motor rating and protection considerations

7/31/2019 Motor Rating and Protection Considerations

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MOTOR RATINGSAND PROTECTION

CONSIDERATIONS

Jose Titus(Engr-EM)


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Induction motor- Widely used inpower plant.

Motor characteristics and insulation

considerations.

NEMA standards and motor ratings.

Mechanical and electricalabnormalities.

Protection considerations

OVERVIEW


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Induction Motor

Constitute more than 95% of drives in thepowerplant as in any other industry.

Wide range of applications in varying power

requirements ( Fractional kW to 10 MW MDBFP)Rugged construction and lesser maintenanceenables use in harsh environments.

Almost constant speed characteristics in itsoperating range.


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Motor equivalent ckt.

Equivalent ckt shows the motor

parameters with mechanical loadseperated.Slip increases with increase intorqueAt pullout torque motor becomesunstableStarting torque much lower thanmax torqueStarting current very high - 5 to 6times rated.Starting torque improved andcurrent decreased by efficientstarting methods


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Motor Ratings

Selection of motor for a particular applicationrequires knowledge about the motor ratings.

Mechanical and electrical losses produce heatwithin the motor.

Deteriorates insulation and cause deformation,stress and fatigue on other components.

Motor should be so chosen that the ratingsguarantee a sufficient expected service life forthe motor.


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HP RatingInsulation is the component that defines the

rated output of the motor.

Insulation grouped into classes depending onthe maximum operating temperature for a

service life of 25-30 yrs.NEMA assigns HP ratings based on temperaturerise under load with an ambient temp of 40 degC.

Motor rated in this way will have a theoreticalservice life equal to that of the insulation

A general rule is that the insulation life getshalved for every 10 deg rise in temperature.


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HP Rating

Motor heating is not homogenous.Temperatures at the hot-spots must not exceedthe max values of the insulation used.

NEMA bases ratings on observable temp risemeasured by resistance method or usingembedded detectors.

Hot spot temperature is then estimated usingan empirical correction depending on themethod of measurement and type of motor.


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HP RatingRating Method Class A Class B Class F Class H

Maximuminsulationtemperature

RTD 105 130 155 180

Allowable risefrom ambient

RTD 65 90 115 140

Upto 1500 HP RTD 65 90 115 140Over 1500 HP,Below 7 kV

RTD 65 85 110 135

Over 1500 HP,Above 7 kV

RTD 60 80 105 125

All ratings Resistance 60 80 105 125

Margin is allowed at higher ratings and higher

voltages to account for hotspots.


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Effect of abnormalities

Operation below rated voltage causes highercurrents at higher slips causing increasedheat.

Operation above rated voltage reducesrunning current but increases core losses andmay also damage insulation.

Operation below rated Hz reduces core losses

but also reduces speed and hence cooling airflow and may possibly result in higher temp.

Operation above rated Hz again increases

core losses and can cause overheating


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Voltage and Hz variationsNEMA standards for the thermal limits in thetable are applicable under the following:

10% voltage variation from rated with ratedHz.

5% Hz variation with voltage at rated value.

A combined variation of V and f such that thesum of their absolute values does not exceed

10%, is also permitted if the frequencyvariation does not exceed 5%.

These criteria set the electrical boundaries for

normal operation


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Motor current

Starting current remains near locked rotor valueuntil speed reaches near 80-90%.

The starting current can be determined from theNEMA locked rotor code stamped on the

nameplate. This denotes KVA/HP of the motor.(See handout).

Thus a 415V, 500 HP motor with code F will havea max starting current

I=5.6*500(1.732*0.415)=3895.5 amps.


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Voltage dip at startingStarting of large motors or a weak system can lead

to dip in voltage and a system collapse.The inverse relation between V and I leads tonegative impacts on systems with large number ofconnected motor loads.

When a large motor is started from a bus havinglarge number of running motor loads, the decreasein V due to starting is magnified by the increased I

in other motors. The resulting dip may causetripping of all running motors.(Uv protn at Simhadriset at 80%).

Hence, advisable to always start large motors from

section having lesser running motor loads.


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Protection considerationsNature of load- Fans, pumps, time-variant loads like

crushers etc.

Same HP requirements can have different torqueand inertial specifications.

Proper operation requires proper load compatibility.

An improper load compatibility may appear asprotective device problems.

Manufacturers provide short term capability in theform of thermal limit curves.

Protection requires a graphical co-ordinationbetween thermal limit curve, starting current curveand the relay characteristics.


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Protection considerationsFig shows the desired

characteristics of an overcurrentrelay.Protection becomes difficult as thestarting time increasesAnother difficulty is that such a

graphic coordination is not valid fortime varying starting currents.

Advances in numerical relays- much easier to have aproper co-ordination.

Mill motors have a time varying starting current curve.Compare the protection settings of 3.3 kV mill motors ofstage -1 electromechanical relays with those in ABBnumerical relays of the same motors of stage-2. (See

the handout)


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Overcurrent protection

Prevents thermal damage- Overloads, stalling, operatorerror, repeated starts, unbalance current.

Insulation failure, mechanical damage to rotor bars and

end rings due to expansion.Proper setting- Thermal limit curve should be known.(IEEE 620 standards- see handout)

Safe times for 3 conditions- Running overloads, Locked

rotor and acceleration heatings.


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Running overload- Small to moderate overloads withmotor at rated speed.

Normal airflow allows operation for several minuteswithout significant temperature rise.

Stator insulation limits operation in this region.

Locked Rotor- Most severe overloading

Current- 6 times rated; Rotor resistance 2.5-3 times

rated due to skin effect; No cooling airflow.

Rotor losses increase to near 100 times normal andstator losses to about 36 times normal.


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Overcurrent protectionMechanical damage to endrings can occur in 10-20

seconds.

Rotor heating limits operation in this portion of thethermal limit curve

Acceleration heating-To be considered only whenstarting time is more than the safe locked rotor timeof the motor.

Rotor heating defines the limits of operation in this

area.TLC of 2800 KW, 11kV PA motors supplied bymanufacturer for stage 2 with 3 regions markedclearly.(See handout)


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Induction disk elements and bimetallic thermalelements commonly used.

Bimetallic- Thermal memory.

Protection attempts to match IR chara of therelay to that of the motor limit curve.

IEEE Standard C37.96-2000 - Min margin b/wthe starting current and the relay chara of 2 sec

for 5 to 10 sec starts and 5 sec for the 40 to 50sec.


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Numerical relaysAll protn funtions included in the same relay.

Mathematical based- Hence setting is easier.

New sophisticated thermal protectionalgorithms.

Algorithm used in stage-II 3.3kV mill motorsprotection using REM relay. (See handout)

Seperate cooling and heating time constants,

integrated NPS protection, real time tempsensing using RTDs.

Basic thermal model- single time constant.


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Phase fault protection

Normally uses instantaneous overcurrentelements set above the starting inrush.

In cases where large motors are connected to

weak systems, LR current can approach thefault current.

In such cases differential relays may be used toprovide protection.

Stage-II MDBFP motor uses instantaneousovercurrent protection in conjunction withdifferential relays


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THANK YOU

motor rating and protection considerations

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