Induction motors duties

Characteristics of a bi-metallic thermal relay The thermal characteristics are almost the same as those of an induction motor. This makes them suitable for protecting a motor by making a judicious choice of the right range for the required duty. (See Figure 12.11 for a typical thermal overload relay and Figure 12.13 for its thermal characteristics.) Ambient temperature com-... 

Consider a process plant having a connected load of 15 000 kW and a running load of 12 500 h.p. at almost 0.65 p.f. lagging. Let a few large induction motors aggregating 2000 h.p. be replaced by as many oversized synchronous machines, with the purpose of improving the system p.f. in addition to performing the motor s duties. 

Use of wound-rotor induction motors has been largely in continuous-duty constant-speed supplications where particularly high starting torques and low starting currents are required simultaneously, such as in reciprocating pumps and compressors. These motors are also used where only alternating current is available to drive machines that require speed adjustment, such as types of fans and conveyors. 

Standard for Petroleum and Chemical Industry Severe Duty TEFC Squirrel Cage Induction Motors—Up To and Including 500 HP... 

IEEE 841, Standard for petroleum and chemical industry— Severe duty totally enclosed fan-cooled (TEFC) squirrel cage induction motors — Up to and including 370 kW (500 hp). )... 

In some cases it is also necessary to consider the fault current contributed by motor consumers, particularly if large synchronous motors are fed from the same busbars as the main generators or main transformer infeeds, see Chapter 11. Induction motors contribute fault current during the sub-transient period and so extra allowance must be made when calculating the making duty. 

The sub-transient impedance determines the initial decay, i.e. in the first cycle or so. Therefore the emfs E" and E, together with the reactances Xj and X j, need to be used for calculating the fault currents. In a similar way to induction motors, the synchronous motors will contribute to fault-making dnty reqnirements. However, they will also contribute towards the fault-breaking duty because of the transient effects. 

DEVELOPMENT OF DUTY CYCLE FOR INDUCTION MOTOR CHARACTERISTICS... 

Figure 1. Hoist duty cycle for induction motor with 25% magnetizing current...

For induction motors, the total current can be approximated by the vector sum of the torque producing component and the magnetising or flux producing component of motor current. These two current vectors are in quadrature, and as the hoist torque approaches zero, the motor current approaches magnetising current. Electrical RMS calculations for hoist motors must be based on motor current, and so the calculation based on torque current only is not accurate for induction motors, and under-estimates the induction motor current. For greatest accuracy the duty cycle must be calculated with the correct motor characteristics. 

In the duty cycle shown in Figure 1, for an induction motor with 25% magnetizing current, the RMS error when using only the torque current plot for RMS calculation is 2% below the actual RMS current. 

Induction motors are popular because of their simplicity and ruggedness. Induction motor characteristics such as magnetizing current affect the RMS current calculation, and must be taken into account when selecting a motor for a mine hoist duty. 

The AC drive has a shorter thermal time constant than the induction motor, and a separate RMS duty cycle calculation is required for both the drive and the motor using the applicable thermal time constant. 

Electric Motors, 615 Terminology, 615 Load Characteristics, 616 Basic Motor Types Synchronous and Induction, 616 Selection of Synchronous Motor Speeds, 619 Duty, 625 Types of Electrical Current, 625 Characteristics, 627 ... 

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