Method and apparatus for symmetrical current starting of polyphase induction motors

A motor starter minimizes transient asymmetrical current to an A.C. induction motor during start-up. Desired timing of closing each phase is calculated using the reactance/resistance ratio (X/R) of the motor and calculated capacitive (X.sub.C) and inductive (X.sub.L) reactances and resistances (R) of the motor circuit cable. A target closing angle of the first phase that crosses zero potential of the voltage cycle is determined based on the system X/R ratio, and that phase of the starter is closed individually. After the first phase is closed, the power frequency is used to determine closing of the other phases at 60 degrees intervals corresponding to the X/R-based target timing.

BACKGROUND OF THE INVENTION 
In recent years, environmental, regulatory, and financial factors have 
prompted new electrical motor designs. In the United States, new laws will 
soon mandate that commercial and industrial users of A.C. induction motors 
use only motors which meet specific requirements for energy efficiency. 
Utilities also provide incentives to commercial and industrial users to 
encourage the use of these more efficient motors. As a result, induction 
motors with a high-efficiency design have become more prevalent. 
A ratio of inductive reactance (X.sub.L) to resistance (R), known as the 
X/R ratio, can be found for any motor. This ratio can generally be 
obtained from the manufacturer, although X/R ratio data is not always 
distributed with motors. In general, the inventor has found that the 
design characteristics of the new highly-efficient motors produce X/R 
ratios greater than those of previous motors. 
Because of certain design characteristics of the energy efficient A.C. 
induction motors, industrial power control circuits may interact with 
these motors in an undesired manner. In particular, nuisance tripping of 
instantaneous trip circuit breakers (provided for short-circuit 
protection) has been a problem during start-up of these motors. It has 
been determined by the inventor that nuisance tripping is caused by an 
additive effect of current asymmetries induced during startup of energy 
efficient A.C. motors. While standard design motors may also draw current 
asymmetrically, the asymmetric components are much larger in energy 
efficient motors, and the substantial asymmetries during startup may be 
misinterpreted by circuit breakers as short circuits between phases 
producing an unnecessary shutdown. 
For safety reasons, it is necessary to maintain exceptional sensitivity and 
provide an instantaneous response in the circuit breakers associated with 
these industrial motors. Ground faults and similar problems may present a 
life-threatening hazard, or may damage costly equipment because of the 
substantial currents present in the motors and their switchgear. 
Phase-to-phase short circuits or ground faults may explosively destroy the 
motors and associated equipment, posing a severe threat to personnel in 
the area. 
Since large startup current asymmetries are inherent in high-efficiency 
motor designs, and instantaneously responsive circuit breakers are 
essential to industrial safety, some amount of nuisance tripping has been 
viewed as unavoidable in industrial applications of high-efficiency 
motors. In general, motor starter circuits have not been viewed as 
providing a substantial solution to this problem. 
Motor starter circuits that attempt to minimize in-rush current to a 
polyphase motor have been provided for other reasons, such as to reduce 
line fluctuations, as disclosed in U.S. Pat. No. 4,628,241 to Bristow. 
Bristow shows a start-up control method for an induction motor where each 
phase has a thyristor or triac to control its firing. The first one or two 
phases are initially fired at a preset angle (35 to 45 degrees) after a 
zero crossing of the phase voltage. Subsequent firings may be timed for 
50-60 degrees after the first firing. The timing angles are successively 
varied to provide a "soft" start. 
Additional motor starters that control phase timing are disclosed in U.S. 
Pat. No. 4,482,853 to Bhavsar, U.S. Pat. No. 5,206,572 to Farag et al., 
U.S. Pat. No. 5,168,202 to Bradshaw et al., U.S. Pat. No. 5,140,247 to 
Verbos, U.S. Pat. No. 4,950,970 to Davis et al., U.S. Pat. No. 4,800,326 
to Usworth, U.S. Pat. No. 4,752,725 to Ominato, and U.S. Pat. No. 
4,470,001 to Resch et al. 
The prior art, however, fails to provide a contactor which can be 
constructed easily and cost-effectively, yet minimizes nuisance tripping. 
In particular, as far as the inventor is aware, the prior art does not 
provide a contactor in which a phase is initially closed in a relationship 
with a zero crossing based on the characteristics of the particular motor 
to minimize nuisance tripping. 
SUMMARY OF THE INVENTION 
Therefore, it is a general object of the present invention to provide a 
novel and improved motor starter for starting energy efficient induction 
motors with minimal nuisance tripping. 
Another general object of the present invention is to provide a novel and 
improved polyphase induction motor starter which starts the motors by 
sequentially and separately applying power to the phase coils. 
A further object of the present invention is to provide a novel and 
improved motor starter which uses a motor-specific electrical parameter 
reflecting the electrical characteristics of the motor to calculate 
desired timing of phase power application. 
Another object of the present invention is to provide a novel and improved 
motor starter which uses a motor-specific parameter reflecting the 
electrical characteristics of the particular motor to calculate a desired 
delay time for applying power to a phase relative to a line cycle. 
Yet another object of the present invention is to provide a novel and 
improved motor starter which uses a parameter reflecting the electrical 
characteristics of the particular motor to calculate a desired delay time 
for applying a first phase of polyphase power, then connects the first 
phase at the appropriate time and subsequently connects the remaining 
phases at intervals based on the timing of the first phase connection. 
An additional object of the present invention is to provide a novel and 
improved motor starter which uses a parameter reflecting the electrical 
characteristics of the particular motor to calculate a desired delay time 
for applying a first phase of three phase power, connects the first phase 
at the calculated time, and connects the remaining phases at successive 
intervals of sixty electrical degrees. 
It is also an object of the present invention to provide a novel and 
improved motor starter which uses the X/R ratio of the controlled motor to 
calculate a desired delay time from a line voltage zero for applying a 
first phase of three phase power, connects the first phase at the 
calculated time, and connects the remaining phases at successive intervals 
of sixty electrical degrees. 
These objects and others are achieved by providing a motor starter that 
minimizes transient asymmetrical current to an A.C. induction motor during 
start-up. Desired timing of closing each phase is calculated using the 
reactance/resistance ratio (X/R) of the motor and calculated capacitive 
(X.sub.C) and inductive (X.sub.L) reactances and resistances of the motor 
circuit cable. A target closing angle of the first phase that crosses zero 
potential of the voltage cycle is determined based on the system X/R 
ratio, and that phase of the starter is closed individually. After the 
first phase is closed, the power frequency is used to determine closing of 
the other phases at 60 degrees intervals corresponding to the X/R-based 
target timing.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention provides an apparatus and method for eliminating or 
greatly minimizing transient asymmetrical current to a polyphase A.C. 
induction motor during start-up. This goal is achieved in the present 
invention by providing a novel and improved three-phase motor starter that 
uses the known X/R ratio of the motor and calculated capacitive (Xc) and 
inductive (X.sub.L) reactances and resistances of the motor circuit cable 
to establish exact closing angles for A.C. voltage cycles, thereby 
producing currents with minimal asymmetrical components. 
Initially, the theoretical underpinnings of the invention will be discussed 
in detail to facilitate a more complete understanding of the exemplary 
embodiments disclosed herein. 
Energy efficient motors are designed to minimize power losses, and in 
general this has been accomplished by increasing the amount and size of 
winding materials. These construction differences increase inductive 
reactance (X.sub.L) and decrease resistance (R), so that more energy 
efficient motors generally have a larger X/R ratio. The larger X/R ratio 
makes the asymmetrical components of current significant for a longer time 
during startup. In particular, a substantial DC transient current is 
generated at startup, and decays exponentially over time. Because this 
transient current adds to the inherent AC motor startup transient, 
instantaneous trip circuit breakers used with industrial induction motors 
may react to the sum transient, falsely detect a short circuit fault, and 
trip out the circuit. 
The inventor has determined that initial motor starting conditions closely 
parallel conditions found in short circuit analysis. A motor is 
essentially a short circuit upon initial application of an A.C. voltage. 
An induction A.C. motor system is made up of reactance and resistance, 
which is cumulatively impedance as given by 
EQU Z=SQRT(X.sup.2 +R.sup.2) (1) 
Induction A.C. motor system loads typically have a more inductive reactance 
level, unless there is an excess of synchronous motors or power factor 
capacitors on-line. Resistive circuits provide current in phase with the 
voltage, but in pure reactance circuits, current lags voltage by 
90.degree.. 
The inventor has found that in a purely reactive circuit, a highly 
symmetrical current can be obtained by closing the circuit at voltage 
maximum, where current will act at its maximum rate of change, but 
symmetrically about zero. In contrast, if circuit closing occurs at 
voltage zero, the current lags the voltage, and the current becomes offset 
"asymmetrically" to provide the 90.degree. lag demanded by the reactive 
circuit. The term "positive zero crossing" will be used herein to describe 
a voltage zero crossing where the voltage moves from negative before to 
positive after the zero crossing. 
In circuits containing both reactance and resistance, voltage zero closure 
still creates maximum asymmetry, but the pure symmetrical current depends 
on the X/R ratio. The point on the voltage wave where a pure symmetrical 
current would be produced, measured from a positive zero crossing, is the 
angle whose tangent equals the X/R ratio of the circuit: 
EQU .THETA.=tan.sup.-1 X/R (2) 
For a typical energy efficient motor, .THETA. will be 80 to 90 degrees. 
This angle is determined for the first phase that crosses voltage zero to 
positive voltage. Knowing the angle .THETA. at which to close, the 
necessary time delay between zero crossing and closing can be calculated 
to establish a closing time for the first phase at .THETA. degrees after 
the positive zero voltage crossing, and the phase can be connected to the 
motor at that time. 
Once the voltage zero crossing of a first phase has been determined, the 
time for voltage zero crossing of phases B and C can be determined based 
on the known system operating frequency. 
Thus, in the method of the present invention, a system X/R ratio (including 
the X/R for the motor and connecting cables) is calculated and used to 
determine the closing angle to produce pure symmetrical current. The 
calculated angle is used to dose each phase separately as that angle is 
met for each individual phase. In particular, current is applied at a 
point in the line cycle .THETA. degrees past a positive zero crossing, 
where .THETA. is the angle whose tangent equals the X/R ratio of the 
circuit. That is, Tan=X/R. The exact closing angle of the first phase that 
crosses zero potential to become a positive voltage is determined based on 
the X/R ratio, and that phase is individually closed at the contactor. The 
other phases are subsequently closed after appropriate time delays. 
Those skilled in this art will appreciate that this broad aspect of the 
invention can be implemented by many alternative designs; thus, variations 
on the preferred embodiments disclosed herein can be readily designed 
within the scope of the invention. 
As an example, the calculations performed by the motor starter, and the 
subsequent switching of the contactors, in a motor startup procedure 
according to the present invention may be performed in accordance with the 
flowchart of FIG. 1. 
First, the X/R ratio of the motor is provided as an input to the motor 
starter in block 100, and conductor specifications sufficient to determine 
the X and R of the conductors (e.g. conductor size and length) are 
provided to the motor starter in block 102. To facilitate this input 
process shown in blocks 100 and 102, the motor starter may have particular 
storage registers for receiving the input quantities. In a preferred 
embodiment, the motor starter may separately receive and have separate 
displays for the X/R ratio and the cable information. In this way, 
maintenance personnel can easily enter and verify the equipment descriptor 
information entered in blocks 100 and 102 of the flowchart. 
In flowchart segment 104, beginning with block 106, the system X/R value is 
calculated. In general, the "system" X/R ratio is calculated by taking 
into account the motor X/R ratio, input by the user, and the ratio of X/R 
for the cable obtained using the total reactance (X) and resistance (R) of 
the motor cable in use. Although the motor circuit conductor does not have 
a defined X/R ratio, the ratio of X/R for the cable accounts for the 
capacitive and inductive reactances X.sub.C and X.sub.L and resistance R 
of the cable. For nonshielded cables typically used in industrial motor 
applications, the capacitive factors can be neglected, so that X.sub.cable 
=X.sub.L of the cable. 
Examining this calculation in greater detail, in block 106, the capacitance 
and inductance of the cable are predicted based on the dielectric constant 
(E) of copper and the input conductor type (solid or stranded). As noted 
above, for an unshielded cable the capacitive effects of the cable can be 
neglected. The inductance L is calculated by L=(0.1404 log.sub.10 
2s/d+0.0153).times.10.sup.-6 H/ft where, s is the center to center 
conductor spacing (inches) and d is the diameter over conductor (inches). 
Using as an example an Okonite FMR Okolon Type TC cable, 500 MCM--3C with 
ground (2 conductors) 37 strand/EPR insulation, triplexed arrangement, 
such as might be used with an MCC IE2-BR3 200 Hp, 226 FLA, 1180 RPM 499T 
frame motor, s=0.92 inches and d=0.788 inches, so L=67.01 nil/ft. 
Then, in block 108 the respective capacitive and inductive reactances 
(X.sub.C and X.sub.L) for the cable are calculated. This calculation may 
be performed using the equation X.sub.L =2.pi.fL where f is the frequency 
(hz), and L is the cable inductance calculated above. For the Okonite FMR 
Okolon 500 MCM Type TC cable at f=60 Hz, X.sub.L =25.26 .mu.H/ft. In cases 
where capacitive reactance must be considered, the total cable reactance 
is found as the vector sum of the capacitive and inductive reactance 
X.sub.cable (Ohms/foot). Cable resistance R.sub.cable is predicted based 
on the input conductor size and type. Using the manufacturer's tables for 
the cable example above, R=27.0 .mu..OMEGA./ft, giving X.sub.cable 
/R.sub.cable =0.9356. 
In block 110, the total system X/R ratio is calculated for use in 
controlling the timing of polyphase power application, based on the 
X.sub.cable /R.sub.cable and X/R for the motor using the following 
equation: 
EQU X.sub.system /R.sub.system =(X.sub.cable +X.sub.motor)/(R.sub.cable 
+R.sub.motor) (3) 
Also in block 110, .THETA. is calculated from the system X/R ratio by 
Equation (2) above. It is desirable to use the system X/R ratio for more 
accuracy, but in some cases it may be sufficient to base the calculations 
described herein only on the motor X/R ratio, particularly in those cases 
where X.sub.cable /R.sub.cable is very close to one, so that the inclusion 
of the cable ratio has little effect on the system X/R ratio. 
Of course, it would also be possible to manually calculate and enter any of 
the input data, or even to calculate .THETA. manually and enter .THETA. as 
a value for operation. However, this method would be less desirable 
because it involves external study and calculation as a part of each motor 
installation. Further, the calculations would have to be repeated upon 
repair or replacement of a motor if any of the parameters were changed. By 
providing data entry and automatic calculation of .THETA., the present 
invention allows fast, effective installation of high efficiency motors 
without the need for particular calculations by those responsible for the 
installation. 
The system operates in a standby, or data receiving mode, until startup of 
the motor is desired, by operation of the status test of block 112. When 
the start button is activated, control is transferred to block 114, 
otherwise control returns to block 100, any data changes are accepted, and 
the calculations are updated as described above with reference to section 
104 upon receipt of new data. 
Upon pressing of the start button, control transfers to block 114. The line 
voltages of the phase power inputs are monitored. Although it would be 
possible to monitor only one phase and wait for a positive zero voltage 
crossing in that phase, preferably all three phases are monitored and the 
first phase to reach a positive zero voltage crossing is selected as the 
first to be started. In this way, a more immediate start-up is provided in 
response to pressing the start button. 
In block 116, the contactor associated with the first phase to be closed 
(the phase for which a zero crossing was detected in block 114) is closed 
after a delay of .THETA. electrical degrees after the zero crossing. The 
timing of the closing is determined by the calculated value of .THETA. and 
the known power frequency (e.g. 60 Hz). At 60 Hz, each degree is 1/60 
seconds/cycle * 1/360 cycle/degrees=0.0000463 seconds/degree. This factor 
(or a similarly calculated factor appropriate for the power frequency in 
the system) is multiplied by .THETA. to obtain the required time delay. 
It is a characteristic of a three phase system that the three phases are 
120 degrees apart from each other. Because the system frequency is known 
(for example, 60 Hz in the United States and 50 Hz in most European 
nations), a corresponding X/R-based closing time can be calculated for the 
other phases by simply adding times corresponding to 60 degrees and 120 
degrees at the system frequency, respectively, to the first calculated 
phase timing. For example, at 60 Hz, 60 degrees corresponds to 2.78 
milliseconds, so the second phase would be connected 2.78 msec after the 
first phase and the third phase would be connected 5.56 msec after the 
first phase. Thus, in block 118, the second phase is connected 60 degrees 
after .THETA., and in block 120, the third phase is started 60 degrees 
later, at .THETA.=120 degrees. In this way, all three phases are applied 
to the motor and the motor is started with minimal current asymmetries. 
A preferred embodiment of a motor control apparatus according to the 
present invention which implements the method described with reference to 
FIG. 1 will now be described in detail, with particular reference to FIG. 
2. 
FIG. 2 shows a motor starter 202 comprising processor 204, input panel 206, 
and phase contactors 208, 210, and 212. The phase contactors 208, 210, and 
212 of motor starter 202 are connected to a motor 220 by three phase lines 
214, 216, and 218 respectively. Phase contactors 208, 210, and 212 are 
connected, through power input lines 224, 226, and 228 respectively, to a 
three phase power source 222. Processor 204 is connected by control lines 
230, 232, and 234 to phase contactors 208, 210, and 212 respectively. When 
activating signals are transmitted by processor 204 through control lines 
230, 232, and 234, phase contactors 208, 210, and 212 respectively are 
made to conduct power from the three phase power source 222 to the 
windings of motor 220. 
Contactors 208, 210, and 212 may be conventional contactors but are 
preferably solid state conducting devices, such as IGBT transistors or 
power MOSFETS, but a separate device is provided for each phase. 
Preferably, the processor is connected to monitor voltage in each phase 
through sensors 236, 238, 240, 242, 244 and 246. These sensors are of a 
generally conventional design and measure voltage flowing through each 
phase, while maintaining complete isolation from the power source. The 
sensors can recognize both positive and negative transitional states of 
the cycles in each phase. 
Processor 204 comprises a microprocessor or microcontroller, and associated 
memory, interfacing, and other supporting components, such as EEPROMs for 
long-time storage of system parameters. Desired calculations, user 
interface functions, and control of each contactor 208, 210, and 212 are 
performed by processor 204. Processor 204 performs the necessary functions 
much faster than the 16.67 msec period of a single 60 Hz A.C. voltage 
cycle. Thus, the calculations and switching can be completed in a short 
period of time relative to a single line cycle, and the three independent 
phase closures can be performed with the exact desired timing, to 
eliminate asymmetrical current components during startup. 
Input panel 206 is a user interface which permits input of system 
parameters to support the novel angle closure method of the present 
invention. Referring briefly to FIG. 3, in a preferred embodiment, two 
displays and two function keys or keypads are provided on the input panel 
for use in establishing necessary data. A first, flush membrane push 
button 302 toggles among the items which are entered using input panel 
206, such as the motor X/R ratio, cable length, cable material, and type 
of cable. A description of the item selected for display and entry is 
provided on display 304 which may be a four digit liquid crystal (LCD) 
display. The current data for the displayed item is provided on display 
306, which may be a four digit liquid crystal (LCD) display. The data for 
the item is entered by the operator through a key or keypad 308. In 
particular, the X/R ratio of the motor will be entered by an installer as 
source data for the calculations. The installer will also input the 
conductor sizes and lengths for the motor "T" leads, as well as any other 
information required to make the calculations. 
A switch input 31.0 may be provided on input panel 206 to set the processor 
to perform calculations appropriately for either a 50 Hz or 60 Hz AC power 
source. In this way, the same contactor can be used with both frequency 
standards. The setting of this switch will determine the time constant 
used to determine a 60 degree starting separation of the phases. 
Individual contactor control switches 312 may also be provided to permit 
independent manual control of power application to the three phases 
controlled by the motor starter 202. 
Referring again to FIG. 2, in general, the processor 204 implements a 
program according to the flowchart of FIG. 1 to receive data and 
sequentially close the three phases. As noted above, the first phase 
closure should always occur at a positive voltage for the phase. It is 
desirable to avoid waiting for an entire line cycle between the first and 
third closures, not only because of the time involved, but also because 
120 degree separations between closings result in a substantial 
correspondence between voltage zero of the second phase and its closing 
time, which will produce asymmetries (and may trip an instantaneous 
circuit breaker). Thus, it is significant in the present invention that 
the closures are each separated by 60 degrees. In this scheme, the second 
phase is closed on a negative voltage cycle and the first and third phases 
are closed on positive voltage cycles. Thus, total time for all three 
phase closures is 5.56 msec. This method of timing closing substantially 
reduces or eliminates problems of asymmetries and nuisance tripping, and 
can be implemented in a cost-effective manner. 
Optionally, the motor starter 202 may also be provided with a signal 
generator 250 for generating a low-voltage (MV) signal that can be 
initiated by the operator manually depressing a control button 3 14 on the 
input panel 206 (shown in FIG. 3 ), and a signal detector 252 for 
receiving and detecting the signal generated by signal generator 250. The 
processor 204 can then measure the propagation time of the low voltage 
signal to the motor and back to the detector 252, adjust accordingly, and 
estimate the length of the motor cables automatically, under program 
control, based on the propagation time. 
Preferably, processor 204 also monitors the power lines to motor 220 
(through sensors 236, 238, and 240) and the operation of the contactors 
208, 210, and 212 (through sensors 242, 244, and 246) to verify that each 
contact closure occurs properly and in correct sequence. Processor 204 
verifies correct closures, and also verifies that all three contactors 
208, 210, and 212 have opened upon a stop condition. If in a start-up 
sequence, one phase is not closed after a command to do so, the start-up 
is cancelled and an error indication is provided. In case of a failure, 
processor 204 will establish an off state for the motor starter 202 in 
which no power is applied to the motor 220. 
Thus, a novel apparatus and method for starting a motor with phase timing 
based on the system X/R ratio has been disclosed. This apparatus and 
method advantageously minimize motor inrush current, and nuisance 
tripping, using ordinary contactors and without complex algorithms.