How Motors Operate Presented by John Freeland

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How Motors OperatePresented by John Freeland

• Reference Audel Electric Motors
• By Rex Miller Mark Miller
• 6th Edition
• Midcontinent Library Call Number 621.46 M617

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Voltage
time

-
Direct Current (DC) e.g. Battery
VA sin (360ft)
A
45
1/60 sec
RMS 120 volt Average 0 volt Peak (A)
170 volt
120 240 360
Three Phase Alternating Current
Series Connection
Parallel Connection
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Basic Motor Operation
N
S
N
S
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DC Motors - General

• The speed is directly proportional to voltage
• The DC motors we see are usually in vehicles,
  toys and cordless tools
• They are fractional horsepower motors
• Reversible by reversing the supply voltage
  polarity

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DC Motors

• 1. Wound rotor (armature) and wound field coil
• Lower efficiency because energy required to
  energize field coils
• Relatively high maintenance because of brushes
  and commutator
• Example First generation cordless drills
• Permanent magnet field motor
• Has wound rotor (armature) with commutator and
  brushes
• Example Some newer cordless drills and car door
  lock motor
• Permanent magnet rotor
• Has a wound field with electronics
• Advantages More efficient
• Lower maintenance
• Longer battery life
• Example New brushless drill motors

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Universal Motors

• Universal motors can operate on AC or DC power
• Has a wound field coil, wound rotor with brushes
  and a commutator
• Speed is proportional to applied voltage
• Series wound universal motor (field coils and
  armature connected in series)
• Can operate up to 20,000 rpm
• Has poor speed regulation with varying load
• Needs feedback circuitry to regulate speed
• Example Vacuum cleaner, router
• Parallel wound universal motor (field coils and
  armature connected in parallel)
• Operates at lower rpm
• Has good speed regulation with varying load

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Single Phase AC Motors

• One common type is a split phase induction motor
• Example Table saw, planer, jointer, drill
  press
• The split phase stator has two windings
  starting (auxiliary) and running
• The starting winding
• Provides starting torque
• Is disconnected by a centrifugal switch between
  2/3 and 3/4 of the synchronous speed.
  15 of synchronous speed is usually sufficient to
  start the motor.
• This switch is a reliability issue. On a quiet
  machine/motor you can usually hear the switch
  open as the motor starts up and close as the
  motor turns off
• Is offset from the running winding ninety
  degrees for a two pole motor and forty-five
  degrees for a four pole motor.
• Has fewer turns of smaller gauge wire.
• Occupies the outer portion of the stator slots.

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Single Phase AC Motors

• The starting and running windings are connected
  in parallel. Their relative connection
  determines if the motor runs clockwise or
  counterclockwise
• If both ends of the starting and running windings
  are accessible, the motor can be reversed
• Induction in a split phase induction motor
  means a current and therefore a magnetic field is
  induced in the rotor by the stator magnetic field
  (transformer action.)
• The rotor usually has aluminum cast into the
  rotor laminations which create one turn coils.
  This arrangement gives rise to calling the motor
  a squirrel cage motor.
• The speed of a squirrel cage, split phase,
  induction motor is nearly constant with load and
  depends on the line frequency, number of poles
  and slip.
• The starting torque is 1.5 to 2.0 times the
  full-load torque
• The starting current is 6 to 8 times the full
  load current.
• Capacitors may be connected in series with the
  starting and/or running windings to increase
  torque and reduce starting current. These
  capacitors are a reliability issue.

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Single Phase AC Motors

• Synchronous speed
• Ns RPM 120 x frequency (f) 7200
• number of poles (p) (p)

Ns f p
3600 60 2
1800 60 4
1200 60 6
900 60 8
3000 50 (England) 2
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Single Phase Motors

• Slip is usually 1-4of the synchronous speed and
  can be expressed three ways
• As a percent of synchronous speed
• As a decimal fraction of synchronous speed
• Directly in RPM

P Ns Actual Speed Slip as Slip as decimal Slip as RPM
2 3600 3450 4.2 0.042 150
4 1800 1725 4.2 0.042 75
6 1200 1150 4.2 0.042 50
8 900 860 4.4 0.044 40
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Single Phase Motors
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Three Phase AC Motors

• Three phase power is industrial power.
• Three phase power is how power is generated and
  distributed from the power plants
• Three phase motors are very reliable because they
  dont use brushes, a commutator, a starting
  centrifugal switch or a starting capacitor
• Motor speed is a function of the number of poles,
  the line frequency and slip
• Reversible by swapping any two of the three phase
  supply wires
• ABCABCABCABCABC
• CBACBACBACBACBA
• Relatively inexpensive solid state controls are
  available to convert single phase power to three
  phase power with variable frequency and voltage
  (speed controls)

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VFD Variable Frequency Drive

• Refer to www.VFDs.com
• A VFD is a motor controller that drives a motor
  by varying the frequency and voltage supply
• Voltage to frequency ratio (V/F) remains constant
  from the VFD
• The strength of the motors magnetic poles is
  proportional to V/F. If the ratio is too low the
  magnetic pole strength is too weak to create
  sufficient torque. If the ratio is too high the
  iron in the poles will magnetically saturate and
  cause overheating.
• Maximum motor speed is 120 of rated speed
• Minimum speed is 20 of maximum rated speed
• Motor bearings have a higher failure rate when
  the motor is powered by a VFD

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VFD Block Diagram