Selsyn (synchro) motors

Chapter 13.11 Selsyn (synchro) motors

Lessons In Electric Circuits Volume II – AC Book
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Summary of Contents

Lessons In Electric Circuits Volume II – AC Book

  • 460CHAPTER 13. AC MOTORS13.10.4Eddy current clutchIf the stator of an induction motor or a synchronous motor is mounted to rotate independently of therotor, an eddy current clutch results. The coils are excited with DC and attached to the mechanicalload. The squirrel cage rotor is attached to the driving motor. The drive motor is started withno DC excitation to the clutch. The DC excitation is adjusted from zero to the desired final valueproviding a continuously and smoothly variable torque. The operation of the eddy current clutch issimilar to an analog eddy current automotive speedometer.Summary: Other specialized motors• The shaded pole induction motor, used in under 50 watt low torque applications, develops asecond phase from shorted turns in the stator.• Hysteresis motors are a small low torque synchronous motor once used in clocks and phono-graphs.• The eddy current clutch provides an adjustable torque.13.11Selsyn (synchro) motorsNormally, the rotor windings of a wound rotor induction motor are shorted out after starting. Duringstarting, resistance may be placed in series with the rotor windings to limit starting current. If thesewindings are connected to a common starting resistance, the two rotors will remain synchronizedduring starting. (Figure 469,13.83) This is usefull for printing presses and draw bridges, where two motorsneed to be synchronized during starting. Once started, and the rotors are shorted, the synchronizingtorque is absent. The higher the resistance during starting, the higher the synchronizing torque fora pair of motors. If the starting resistors are removed, but the rotors still paralleled, there is nostarting torque. However there is a substantial synchronizing torque. This is a selsyn, which is anabbreviation for “self synchronous”.M1StatorRotorStart resistanceφ2φ1φ3M2φ2φ1φ3RotorStatorFigure 13.83: Starting wound rotor induction motors from common resistors.The rotors may be stationary. If one rotor is moved through an angle θ, the other selsyn shaftwill move through an angle θ. If drag is applied to one selsyn, this will be felt when attempting torotate the other shaft. While multi-horsepower (multi-kilowatt) selsyns exist, the main appplicationis small units of a few watts for instrumentation applications– remote position indication.Instrumentation selsyns have no use for starting resistors. (Figure 470,13.84) They are not intendedto be self rotating. Since the rotors are not shorted out nor resistor loaded, no starting torque is