Alternating Current Machines by M.G. Say

Author:M.G. Say
Language: eng
Format: epub, pdf
Tags: Electronics, Electrical Engineering, Transformers, Magnetic Circuits, Windings, Induction Machines, Synchronous Machines, AC

0-5 10 1-5

Output power (p.u)

8.18 Motor performance curves.

2-0

more rapidly, until at maximum load (about 100% overload) the current may be about three times its full-load figure, at a power factor in the neighbourhood of 0.7. At this stage a small increase in overload will cause the motor to stop suddenly. The current will rise to e.g. five times rated full-load value, the torque will be small, and the power factor lower, 0.2-0.4. The machine will not restart until the load torque has been reduced below that available at starting. The rating of a motor must be reckoned in relation to its pull-out or stalling (maximum) torque, a usual relation of maximum to full-load torque being 2:1.

A typical set of characteristic curves over the complete range is given in Fig. 8.18.

The efficiency and power factor at rated load depend on the size and application, and particularly on the nominal speed nx = f/p (r/s) or 60 f/p (r/min) for a machine with p pole-pairs and a supply of frequency/. Because of less effective use of material and greater leakage inductance a low-speed machine is heavier, less efficient and more reactive than a fast machine. The figures below compare the data for a 5 kW machine at various speeds, and a range of cage and slip-ring machines.

5 kW 50 Hz 3-ph Cage Motor

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