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One horse power Brake horse power is the net effective mechanical horse power available
at the pulley or coupling of a motor, i.e., the actual horse power available
after all losses in the motor itself have been deducted. The power factor is usually taken at 0.8 (as an all round figure) but this varies with the size and speed of the motor. The current required to supply a three phase motor at full load is:
Where E is the voltage between phases. Electric motors are rated in two ways - continuous maximum rating and short-time rating. Unless otherwise stated, the rating is continuous. But in the case of railway and crane motors, which work under intermittent service, and where heavy currents are drawn followed by coasting or standstill which allow the motor to cool, the short-time rating is also stated. This short-time rating is the horse power the motor is built to deliver without danger for a specified period, details of which will be given on the rating plate. For instance, a railway motor with 50 h. p. short-time rating will deliver 50 h. p. for one hour without dangerous temperature rise, but might be suitable for, say, only 30 h. p. continuously. Similarly, where a 200 h. p. load, say, is to be handled only occasionally, and for short periods, a much smaller motor than 200 h. p. continuous rating is used: and, in such a case, the motor is given a short-time rating, such as 200 h. p. for ten minutes. From this it follows that the size of cable required to supply a motor also depends on the rating of the motor, i.e., on the continuous or intermittent nature of the load.
POWER FACTOR IMPROVEMENT Power factor at a particular point in a circuit or installation is most easily obtained by means of a power factor indicator. Where such an instrument is not available, the following method may be used. From the tradings of a voltmeter, ammeter and wattmeter suitable connected, the power factor for a balanced three phase circuit may be calculated as follows:
DETERMINATION OF CAPACITOR RATINGS 1. Individual motor correction For direct connection to induction motor terminals it is recommended that the ratings of the capacitor should be approximately 85 per cent of the no-load magnetizing kvar of the machine. 2. Line correction Where individual motor correction is not suitable, it is recommended practice to install a capacitor to deal with a group of motors by line correction at either the main busbars or at suitable distribution boards. The rating of such capacitors may be ascertained if the existing power factor and the degree of power factor correction desired is known. For example, to improve the power factor of a 100 kilowatt load from 0.65 to 0.95 power factor the multiplying factor is 0.840, therefore the rating of capacitor necessary is 84 kvar.
Keystone Technology Limited
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