Motors

1
Motors Motor Starters

• Prepared By
• Erik Redd
• Jeremy Roberts

2
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.

3
Diagram of an ac-motor

• This shows a three phase, two pole stator.
• Where A, B, and C are the three phases

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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.

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Synchronous Speed

• Speed at which it takes the motor to go one cycle
  and one revolution.
• S120frequency
• ( poles)
• Example
• For a three-phase, 60 Hertz, 2 pole motor
• S12060/23600 revolutions per minute

6
Polyphase Squirrel-Cage Induction Motors

• 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

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• Example.
• A 2 pole, 60 Hz motor runs at a full-load speed
  of 1760 rpm.
• What is the slip?

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• Ans. slip 3600-1760100
• 3600
• 51.1

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Single-Phase Motors

• 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
  split-phase, capacitor start, permanent
  split-capacitor, and shaded-pole motors

10
Resistance Split-Phase Motors

• 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

11
Capacitor-Start Motors

• 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

12
Permanent Split-Capacitor Motors

• Winding arrangement is the same as the capacitor
  and split-phase motors
• Capacitor can run continuously, rated in
  microfarads for high-voltage ratings
• No centrifugal switch is needed
• IM lags VL, while IS leads VL
• Efficiency is between 50-70

13
Shaded Pole Motors

• 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

14
DC Motors

• 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

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Armature Voltage Control

• Is used for motor speeds below base speed
• Output torque TkøIA
• k is machine constant
• ø is the main pole flux
• IA is the armature current

16
Shunt Field Control

• Is used for motor speeds above base speed
• Horsepower, (HP) Torquerpm
• 5252
• Where torque is in lb-ft

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Speed Regulation

• Speed Regulation
  (IR) no load rpm- full load rpm
• full load rpm

18
Brushless DC Motors

• 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

19
Brushless DC Motors

• 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

20
Brushless DC Motors

• 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

21
Picture of a Brushless Motor
22
Motor Control Starters

• Motor will draw high inrush current while the
  starter will slow current down
• Starter reduces the amount of torque needed to
  start the motor

23
Magnetic Motor Starter

• 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

24
Full-Voltage Starter

• Contains one set of contacts
• Motor is directly connected to the line voltage

25
Reversing Motor Starter

• 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

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Reduced-voltage Motor Starter

• 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

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Autotransformer Starter

• 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

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Primary Resistor Starter

• 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

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Wye Delta Starter

• 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

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Part Winding Starter

• 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

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Solid-State Motor Starter

• For lower starting torque and smooth acceleration
• Used on conveyors, pumps, compressors, etc.

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Standard Modes of Operation

• 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