Class Notes - Year 4 - 2020-2021
Year 4

Lesson 410

Motor Starting Methods

IEC Manual 410

Problems with Full Voltage Starting

Full voltage works for smaller motors
Service would have to be extremely oversized to work with larger motors
Current drawn by motor causes voltage sag
This affects all loads on the service
Life of motor is decreased if voltage is less than nameplate
10HP tries to deliver 10HP, regardless of voltage
More amperage will increase to account for voltage decrease

Reduced Voltage Starting

Used to reduce starting current and torque
Reduced voltage starting will affect speed, but it is not used as a speed control method
Motor drive should be used

Hard Start vs Soft Start

Hard Starting

Because rotor is at rest, starting current can be 6 times FLC
0 RPM to full speed in minimal time
Could cause breakers and fuses to trip
Can cause voltage drop on feeder sand services
Could lead to:
Difficulty starting motor
Dimming lights
Other motors stalling
Malfunction of sensitive equipment
Thermal stress to motor caused by rapid rise in internal temp
Soft starting will not lower overall heat in motor, but ensure that it is gradual
Stress in mechanical couplings
Fire pumps utilize hard starting
Rarely run
Better to protect building at the detriment of pump

Soft Start

Power is applied more gradually
Initial surge current is reduced
Motor takes longer to reach full speed
Traditional methods:
Part winding
Series resistor or reactor
Wye-start, delta-run
Modern Methods
Programmable solid-state soft starters
Use silicon controlled rectifiers, thyristors, or other power semiconductors
Generally available for 3 phase or single phase, up to 600V and 700HP
Two primary methods:
Current limit
Limit maximum starting current to fixed, preset level
Current level is selectable
Setting of 300% FLA is typical
This is still somewhat abrupt
Current ramp
Much more gradual than limit method
User selects initial voltage and current so gradual rise to max selected value is achieved
Max is maintained until motor reaches full speed
Another method is adjustable speed drive
VFD is a specific type of ASD
Studied in future lesson
Soft starting reduces maintenance but will not necessarily save money on electric bill
Slowly starting motor still takes about the same amount of energy as quickly starting

EMC5 19-1 - Reduced-Voltage Starting

Power Source

Used to reduce large current drawn from utility lines
Current drawn at startup can be 2 to 6 times FLC
Electric utilities limit inrush current drawn from their lines to a max amount for a specified time period
Utility is not limiting total max current, but dividing amount of current into steps
This permits incremental start, allowing voltage regulators time to compensate
Increment current
Maximum current permitted by utility in any one step of an incremental start
Reduced voltage starting provides incremental draw over longer period of time

EMC5 19-2 - DC Motor Reduced-Starting

DC motors known for high torque and good speed control
DC motors often used where powered with batteries
Example: Forklift
DC motor drives used for starting DC motors with a reduced voltage
When motor is started, high inrush current is produced
A decrease in current results as a motor accelerates because motor generates voltage that is opposite to applied voltage
This is called counter electromotive force (CEMF)
Depends on speed of motor
CEMF is 0 at stand still
Increases with motor speed

EMC5 19-3 - Reduced Voltage Starting for 3 Phase Induction Motors

Majority of industrial applications use 3 phase induction motors
Reduced voltage typically not used for 1 phase because they are usually smaller

AC Motor Reduced-Voltage Starting

High inrush can cause voltage sags
Locked rotor current (LRC) is steady-state current taken from power line with rotor locked and voltage applied
Full load current (FLC) is current required to produce full load torque at rated speed

EMC5 18-5 - Open and Closed Circuit Transition

Motors started at reduced voltage must be switched to line voltage before reaching full speed
Should occur as close to full speed as possible
If not, surge current can still occur
Two methods:
Open circuit transition
Closed circuit transition

Open Circuit Transition

Motor is temporarily disconnected from voltage source when switching from a reduced starting voltage level to a running voltage level before reaching full speed
Not preferred because it causes high-current transient surge
High current surge based on motor speed at time of transition
Less expensive

Closed Circuit Transition

Motor remains connected to the voltage source when switching from a reduced voltage level to a running voltage level before reaching full speed
This is preferred because it does not cause high-current transient surge
More expensive

EMC5 19-4 - Primary Resistor Starting

Reduced starting method that uses resistor connected in each motor line (or in 1 line for single phase) to produce voltage drop
This reduces starting current as it passes through resistor
Timer is provided in control circuit to short the resistors after motor accelerated to specified point

Primary Resistor Starting Circuits

External resistance is added and taken away from motor circuit

EMC5 19-5 - Autotransformer Starting

Uses a tapped 3 phase autotransformer to provide reduced voltage starting
One of the most effective methods
Motor terminal voltage does not depend on load current
Motor current and line current are not equal
This avoids utility company limitations

Autotransformer Starting Circuits

Various windings of the transformer are added and taken away from motor circuit
Step by Step
Full Speed

EMC5 19-6 - Part-Winding Starting

Starting a motor by first applying power to part of the motor coil windings for starting and then applying power to the remaining coil windings for normal running
Motor windings must be divided into two or more equal parts
Usually wye connected motor is used but delta connected can also be used

Wye-Connected Motors

When control circuit is energized, one winding of motor is connected directly to line
Gives about 65% or LRC and 45% torque
After about 1 second, second winding is connected in parallel with first winding
Not true reduced voltage method
Usually classified as reduced voltage because of resulting reduced current and torque

Advantages and Disadvantages

Less expensive than most because it doe not require voltage reducing components
Transition is inherently closed
Poor starting torque

Part-Winding Starter Circuits


EMC5 19-7 - Wye-Delta Starting

Accomplishes reduced voltage starting by first connecting motor leads in wye for starting motor
When started as wye, 58% of normal voltage and 33% or normal torque
These motors are specifically wound with 6 leads
When wye-delta started is energized, 2 contactors close
One connects windings in wye configuration
Other connects motor to line voltage
After time delay, wye contactor opens, momentarily de-energizing motor, and 3rd contactor closes to reconnect windings in delta
Inherently open transition

Wye-Delta Motors

Each winding receives 208V if connected as delta
Each winding receives 120V if connected as wye

Wye-Delta Starting Circuits


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