Motor Circuit Analysis –
Principles & Case Studies
Treadstone Rubber Pvt. Ltd.
MCA Capabilities
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Test directly from the MCC
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Check connections, cable & motor
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Detect broken bars, inter-turn shorts, winding contamination,
insulation problems & air-gap eccentricity
Check AC/DC motors, generators, servos & transformers
Motor basics - stator construction
Motor basics - stator failure modes
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Turn to Turn
Coil to Coil
Open Circuit
Phase to Phase
Coil to Ground
What is Motor Circuit Analysis?
Measurement & analysis of the following parameters:
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Resistance
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Inductance
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Impedance
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Phase Angle
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Current/Frequency Response (I/F)
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Capacitance
What are Phase Angle & Current/
Frequency Response?
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Phase Angle
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The relative angle between the two vectors of the applied
test voltage & drawn current
Current/Frequency Response
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Voltage is applied to the winding at different frequencies &
the change in the current drawn is assessed
Evaluation of Windings
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Winding Shorts
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Phase Angle and Current/Frequency Response
Loose Connections
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Resistance
Winding Contamination/Overheating
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Inductance, Impedance & Capacitance
Rotor Condition and Severity
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Inductance and Impedance Waveforms
Setting the Rules
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Phase Angle and I/F readings
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Both Fi and I/F > +/- 2 – Shorted Turns
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Fi > +/- 1, I/F Balanced – Shorted Coils in the same phase
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Fi Balanced, I/F > +/- 2 – Shorted Phase to Phase
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Rules stand regardless of motor size
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Resistance ~ +/- 5%
Setting the Rules (Cont.)
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Impedance, Inductance & Capacitance
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If impedance and inductance are parallel, winding is
clean.
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If impedance and inductance are not parallel, winding
contamination or overheating has occurred.
If capacitance changes over time, winding contamination
&/or partial discharges are indicated.
Setting the Rules (Cont.)
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Inductance and Impedance Rotor Tests
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Measurements of inductance at different rotor positions will
indicate the condition of the rotor bars.
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Rotor tests should be symmetrical and not necessarily
‘perfect’ sine-waves
Insulation Diagram of Motor
Phase A
Ground
Phase B
Phase C
Circuit capacitance
changes due to charge
effects of molecules in an
insulation medium.
Dipoles are created
as electric field crosses the
molecules. As they align,
capacitance increases.
The Dipole
Neg Potential
-
+
-
Pos Potential
Dipolar Motion in Operation
Capacitance
High
Wire
Low
GRND
High
MegOhms
Wire
High
Ground Insulation
Low
Voltage
High
Dipolar Motion in DC Tests
Capacitance
High
Neg
GRND
Pos
Neg
Low
MegOhms
High
Low
Meg
Ohms
Time
Dipolar Motion in Surge Test
Conductor 1
Conductor 2
Overcomes Dipolar Spin.
Impulse
Requires Higher Voltage
in order to cross air gap.
Potentially Destructive!!!
Voltage
Time
Dipolar Motion in MCA - 1
wire
wire
wire
wire
Good Phase
Phase Angle: 77 degrees
Current/Frequency: -44%
wire
wire
Dipolar Motion in MCA - 2
wire
wire
wire
Capacitive
Defect
wire
Bad Phase
Phase Angle: 73 degrees
Current/Frequency: -40%
wire
wire
Insulation Test Method Overview
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Electrical Insulation is a dielectric.
Electric fields cause atoms to polarize with a change to capacitance
as they polarize.
DC insulation tests measure change to capacitance as charge
crosses in one direction.
Surge test uses a high voltage impulse in order to detect defects by
causing an arc. Voltages too high to detect small changes in
capacitive reactance due to defects in insulation.
MCA fully excites the insulation at low voltage. Small changes will
effect circuit capacitance, which will not be ‘masked’ by high
voltage.
MCA method also allows early detection of faults and, as a result,
can trend winding faults over time.
Case Study 1 – 650 KW motor with interturn
short in stator
Motor is not grounded but trips on start-up !
Data courtesy – Rajashree Cement, Malkhed
Case Study 2 – 900 KW motor with winding
contamination & loose overhang
Motor had significant contamination,
also the overhang lacing was completely broken !
Data courtesy – Reliance Industries, Hazira
Case Study 3 – 6000 KW synchronous motor
with interturn short in rotor
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6000 KW, 200 RPM Synchronous
Motor – 36 rotor coils and 244 stator
coils.
Failed on start-up but unable to
determine cause over two days
with surge testing and other
winding test technologies.
Motor circuit analysis applied,
which identified faults within the
rotor.
Four rotor coils found to be directly
shorted, causing a fault in the
motor secondary circuit.
Repaired and returned to service.
Data courtesy – BJM Corporation, USA
Case Study 4 – 300 KW motor with repetitive
bearing failures
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300 KW, 3000 RPM motor with
continuous bearing faults
MCA rotor test shows combination
of casting voids (resulting in severe
rotor heating), and eccentric rotor
Fault detected on inspection combined
casting
void
and
eccentric end-shield bearing fit.
Data courtesy – BJM Corporation, USA
Case Study 5 – Noisy 150 KVA transformer
Data courtesy – BJM Corporation, USA
Software Evaluation: EMCAT
EMCAT software for MCA automatic data interpretation from direct data access from
ALL-TEST IV PRO Data collector or manual data entry of combined technologies.
EMCAT: Automated analysis of AC motors
EMCAT: Automated analysis of AC rotors
EMCAT: Trending charts
TFE Software – estimation of residual life!
All-Test: Which one suits your needs?
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All-Test 31
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Impedance
Phase Angle
Current/Frequency Response
Insulation Resistance
Automated Rotor Test
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All-Test IV Pro
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Impedance
Phase Angle
Current/Frequency Response
Insulation Resistance
Manual Rotor Test
Resistance
Inductance
Capacitance
Data logging
Routes
For more details, contact:
TREADSTONE RUBBER PVT. LTD.
COMMERCE HOUSE
ROOM#307
SHARDA BABU STREET,
RANCHI-834001
JHARKHAND
Tel: 91-651-3290691
Fax: 91-651-2305307
E-mail: treadsto@rediffmail.com