ELECTRIC MOTOR DESIGN Page 1 of 11 Motor Design 2008/12/22 ...

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Motor Designhttp://www.energymanagertraining.com/equipment_all/electric_motors/eqp_motor_d...Page 6 of 112008/12/22Figure - Capacitor start motor winding circuit. TROUBLESHOOTING AND REPAIRThe procedure for troubleshooting and repair for the capacitor motor is the same as for the split-phase motorexcept for the capacitor. Capacitors are rated in microfarads and are made in various ratings, according tothe size and type. Acapacitor may be defective due to moisture, overheating or other conditions. In such acase it must be replaced with another one of the same value of capacity. To test a capacitor, remove themotor leads from the capacitor and connect the capacitor in series with a 10-amp fuse across a 110-voltline. If the fuse burns out, the capacitor is short-circuited and must be replaced. If the fuse does not burnout, leave the capacitor connected to the line for a few seconds to build up a charge. Do not touch theterminals after the charging process as serious injury may result from the stored charge. Short the terminalswith an insulated handle screw driver. A strong spark should show if the capacitor is good. If no spark or aweak spark results, the capacitor must be replaced.The procedure for rewinding a capacitor motor is the same as for the split-phase motor except for thecapacitor.UNIVERSAL MOTORSA universal motor is one that operates on either single-phase ac or dc power. These motors are normallymade in sizes ranging from 1/ 200 to 1/ 3 horsepower.You can get them in larger sizes for special conditions. The fractional horsepower sizes are used on vacuumcleaners, sewing machines, food mixers, and power hand tools. The salient-pole type is the most populartype of universal motor. The salient-pole type consists of a stator with two concentrated field windings, awound rotor, a commutator, and brushes. The stator and rotor windings in this motor are connected in serieswith the power source. There are two carbon brushes that remain on the commutator at all times. These twobrushes are used to connect the rotor windings in series with the field windings and the power source (fig. 7-35). The universal motor does not operate at a constant speed. The motor runs as fast as the load permits;i. e., low speed with a heavy load and high speed with a light load. Universal motors have the highesthorsepower-to-weight ratio of all the types of electric motors.The operation of a universal motor is much like a series dc motor. Since the field winding and armature areconnected in series, both the field winding and armature winding are energized when voltage is applied tothe motor. Both windings produce magnetic fields which react to each other and cause the armature torotate. The reaction between magnetic fields is caused by either ac or dc power.SHADED-POLE MOTORSThe shaded-pole motor is a single-phase induction motor that uses its own method to produce startingtorque. Instead of a separate winding like the split-phase and capacitor motors, the shaded-pole motor'sstart winding consists of a copper band across one tip of
Motor Designhttp://www.energymanagertraining.com/equipment_all/electric_motors/eqp_motor_d...Page 7 of 112008/12/22Figure - Universal motor schematic.each stator pole. This copper band delays the magnetic field through that portion of the pole. When acpower is applied, the main pole reaches its polarity before the shaded portion of the pole. This action causesthe shaded poles to be out of phase with the main poles and a weak rotating magnetic field is produced.Because of the low-starting torque, it isn't feasible to build motors of this type larger than 1/ 20 horsepower.They are used with small fans, timers, and various light-load control devices. Remember, all single-phaseinduction motors have some auxiliary means to provide the motor with starting torque. The method used forthis starting torque depends on the application of the motor.CONNECTING THREE-PHASE MOTORSConnecting a three-phase motor is a simple operation. All three-phase motors are wound with a number ofcoils, with a 2-to-1 ratio of slots to coils.These coils are connected to produce three separate windings called phases, and each must have the samenumber of coils. The number of coils in each phase must be one-third the total number of coils in the stator.Therefore, if a three-phase motor has 36 coils, each phase will have 12 coils. These phases are usuallycalled Phase A, Phase B, and Phase C. All three-phase motors have their phases arranged in either a wyeconnection or a delta connection.WYE CONNECTIONA wye-connected three-phase motor is one in which the ends of each phase are joined together parallelingthe windings. The beginning of each phase is connected to the line. Figure 7-42 shows the wye connection.DELTA CONNECTIONA delta connection is one in which the end of each phase is connected in series with the next phase. Figure7-43 shows the end of Phase A connected to the beginning of Phase B. The end of Phase B is connected tothe beginning of Phase C, and the end of Phase C is connected to the beginning of Phase A. At eachconnection, a wire is brought out to the line
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ELECTRIC MOTOR DESIGN Page 1 of 11 Motor Design 2008/12/22 ...

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