2. TABLE I
LIST OF SPIM PARAMETERS
rs (rS) Resistance of main (auxiliary) winding
Lls (LlS) Leakage inductance of main (auxiliary) winding
Lms (LmS) Self inductance of main (auxiliary) winding
rr (r R) Resistance of rotor winding referred to
main (auxiliary) winding
L lr (L lR) Leakage inductance of rotor winding
referred to main (auxiliary) winding
C1 Capacitance of start capacitor
r1 Resistance of start capacitor
Fig. 1. The equivalent circuit of an SPIM with only the main winding.
Section IV, and the paper is wrapped up in Section V.
II. SPIM EQUIVALENT CIRCUITS
This section introduces the steady-state equivalent circuits
for SPIMs, which will be used in the proposed parameter
estimation method.
We consider squirrel-cage SPIMs. The stator has two wind-
ings, a main winding and an auxiliary winding, that are placed
90 electrical degrees apart in space. The two windings usually
have different number of turns. The external circuit typi-
cally consists of capacitors with/without a centrifugal switch,
connected in series with the auxiliary winding to enhance
the SPIM start-up torque characteristics. Depending on the
external circuit, the SPIMs can be classiļ¬ed into three types
of topology, namely, the capacitor-start (CS), the capacitor-
startācapacitor-run, and the permanent-split ones; see e.g., [9].
In this paper, we will focus on the CS topology because it is
the most popular type for residential applications such as air-
conditioner compressors. The list of parameters in the SPIM
steady-state equivalent circuit is summarized in Table I.
Development of the SPIM steady-state equivalent circuit
is very similar to the one for three-phase induction motor.
However, due to the winding arrangement and the single-
phase sinusoidal power supply, one has to consider two mmf
waves traveling in opposite directions for each of the stator
windings. If only the main winding is connected, the air-gap
mmf can be decomposed into two traveling waves: one travels
in the positive rotor angle direction, and the other one travels
in the negative direction. With sinusoidal current input, the
Fig. 2. The equivalent circuit of an SPIM with only the auxiliary winding.
fundamental component of magnetomotive force (mmf) in the
air-gap at time t and angular position Īø can be written as:
Fag(t, Īø) =
1
2
F0 cos(Ļet ā Īø + Ļ)
+
1
2
F0 cos(Ļet + Īø + Ļ), (1)
where F0 is the peak of mmf at the fundamental harmonic in
the air-gap, Ļ is the phase angle of the main winding current,
and Ļe is the electrical frequency (e.g., 120Ļ Rad/s). The ļ¬rst
summand with ĻetāĪø is the mmf wave traveling in the positive
Īø direction with the angular speed Ļe, while the other one is
the mmf wave in the negative direction at the same speed. Let
s be the slip between the SPIM rotorās mechanical speed and
the mmf wave in the positive direction. It can be shown that the
slip between the rotor and mmf wave in the negative direction
would be 2 ā s. For each of the mmf waves, the reļ¬ected
effect of the SPIM rotor on each stator winding is similar to
that of the three-phase motor. Hence the equivalent circuit of
one SPIM winding consists of two equivalent circuits of three-
phase induction motor connected in series with the equivalent
parameters scaled by the 1
2 factor as described in Fig. 1 and 2.
The top parts in each of the equivalent circuits correspond to
the mmf wave in the positive rotor angle direction, and thus
are termed as the forward branch. Accordingly, the bottom
parts for the mmf in the negative direction are referred as
the backward branch. Last, note that in Fig. 2 the auxiliary
winding is connected in series to the external start-capacitor
circuit, consisting of C1 and r1 and a centrifugal switch.
As mentioned in Section I, earlier work in [5] and [6]
ignores the core-loss resistance term rcs in these equivalent
circuits, assuming its value is signiļ¬cantly bigger compared
to those of other components connected in parallel. The rest
of the paper will develop the parameter estimation method
by accounting for the rcs term with improved performance in
ļ¬tting the test measurement data.
III. PARAMETER ESTIMATION METHOD
This section will present the parameter estimation method
using the tests with the main winding connected while the
3. Fig. 3. The equivalent circuit with only the main winding under the locked-
rotor test.
auxiliary winding disconnected; i.e., for the SPIM equivalent
circuit in Fig. 1. For the auxiliary winding circuit in Fig. 2,
one can ļ¬rst disconnect the external capacitor (with C1 and
r1 shorted). This way, the two equivalent circuits become the
same, and the method presented for only the main winding
can be directly adopted to estimate the motor parameters in
Fig. 2. The goal of the proposed estimation method is to
ļ¬nd the circuit parameter values in order to match the active
and reactive power measurements of the motor under various
test conditions. Certain parameters, such as the main winding
resistance rs, can be directly measured using the dc test.
For some of other parameters, including the inductance terms
Lls, Llr, and Lms, as well as the rotor resistance rr, one
can use additional test measurements to directly calculate a
rough estimate of their values. This stage is similar to the
method in [5] and [6] by ignoring the rcs term. To improve
the estimation accuracy, the Newton-Raphson (N-R) algorithm
will be adopted to ļ¬t all the test measurements by iteratively
updating the estimates for all these parameters together with
the core-loss resistance rcs.
A. SPIM tests
To collect the measurements, the following three tests will
be performed:
1) dc test: resistance rs is directly measured.
2) locked-rotor test: in this test, the rotor is locked from
rotation. An appropriate voltage source will be applied
to the stator winding such that the SPIM terminal current
is close to its rated value. The terminal rms voltage V m
LR,
rms current Im
LR, active power Pm
LR, and power factor
pfm
LR are measured.
3) no-load test: in this test, the rotor is rotated at the
synchronous speed with the assistance of an external
mechanical source. The applied voltage source to the
stator is at the rated level. Same measurements as those
in the locked-rotor test will be collected, denoted by
V m
NL, Im
NL, Pm
NL, and pfm
NL, respectively.
Fig. 4. The equivalent circuit with only the main winding under the no-load
test.
One can simplify the equivalent circuits in Figs. 1-2 under
the locked-rotor and no-load test conditions. The simpliļ¬ed
circuits make it possible to directly approximate some param-
eters such as Lls, Llr, Lms, and rr. The parameter estimates
obtained by this approach will be used as the initial guess for
the N-R updates later on.
The following assumptions can be made to simplify the
SPIM equivalent circuits:
ā¢ The core-loss resistance rcs is signiļ¬cantly larger than
the magnetizing reactance ĻeLms. Both of them are much
larger than the rotor leakage reactance ĻeLlr and the rotor
resistance rr.
ā¢ The stator leakage inductance and rotor leakage induc-
tance are the same; i.e., Lls = Llr.
ā¢ There is no coupling between the two stator windings.
Under these assumptions, the simpliļ¬ed equivalent circuit
for the locked-rotor test can be constructed using the fact that
the slip s = 1, as shown by Fig. 3. The core-loss resistance rcs
and the winding magnetizing reactance ĻeLms are eliminated
from the simpliļ¬ed circuit. The resultant circuit consists of
three resistance and three inductance terms, all connected in
series.
Similarly for the no-load test, one can set the slip s = 0
and construct the simpliļ¬ed equivalent circuit as illustrated
in Fig. 4. As the rotor effective resistance in the forward
branch is inversely proportional to the slip s and goes to
inļ¬nity in this case, the main winding magnetizing reactance
ĻeLms part needs to be kept with all other terms eliminated.
The simpliļ¬cation of the backward branch follows similarly
from the locked-rotor one. The resultant circuit consists of two
resistance and two inductance terms, all connected in series.
Having the simpliļ¬ed equivalent circuits for different tests,
the SPIM parameters can be calculate using the following
6. 0 1 2 3 4
0
5
10
15
20
25
Iteration index
Residualnorm||f(x)||2 Main winding
Auxiliary winding
Fig. 6. Iterative residual error norm obtained by the N-R algorithm for the
main and auxiliary winding tests, respectively.
TABLE VI
ACTIVE AND REACTIVE POWER COMPARISON FOR THE SPIM
WITH ONLY MAIN WINDING
Test
Active power [W]
Measured Direct Calc. N-R Alg.
Locked-rotor test 105.03 100.07 105.03
No-load test 85.79 41.97 85.79
Test
Reactive power [VAR]
Measured Direct Calc. N-R Alog.
Locked-rotor test 136.55 135.15 136.55
No-load test 522.13 528.20 522.13
TABLE VII
ACTIVE AND REACTIVE POWER COMPARISON FOR THE SPIM
WITH ONLY AUXILIARY WINDING
Test
Active power [W]
Measured Direct Calc. N-R Alg.
Locked-rotor test 227.11 222.01 227.11
No-load test 78.18 45.96 78.18
Test
Reactive power [VAR]
Measured Direct Calc. N-R Alog.
Locked-rotor test 133.04 133.76 133.04
No-load test 295.99 305.94 295.99
W and 19.86 VAR. Although it is reasonable to neglect the
core-loss resistance as corroborated by its large value, it is
truly necessary to include this term for better ļ¬tting the motor
terminal power measurements under various steady-state test
conditions.
V. CONCLUSIONS
In this paper, we have presented a methodology to es-
timate the parameters for the SPIM steady-state equivalent
circuit. Based on the motor terminal active and reactive power
measurements under different tests, it is possible to directly
calculate several parameter values by eliminating the core-
loss resistance in the equivalent circuit. Although the resultant
estimates provide a good initial guess, there exists a noticeable
error in matching the active power output under the no-load
test. To tackle this issue, we solved the parameter estimation
problem with the core-loss term as an additional unknown
TABLE VIII
ACTIVE AND REACTIVE POWER COMPARISON UNDER
DIFFERENT OPERATING CONDITIONS
Slip Measured Direct Calc. N-R Alg.
Active
power
[W]
0.0083 202.75 124.50 164.30
0.0150 280.28 188.85 227.56
0.0231 362.37 263.83 301.18
0.0306 364.30 328.11 363.11
0.0350 444.58 369.87 405.24
Reactive
power
[VAR]
0.0083 445.76 124.50 164.30
0.0150 433.06 188.85 227.56
0.0231 426.65 263.83 301.18
0.0306 422.47 328.11 363.87
0.0350 425.09 369.87 405.23
using the Newton-Raphson iterative updates. Experimental
results demonstrated that the N-R estimates improve the ļ¬t
of both real and reactive power measurements upon the direct
calculation ones at different operating conditions.
Future research directions include extension of the current
steady-state testing framework to the dynamic setting because
the dynamic transient behavior of SPIMs have increasingly af-
fected the operations of power systems under fault conditions;
see e.g., [3]. We are interested in investigating the performance
analysis and equivalent circuit parameter estimation of SPIMs
during start-up time. Those investigations are anticipated to use
the transient measurement data under normal motor operating
conditions in a non-intrusive fashion.
VI. ACKNOWLEDGMENT
This research is partially supported by an unrestricted gift
from Texas Instruments (TI). P. Huynh and H. Zhu also would
like to thank Dr. R. Narasimha at the TI ESP group for
valuable inputs and discussions.
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