Download Causes of Motor Failures

RMM Systems, LLC
By Sam Rietta
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Have you ever lost a motor?
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Caused a loss of production?
After a storm?
Same motor over and over again?
While you were away from the plant?
Something happened on the power company
side.
• Didn’t know why?
Why Do Motors Fail?
Common Causes of Motor Failures
Costs Associated with Motor Failures
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Equipment downtime $$$$
Loss of production
Collateral equipment damage
Equipment replacement
Work in process damage
Overtime for repair crews
Motor Windings Breakdown
• Thermal
• Mechanical
• Environmental
Thermal Breakdown
• Rule of thumb:
A 10o C temperature rise above rated
temperature will half the life of a motor
• Motor operating temperature is based on:
• 40o C ambient temperature
• Class B Insulation limit is 130o C
• Class F Insulation limit is 155o C
Thermal Breakdown Causes:
• Overload
• Voltage Variations
• Single Phasing
• Short Cycle
• Lightning and Surges
• Improper Ventilation
• Dirt & Dust Accumulation
Mechanical Breakdown Causes:
• Bearing failure resulting in rotor or stator
rubbing insulation off
• Vibration
• Improper Alignment
Bearing Failure Causes
• Improper lubrication
• Excessive belt tension
• Mis-alignment
Environmental Breakdown Causes:
• Moisture / Water
• Corrosion/ Chemicals
• Rodents & Insects
• Age
Causes of Motor Failures
Overcurrent
30%
Mechanical
44%
Other
12%
44 % of all motors
failures are due to
overloads and
Single
Phasing
single-phasing.
14%
Why Motors Fail...
Bearing
Failures
13%
Rotor Failures
5%
Contaminants
19%
Misc
9%
Old Age
10%
Overload
30%
Single Phasing
14%
* Based on 9,000 failure events researched by the Electric Motor Manual which
was written by Robert Lawrie in conjunction with the staff at Electric
Construction and Maintenance Magazine.
Number One Motor Failure…
Overload!
Overload
30%
Overloads
“ It is practically impossible to burn out a motor
from overload when it is protected by properly
sized and maintained motor protection devices”.
William P. Maples, P.E.
Motor Protection
Device
% FLA
Overloads
Fuses (Dual Element)
Fuses (Fast Acting)
Circuit Breakers
MCP
115%
125%-225%
400%a
400%b
800%-1700%
NEC 430-32,52
A
300% FLA for non time delay fuses exceeding 601 amps
B
300% FLA for inverse time circuit breakers for motor FLA exceeding 100 amps
Motor Service Factor
Motor service factor of 1.15 means that the
temperature rise of a motor will not exceed
design temperature even if the motor is
carrying a continuous overload of 15%
U.L and IEC Test Procedures
• U.L. 508 Determines test procedures for
motor starters.
• U.L. Short Circuit test procedures
– Starter door cannot be blown open
– Contacts must not disintegrate
– Overload burnout is allowed with breaker tests.
Short Circuit Protection of Motor Controllers
Motor Controller
HP Rating
Test Short Circuit
Current Available
1 HP or less and 300V or less
1,000 A
50 HP or less
5,000 A
Greater than 50 HP to 200 HP
10,000 A
201 HP to 400 HP
18,000 A
IEC Standards
• IEC Publication 947 “Low Voltage
Switchgear and Control”
IEC Levels of Motor Starter Protection
• Type 1 Protection.
– No discharge of parts beyond the enclosure is
allowed
– Damage to the contactor and overload relay is
acceptable
IEC Levels of Motor Starter Protection
• Type 2 Protection.
– No damage is allowed to either the contactor or
overload relay
– Light contact welding is allowed, but must be
easily separable
Voltage
• Voltage Variations
• Phase imbalance
• Phase loss (Single Phasing)
Voltage Variation
Nema Standards states that motors shall operate successfully
at rated load under the following conditions:
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+/- 10% variation of rated voltage at rated frequency
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Any voltage drop below 10% will reduce starting torque which
can be serious because starting times may increase beyond
allowable limits.
Winding failure from extreme voltage variations are identical to
those of overloads because the input current is uniformily
excessive.
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Low Voltage… Cause
• Demand for power is greater than the
supply
• Long cable runs
• Overloaded transformers
• Distribution system
Low Voltage…
Effect
• Reduced efficiency
• Reduced life expectancy
• Increased temperatures
• Increased current
• Lower torque
High Voltage…
Definition
Operating a motor above 110%
of nameplate voltage
High Voltage…
Cause
• Poor line regulation
• Incorrect transformer tap
• Utility demand down
High Voltage… Effect
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Reduced efficiency
Reduced power factor
Increased power consumption
Increased current
Increased temperature
Voltage Unbalance
Percent voltage unbalance
= 100 x Maximum voltage deviation from average voltage
average voltage
Voltage Unbalance
example:
Assume voltages to a motor are 228 V to Phase A, 220V to
Phase B and 236 V to Phase C. The average voltage is
228 V. The voltage unbalance is as follows:
Voltage imbalance = 236-228 x 100 = 3.5%
228
Effect of
Voltage
Unbalance
on Motor
Longevity
Effects of Voltage Unbalance
Horsepower Derating
Another Rule of Thumb………….
A 3.5% voltage unbalance will cause a 25%
temperature rise.
An increase in temperature rise in the phase with the highest
current is 2 times the square of the voltage imbalance
Causes of Voltage Unbalance:
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Unequal loading of a three phase system
Unequal tap settings on transformers
Loose connections in the power system
Damaged starter contacts
Open delta transformer systems
Storms (wind, fallen trees, lightning)
Poor regulation by utility
Example of heat rise due to voltage
variations
50 HP Motor
Heat rise
Ambient Temperature
8% Unbalance Effect:
60oC
40oC
76oC
(8x8=64 x 2= 128% x 60oC= 76oC)
176oC
Result: Motor Failure
14% of All Motor Failures Single Phasing
Single Phasing
14%
Secondary Single Phasing
Secondary Single Phasing
Causes of Secondary Single Phasing
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Damaged motor starter contact
Burned open overload relay
Damaged switch or circuit breaker
Blown fuse
Open cable or bus on transformer secondary
Open motor winding
Open transformer winding
Primary Single Phasing
Primary Single Phasing
Causes of Primary Single Phasing
• Open line on primary
– Storm (wind, ice, fallen trees, lightning)
– Vehicle striking utility pole
– Construction mishaps
• Blown Primary fuse
• Open contact on utility contactor or re-closer
• Open winding on transformer
RMM Systems
In conclusion:
• Motors will last a long time if operated
within their design parameters.
( Voltage, current & temperature)
• Identify the cause of your motor failure
• When replacing motors also consider
changing overloads and motor start contacts
• For critical motors incorporate the latest
motor monitoring products.
Thank you!
From
RMM Systems