projected to fall below the target temperature 148, technique 106 continues to
optional time delay 144 and returns to step 108.
[0045] If, however, the winding temperature is lower than the target temperature 150
or the winding temperature is projected to fall below the target temperature 152,
technique 106 enters motor heater mode 154. During motor heater mode 154, technique
106 consults the temperature database for the current stator winding temperature
determined during steps 126-138 and calculates heating parameters 156 required to heat
the stator windings above the target temperature. Heating parameters may include
thyristor firing angle, triggering frequency, and triggering duration. Technique 106 then
injects a DC signal 158 using the heating parameters. Following DC injection at step
158, technique 106 re-enters temperature estimation mode at step 126 and proceeds
through steps 130-138 to determine if the injected DC signal adequately heated the
stator windings.

[0046] A technical contribution for the disclosed method and apparatus is that it
provides for a controller-implemented technique for determining stator winding
resistance and heating stator windings for thermal protection of AC motors in an idle or
shutdown condition. The technique controls switching time of a series of switches in a
motor control device to generate a DC signal in an output of the motor control device
corresponding to an input to the AC motor and determines a stator winding resistance
from the DC signal. A temperature of the stator windings may also be determined based
on the stator winding resistance and the switching time of the series of switches may be
controlled to heat the stator windings.
[0047] Therefore, in accordance with one aspect of the present invention, a motor
control device includes a circuit having an input connectable to an AC source and an
output connectable to an input terminal of a multi-phase AC motor. The circuit further
includes a plurality of switching devices to control current flow and terminal voltages in
the multi-phase AC motor and a controller connected to the circuit. The controller is
configured to activate the plurality of switching devices to create a DC signal in an
output of the motor control device corresponding to an input to the multi-phase AC
motor, determine a stator winding resistance of the multi-phase AC motor based on the
DC signal, and estimate a stator temperature from the stator winding resistance.
[0048] In accordance with another aspect of the invention, a method for monitoring
and controlling a multi-phase AC motor includes the step of configuring a motor control
device with a plurality of switching devices to condition voltage and current to the
multi-phase AC motor. The method also includes the steps of disposing the motor
control device in series between an AC power source and the multi-phase AC motor and
selectively operating the motor control device in a temperature estimation mode.
Operating the motor controller in the temperature estimation mode also includes
transmitting a gate drive signal having a firing angle therein to a switching device on
each of two phases of the multi-phase AC motor, thereby triggering the switching
devices to introduce a DC signal into a current path formed by the two phases of the
multi-phase AC motor, measuring the DC signal introduced to the multi-phase AC
motor, determining the resistance of the stator winding based on the measured DC

signal, and calculating a temperature of the stator winding based on the determined
resistance.
[0049] In accordance with yet another aspect of the invention, a soft-starter to
control transmission of voltage and current from an AC power source to an induction
motor having a stator winding includes a plurality of supply lines, each supply line
corresponding to a phase in the induction motor. The soft-starter also includes at least
one solid-state switch located on each of the plurality of supply lines to condition a
motor line voltage and a phase current to the induction motor and a processor. The
processor is programmed to simultaneously trigger one of the at least one solid-state
switches on each of two of the plurality of supply lines to inject a DC current and
determine a resistance of the stator winding based on the DC current.
[0050] In accordance with yet another aspect of the invention, a motor control
device is electrically connected to a multi-phase AC motor. The motor control device
includes at least one solid-state switch corresponding to each phase of the multi-phase
AC motor to control current flow and terminal voltages. The motor control device also
includes a controller configured to trigger a solid-state switch on each of two phases of
the multi-phase AC motor to inject a DC signal into a current path formed by the two
phases. The controller is also configured to selectively trigger additional solid-state
switches such that the solid-state switch triggered on each of two phases of the multi-
phase AC motor is alternated between phases of the multi-phase AC motor thereby
injecting a DC signal having a substantially equal magnitude into each phase of the
multi-phase AC motor.
[0051] The present invention has been described in terms of the preferred
embodiment, and it is recognized that equivalents, alternatives, and modifications, aside
from those expressly stated, are possible and within the scope of the appending claims.

We.claim

1.A motor control device comprising:
a circuit having an input connectable to an AC source and an output
connectable to an input terminal of a multi-phase AC motor, the circuit comprising:
a plurality of switching devices to control current flow and
terminal voltages in the multi-phase AC motor; and
a controller connected to the circuit and configured to:
activate the plurality of switching devices to create a DC
signal in an output of the motor control device corresponding to an input to the multi-
phase AC motor;
determine a stator winding resistance of the multi-phase
AC motor based on the DC signal; and
estimate a stator temperature from the stator winding
resistance.
2.The motor control device of claim 1 wherein each of the plurality of
switching devices comprises a pair of solid-state switches, wherein each pair of solid-
state switches correspond to a phase in the multi-phase AC motor, and wherein the
controller activates a first solid-state switch in a first pair of solid-state switches and a
second solid-state switch in a second pair of solid-state switches.
3.The motor control device of claim 2 wherein each pair of solid-state
switches comprises a pair of thyristors in an anti-parallel arrangement.
4.The motor control device of claim 2 wherein the controller is configured
to selectively activate the first and second solid-state switches and wherein the first and
second solid-state switches correspond to a first phase and a second phase of the AC
motor, respectively.

5.The motor control device of claim 4 wherein the controller is configured
to transmit gate drive signals to the first and second solid-state switches at a firing angle
before a falling zero-crossing of a phase-to-phase voltage to activate the first and second
solid-state switches.
6.The motor control device of claim 5 wherein the controller is further
configured to:
transmit the gate drive signals to the first and second solid-state switches
at an identical firing angle; and
control the firing angle of the first and second solid-state switches to vary
the magnitude of the DC signal.
7.The motor control device of claim 2 wherein the controller is further
configured to selectively activate each of the solid-state switches in each of the plurality
of switching devices in one cycle of the multi-phase AC motor such that the pair of
activated solid-state switches alternates between phases of the multi-phase AC motor.
8.The motor control device of claim 7 wherein the selective activation of
each of the solid-state switches in one cycle of the multi-phase AC motor injects the DC
signal into each phase of the multi-phase AC motor, the injected DC signal
approximating a periodic AC signal; and
wherein the controller is further configured to modify a magnitude of the
periodic AC signal to regulate the stator temperature.
9.The motor control device of claim 8 wherein the controller is further
configured to control a firing angle of the pair of switching devices to vary the
magnitude of the periodic AC signal.
10. The motor control device of claim 1 wherein the controller is further
configured to:

compare the estimated stator temperature to a first stator temperature
threshold; and
selectively activate each of the plurality of switching devices in one
cycle of the multi-phase AC motor to inject the DC signal if the estimated stator
temperature is below the first stator temperature threshold.
11. The motor control device of claim 1 wherein the controller is configured
to activate the plurality of switching devices when the multi-phase AC motor is in a
standstill condition to create a current path from less than all phases of the multi-phase
AC motor.
12. The motor control device of claim 1 wherein the controller is further
configured to:
compare the estimated stator temperature to a second stator temperature
threshold; and
maintain the AC motor in the standstill condition if the estimated stator
temperature is greater than the second stator temperature threshold.
13. A method for monitoring and controlling a multi-phase AC motor
comprising:
configuring a motor control device with a plurality of switching devices
to condition voltage and current to the multi-phase AC motor;
disposing the motor control device in series between an AC power
source and the multi-phase AC motor;
selectively operating the motor control device in a temperature
estimation mode, wherein operating the motor controller in the temperature estimation
mode comprises:
transmitting a gate drive signal having a firing angle to a
switching device on each of two phases of the multi-phase AC motor, thereby triggering
the switching devices to introduce a DC signal into a current path formed by the two
phases of the multi-phase AC motor;

measuring the DC signal introduced to the multi-phase AC
motor;
determining the resistance of the stator winding based on the
measured DC signal; and
calculating a temperature of the stator winding based on the
determined resistance.
14. The method of claim 13 wherein calculating the stator winding
temperature comprises calculating the stator winding temperature according to:

where Tso and ./?so represent the stator winding temperature and stator winding resistance
at room temperature, - is the determined stator winding resistance; and \i is a
temperature coefficient of resistivity.
15. The method of claim 13 wherein transmitting the gate drive signals to the
switching device on each of two phases comprises transmitting the gate drive signals at
a desired firing angle to one of a pair of thyristors in each of the switching devices on
the two phases.
16. The method of claim 13 further comprising modifying the desired firing
angle of the gate drive signal to control a magnitude of the DC signal.
17. The method of claim 13 further comprising:
comparing the calculated temperature of the stator winding to a pre-
determined stator winding temperature threshold; and
if the calculated temperature of the stator winding is below the pre-
determined stator winding temperature threshold, then operating the motor control
device in a heater mode.
18. The method of claim 13 wherein configuring the motor control device
with the plurality of switching devices comprises configuring the motor control device
with a switching device on each of a first phase, a second phase, and a third phase of the
multi-phase AC motor; and
wherein operating the motor control device in the heater mode
comprises:
selectively triggering the switching device on each of the second
phase and the third phase to introduce the DC signal therein;
selectively triggering the switching device on each of the first
phase and the third phase to introduce the DC signal therein; and
selectively triggering the switching device on each of the first
phase and the second phase to introduce the DC signal therein.
19. The method of claim 13 wherein operating the motor control device in
the heater mode introduces a periodic DC signal into each of the first, second, and third
phases, thereby introducing an AC signal into the multi-phase AC motor and evenly
heating each winding of the multi-phase AC motor.
20. The method of claim 19 wherein operating the motor control device in
the heater mode further comprises controlling a magnitude of the AC signal injected
into the multi-phase AC motor based on the calculated temperature of the stator winding
to control heating of the windings of the multi-phase AC motor.
21. A soft-starter to control transmission of voltage and current from an AC
power source to an induction motor having a stator winding, the soft-starter comprising:
a plurality of supply lines, each supply line corresponding to a phase in
the induction motor;
at least one solid-state switch located on each of the plurality of supply
lines to condition a motor line voltage and a phase current to the induction motor; and
a processor programmed to:

simultaneously trigger at least two solid-state switches, one on
each of at least two of the plurality of supply lines to inject a DC current; and
determine a resistance of the stator winding based on the DC
current.
22. The soft-starter of claim 21 wherein the processor is further programmed
to:
determine a temperature of the stator winding based on the determined
resistance of the stator winding;
determine if the temperature of the stator winding is below a stator
winding temperature threshold; and
if the temperature of the stator winding is below the stator winding
temperature threshold, then selectively trigger each of the at least one solid-state
switches such that the two of the plurality of supply lines having triggered solid-state
switches is sequentially alternated between phases of the induction motor.
23. The soft-starter of claim 22 wherein the processor is further programmed
to inject a DC current of equal magnitude into each of the phases of the induction motor
based on the selective triggering of each of the plurality of solid-state switches, the
injected DC current approximating an AC current injected in the phases of the induction
motor.
24. The soft-starter of claim 23 wherein the processor is further programmed
to control the magnitude of the AC current to regulate a temperature of the stator
winding.
25. The soft-starter of claim 21 wherein the at least one solid-state switch
located on each of the plurality of supply lines comprises two solid-state switches
arranged in an anti-parallel configuration.
26. The soft-starter of claim 21 wherein the processor alternates the
simultaneous triggering of two solid-state switches between each of the plurality of
supply lines so as to heat each phase of a three-phase motor.
27. A motor control device electrically connected to a multi-phase AC
motor, the motor control device comprising:
at least one solid-state switch corresponding to each phase of the multi-
phase AC motor to control current flow and terminal voltages; and
a controller configured to:
trigger a solid-state switch on each of two phases of the multi-
phase AC motor to inject a DC signal into a current path formed by the two phases; and
selectively trigger additional solid-state switches such that the
solid-state switch triggered on each of two phases of the multi-phase AC motor is
alternated between phases of the multi-phase AC motor thereby injecting a DC signal
having a substantially equal magnitude into each phase of the multi-phase AC motor.

A system and method for measuring and controlling stator winding
temperature in an AC motor while idling is disclosed. The system includes a circuit
having an input connectable to an AC source and an output connectable to an input
terminal of a multi-phase AC motor. The circuit further includes a plurality of
switching devices to control current flow and terminal voltages in the multi-phase AC
motor and a controller connected to the circuit. The controller is configured to activate
the plurality of switching devices to create a DC signal in an output of the motor control
device corresponding to an input to the multi-phase AC motor, determine or estimate a
stator winding resistance of the multi-phase AC motor based on the DC signal, and
estimate a stator temperature from the stator winding resistance. Temperature can then
be controlled and regulated by DC injection into the stator windings.