Motor Analysis (2) - Finish

2. Motor Analysis Test

2.1 Electrical Surge Comparison
In addition to ground wall insulation resistance, one of the primary concerns related to motor condition is winding insulation. Surge comparison testing can be used to identify turn-to-turn and phase-to-phase insulation deterioration, as well as a reversal or open circuit in the connection of one or more coils or coil groups.

Recent advances in the portability of test devices now allow this test technique to be used in troubleshooting and predictive maintenance. Because of differences in insulation thickness, motor winding insulation tends to be more susceptible to failure from the inherent stresses existing within the motor environment than ground wall insulation. Surge comparison testing identifies insulation deterioration by applying a high frequency transient surge to equal parts of a winding and comparing the resulting voltage waveforms. Differences seen in the resulting waveforms are indicative insulation or coil deterioration. A properly trained test technician can use these differences to properly diagnose the type and severity of the fault. In addition to utilization of this motor analysis technique in a predictive maintenance program, it can also be used to identify improper motor repair practices or improper operating conditions (speeds, temperature, load).
Surge comparison testing is a moderately complex and expensive predictive maintenance technique. As with most predictive maintenance techniques, the greatest saving opportunities do not come directly from preventing a catastrophic failure of a component (i.e., motor) but rather the less tangible cost saving benefits. Reduced downtimes, ability to schedule maintenance, increased production, decreased overtime, and decreased inventory cost are just a few of the advantages of being able to predict an upcoming motor failure.

2.2 Motor Current Signature Analysis
Another useful tool in the motor predictive maintenance arsenal is motor current signature analysis (MCSA). MCSA provides a non-intrusive method for detecting mechanical and electrical problems in motor-driven rotating equipment. The technology is based on the principle that a conventional electric motor driving a mechanical load acts as a transducer. The motor (acting as a transducer) senses mechanical load variations and converts them into electric current variations that are transmitted along the motor power cables. These current signatures are reflective of a machine’s condition and closely resemble signatures produced using vibration monitoring. These current signals are recorded and processed by software to produce a visual representation of the existing frequencies against current amplitude. Analysis of these variations can provide an indication of machine condition, which may be trended over time to provide an early warning of machine deterioration or process alteration.
Motor current signature analysis is one of the moderately complex and expensive predictive techniques. The complexity stems in large part from the relatively subjective nature of interpreting the spectra, and the limited number of industry-wide historical or comparative spectra available for specific applications.

3. System Applications

• Stem packing degradation
• Incorrect torque switch settings
• Degraded stem or gear case lubrication
• Worn gear tooth wear
• Restricted valve stem travel
• Obstructions in the valve seat area
• Disengagement of the motor pinion gear
• Improper seal/packing installation
• Improper bearing or gear installation
• Inaccurate shaft alignment or rotor balancing
• Insulation deterioration
• Turn-to-turn shorting
• Phase-to-phase shorting
• Short circuits
• Reversed or open coils.

4. Equipment Cost/Payback

As indicated earlier, motor analysis equipment is still costly and generally requires a high degree
of training and experience to properly diagnosis equipment problems. A facility with a large number of motors critical to process throughput may find that ownership of this technology and adequately trained personnel more than pays for itself in reduced downtime, overtime cost, and motor inventory needs. Smaller facilities may find utilization of one of the many contracted service providers valuable in defining and maintaining the health of the motors within their facility. As with most predictive maintenance contract services, cost will range from $600 to $1,200 per day for on-site support. Finding a single motor problem whose failure would result in facility downtime can quickly offset the cost of these services.

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Motor Analysis (1)

1. Introduction

When it comes to motor condition analysis, infrared (IR) and vibration will not provide all the answers required to properly characterize motor condition. Over the past several years, motor condition analysis techniques have evolved from simple meggering and hi-pot testing into testing techniques that more accurately define a motor’s condition.

Motor faults or conditions like winding short-circuits, open coils, improper torque settings, as well as many mechanically related problems can be diagnosed using motor analysis techniques. Use of these predictive maintenance techniques and technologies to evaluate winding insulation and motor condition has not grown as rapidly as other predictive techniques. Motor analysis equipment remains fairly expensive and proper analysis requires a high degree of skill and knowledge. Recent advances in equipment portability and an increase in the number of vendors providing contracted testing services continue to advance predictive motor analysis techniques. Currently, more than 20 different types of motor tests exist, depending on how the individual tests are defined and grouped. The section below provides an overview of two commonly used tests.

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Motor Trouble-Shooting (11) - FINISHED

Problem:
Run capacitor fail.


Like Causes:
1. Ambient temperature too high.
2. Possible power surge to motor, caused by lightning strike or other high transient voltage.

What To Do:
1. Verify that ambient does not exceed motor’s nameplate value.
2. If a common problem, install surge protector.


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Motor Trouble-Shooting (10)

Problem:
Start capacitors continuously fail.


Like Causes:
1. The motor is not coming up to speed quickly enough.
2. The motor is being cycled too frequently.
3. Voltage to motor is too low.
4. Starting switch may be defective, preventing the motor from coming out of start winding.

What To Do:
1. Motor may not be sized properly. Verify how long the motor takes to come up to speed. Most single phase capacitor start motors should come up to speed within three seconds. Otherwise the
capacitors may fail.
2. Verify duty cycle. Capacitor manufacturers recommend no more than 20, three-second starts per hour. Install capacitor with higher voltage rating, or add bleed resistor to the capacitor.
3. Verify that voltage to the motor is within 10% of the nameplate value. If the motor is rated 208-230V, the deviation must be calculated from 230V.
4. Replace switch.

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Motor Trouble-Shooting (9)

Problem:
The motor, at start up, makes a loud rubbing or grinding noise.


Like Causes:
Rotor may be striking stator.

What To Do:
Ensure that motor was not damaged in shipment. Frame damage may not be repairable. If you cannot see physical damage, inspect the motor’s rotor and stator for strike marks. If signs of rubbing are present, the motor should be replaced. Sometimes simply disassembling and reassembling motor eliminates rubbing. Endbells are also sometimes knocked out of alignment during transportation.

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Motor Trouble-Shooting (8)

Problem:
Bearings continuously fail.


Like Causes:
1. Load to motor may be excessive or unbalanced.
2. High ambient temperature.

What To Do:
1. Besides checking load, also inspect drive belt tension to ensure it’s not too tight may be too high. An unbalanced load will also cause the bearings to fail.
2. If the motor is used in a high ambient, a different type of bearing grease may be required.You may need to consult the factory or a bearing distributor.

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Motor Trouble-Shooting (7)

Problem:
Motor vibrates.


Like Causes:
1. Motor misaligned to load.
2. Load out of balance (Direct drive application.)
3. Motor bearings defective.
4. Rotor out of balance.
5. Motor may have too much endplay.
6. Winding may be defective.

What To Do:
1. Realign load.
2. Remove motor from load and inspect motor by itself. Verify that motor shaft is not bent. Rule of thumb is .001" runout per every inch of shaft length.
3. Test motor by itself. If bearings are bad, you will hear noise or feel roughness. Replace bearings. Add oil if a sleeve of bearing. Add grease if bearings have grease fittings.
4. Inspect motor by itself with no load attached. If it feels rough and vibrates but the bearings are good, it may be that the rotor was improperly balanced at the factory. Rotor must be replaced or rebalanced.
5. With the motor disconnected from power turned shaft. It should move but with some resistance. If the shaft moves in and out too freely, this may indicate a preload problem and the bearings may need additional shimming.
6. Test winding for shorted or open circuits. The amps may also be high. Replace motor or have stator rewound.

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Motor Trouble-Shooting (6)

Problem:
Motor overload protector continually trips.

Like Causes:
1. Load too high.
2. Ambient temperature too high.
3. Protector may be defective.
4. Winding shorted or grounded.

What To Do:
1. Verify that the load is not jammed. If motor is a replacement, verify that the rating is the same as the old motor. If previous motor was a special design, a stock motor may not be able to duplicate the performance. Remove the load from the motor and inspect the amp draw of the motor unloaded. It should be less than the full load rating stamped on the nameplate.
2. Verify that the motor is getting enough air for proper cooling. Most motors are designed to run in an ambient temperature of less than 40°C. (Note: A properly operating motor may be hot to the touch.)
3. Replace the motor’s protector with a new one of the same rating.
4. Inspect stator for defects, or loose or cut wires that may cause it to go to ground.

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Motor Trouble-Shooting (5)

Problem:
Motor runs in the wrong direction.


Like Causes:
Incorrect wiring.

What To Do:
Rewire motor according to wiring schematic provided.


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Motor Trouble-Shooting (4)

Problem:
Motor takes too long to accelerate.


Like Causes:
1. Defective capacitor
2. Faulty stationary switch.
3. Bad bearings.
4. Voltage too low.

What To Do:
1. Test capacitor per previous instructions.
2. Inspect switch contacts and connections. Verify that switch reeds have some spring in them.
3. Noisy or rough feeling bearings should be replaced.
4. Make sure that the voltage is within 10% of the motor’s nameplate rating. If not, contact power company or check if some other equipment is taking power away from the motor.

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Motor Trouble-Shooting (3)

Problem:
Motor runs but dies down.


Like Causes:
1. Voltage drop.
2. Load increased.

What To Do:
1. If voltage is less than 10% of the motor’s rating contact power company or check if some other equipment is taking power away from the motor.
2. Verify the load has not changed. Verify equipment hasn’t got tighter. If fan application verify the air flow hasn’t changed.

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Motor Trouble-Shooting (2)

Problem:
Motor has been running, then fails to start.


Like Causes:
1. Fuse or circuit breaker tripped.
2. Stator is shorted or went to ground. Motor will make a humming noise and the circuit breaker or fuse will trip
3. Motor overloaded or load jammed.
4. Capacitor (on single phase motor) may have failed.
5. Starting switch has failed.

What To Do:
1. Replace fuse or reset the breaker.
2. Disassemble motor and inspect windings and internal connections. A blown stator will show a burn mark. Motor must be replaced or the stator rewound.
3. Inspect to see that the load is free. Verify amp draw of motor versus nameplate rating.
4. First discharge capacitor. To check capacitor, set volt-ohm meter to RX100 scale and touch its probes to capacitor terminals. If capacitor is OK, needle will jump to zero ohms, and drift back to high. Steady zero ohms indicates a short circuit; steady high ohms indicates an open circuit.
5. Disassemble motor and inspect both the centrifugal and stationary switches. The weights of the centrifugal switch should move in and out freely. Make sure that the switch is not loose on the shaft. Inspect contacts and connections on the stationary switch. Replace switch if the contacts are burned or pitted.

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Motor Trouble-Shooting (1)

Caution:
1. Disconnect power to the motor before performing service or maintenance.
2. Discharge all capacitors before servicing motor.
3. Always keep hands and clothing away from moving parts.
4. Be sure required safety guards are in place before starting equipment.

Problem:
Motor fails to start upon initial installation.

Like Causes:
1. Motor is miswired.
2. Motor damaged and rotor is striking stator.
3. Fan guard bent and contacting fan.

What To Do:
1. Verify motor is wired correctly.
2. May be able to reassemble; otherwise, motor should be replaced.
3. Replace fan guard.
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Motor Diagnostics (3)

3. COUPLING (Direct & Belted)


The coupling between the motor and load also has faults due to wear and application:
•Direct drive misalignment
•Insert wear

The most accurate system for fault detection is vibration analysis then motor current signature analysis then infrared analysis.

4. LOAD (Fans, Pumps, Compressors, Gearboxes, Etc.)

The primary load faults are worn parts such as the seal, bearings and impellor.

Test instruments capable of testing load problems include:
•MCSA
•Vibration
•Infrared Analysis, and
•Ultrasonics

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Motor Diagnostics (2)

2. Electric Motor

The primary purpose of the electric motor is to convert electrical energy to mechanical torque. As a result, it has both electrical and mechanical components that must be considered.

Electric motors include mechanical and electrical components.

The first review is mechanical.
The primary mechanical problems include:

• Bearings
• Bad or worn shaft or bearing housings
• Vibration issues

Each of these can be detected using:

• Motor current signature analysis will detect the more severe problems.
• Vibration analysis will detect the faults earliest but requires a fair amount of experience.
• Infrared will detect problems when they are severe.
• Ultrasonics will detect the more severe problems

Electrical faults include:

• Winding shorts including turn to turn and coil to coil
• Insulation to ground faults
• Winding contamination
• Rotor faults including casting voids and broken rotor bars
• Air gap faults including an eccentric rotor

The winding tests to detect these problems include:

• MCA and MCSA will detect all of the faults
• Vibration will detect late-stage faults
• Insulation to ground will only detect ground faults
• Surge testing will only detect winding shorts in the first few turns of the winding
• All other testing will only detect late stage faults.

to be continued.....

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Motor Diagnostics (1)

Before start of motor diagnostics, we must know the scope of motor diagnostics itself.

The electric motor systems are consist of :
1. Controls
2. Motor
3. Coupling
4. Load

1. CONTROLS

Controls are consist of :

• MCCs (Motor Control Centers) and Disconnects (Starters, Overloads)
• Cabling

Common MCC/Disconnects problems :

• Loose connections
• Bad contacts
• Bad PF correction capacitors

Test methods that can be utilized to review the condition of these components include:

• Electrical Signature Analysis (ESA)
• Motor Circuit Analysis (MCA)
• Infrared Analysis
• Ultrasonics
• Volt/Amp Meter testing
• Ohm Meter testing
• Visual inspection

Common cable problems include:

• Thermal breakdown due to overloads or age.
• Contamination which can be even more serious in cables that pass underground through conduit.
• Phase shorts can occur as well as grounds. These can be caused by ‘treeing’ or physical damage.
• Opens due to physical damage or other causes.
• Physical damage is often a problem in combination with other cable problems.

Test instruments include motor circuit analysis, motor current signature analysis, infrared and insulation to ground testing.

Problems in the system before the electric motor can be broken down in the following order:

• Poor power factor
• Poor connections
• Undersized conductors
• Voltage unbalance
• Under or over voltage conditions

The most common equipment that covers all of these include motor circuit analysis, motor current signature analysis and infrared analysis.

to be continued...........

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