Transcript MOTOR CONTROL STARTER

Motors & Motor Starters
Prepared By:
Erik Redd
&
Jeremy Roberts
Motors
AC-Motors
Parts of an Electric Motor
A. Stator : Stationary Frame
B. Rotor : Revolving Part
The rotary motion in an ac-motor is caused by the
fundamental law of magnetism.
This law states that like poles repel and unlike poles attract.
Diagram of an ac-motor
This shows a three
phase, two pole stator.
Where A, B, and C are
the three phases
Diagram of the Three Phases
Fig. 13-2 Pg. 244
Poles 1 and 4 are at their greatest magnetic field at
time equal to one, because phase A (red line) is
connected to those poles, and the same for the
other poles when their corresponding phases are at
maximum current magnitude.
Synchronous Speed
Speed at which it takes the motor to go one cycle and
one revolution.
S=[120*frequency}]
(# poles)
Example:
For a three-phase, 60 Hertz, 2 pole motor:
S=[120*60]/2=3600 revolutions per minute
Polyphase Squirrel-Cage
Induction Motors
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The most common three-phase
motor
Does not have solid poles
Instead, it has laminations:
numerous flat sheets held together
in a package. They are insulated
from each other (this reduces Eddy
currents) making up the stator
The difference between induction
and synchronous motors is that the
rotor for an induction motor can
travel at a different speed than the
stator. This is called Slip.
slip= Syn. rpm – Motor rpm *100
Syn. rpm
Example.
A 2 pole, 60 Hz motor runs at a full-load
speed of 1760 rpm.
What is the slip?
Ans. %slip= 3600-1760*100
3600
=51.1%
Single-Phase Motors
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Supplied by single source of ac voltage
 Rotor must be spun by hand in either direction,
does not have a starting mechanism
 Has no starting torque
 Three different types of single-phase motors: splitphase, capacitor start, permanent split-capacitor,
and shaded-pole motors
Resistance Split-Phase Motors
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Has a start winding and a main
winding
Winding currents are out of
phase by 30 degrees, this
produces a flux field that starts
the motor
Main winding current (IM) and
start winding current (IS) lags
supply voltage (VL)
Start (inrush) current is high
Needs centrifugal starting
switch or relay to disconnect
the start winding (protects it
from over heating)
Efficiency is between 50-60%
Capacitor-Start Motors
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Has the same winding and
switch mechanism arrangement
as split-phase but adds a short
time-rated capacitor in series
with the start winding
The time shift phase between
the main and start winding is
close to 90 degrees
IS leads VL
Efficiency is between 50-65%
Capacitor controls the inrush
current
Permanent Split-Capacitor
Motors
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Winding arrangement is the
same as the capacitor and splitphase motors
Capacitor can run continuously,
rated in microfarads for highvoltage ratings
No centrifugal switch is needed
IM lags VL, while IS leads VL
Efficiency is between 50-70%
Shaded Pole Motors
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Simple construction, least
expensive
Has a run winding only, shading
coils are used instead of the
start winding
Stator is made up of a salient
pole, one large coil per pole,
wound directly in a single large
slot
A small shift in the rotor causes
torque and starts the motor
Efficiency is between 20-40%
DC Motors
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Consists of an armature winding and a stator
winding
Armature windings act as the rotor
Has three different classifications: constant torque,
constant horsepower, or a combination of the two
Standard industrial dc motors are shunt wounded
Modifications of the dc motor are: shunt wound,
stabilized shunt exciting fields, compound wound
motors, and series wound motors
Armature Voltage Control
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Is used for motor speeds below base speed
 Output torque= T=k*ø*IA
k is machine constant
ø is the main pole flux
IA is the armature current
Shunt Field Control
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Is used for motor speeds above base speed
 Horsepower, (HP)= Torque*rpm
5252
Where torque is in lb-ft
Speed Regulation
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Speed Regulation
(IR)= no load rpm- full load rpm
full load rpm
Brushless DC Motors
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Three phase ac power is converted into dc
by the input side of the motor to charge up a
bank of storage capacitors
 These capacitors are called the Buss
 The purpose of the buss is to store energy
and supply dc power to transistors in the
output side as the motor requires the power
to start up
Brushless DC Motors
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Figure 13-21, page 264 shows the input power
section
It consists of three fuses, six diodes, a choke, and
two capacitors
The fuses protect the diodes
The choke protects against line transients
The motor control may run at very low speeds at
very high torques while drawing little current from
the ac line
Brushless DC Motors
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This picture is a
representation of the
encoders (rotor part of
the motor) telling the
corresponding
transistors (stator) to
turn on in order to get
maximum torque from
the motor
Picture of a Brushless Motor
Motor Control Starters
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Motor will draw high inrush current while the
starter will slow current down
 Starter reduces the amount of torque needed to
start the motor
Magnetic Motor Starter
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Normally open contacts
 Not always possible to control amount of
work applied to the motor
 Has overloads
– Motor may be overloaded resulting in damage
to the motor
– Open due to excessive motor current, high
temperature, or a combination of both
Full-Voltage Starter
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Contains one set of
contacts
 Motor is directly
connected to the line
voltage
Reversing Motor Starter
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Contains two starters of equal size
 Two starters connect to the motor
 Interlocks are used to prevent both starters from
closing their line contacts at the same time
 Figure 14-4A
Reduced-voltage Motor Starter
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Applies a percentage of the total voltage to start
(50% - 80%)
 After motor rotates, switching is provided to apply
full voltage
 Torque will be reduced when starting
 Four types:
1) Autotransformer
2) Primary Resistance
3) Wye – Delta
4) Part Winding
Autotransformer Starter
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Two contactors are used:
1) Start contactor
- Closes first and connects motor to the line
through an autotransformer
- Deenergizes
2) Run contactor
- Motor switches to this contacter which has
full voltage
Primary Resistor Starter
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Two contactor
1) Line contactor
- First to energize connecting motor to the
line voltage through a resistor
- After preset time, contactor opens
2) Accelerating contactor
- Energizes
- Causes smooth acceleration to full voltage
Wye – Delta Starter
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Three contactors are used
1) Line contactor and start contactor
- Energizes first and connects motor in wye
putting about 58% of line voltage across
each motor phase
- Contacts open after preset time
2) Run contactor
- Energizes connecting motor in delta and
putting full voltage on the motor
Part Winding Starter
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Starter supplies about 48% of normal starting torque
Not truly a reduced-voltage means
Two Types
1) Two-Step - one winding connected to
full voltage line and, after a preset time,
the other connects
2) Three-Step – one winding is connected in series
with a resistor to the voltage line; after interval, resistor
is shorted out and then second line is connected to
full voltage line
Solid-State Motor Starter
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For lower starting
torque and smooth
acceleration
 Used on conveyors,
pumps, compressors,
etc.
Standard Modes of Operation
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Motor voltage gradually increases during
acceleration
 Creates a kick start pulse of 500% of full
load amperage for high friction
 Used when necessary to limit current
 Used when motor requires a full voltage
start