RMM Systems, LLCBy 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 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

Mechanical44%

Overcurrent30%

Single Phasing

14%

Other12%

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 Failures5%Failures

Bearing

13%

Contaminants19%

Overload30%

Misc9%

Single Phasing14%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 ProtectionDevice % FLA

Overloads 115%Fuses (Dual Element) 125%-225%Fuses (Fast Acting) 400%a

Circuit Breakers 400%b

MCP 800%-1700%

NEC 430-32,52A 300% FLA for non time delay fuses exceeding 601 ampsB 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 ControllerHP Rating

Test Short CircuitCurrent 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:

• +/- 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

Effect of Voltage

Unbalance on Motor Longevity

Effects of Voltage UnbalanceHorsepower 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 60oC

Ambient Temperature 40oC

8% Unbalance Effect: 76oC(8x8=64 x 2= 128% x 60oC= 76oC)

176oC

Result: Motor Failure

14% of All Motor Failures - Single Phasing

Single Phasing14%

Secondary Single Phasing

Secondary Single Phasing

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

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

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.

RMMRMM Systems Systems

Thank you!

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