Electric Motors

  1. Motor Principle
    1. Magnetism
      1. Direction of the flow of electrons
      2. Left –hand conduction rule
    2. Motor Rotation
      1. Two interacting magnetic fields
      2. Attraction repulsion used to start motor rotation
      3. Right hand motor rule.
  2. Types of motors
    1. DC Motors
      1. Permanent-Magnet
      2. Series: Since the series field winding will carry the same amount of current as the armature it is wound with heavy-gauge wire that is large enough to carry the full motor load current. They will have a lower resistance.
      3. Shunt
      4. Compound
      5. Motor Counter Electromotive Force (CEMF)
        Counter EMF is a form of resistance that opposes and limits the flow of motor armature current.
        Problem: The armature of a 250 VDC Motor draws 15 Amps when operating at full load and has a resistance of 2 Ohm. Determine the Counter EMF Produced by the armature when operating at full load.  
        Solultion: IA = VMTR  - CEMF / RA 
        CEMF = VMTR  - (IA X RA)
        = 250V -( 15A X 2 Ohms)
        = 250 - 30
        = 230 V
    2. Type Of Three Phase Electric Motors
      1. Induction
      2. Squirrel Cage
        Percent Slip = (Synchronous speed -Actual Speed/
        Synchronous speed) X100
        (1800-1750)/1800 X 100 = 2.9 %
      3. Wound –Rotor Induction
      4. Synchronous
    3. Single-Phase AC Motors
      1. Split-Phase
      2. Split-Phase Capacitor
      3. Shaded-Pole
      4. Universal
    4. Motor Selection
      HP = (Torque X Speed)/Constant
      The  constant = 5,252
      LR Current (Single-Phase Motor)
      =Code Letter Valve X HP X 1000/Rated Voltage
      LR Current (Three Phase Motor)
      = Code Letter Valve X HP X 577/Rated Voltage
  3. Troubleshooting
    1. Voltage Imbalance: Measure Voltage and Calculate Valve
      Three Voltmeters 
      No more that 1 to 2 percent Allowed
      Average Voltage Deviation = L1 + L2 +L3 / 3
      Max Voltage Deviation From the Average Voltage /Average Voltage X 100 = Percent of Voltage Imbalance
    2. Go over Troubleshooting Charts for both AC and DC Motors