ALTERNATING CURRENT (AC) MOTORS TEMINOLOGY GLOSSARY BASIC INFORMATION

1 accelerating thermal limit (loci): The time-current relationship limit based on the allowable temperature limit of the rotor during the accelerating (starting) process.

2 accelerating time-current curve: The characteristic starting current vs. time curve representing the motor acceleration at a given applied voltage.

3 accelerating torque: The net torque between the motor capability torque and load torque during the
starting process, which accelerates the motor and load to operating speed.

4 breakdown torque: The maximum torque that a motor will develop with rated voltage at rated frequency, where an abrupt drop in speed will not occur.

5 damper winding limited: A synchronous motor that is thermally limited by its damper winding.

6 field winding: The rotor circuit on an ac synchronous motor, which consists of winding connected to a dc source and produces the main electromagnetic field of the motor.

7 high-inertia load: A load that has a moment of inertia that exceeds normal values, as described in NEMA MG-1-1993, and for which the motor needs to be designed to have both thermal and mechanical capability to accelerate the load to operating speed without exceeding its limits.

8 load torque: The torque required by mechanically driven equipment across the operating speed range.

9 locked-rotor current: The steady-state motor current with the rotor locked, when supplied from a
source at rated voltage and frequency.

10 locked-rotor impedance: The impedance at the motor terminals during the locked-rotor condition.

11 locked-rotor thermal limit (permissible locked-rotor time): The maximum permissible safe time vs. locked-rotor current flowing in the windings at rated voltage and frequency.

12 locked-rotor torque: The minimum torque that a motor will develop at rest, for all angular positions of the rotor, at rated voltage and frequency.

13 loss of synchronism (out of step): A condition that exists when the synchronous machine has lost synchronism with respect to the supply system.

14 motor torque capability: The torque capability of the motor, supplied with rated voltage and frequency across the operating speed range, during the starting and running processes.

15 pull-in torque: The maximum constant torque of a synchronous motor under which the motor will pull its connected inertia load into synchronism, at rated voltage and frequency, when its field excitation is applied.

16 pull-up torque: The minimum torque developed by the motor during the period of acceleration from rest to the speed at which breakdown torque occurs.

17 rotor: The rotating component of an ac motor that contains the field winding, which may be wound or of the squirrel-cage type. In the case of the synchronous motor, the field winding may be energized from a dc source.

18 rotor limited: A motor that is thermally limited by its rotor system.

19 running thermal capability: A plot of maximum permissible time vs. percent of rated current flowing in the motor winding when the motor is running.

20 service factor: A multiplier which, when applied to the rated horsepower, indicates a permissible
horsepower loading that may be carried under the conditions specified for the service factor.

21 slip cycle: When a synchronous motor slips out of step (loss of synchronism) a complete 360°
with respect to the supply source.

22 slip frequency: The frequency of a induction motor rotor’s voltage and current when the rotor is rotating at some value of slip.

23 starting current: The current required by the motor during the starting process to accelerate the motor and load to operating speed. Maximum starting current at rated voltage is drawn at the time of energizing.

24 starting time: The time required to accelerate the load to operating speed.

25 starting torque: The rated motor torque capability during start at rated voltage and frequency.

26 stator: The stationary component of an ac motor that contains the armature winding and stator core.

27 stator-limited: A motor that is thermally limited by its stator system.

28 squirrel-cage winding: A rotor circuit that consists of conducting bars, connected with an end ring on each end, which constitute the rotor winding. Voltages are induced on this circuit which, in turn, produces current flow. Interaction between the stator and rotor fields creates the electromagnetic torque of the motor.

29 thermal limit curve (cold): A plot of maximum permissible time vs. percent of rated current flowing in the motor winding when the motor is started from ambient temperature.

30 thermal limit curve (hot): A plot of maximum permissible time vs. percent of rated current flowing in the motor winding when the motor is started from rated operating temperature.

31 wound rotor winding: The rotor circuit on a wound rotor induction motor, which consists of a
polyphase winding that carries the alternating current produced by induction.

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