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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