Electromagnets

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Electromagnets
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Motor Operation 101
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Motor Operation 101
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Motor Operation 101
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Motor Operation 101
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Motor Operation 101
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Motor Operation 101
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Motor Operation 101
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Motor Operation 101
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Motor Operation 101
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Motor Operation 101
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Work

• Work means force multiplied by distance.
• For example, if you lift a 50-pound weight two
  feet, you have performed 100 foot-pounds of work.
• The time required to lift it is not considered in
  figuring units of work.

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Torque

• Torque is a force that tends to produce rotation.
• If a force of 50 pounds is applied to the handle
  of a 2' crank, this force produces 100 pounds of
  torque (twist-ability) when it is at right angles
  to the crank arm.
• Torque may be converted into horsepower when the
  element of time is considered.

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Torque
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Torque
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Horsepower

• When torque, in foot pounds, is multiplied by the
  speed in revolutions per minute, it may be
  divided by the constant 5250 to find horsepower
• Horsepower Foot pounds torque x revolution per
  minute
• 5250

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Horsepower

• Horsepower has been the common measurement for
  mechanical power since the 1760s
• English scientist named James Watt proved that a
  horse hitched to a pulley could lift 550 pounds
  at the rate of one foot per second. He called
  this one horsepower.
• In terms of a minute, one horsepower is the power
  required to lift 33,000 pounds one foot. A unit
  of power, then, is equal to a rate of 33,000
  foot-pounds of work per minute, or 550 per second.

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Horsepower and Watts

• All of us are familiar with the wattage of the
  electric light bulbs we used at home.
• Electric power is usually expressed in Watts or
  1000-Watt units (or kiloWatts) instead of
  horsepower.
• One horsepower is equal to 746 watts, or about
  3/4 of one kilowatt.

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Motor Mountings
Rigid Mount
Resilient Mount
Stud Mount
Flange Mount
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Insulation

• Trend in motor construction has been to small
  designs running at higher temperatures.
• Improved insulation systems have made these
  designs possible.
• Insulating materials or combinations of such
  materials are grouped into temperature tolerance
  classes,
• At present, four classes are recognized by the
  American Institute of Electrical Engineers

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Classes of Insulation

• Class A 105C Rating
• Class B 130C Rating
• Class F 155C Rating
• Class H 180C Rating

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Common Motor Enclosures
Open Motor
Totally Enclosed Motor (Not air tight)
Drip Proof Motor
Totally Enclosed, Fan Cooled Motor (Not air tight)
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Less Common Motor Enclosures
Totally Enclosed, Non- Ventilated Motor (Not
air tight)
Explosion Proof (Explosion Contained)
Dust Ignition Proof Motor
Totally Enclosed, Fan Cooled, Guarded Motor
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Motor Nameplate
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Motor Nameplate

• Style or model number
• Number of horsepower
• Speed in RPM for ratings
• Operating voltage
• Normal current
• Frequency operation
• Frame size
• Time for continuous operation
• Thermoguard protection
• Manufacturers code number
• Motor type letter code

• Horse power service factor
• Power type
• Max current at service factor
• Code for locked rotor kV?A
• Max ambient temperature (C)
• Insulation class
• Housing or type of enclosure
• Wiring diagram
• Stock number (replacement )
• Underwriter Lab. symbol
• Canadian Stand. Assoc. symbol

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AC vs. DC Motors

• DC motor advantages
• Simple design
• Easy to control speed speed and armature
  voltage are linear
• Easy to control torque torque is proportional
  to current
• Simple, cheap drive design can be
  potentiometer

• AC motor advantages
• Simple
• Low cost
• Reliable

• AC motor disadvantages
• Expensive speed control AC inverter drive
  more expensive
• Not good at low speed thermal considerations
• Poor positioning unless used in servo system

• DC motor disadvantages
• More expensive
• Larger
• High maintenance unless brushless

http//www.oddparts.com/acsi/motortut.htmBLDC
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AC Series (Universal) Motor

• Called universal motors because they can run on
  either alternating current or direct current.
• They operate at speeds as high as 35,000 rpm or,
  in other words, 35,000 complete turns of the
  shafts in one minute.
• Their high starting torque and their ability to
  adjust to widely varying loads make them ideal
  for electric shavers, drills, saws, sanders,
  blenders, vacuum cleaners, and other portable
  household appliances.

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Single, Split Phase, Induction Motor

• Motor has two types of coils, for starting and
  for running
• To start the motor, the smaller coils are used
• As the motor starts to spin, centrifical force
  disconnects the starting coils and connects
  the running coils
• As the motor slows down, the end of the
  centrifical force disconnects the running coils
  and connects the starting coils

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Capacitor Motor

• Similar to the single, split phase induction
  motor
• Capacitor (charge bank) is used to smooth out any
  sudden changes in current draw

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Polyphase Induction Motor

• Simple operation due to the three-phase power
  supplied to the stator windings.
• Three identical sets of running windings are
  mounted in the stator, and each set of windings
  is connected to a different phase of the power
  source.
• The relationship of the rise and fall of the
  current in each phase with that of the other
  phases produces the rotating magnetic field that
  in turn produces the twisting motion in the motor
  shaft.

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Polyphase Induction Motor

• When phase current positive, winding is north
  pole
• When phase current negative, winding is south pole

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Selecting Proper Motor

• What are the characteristics of the power supply?
• What horsepower and speed are required?
• What type of motor will best do the job?
• What frame size is required?
• What enclosure is needed?
• What kind of bearings are required?
• In what direction will the motor rotate?
• How will the motor be mounted?
• Should thermal protection be applied?

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1. Characteristics of the power supply?

• AC or DC?
• Single or polyphase?
• Most homes, farms, and offices have single phase
• Factories and industrial plants will have
  polyphase
• 50 or 60 Hz?
• Voltage?

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2. What horsepower and speed are required?

• 1hp 746W
• Determine full load speed
• 3450rpm, 1725rpm, 1140rpm, 850rpm typical
• Determine desired horsepower at that speed

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3. What type of motor will best do the job?

• Different motors due different jobs better

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4. Frame size?

• Three common frame sizes are 42, 48, and 56
• Shaft diameters of 3/8 , 1/2 , 5/8
• Frame diameter of 4-7/8 , 5-7/8 , 6-3/4

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5. Type of enclosure

• Most small motors are drip proof guarded
• If this is not sufficient, most common is
  enclosed motors

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6. Type of bearings

• Two common types of bearings sleeve bearings
• ball bearings
• Sleeve bearings are less expensive, usually
  quieter
• Ball bearings recommended for larger, more
  powerful motors

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7. Direction of motor rotation

• Many small motors can rotate in both directions
• May not be true for larger motors
• Look for CW or CCW designations

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8. How will it be mounted?

• Horizontal or vertical mounting?
• Rigid, resilient, flange, and stud mounting types

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9. Thermal protection?

• Thermal protection in a motor is provided by a
  temperature sensitive element which activates a
  switch.
• This switch will stop the motor if the motor
  reaches the pre-set temperature limit.
• Two major types of thermal protection switches
  are available.
• One will re-start the motor when the temperature
  has been reduced. This type is called Automatic
  Reset
• The other type is called Manual Reset (this
  type usually is in the form of a small
  pushbutton on the end of the motor opposite the
  shaft). When the motor has cooled sufficiently,
  the button is pushed and the motor will start.

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9. Thermal protection?

• There are times when automatic reset protection
  would be very dangerous. For example, on a drill
  press.
• The motor may have cooled to the point where it
  will automatically re-start, just as the operator
  is loosening the chuck with a chuck key.
• For applications that could be harmful if started
  without warning, always use manual restart.

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Why does a larger motor cause lights to blink
when started?

• Because of the higher starting current required
  to produce high torque and the increase in
  allowable operating temperatures, the copper wire
  which forms the coils of the stator must be of a
  larger diameter, or of a shorter length.
• For this reason, the larger motor will draw a
  greater amount of current when starting than will
  a smaller motor.
• The extra current draw could cause a blink in the
  lighting system.

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Can low voltage burn out a motor?

• In order to turn out its rated horsepower, a
  motor operating at maximum efficiency must draw
  746 watts for each horsepower it produces.
• The number of watts a motor requires is
  determined by multiplying the voltage and the
  amperage.
• Therefore, if the voltage is reduced, the
  amperage must be increased to maintain the same
  wattage or horsepower.
• The increased amperage results in excessive heat
  and can cause motors to burn out.

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What are the advantages of 3-phase motors?

• The main advantage is the lower power rates for
  3-phase service.
• Another advantage is in motor construction since
  a 3-phase motor needs no centrifugal switch or
  starting windings. The interaction of the three
  phases produces the rotating magnetic field
  within the motor.
• As a general rule, 3-phase motors require much
  less starting current and are quite easily
  reversed by switching any two of the three power
  supply leads.

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What is meant by single phasing?

• Single phasing is the term used to describe the
  condition which occurs on 3-phase when a supply
  line failure causes one of the three power lines
  to become inoperative.
• The amps will increase substantially on the other
  two motor legs and a motor Burn Out can occur.

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What are all angle sleeve bearings?

• Simply stated, by using them a motor may be
  mounted at any angle, even vertically.

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What advantages are gained with sleeve bearings?

• In addition to being lower priced, sleeve
  bearings are not as easily contaminated as
  rolling elements or ball bearings.
• Sleeve bearings are not as likely to become
  seized if they are contaminated, and they are
  generally quieter than a normal-quality ball
  bearing.

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What is meant by 350 or 500 starting torque?

• This means that on start-up, the motor will have
  3-1/2 or 5 times the toque (twisting ability)
  that it has at full speed.
• This is a very desirable characteristic when
  heavy loads must be accelerated.

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What is meant by service factor?

• The service factor is the amount by which a motor
  can be overloaded without causing a serious drop
  off in speed or dangerously high temperatures.
• If a motor has a service factor of 1.25, then it
  can be loaded up to 25 greater than its rated
  horsepower.
• It is a compensation used to insure continued
  performance of a motor which must operated under
  varying load conditions.
• The service factor can be compared to a safety
  factor or a safety precaution.

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What is a resilient motor mount?

• A resilient motor mount is a rubber ring on each
  end of the motor which cushions the motor base
  from the vibrations and load shocks which the
  motor experiences in service.
• This type of mount is effective in cutting down
  noise produced by motors which are mounted on or
  near sounding-board-like materials, sheet metal,
  thin plywood, or even air distribution systems.

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