Motor Starting Analysis

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Motor Starting Analysis
Dan HarmsLa Plata Electric Association
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Overview

• Starting a motor
• Why run motor analyses?
• Available fault current and voltage flicker
• Using motor analysis in WindMil
• VFD and soft starting options
• Communicating results to customer

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Starting a Motor

• Motor must overcome its own rotor inertiaplus
  that of the load
• Locked rotor starting current may be 6 to 9 times
  normal
• running current

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System Voltage Drop
Current Flow
Voltage Drop

• Inrush of current causes additional voltage dip
  from the source to the motor

Source
115 kV 60 mi of 477 ACSR
0.2
9 A
Substation 10 MVA 6.5
0.3
12.47 kV 3 mi of 4/0 ACSR
83 A
1.9
Dist Xfmr 500 kVA, 3.5
2.0
2.1 kA
1.8
480 V 100 ft of 2-500 MCM
300 HP
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Why Run Motor Analysis?

• Customer perspective
• Will the motor start?
• Dropout voltage
• Typically 80 - 85 of (of what we consider)
  nominal
• Motor heating is proportionate to the square of
  the operating current
• You will likely hit a voltage flicker constraint
  before hitting a dropout voltage constraint

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Why Run Motor Analysis?

• Utility perspective
• Transformer sizing / impedance
• Voltage flicker

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Voltage Flicker
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Point of Common Coupling

• Distribution transformer secondary
• If other customers share distribution transformer
• Distribution transformer primary
• If distribution transformer is dedicated to motor
  load
• Substation transformer secondary
• If large motor has its own feeder
• Substation transformer primary
• If large motor has its own substation

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System Voltage Drop
Voltage Drop
Current Flow
Cumulative Voltage Drop
Source
115 kV 60 mi of 477 ACSR
9 A
0.2
0.2
Substation 10 MVA 6.5
0.3
0.5
12.47 kV 3 mi of 4/0 ACSR
83 A
1.9
2.4
Dist Xfmr 500 kVA, 3.5
2.0
4.4
2.1 kA
1.8
6.2
480 V 100 ft of 2-500 MCM
300 HP
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Available Fault Current and Voltage Flicker

• Maximum allowable inrush current approximately
  equals maximum allowable flicker times available
  fault current

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Motor Analysis in WindMil
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WindMil Examples

• Locked rotor motor start on a distribution system
• Custom data boxes
• Soft starting options
• Add capacitor for motor start assist
• Large motor analysis on looped transmission
  system
• Variable frequency drives
• Color by maximum allowable kVA

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Communicating Results

• Customer needs to be capable of interpreting the
  results you provide them
• Maximum starting kVA
• Best format to present results
• Independent of voltage
• Maximum starting amps
• Voltage needs to be specified
• Maximum horsepower
• May be the only thing the customer comprehends
• Many other variables could increase the amount of
  starting current above what you projected

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Follow Up

• Make a file of your studies
• Dont be afraid to spot check installations
• Keep an eye on harmonics

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QUESTIONS?

• Dan Harms
• La Plata Electric Association
• 970-382-3514
• dharms_at_lpea.coop