Distinguishing Motor Starts From Short Circuits

Through Phase-Angle Measurements

Michael R. Yenchek, James C. Cawley, Albert L. Brautigam, and Jeffrey Shawn Peterson

Abstract—The National Institute for Occupational Safety and

Health investigated how the starting of induction motors may

cause nuisance tripping of short-circuit protection on coal mine

power systems. This research had a dual purpose: 1) to identify

how motor-start waveforms differ from those for short circuits

and 2) to devise a method to provide short-circuit protection

without intentional time delays to account for motor starts. This

technology will help ensure that surface temperatures of energized

electrical apparatus will not exceed gas or dust ignition thresholds

when short circuits occur.

Index Terms—Longwalls, mining, motor, phase angle, short circuit.

I. INTRODUCTION

PROPOSED rules for high-voltage electrical equipment

used in longwall face areas of underground coal mines

were published on August 27, 1992 by the Mine Safety and

Health Administration (MSHA) [6]. The intent of these regulations is to reduce the likelihood of fire, explosion, and shock

hazards by citing requirements for electrical enclosures, circuit

protection, testing, and personnel protection. The operational

limits for short-circuit protective devices are proposed within

section 75.814 of these rules.

Short-circuit protection for electrical apparatus in underground coal mines is critical. When different phases of an

electrical circuit inadvertently come in contact, thousands

of amperes may flow. Currents of this magnitude can cause

explosions and fires if permitted to exist even for periods as

brief as 1 s. The thermal energy expended is directly related to

(the square of the current times the time) [2]. Consequently,

protective device settings must be specified with a maximum

sensitivity to current ( ) and a minimum reaction time ( ).

Unfortunately, extremely low current settings may interfere

with mining operations by reacting to normal transient events,

such as the starting of motors. In response, the proposed MSHA

rules specify maximum intentional time delays for short-circuit

 

M. R. Yenchek, J. C. Cawley, and A. L. Brautigam are with the Mining

Injury Prevention Branch, Pittsburgh Research Laboratory, National Institute

for Occupational Safety and Health, Pittsburgh, PA 15236-0070 USA (e-mail:

mby5@cdc.gov; gzc4@cdc.gov; aib4@cdc.gov).

J. S. Peterson is with the Hearing Loss Prevention Branch, Pittsburgh Research Laboratory, National Institute for Occupational Safety and Health, Pittsburgh, PA 15236-0070 USA (e-mail: jnp3@cdc.gov).

protection of cables extending from the power center to motor

starters. In the future, the trend toward higher efficiency motors

with greater peak starting currents may necessitate even longer

delays. However, in background discussion of the rules, MSHA

solicits comments regarding the elimination of intentional time

delays with a conjunctive increase in current settings. This reflects the dilemma of short-circuit protection, namely, that circuit protective devices should have high sensitivity to faults,

but not interfere with normal mining operations. Accordingly,

MSHA requested a high-priority research effort aimed at eliminating intentional time delays in short-circuit protection.

The Pittsburgh Research Laboratory of the National Institute

for Occupational Safety and Health (NIOSH) investigated how

the starting of induction motors may cause nuisance tripping of

short-circuit protection on coal mine power systems.1 The specific objectives of this research project were to: 1) identify how

motor-start waveforms differ from those for short circuits and

2) devise a method to provide short-circuit protection without

intentional time delays to account for motor starts. Minimal

reaction times to short circuits will help ensure that the temperatures of energized electrical apparatus will not exceed gas

or dust ignition thresholds when such faults occur. Additionally, the capability of distinguishing between motor-start and

short-circuit events precludes nuisance protective device activation. Although the project focused on high-voltage longwalls,

the technology subsequently developed may be applicable to

low- and medium-voltage mine power systems as well.

II. BACKGROUND

Protection against short circuits on high-voltage mine power

circuits is typically provided by vacuum interrupters. The current magnitude thresholds or settings at which these devices operate are specified through the MSHA approval process [3] for

high-voltage longwalls. The initial inrush currents, demanded

by large induction motors starting across the line, may exceed

these settings and activate the protection devices needlessly.

Consequently, to prevent nuisance tripping of the short-circuit

protection, it is desirable to seek a means to momentarily disable or inhibit the protection device activation for a finite period

following motor energization.

A speed-sensing switch could logically be utilized to signal

motor start, but requires direct access to rotating parts, which is

sometimes not feasible or economical. In addition, this sensor

1This research originated under the U.S. Bureau of Mines prior to its transfer

to the Office for Mine Safety and Health Research, Pittsburgh Research Laboratory, National Institute for Occupational Safety and Health, in 1996.
 

Fig. 1. Measuring motor-start waveforms on a 950-V continuous mining

machine.

may be slow to operate where load inertia is high. This is also

true for induction-disk impedance or distance relays used primarily for fault protection on transmission lines [8], [10], [5]. In

the late 1970s, a system was developed in the U.K. that monitored the phase angle between voltage and current to distinguish

between faults and motor starts [7]. Despite promising results,

this technology has not been incorporated in short-circuit protection for U.S. mines. Given the catastrophic potential of inadequate electrical protection, it is imperative to reexamine this

problem to minimize any intentional time delays while maximizing current sensitivity.

III. MOTOR-START EVALUATIONS

Electrical safety can be enhanced if the circuit-protection device recognizes and reacts only to short circuits. To devise a

means to discriminate between motor starts and short circuits,

one must first investigate how the waveforms of each event

differ. Any distinguishing characteristics may then be keyed

upon through the design of sensing circuitry. Accordingly, it was

decided to record the voltage and current waveforms of mine

induction motors of various voltages and evaluate their salient

characteristics.

A. Field Tests of Mine Motors

To gain insight into motor-start signatures, field tests of a

variety of mine motors were conducted. These included motors with application in both longwall and continuous mining,

with voltages ranging from 440 to 4160 V and power ratings

from 10 to 450 hp. Because mine motors are typically started

across the line with full voltage, it was necessary to select test

sites with this capability. These included a rebuild shop with

440- and 550-V motors up to 150 hp. At another facility, a

4160-V conveyor motor was evaluated. Finally, the pump, conveyor, and cutter motors of a 950-V continuous miner were analyzed (Fig. 1). To ensure random motor energization relative to

the voltage cycle, 12 successive start recordings were made of

each motor. Recordings were usually made at the terminals of

the motor. However, recordings of the continuous miner motors

Fig. 2. Phase-to-phase voltage waveform for start of 950-V 165-hp motor.

were made at both the motor terminals and in the load center

that fed the 153-m trailing cable.

B. Data Acquisition Hardware and Software

Transducers were used to record three-phase currents

during motor starts. The Hall-effect current sensors featured

a frequency response from dc to 1000 Hz to capture higher

frequency transients that may be present. The 1000-A split-core

configurations had 5000-V line-to-output isolation. They

were used in conjunction with matched signal conditioners

having 10-V outputs directly proportional to the amplitude

of the input signal. Tests with low-voltage motors utilized

0 to 1000-V voltage transducers with a proportional 0 to

10-V output. These sensors had a frequency range of dc to

5000 Hz. At 1000 V and above, custom potential transformers

were used to acquire the voltage waveform. The current

signal conditioners and voltage sensors were connected to

ground-isolation boards, 1000-Hz programmable filters, and

a 12-bit analog-to-digital board. This board had the capability

of scanning up to eight separate channels and was equipped

with simultaneous sample-and-hold circuitry that eliminated

phase shifts among channels. The output of this board was

digitally recorded in a multichannel file using a 486-based

laptop personal computer that was made rugged for field use. A

custom computer program was written that allowed the user to

choose the number of channels to record at sampling rates up

to 30 720 Hz per channel. During acquisition, data were written

directly to disk with data files saved in binary format.

C. Motor Waveform Analyses

The motor-start voltage and current data were subsequently

imported into a commercial data analysis package. Three-phase

waveforms of voltage and current were plotted versus time for

each of the motor tests. Typical plots are shown in Figs. 2 and

3. The inrush current following circuit energization has the potential to cause nuisance tripping of the short-circuit protection. It lasts for a time period that depends on the motor characteristics and loading. Closer examination of the time-varying

signal revealed high-frequency oscillations within the first few
 

Fig. 3. Line-current waveform for start of 950-V 165-hp motor.

Fig. 4. Superposition of motor-start voltage and current waveforms.

milliseconds following energization. These perturbations were

later attributed to contact bounce. A fast Fourier transform of

the current waveforms displayed no frequencies other than the

fundamental and its harmonics. Consequently, the examination

of individual motor-start waveforms revealed no unusual characteristics upon which a motor-start detection scheme could be

keyed.

Attention then turned to a study of the induction motor phase

angle during starting. The phase angle may be obtained by superposing the voltage and current waveforms of a particular

phase on a common time scale (Fig. 4) and recognizing that

nominal 30 phase shifts were inherent with the phase-to-phase

connections of the voltage transducers. It should be noted that

the phase angles observed were not equivalent to those associated with power factor. The angle in degrees between two corresponding points of the waveforms may be derived by considering that, at a frequency of 60 Hz, one 360 waveform cycle is

completed every 0.016 67 s.

Typical inductive reactance-to-resistance ( ) ratios for induction motors, published in the IEEE “Red Book” [1], are in

the range of 10 to 20. A fault on a circuit feeding an induction motor essentially shunts that inductive load, decreasing the

phase angle between the voltage and current. Further, it is known

that the phase angle of an induction motor during start is larger

Fig. 5. Current waveform distortion of pump motor on continuous mining

machine.

than that at full load [10]. Accordingly, the phase angle between

the voltage and starting current must theoretically be larger on

any given mine motor circuit than that resulting from a fault.

D. Motor-Start Phase-Angle Observations

Phase angles from start to full speed were derived from the

motor waveforms recorded during field tests. The voltage and

current waveforms were superposed and the time lag calculated

at the zero crossings from energization to the point where the

motor had attained full speed. Initial analyses involved nine

unloaded, Class H, 440- and 550-V induction motors at a rebuild facility. The angle between voltage and current for the

first and second zero crossings was found to vary by as much

as 50 , not only among each of the three phases for a given

test, but also for any given phase in successive tests. In addition, phase-angle variations were observed even after the motor

attained full speed. A number of factors contributed to these

phenomena. First, in many cases immediately following energization, the current waveforms exhibited a transient distortion

that affected the exact time of the zero crossing. High-frequency

oscillations due to contact bounce enhanced these irregularities.

More importantly, the position of the voltage wave of a particular phase relative to the time of energization had an even

greater impact on the phase angle. For example, if the voltage

of a given phase was approaching a zero crossing and about to

change polarity at the moment of motor energization, the resultant current wave for that phase would exhibit distortion during

the first cycle (Figs. 2 and 3). Further, some of the inconsistencies may be attributed simply to the fact that all of the motors

were tested on the shop floor without any mechanical load on

their shafts. Repeatable phase angles were observed in subsequent tests of a loaded 4160-V 450-hp motor at another facility

and motors under load on a continuous miner. Finally, as expected, there were differences observed in phase angles during

starts with motors of different horsepower or manufacturers.

Next, the results of tests on continuous mining machine motors were analyzed. During data acquisition, considerable waveform distortion of the pump motor current was noted (Fig. 5).
 

Fig. 6. Phase angle versus time during start of cutter.

Again, there was some variation among calculated phase angles during the first two cycles, but not to the degree observed

with unloaded motors. Despite the waveform distortion, following the first two cycles and continuing to running speed,

phase-angle magnitude behaved much more predictably over

time among phases and successive tests. Fig. 6 shows that the

phase angle from start to full speed for the cutter motor declined

almost linearly as measured at the power center. Not surprisingly, because trailing cables exhibit a lower ratio compared with that of motors, the phase angles calculated at the

motor terminals during starts were on average 5 greater than

those obtained from tests at the power center.

These analyses provided insight into the behavior of the phase

angle between the current and voltage of a motor during start.

They showed that, for a loaded motor after the first two cycles,

the phase angle was relatively predictable. Consequently, the

phase angle can be a reasonable basis for a methodology designed to distinguish between motor starts and short circuits on

mine power systems.

IV. MOTOR-START DETECTION SCHEME

The proposed MSHA rules for high-voltage longwalls permit

a maximum intentional time delay in short-circuit protection

of 0.25 s or 15 cycles [6]. To minimize electrical fire hazards,

a motor-start detection scheme should react much faster, ideally with no intentional time delay. Initially, it was believed that

commercial phase-angle meters might be suited for incorporation into a detection system. However, the typical response times

specified for these instruments was found to be 100 ms, which is

too slow for consideration. Minimal time delay is best achieved

through a digital-based design that does not rely on computational processes or integration of 60-Hz-power waveforms. Accordingly, the approach was to devise a means to detect the time

between each zero crossing of the voltage and current signals

beginning with the first half-cycle.

A. Prototype Circuit Operation

The prototype circuit is shown in Fig. 7. The circuit can monitor the current and voltage waveforms for one phase of a threephase circuit. A suitable potential transformer, or equivalent

transducer is required for a line-to-line voltage measurement

and a current transducer is required for the phase current measurement. After detection of a high-current condition, the digital

logic performs the following operations:

• starts an initial delay;

• at the conclusion of the initial delay, measures the actual

phase angle and compares it against a desired minimum

set point;

• at the conclusion of the initial delay, initiates a motor

locked (stalled) rotor timeout interval;

• controls the main circuit breaker interface relay.

To accurately measure the phase-angle electrical degrees

during each half-cycle, a crystal-controlled oscillator was

selected as the time reference. Oscillator pulses generated

during zero crossings of voltage and current are counted and

compared with a preset (phase-angle) delay. The inherent 30

shift between the line-to-line and phase voltage waveforms is

accounted for by the detector circuitry. If the angle between

voltage and current exceeds the setting of the circuit and

current magnitude is high, the device sends a signal to inhibit

circuit breaker action. An adjustable initial time delay was

incorporated to account for switching transients which can

distort phase angle measurements during the first few cycles.

Situations where high current is detected and the phase angle

is equal to or greater than the set-point minimum, could be

indicative of a motor with a locked (stalled) rotor. This situation

should be detected and cleared by tripping the main breaker

after a finite timeout interval. A timeout interval of from zero

to 255 voltage half-cycles is incorporated to accomplish this.

Three prototypes (one per phase) were then constructed and

tested in the laboratory.

B. Test Results

The detector circuit was tested using a 50-hp 460-V induction motor in the laboratory with an unloaded 95-kVA generator coupled to its shaft. The phase shift between motor voltage

and startup current, was observed to be approximately 90 , implying a 60 phase angle considering the phase-to-phase voltage

connections. The performance of the three-phase detector circuitry was evaluated by adjusting initial time delays, phase angle

thresholds, and locked-rotor timeout. Three-phase motor voltages and currents were recorded along with detector outputs.

A typical test result is shown in Fig. 8. The current detection

threshold for the three-phase detection circuits was set at 300

A. In practice, this value would be fixed at the required short

circuit setting of the circuit breaker. Since the 50-hp laboratory

motor start currents exceeded this value, the detectors were activated. The intentional time delay for all three phase detectors

was set at one cycle. In application, this value is adjustable up to

four cycles to account for switching transients that may distort

phase angle measurements. For test purposes, the phase angle

detection thresholds for phases A, B, and C were set at 50 , 60 ,

and 70 , respectively. In actuality, the thresholds would be set

to the same value. Fig. 8 depicts voltage, current, and detector

output signals for all three phases.
 

Fig. 7. Prototype phase-angle detection circuitry (only one phase shown).
 

Fig. 8. Laboratory tests of prototype circuit with a 50-hp motor.

Throughout the startup of the motor, the phase voltages remained constant while the phase currents could be seen to decay

toward running values. The motor phase angle of 60 is greater

than the appropriate 50 setting of phase A, so the phase-A detection circuitry correctly identifies the input voltage and current waveforms as a motor start and inhibits the circuit breaker

from activating. The angle threshold for phase C has been intentionally, but inappropriately, set at 70 . Since the motor’s

phase angle is less than this level, the phase-C detector circuit

considers this a short-circuit event and outputs a signal to enable circuit breaker action. The angle detection level setting of

60 for phase B is close to the actual motor signal. Initially the

motor’s angle exceeds 60 and the detector correctly identifies

the motor start and inhibits circuit breaker action. However, as

discussed previously, motor phase angles can decrease slightly

during starts. Apparently, after about 36 cycles (0.61 s), the measured angle drops below 60 , causing the detector output to send

an enabling signal to the circuit breaker as long as the B current

exceeds 300 A. It must be emphasized that in application the

phase-angle detection thresholds for all phases would be identical and fixed below the phase angle of the motor voltage and

current. This level would be set by starting the protected motor

a dozen times and adjusting the angle detection level. Nevertheless, should the detector output conflicting signals from two

phases, the circuit breaker will always be allowed to trip. Conversely, to prevent circuit breaker action, inhibit signals must be

outputted for all three phases.

C. Application

The prototype detector can have utility on three-phase power

systems where the starting of induction motors causes nuisance

tripping of the short-circuit protection. In lieu of incorporating

an intentional time delay or increasing the trip setting of the circuit breaker, the detector may be used to momentarily inhibit

circuit breaker action during motor starts. It is envisioned that

the detector would be located near the circuit breaker. The device would temporarily inhibit breaker action only if high phase
 

angles and high currents were indicated for all phases. It is recommended that the activation point of the current magnitude detector be adjusted to the required magnetic setting of the circuit

breaker. Consequently, the device would remain inactive for currents less than the circuit breaker setting. When phase currents

exceed this level, a built-in delay in detector activation would

prevent false indications for the initial period of motor starts. Because the phase-angle characteristics of motors depend on motor

voltage, class, manufacturer, and horsepower, presetting of the

detector would be impractical. In application, the device activation point plus some margin would be determined by starting

the protected motor a dozen times. This would account for any

transients resulting from the randomness of the instant of energization relative to the voltage cycle.

V. SUMMARY AND CONCLUSIONS

The NIOSH Pittsburgh Research Laboratory investigated

electrical safety in coal mines through research into short-circuit protection of mine power systems. Voltage and current

waveforms of mine induction motors were recorded during

field tests, and their salient characteristics were evaluated. An

attribute of motor-start signatures that distinguished them from

short circuits was the relatively large phase angle between

voltage and current. This was the focus of subsequent efforts

to devise a means to discriminate between motor starts and

short circuits. Electronic circuitry was designed to differentiate

between phase angles associated with motor starts and faults

and react to momentarily disable circuit breaker action for

motor starts. A prototype was successfully evaluated with a

three-phase induction motor in the laboratory.

By devising a method to distinguish between motor starts and

short circuits, any intentional time delays in response to short

circuits can be significantly reduced from up to 0.25 s to 66 ms.

Rapid response to short circuits will help ensure that the surface

temperatures of energized electrical apparatus will not exceed

gas or dust ignition thresholds when such faults occur. In addition, the capability of differentiating motor starts from short

circuits will preclude nuisance protective device activation. Ultimately, this work may result in revisions to high-voltage longwall approval guidelines and improvements in circuit breaker

designs. Moreover, the technology may be applicable to motor

circuits at all voltage levels.

REFERENCES

[1] Recommended Practice for Electric Power Distribution for Industrial

Plants (IEEE Red Book), ANSI/IEEE Standard 141, 1993.

[2] IEEE Recommended Practice for Protection and Coordination of Industrial and Commercial Power Systems (IEEE Buff Book), ANSI/IEEE

standard 242, 1986, p. 344.

[3] C. M. Boring and K. J. Porter, “Criteria for approval of mining

equipment incorporating on-board switching of high-voltage circuits,”

in Proc. Ninth WVU Mining Electrotechnology Conf.. Morgantown,

WV, 1988, pp. 267–274.

[4] Code of Federal Regulations, Office of the Federal Register, U.S. Government Printing Office, Washington, DC, <AUTHOR: YEAR?>.

[5] J. E. DeCastro, R. T. Beck, C. Cai, and L. Yu, “Stall protection of large

induction motors,” IEEE Trans. Ind. Applicat., vol. 31, pp. 1159–1166,

Sept./Oct. 1995.

[6] “Mine safety and health administration: Proposed rules,” Federal Register, 57 Fed. Reg. 39036, 1992.

[7] H. Lord H and F. K. Pearson, “Ultra-sensitive instantaneous fault protection for cage inductionmotor supplies,” Mining Technol., vol. 62, pp.

219–223, 1980.

[8] L. A. Morley, J. N. Tomlinson, G. Moore, and D. E. Kline, “Portable

trailing cables, splices, and couplers design and installation considerations,” The Pennsylvania State Univ., University Park, PA, U.S. Bureau

of Mines Contract J0199106, 1982.

[9] L. A. Morley, T. Novak, and I. Davidson, “The application of 2,400 V

to longwall face equipment,” IEEE Trans. Ind. Applicat., vol. 26, pp.

886–892, Sept./Oct. 1990.

[10] J. M. Schulman, W. A. Elmore, and K. D. Bailey, “Motor starting protection by impedance sensing,” IEEE Trans. Power App. Syst., vol. PAS-97,

pp. 1689–1695, <AUTHOR: MONTH?> 1978.

Michael R. Yenchek (S’70–M’71–SM’89) received

the B.S.E.E. and M.S.E.E. degrees from the University of Pittsburgh, Pittsburgh, PA, in 1971 and 1977,

respectively.

From 1971 to 1980, he was an Electrical Engineer

with the Coal Mine Safety and Health Division,

Mine Safety and Health Administration. In 1980,

he joined the Pittsburgh Research Laboratory,

U.S. Bureau of Mines (now National Institute for

Occupational Safety and Health), Pittsburgh, PA,

where he is currently the Chief of the Protective

Equipment Section, Mining Injury Prevention Branch. He has been responsible

for research programs concerning grounding and ground fault, dc haulage,

motor, and trailing cable protection.

Mr. Yenchek is a member of Eta Kappa Nu and a Registered Professional

Engineer in the Commonwealth of Pennsylvania.

James C. Cawley (M’82–SM’83) received the

B.S.E.E. degree from The Pennsylvania State

University, University Park, and the M.B.A. degree

from the University of Pittsburgh, Pittsburgh, PA.

He designed vibration monitoring instrumentation

for Reliance Electric Company, Columbus, OH, and

industrial communications systems for Marshall

Electronics Company, Pittsburgh, PA, prior to

joining the Pittsburgh Research Center, U.S. Bureau

of Mines, Pittsburgh, PA. In positions there as an

Electrical Engineer and later as an Engineering

Supervisor, he was active in the fields of intrinsic safety, explosion-proof

enclosures, and mine electrical systems and diagnostic systems for mine

machinery. Since the activities of the former U.S. Bureau of Mines were

assumed by the National Institute for Occupational Safety and Health, he has

worked on electrical safety issues related to overhead power-line contact with

cranes and trucks and in the causal analysis of electrical incidents.

Mr. Cawley is a Registered Professional Engineer in the Commonwealth of

Pennsylvania.

Albert L. Brautigam received the B.S. and M.S. degrees in electrical engineering from the University of

Pittsburgh, Pittsburgh, PA.

He has more than 30 years of experience in the

design of control, measurement, and data acquisition

systems. He was a Research Engineer at the Graham

Research Laboratory, Jones & Laughlin Steel

Corporation, Senior Engineer for the Westinghouse

Electric Corporation Water Reactors Division, and

Specialist Engineer for Boeing Services International, Inc. Currently, he is an Electrical Engineer

with the Pittsburgh Research Laboratory, National Institute for Occupational

Safety and Health (formerly the U.S. Bureau of Mines), Pittsburgh, PA. He

is responsible for the development of electronic control and measurement

systems for mining equipment.
Jeffrey Shawn Peterson received the B.E. degree in

electrical engineering from Youngstown State University, Youngstown, OH, and the M.S. degree from

the University of Pittsburgh, Pittsburgh, PA, in 1987

and 1998, respectively.

Since August 1987, he has been with the U.S. Bureau of Mines/Department of Energy/National Institute for Occupational Safety and Health, Pittsburgh,

PA. He has conducted research in a variety of mining

areas, including the detection of downed trolley lines,

short-circuit protection of high-voltage longwall circuits, and intrinsic safety. During the previous three years, his research focus

has shifted to hearing loss prevention, in which area he has worked extensively

in the areas of signal collection and analysis and engineering controls.