AC

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AC
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AC Motors Summary
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AC
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AC Motor Theory

• Rotating flux field created by alternating field
  current.
• Rotating field induces current within copper
  conductors in rotor.
• Rotor rotates due to torque exerted on
  conductors.

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Induction Motor Stator
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Induction Motor Rotor
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Single-Phase AC Motors

• Used for low-hp applications only (typically less
  than 5 hp).
• Low operating efficiency (typ. 60-80).
• Same basic operating principles as 3-phase, but
  field alternates rather than rotating.
• Direction of rotation not defined as startup.
  Must use supplemental method to create rotating
  field.

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Single Phase Induction Motors

• 1.) Resistance Split Phase Motor
• Secondary winding of fine high resistance wire
  used.
• Change in inductive characteristics causes AC
  phase shift between primary and secondary
  windings, creating a rotating field.
• I2R losses in secondary winding overheat quickly,
  suited for low loads only.
• Centrifugal switch used to disconnect start
  winding once motor reaches about 75 of
  synchronous speed.

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Single Phase Induction Motors

• 2.) Capacitor Start Motor
• Motor wired with two identical windings 90
  degrees apart.
• Capacitor places in series with one winding,
  creating phase shift to generate rotating
  magnetic field
• Lower I2R losses during start
• Better suited to starting high inertial loads
• Utilizes a centrifugal switch to drop start
  winding when motor speed reaches 75 of
  synchronous speed.

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Single Phase Induction Motors

• 3.) Shaded Pole Motor
• Part of pole surrounded by copper ring.
• Ring delays formation of magnetic field in that
  portion of the pole, causing rotor to turn in
  that direction.
• Fixed direction of rotation, cannot be changed
  after mfg.
• Very inefficient, as start function cannot be
  disconnected once motor is as speed.
• Used for fractional horsepower motors only (typ lt
  .05hp)

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Shaded Pole Motor Contd
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Single-Phase AC Induction Motor Performance
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Universal Motors

• Operate on single phase AC or DC power
• Essentially a series-wound DC motor, complete
  with armature, commutator, and brushes.
• Operates on AC as both field and armature
  reverse, rotating in the same direction.
• Applications Vacuum cleaners, jig saws, drills
  
• High-speed at no-load, similar to the series DC
  motor.
• Low efficiency

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Synchronous Motors

• Runs at synchronous speed, no slip.
• Magnets on rotor lock-on to rotating flux
• Motor must used external start assist such as
• External start motor
• Squirrel cage built into rotor, to start as an
  induction motor
• Variable frequency supply slowly increase
  speed.
• Very high power (200 20,000 hp).
• Rock crushers, rolling mills, paper mills.
• Very low power
• Timing devices (wall and oven clocks)
• Sheet feed devices
• Well suited to high-torque, low rpm applications
  may eliminate the need for speed reducers

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Reading the Nameplate AC Induction Motors
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Reading the Nameplate

• Design

• NEMA specifies minimum values of locked-rotor,
  breakdown, and pull-up torque.

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3 Phase Motor Design
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Torque Curve Terminology
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Reading the Nameplate contd

• Insulation

• Indication of motors resistance to temperature.
• NEMA ratings A,B,F, or H, A being lowest
  resistance, H being highest.
• Operating temperature is a result of ambient
  temperature (air temp in area surrounding motor)
  and heat generated by motor itself (I2R losses,
  startup losses).
• Altitude limits may also be specified in
  combination with thermal limits due to reduced
  heat rejection capability at higher altitudes.
  Motors run cooler at lower altitudes.

• Max. Ambient.

• Maximum ambient temperature, degrees C, at which
  motor can operate at nameplate ratings with
  nameplate insulation class.

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Reading the Nameplate contd

• Supply Voltage, Frequency, Phase

• Voltage designed operation voltage for motor.
  May be single or multiple-voltage design. Most
  motors are designed to operate with a supply
  voltage tolerance of - 10, but nameplate
  ratings are only achieved with nameplate supply
  conditions. Typical supply voltages 208, 230,
  460, 575
• Frequency Input frequency of AC supply. North
  American standard is 60 Hz (cycles per second).
  In Europe 50 Hz is more common.
• Phase Number of AC supply lines powering
  motor. Typically 3 or 1.

• F.L.A.

• Full-Load Amps Current required to operate at
  its nameplate horsepower with nameplate supply
  voltage and frequency.

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Reading the Nameplate contd

• Duty Cycle

• On-Time Allowed
• Continuous, or 15, 30, 45 minutes per hour
  operation.
• Most smaller motors are designed for continuous
  duty..

• Service Factor

• Continuous overload capability of motor.
• If motor is kept within temperature limits, and
  electrical supply can maintain nameplate voltage
  level, motor may be run beyond nameplate power
  output safely.
• 1.15 indicates 115 service factor, or 15
  overload capability.

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Reading the Nameplate contd

• Mounting

• C-Face Motor is mounted to machine face of
  motor.
• Rigid Base Motor has feet integral to the
  motor body that allow it to be rigidly fastened
  to the machine.
• Resilient Base Motor has rubber-isolated feet
  integral to the motor body that allow it to be
  mounted but isolated from the machine.

• Enclosure

• ODP Open Drip-Proof Motor open, free
  exchange of cooling air, enclosure provides
  protection against liquid within 15 degrees of
  vertical.
• TEFC Totally Enclosed-Fan Cooled Better
  protection against external contamination, and
  includes an integral fan to force cooling air
  across motor.

• Frame

• Frame size indicates dimensional standard to
  which motor was manufactured. Standards specify
  external dimensions, mounting, shaft diameter and
  length, c-face or foot mounting dimensions, etc.

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C-Face Capacitor Start/Run Motor
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Foot-Mount