Industrial Motor Testing, Measurements, Failures, etc.

Motor testing

Advances in a number of technological fields have contributed greatly to the development of testing equipment. This has made the maintenance of electric motors a more precise science. The task of keeping all 'systems on the go' is now the responsibility of a 'cut to the bone' staff of technicians and engineers.

Despite all the hi-technology, the proper maintenance of equipment like motors is dependent on training, motivation, and awareness of the man behind the machine. The following tools and equipment are now available for monitoring, measuring, and metering the various parameters of the motors in a motor performance/operation.

Different tests provide various data, related to the motor, which give an idea about the probable performance of the motor and its efficiency.

Methods of testing DC machines

DC motors are tested by using the following methods:

1. Direct loading or brake test:

This test is meant for calculating the efficiency of a motor and is a direct loading test. It’s used for small motors only. The motor is loaded directly by a mechanical rope or a belt brake.

A typical arrangement for the direct loading of the motor for a brake test.

In this method, a force is applied by adjusting the brake and is measured in Newtons using an attached spring balance.

++++24 Test arrangement of direct loading of motor for brake test -- Spring balance.

Torque is then calculated as, T Fr =× Where:

T = Torque in Newton meters; F = Force applied by brake; r = Radius from center of motor shaft to point of force application.

A voltmeter and an ammeter are connected for measuring the supply voltage and the current taken by the motor.

A load is applied gradually to the motor and the corresponding current, voltage, torque, and speed-reading are noted. In the case of testing a series motor, it’s better to take the readings from a high load to gradually reducing load.

As the whole output is wasted in heat, it’s used only for small capacity motors.

A measurement of force, exerted by the arm and the radius or length of the arm, can give the torque. If R is the radius of the pulley, and W and w are weights on either side of the brake arrangement, then Net torque = ( ) kgm = 9.81 ( ) N.m.

Output = 2 p n T/60 W

The input to the motor is measured. If V is the measured applied voltage and I the input current, then

Input = V × I W

The efficiency of the motor:

Efficiency = Output /Input

A dynamometer arrangement can be used alternatively to measure the pull and the torque on the motor under braking conditions.

2. Swinburne test:

This test is for shunt and compound type of DC machines and gives the efficiency of the motor on any load, from the data calculated on 'no load' conditions.

This is an indirect method of testing the efficiency of a DC shunt motor, by measuring losses, like iron loss and mechanical losses. In addition, it’s assumed that these losses remain constant during all load conditions.

The DC motor is run on 'no load'. The voltage and speed are adjusted to the rated values. A connection diagram for the Swinburne test.

The armature resistance (Ra) is measured with an ammeter/voltmeter method, including all of the series and interpole winding. Similarly, the shunt field resistance (Rf) is also measured.

The motor is run at a rated speed and at a rated voltage without any load.

++++25 --- Test arrangement for Swinburne test

At no load: Armature current = Io Input to the armature = V Io Since motor is at no load, this is the power required to overcome losses.

So, power loss

oo PVI =×

Field copper losses is constant at all loads and is given by:

The efficiency of the DC motor is determined as:

Where PL = Total losses at any armature current.

The following are the advantages of the Swinburne test:

• The efficiency of the machine can be determined without any direct loading.

Energy is saved during the testing. This is quite useful, particularly for large machines.

• The efficiency can be determined at any load.

The following are the disadvantages of the Swinburne test:

• Only shunt and compound machines can be tested by this method. The series machines cannot be tested as they cannot run on no load.

• The effect of commutation and armature reaction is not considered or tested in this method. The errors due to assumption of constant iron losses at all the loads are insignificant.

3. Hopkinson test on shunt motors:

The direct loading method for large machines involves huge power losses. To avoid this, the regenerative method of testing is used. This test can be performed on two identical shunt machines, mechanically coupled to each other. One machine is run as the motor and the other as a generator. A typical arrangement for the Hopkinson test on two similar shunt machines.

One machine starts as a motor taking supply from the mains. The field current is adjusted to run the machine at the rated speed. The second machine is mechanically coupled and is run at the same speed. The excitation of the machine is so adjusted, that the voltage across the armature is slightly higher than the supply voltage. This is checked with a Voltmeter V. The polarities of the machines should be suitable for a parallel operation. When the voltmeter reads zero or a slightly higher 1 or 2 V indicating that it would be generator action, the switch is closed so that both the machines are in parallel across the supply. As both the machines are similar and equal in size and rating, the constant losses - friction, windage, and iron - are assumed to be equal.

++++26 Test arrangement for the Hopkinson test on two similar shunt machines.

The efficiency of the shunt machine acting as a motor and as a generator can be determined as follows:

For motor:

Where Pm = Constant loss.

Total losses = armature copper loss + Field copper loss + Constant loss Output = Input - Losses E

4. Retardation test:

If a motor is brought up to the speed and then switched off, it will slow down typically.

++++27 Retardation test on a motor

The angular retardation dowdy at any instant is directly proportional to the rotating torque and it varies inversely as the moment of inertia (J) of the motor. The power consumed in overcoming the losses due to rotation is given as follows:

The test can be carried out on a separately excited motor or shunt motor. The friction losses and iron or core losses can be determined by this test.

5. Field's series test:

This test is used for obtaining the efficiency of two similar motors. This test avoids the difficulty of obtaining readings on light loads, by using the motor current to excite the field as a generator. The generator armature is connected to the load resistance.

The connection diagram for Field's test. As both the machines have the same excitation, iron losses in both machines are considered equal.

++++28 Field's efficiency test on two series motors

The total losses in each machine is given as:

Motor output = Motor input - Losses

Motor efficiency = Output/Input

Measurements used for a motor

The following are the measurements used for a motor:

• Temperature: Thermocouples/Resistance elements/thermistors measure temperatures of windings, bearings, etc.

• Voltage and current: Volts and Amps are measured using portable voltmeter, recording voltmeter, Ammeters - clamp type, recording, CRO, etc.

• Insulation resistance: Meggers - hand/motor-operated.

• Winding resistance: Kelvin bridge, wheatstone bridge, resistance meters, etc.

• Vibration: Vibration metering, monitoring, and analyzing equipment.

• Speed: Stroboscopes, tachometer, etc.

• Dielectric loss angle measurement: Tan-delta measurement.

Motor failures and methods to extend its life

The trouble free operation of induction motors over the greater part of their service life, which in many instances, exceeds that of driven equipment, requires little more than regular and routine maintenance chores. Regular cleaning, correct lubrication, and proper maintenance, is all that is required to ensure a consistently high level of performance from a motor, that is correctly selected and properly installed. In essence, the useful service life of a motor is largely a function of the quality of maintenance. Maintenance is all the more important, in the context of the present-day motors as they are precisely designed to exact ratings and optimized parameters. Hence, any lapse in the proper maintenance of the motors is likely to affect the performance. It has been established through field experience that the majority of the failures occur because of the following:

• Insulation failures
• Rotor-bar failures
• Mechanical problems

The maintenance program for a motor is given as follows:

1. Periodic inspection of motor. Accurate shaft alignment. For directly coupled motors, shaft alignment between load and motor shaft should be proper. In case of belt-type system, check for belt condition, belt tension.

2. Check motor heating. If motor heats up quickly, check and clean air filters.

Therefore, the airflow will be adequate.

3. Keep motor clean and free from dirt and oil.

4. Check for dampness around the motor or inside the motor. This can reduce insulation strength of motor winding. As far as possible, keep motor dry internally as well as externally. Also, run motor for few hours if not in use for a long time so that moisture dries.

5. Check bearing condition on a regular basis. Bearing should be lubricated with prescribed lubricant. At the same time, keep in mind that lubrication should be always done in proper quantity. Excess as well as lesser quantity can do harm.

6. Check for any abnormal noise or excess vibrations from motor or coupling.

Do vibration analysis if necessary.

If the above guidelines are followed, the motor will remain problem free.

Motors driven by variable speed drives have specific requirements for correct performance such as special cooling, bearing insulation, use of terminal filters to absorb high voltage pulses, etc. In the case of retrofitted drives, the manufacturer needs to be consulted and any additional measures recommended need to be incorporated to avoid failures of motor.

Motor control trouble-remedy table

Trouble:

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Contacts chattering

Welding or freezing

Short tip life tip overheating

Coils open circuit; Overheated coil

Overload relay; Tripping; Trip failure; Magnetic and mechanical parts noisy magnet; Pickup and sealing failure; Failure to drop out; Pneumatic timers erratic timing; Failure of contact operation; Limit switches damaged parts; Manual starters failure to reset

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Cause:

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1. Poor contact in control circuit

2. Low voltage

1. Current inrush abnormal

2. Tip pressure low

3. Low voltage

4. Ingressed foreign matter preventing contact closing

5. Short-circuit/ground fault

1. Filing or dressing

2. High current interruption

3. Low tip pressure

4. Foreign matter ingress

5. Short-circuits/ground fault

6. Loose power circuit connection

7. Persistent overload

1. Mechanical damage.

1. Overvoltage or high ambient temperature

2. Coil unsuitable

3. Shorted turns due to mechanical damage

4. Undervoltage/magnet seals in failure

5. Pole faces dirty

6. Obstruction to moving elements

1. Persistent overload

2. Corrosion or loosening

3. Unsuitable thermal units

4. High coil voltage

1. Thermal units not suitable

2. Mechanical bindings, dirt, corrosion, etc.

3. Damaged relay

4. Relay contact welded

1. Shading coil broken

2. Magnet faces dirty/rusty

3. Low voltage

1. Loss of control voltage

2. Low voltage

3. Moving part obstruction

4. Open or overheated coil

5. Coil unsuitable

1. Sticky substance on pole face

2. Voltage persistence

3. Worn or corroded parts failing to separate

4. Residual magnetism caused by lack of air gap in magnet path.

5. Welding of contacts.

1. Ingress of foreign matter in valve

1. Actuating screw not correctly adjusted

2. Worn/broken parts in snap switch

1. Actuator overtravel

1. Latching mechanism damaged

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Remedy:

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1. Replace the device

2. Check coil terminal voltage/general voltage fluctuation/voltage dips during starting

1. Check shorts/grounds. Check motor load current. Use higher size contactor

2. Replace contacts/springs. Contact carrier may be damaged

3. Check coil terminal voltage and voltage dips during starting

4. Clean contacts with Freon

5. Remove fault. Ensure correctly rated fuse/circuit breaker is used

1. Ensure silver tips are not filed. Rough spots or discoloration don’t harm tips or cause malfunction

2. Replace with higher size device.

Check current levels/faults

3. Replace contacts/springs. Contact carrier may be damaged

4. Clean contacts with Freon. Check enclosure for ambient condition suitability

5. Remove fault. Check correctly rated fuse/circuit breaker is used

6. Tighten

7. Check motor load current. Install larger device

1. Handle/store coils with care

1. Check terminal voltage less than 110% of rated voltage

2. Replace with correct coil

3. Replace coil

4. Check coil terminal voltage. This should be at least 85% of rated voltage

5. Clean pole faces

6. Check free movement of contact and armature assembly

1. Check excessive motor currents, current unbalance. Take corrective action

2. Clean/tighten

3. Replaced with correct size for the application and conditions

4. Check coil voltage is within 110% of rated capacity

1. Apply proper thermal units

2. Clean/remove particles/obstruction, etc. to restore to proper functioning condition. Replace relay/thermal unit if not possible

3. Replace relay and thermal units

4. Replace contact or entire relay as necessary

1. Replace magnet and armature assembly

2. Clean

3. Check coil terminal voltage/voltage fluctuation/motor starting voltage dips

1. Check control circuit. For loose connection/poor contact continuity

2. Check coil terminal voltage/voltage fluctuation/ motor starting voltage dips

3. With power off, check contact and armature assembly movement

4. Replace

5. Replace

1. Clean

2. Check coil terminal voltage/control circuit

3. Replace defective parts

4. Replace magnet and armature

5. See 'Contacts - Welding or freezing'

1. Replace complete timing head.

Return timer to factory for repair and adjustment

1. Adjust as per manual service instructions

2. Replace switch

1. Use resilient actuator. Operate within device tolerance limits

1. Replace starter

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Motor starter check chart

[Trouble:]

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Contactor/Relay closing failure

Contactor or relay fails to open

Contact corrosion/welding

Arc lingers across contacts

Noisy AC magnet I Frequent coil failure

Burning of panel/equipment due to starting resistor heat

[Cause:]

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Supply voltage failure; Low voltage; Open-circuited coil Pushbutton, interlocks, or relay contact not making; Loose connections or broken wire; Incorrect pushbutton connection; Open o/l relay contact Mechanical parts damaged, corroded, not properly aligned/assembled, etc.

Incorrectly connected pushbutton; Worn shim in magnetic circuit; Residual magnetism holds armature closed; Pushbutton, interlock, or relay contact fails to open coil circuit; 'Sneak' circuits; Welding of contacts

Mechanical part malfunction due to damage corrosion, etc.

Contact spring pressure not adequate. Overheating or arcing on closing Reduction of effective contact surface area due to pitting, etc. Abnormal operating conditions

Chattering of contacts due to external vibrations

Sluggish operation; Blow out problem; Series blow out may be short circuited Shunt blow out may be open Ineffective blowout coil

Note travel of contacts, in case blowout is not used; Arc box might be left off or not in correct position if blow out is used; Overload

Improper assembly

Broken shading coil Low voltage

High voltage

Gap in magnetic circuit.

Ambient temperature may be high Frequent starting U

[Corrective Action ]

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Check fuses/disconnect switch.

Check power supply. Ensure correct size of wire Replace Adjust to ensure correct movement, easy operation, and correct contact pressure Check circuit. Isolate circuit first Check with wiring diagram

Reset relay

Clean/align and adjust for proper operation

Check connection with wiring diagram and rectify Replace shim

Make adjustment for correct movement, ease of operation, and proper opening Check for insulation failure See 'Excessive corrosion of contacts' Clean mechanical parts. Check for free movement Remove obstruction/ingressed matter. Repair or replace worn or damaged parts Adjust for correct contact pressure. Replace spring or worn contacts if necessary Dress up contacts with fine file.

Replace if badly worn-- Check rating and load. In case of severe operating condition replace open contactors with oil-immersed or dust-tight equipment Instruct operator in proper control of manually operated device--Check control switch contact pressure. Replace spring if it fails to give rated pressure Tighten all connections. If problem persists mount/ move control, so that vibrations are decreased Clean and adjust mechanically.

Align bearings. Check free movement-- Check blow out type with wiring diagram. Check blow out circuit --Check rating. Replace in case of improper application.

Check polarity and reverse coil if necessary

Increasing travel of contacts increases rupturing capacity

Ensure that arc box is fully in place

Check rating against load

Clean pole faces. Adjust mechanical parts; Replace

Check power supply. Check wire size

Check supply voltage against controller rating Check travel of armature.

Adjust magnetic circuit.

Clean pole faces Check controller rating against ambient temperature.

Replace coil with correctly rated coil for ambient, from manufacturer

Use higher-capacity resistor

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