6d Motor Protection Lecture.325

7/28/2019 6d Motor Protection Lecture.325

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Motor Control

Starting Disconnect Means,

Controller, StartingMethod

Running

Motor Protection, WireProtection, SpeedControl

Stopping

Coasting, Braking

Motor Starters

The starting mechanism that energizes the

circuit to the motor

There are varying devices that can be used

but only two starting methods for motors. Across the Line

Reduced Voltage


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Across the Line Start

Connect motor

windings to linevoltage.

Simplest & lowest cost

way to start motor

High starting torque

and high starting

current.

Two Types

Manual Motor Starter Magnetic Motor Starter

Reduced Voltage Starting

Apply below line voltage

to motor windings during

starting period then

connect to line voltage

once started and up tospeed.

Reduced starting current

& reduced starting torque.

Reduce Mechanical

Starting Shock

More equipment and

higher cost


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Reduce Inrush Current

Increase Power Quality

Reduces the in-rush current anvoltage flicker on the electrical

system when starting the motor.

(good)

Reduces the motors starting

torque. (not so good)

Consideration for shutting

down the load/system must be

made to ensure it can bestarted the next time.

Reduce Mechanical Starting Shock

Applying full startingtorque immediately tends tojerk the system to speedquickly.

Breakable items can bejostled.

Drivelines and parts can bedamaged.

Reduced torque applied tothe system at starting resultsin a smoother soft start.

Conveyor belts

Complicated drive trains withmechanical connections.


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Motor Protection

Overcurrent Protection

Protect motor & circuit fromvery high short circuit

currents.

Overload Protection

Protect motor & circuit from

overload currents.

Other

Low/high voltage, phase failure,

phase reversal, ground fault,bearing temperature monitors,

winding temperature monitors,

surge suppression, etc.

Overcurrent Protection

Must size a fuse/breaker

large enough to start the

motor but protect from

dangerous overcurrents

caused by shorts and

ground faults.

Generally results in too

large a device to protect

from overloaded conditions.


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Sizing Motor Protection

Find the motors Design FLA in the NEC

1 Phase: Table 430-148

3 Phase: Table 430-150

NEC Requirements, Table 430-152

Standard Fuse 300% of FLA

Time-delay Fuse 175% of FLA

Instantaneous Trip Breaker 800% of FLA

Inverse Time Breaker 250% of FLA

Standard Fuse

Sizing to 300% of NEC FLAthen the next highest

standard size.

Exception allows the next size larger when the motor

will not start.

Will hold 500% of rating for approximately

second.


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Time Delay Fuse

Size to 175% of NEC FLA, select next standard higher size.

Hold 500% of their rating for up to 10 seconds.

Can be sized much lower than NEC requires and providebackup overload protection.

Inverse Time Breaker

Have both thermal and

magnetic features set to strip

at standardized levels.

Most common breaker used

in the industry.

Time to trip varies with amp

& volt rating of breaker.

Will start any motor with

Locked Rotor Current below

300% of its rating.


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Instantaneous Trip Breaker

Used where time-delay fuses or standard

circuit breakers will

not hold the starting

current of the motor.

NEC allows sizing up

to 800% of FLA.

Some have adjustable

settings.

Overload Protection

Protects the motor circuit &motor from overloadingconditions when the motor isrunning.

The larger the overload, the

more quickly thetemperature will increase toa point of damaging theinsulation on the motorwindings.

NEMA: motor survive a150% overload for 2 minuteswhen motor is at normaloperating temperature

0

100

200

300

400

500

600

FullLoad

Amps(%)

0 1 2 3 4 5 6 7 8 9 10 11 12

Time (Minutes)

Motor Heating Curve

Motor Damage Area

Allowable Operation Area


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Overload Types

Bimetallic & Melting AlloyOverloads

Heaters & or Heat cause acontactor to open

Solid State Overloads

Respond to current goingto motor, preprogrammedfor certain characteristicssimilar to standardoverloads

Electronic Overloads

Microprocessor Drivenmonitoring of current onall phases to motor.

Wide range of adjustmentand calibration as well ascommunication

1

10

100

1000

TripTime(Seconds)

0 200 400 600 800 1000 1200

Rated Current (%)

Heater Trip Characterist

Bimetallic & Melting Alloy

Advantages

Responds to total heat it sees

(combination of ambient &

temperature rise)

Simple, known technology

Low cost

Simple troubleshooting Disadvantages

Located at starter and not motor (may

be different temperatures

Trip Variability (slow/accuracy)

No additional protection other than

overloads (low/high voltage, single

phasing, phase unbalance, etc.)


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Solid State Overloads Advantages

Ambient insensitive

Wide FLA adjustment toindividual motors

No heaters

Wide temperature range

Disadvantages

Set points are still extreme

Generically protects while

trying to prevent nuisance

tripping. Many dont protect against single

phasing or unbalance

Cost

Electronic Overloads

Advantages

Set trip levels, trip classes and

time delays

Protection from all types of

voltage variations including

low/high, ground fault, single

phasing, phase reversal and

phase unbalance.

Communications with

controllers & PLCs

Disadvantages

Complexity

Cost


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Sizing Overload Devices

NEC rules allow

two methods:

Calculation

Based on Motors

FLA

Mnfg. Chart in

Starter Cover

140%140%Will Not

Start?

125%115%50 Deg C

125%115%40 Deg C

> 1.15up to 1.15

Service FactorTemperature

Rating

Example

FLA = 22, S.F. = 1.00, AMB = 40 Deg C

Whats the minimum overcurrent protection device size?

Whats the minimum overload protection device size?


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Step 1. NEC Full Load Amps

Find NEC Design Amps:

NEC Table 430-150

3 Phase, 10 Hp, 230 Volt

Amps = 28 amps

Step 2. Conductor Size

Whats the minimum conductor size?

NEC 430-22: Ampacity = 125% of FLA

125% X 28 amps = 35 amps

Need a conductor with an ampacity of 35 amps.

From NEC 310-16: #8 AWG Copper

NEC does this so #8 AWG copper will be largeenough for any 10 Hp motor in the future.


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Step 3. Overcurrent Device

NEC Table 430-152: Standard Fuse

28 amps X 300% = 84 amps, 90 amps

Time Delay Fuse

28 amps X 175% = 49 amps, 50 amps

Instantaneous Breaker

28 amps X 800% = 224 amps, 225 amps

Inverse Time Breaker 28 amps X 250% = 70 amps, 80 amps

Step 4. Overload Device

Use the Nameplate FLA:

For S.F. = 1.0, AMB = 40 deg C

22 amps X 1.15 = 25.3 amps

If this does not allow the motor to start,

increase to a maximum of:

22 amps X 140% = 30.8 amps


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Ambient Compensated Overloads

Standard overload

devices are designed formaximum ambient temp

of 104 deg F.

Above: Nuisance

Tripping

Below: No Protection

Ambient Compensated

Devices have a flattertemperature response and

should be used in

outdoor locations.

60

70

80

90

100

110

120

130

140

RatedCurrent(%)

20 40 60 80 100 120 140 160

Ambient Temperature (F)

Ambient Temperature Correction

Standard Rating, 40 C

Non-Compensated

Compensated

Other Protection Devices

Low/High Voltage

Protection

Phase Failure Protection

Phase Reversal Protection

Ground Fault Protection

Monitors

Bearing Temperature

Monitors

Winding Temperature

Monitors

Vibration Monitors

6d Motor Protection Lecture.325

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