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RMM Systems, LLC By Sam Rietta RMM Systems, LLC is a manufacturer's representative firm focused on providing value added sales solutions by providing customer service, technical support and application assistance to our customers, design engineering firms and specification authorities. It is our goal to provide a high degree of professionalism to complement the image and objectives of the leading manufacturer's of electrical equipment we represent.
Have you ever lost a motor? 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 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 10 o C temperature rise above rated temperature will half the life of a motor Motor operating temperature is based on: 40 o C ambient temperature Class B Insulation limit is 130 o C Class F Insulation limit is 155 o 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 44 % of all motors failures are due to overloads and single-phasing.
Why Motors Fail... * 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. Rotor Failures 5% Failures Bearing 13% Contaminants 19% Overload 30% Misc 9% Single Phasing 14% Old Age 10%
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 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.
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: +/- 10% variation of rated voltage at rated frequency 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.
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 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
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: 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 60 o C Ambient Temperature 40 o C 8% Unbalance Effect: 76 o C (8x8=64 x 2= 128% x 60 o C= 76 o C) 176 o C Result: Motor Failure
14% of All Motor Failures - Single Phasing Single Phasing 14%
Causes of Secondary Single Phasing 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
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
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. RMM Systems