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Chapter 8: Synchronizing Induction Motors

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FieldValue
SourceTheory and Calculation of Electric Apparatus
Year1917
Section IDtheory-calculation-electric-apparatus-chapter-07
Locationlines 13956-14465
Statuscandidate
Word Count1860
Equation Candidates In Section0
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CHAPTER VIII SYNCHRONIZING INDUCTION MOTORS 94. Occasionally two or more induction motors are operated in parallel on the same load, as for instance in three-phase rail- roading, or when securing several speeds by concatenation. In this case the secondaries of the induction motors may be connected in multiple and a single rheostat used for starting . and speed control. Thus, when using two motors in concatena- tion for speeds from standstill to half synchronism, from half synchronism to full speed, the motors may also be operated on a single rheostat by connecting their secondaries in parallel. As in parallel connection the frequency of the secondaries must be the same, and the secondary frequency equals the slip, it follows that the motors in this case must operate at the same slip, that is, at the same
... speed control. Thus, when using two motors in concatena- tion for speeds from standstill to half synchronism, from half synchronism to full speed, the motors may also be operated on a single rheostat by connecting their secondaries in parallel. As in parallel connection the frequency of the secondaries must be the same, and the secondary frequency equals the slip, it follows that the motors in this case must operate at the same slip, that is, at the same frequency of rotation, or in synchronism with each other. If the connection of the induction motors t ...
... e. 95. Assume two equal induction motors with their primaries connected to the same voltage, supply and with llieir seeondarioi connected in multiple with each other to a common resistance, r, and neglecting for simplicity the exciting current and the vol- tage drop in the impedance of the motor primaries as not mate- rially affecting the synchronizing power. Let Zi — n + ./j-t = secondary self-inductive impedance at full frequency; s = slip of the two motors, as fraction of syn- chronism; Co = absolute value of impressed voltage and thus, when neglec ...
... used in the motor secondaries, so as to allow different slips. When rigidly connect- ing the two motors with each other, it is essential to take care that the motor secondaries have exactly the same relative posi- tion to their primaries so as to be in phase with each other, just as would be necessary when operating two alternators in parallel with each other when rigidly connected to the same shaft or when driven by synchronous motors from the same supply. As in the induction-motor secondary an e.m.f. of definite fre- quency, that of slip, is gene ...
... ve in the second or leading motor; that is, the motor which lags in position behind gives more power and thus accelerates, while the motor which is ahead in position gives less power and thus drops back. Hence, the two motor armatures pull each other into step, if thrown together out of phase, just like two alternators. The synchronizing torque (14) is zero if t = 0, as obvious, as for r = 0 both motors are in step with each other. The syn- chronizing torque also is zero if r = 90°, that is, the two motor armatures are in opposition. The position of ...
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