cement clinker at high temperatures. If tbe kiln receives finely ground and precisely composed dry feed as mentioned above, the cement plant is called as a dry process one. In wet process, the raw materials are ground with water to produce slurry before entering the kiln feed tank. Dry process is preferred to wet process because less fuel is required by such kilns. Wet process is necessitated sometimes, since, certain materials contain so much water that adding a little more water and using wet process is better than trying to dry the raw materials. The clinker coming out of the kilns is air cooled in special types of coolers and then transported to the storage. After aging in storage for at least three days the clinker, mixed with. The right amount of gypsum is fed to the cement grinding mills and ground to required fineness. The cement is stored in silos, drawn for packing in gunny bags and dispatched by wagons or trucks to the dealers. Types Drives The driving motors used in the cement industry can be broadly classified is follows: Raw mill and cement mill drives Kiln drives Crusher drives Waste gas fan drives Compressor drives etc. Raw mills and cement mill drives: Slip ring induction motors of 6.6 kV are widely used. In order to improve the power factor of the line current drawn, high voltage capacitors of adequate reliability and automatic capacitor control switchgear and circuit breakers are to be used. Even after adding the price of the capacitors and the control gear, the slipping motor is cheaper than the synchronous motor of the same rating. Liquid resistance starters are, usually employed to start the motor and to bring it up to full speed. Gear boxes are also attached in order to get the desired mill speed of about 15 rpm. From the point of view of voltage dips during starting. The starting current of mill drives for large cement plants is normally restricted to 1.75 times the full load current. The starting torque for the mill motors for large cement plants is limited to 125 per cent of the rated torque and the pull out torque is restricted to nearly 240 per cent of the rated torque. Normally, the motors should be able to withstand 50 per cent overload for one minute occurring four times per hour at equal intervals. The motors for such drives are generally designed for
motor drive system must be designed such that motive power is supplied in equal parts by the two motors to prevent overloading of either one of them or its mechanical transmission system. The speed of the two motors must necessarily be the same because they are coupled through the gear system. This can be achieved either with series connection of the two de motors or with parallel connection by means of a closed loop control system. Crusher drives: The motors used in crushers are of the slip ring type. Stalling considerations play a very important part in the design of these motors. Normally, the motors are designed to withstand locked rotor current during running without any external resistance introduced in the rotor circuit, for one minute. This is quite important, since very often the crushers tend to get jammed, when a big sized boulder gets trapped between the jaws of the crushers. Generally, the starting torque for such drives is limited to 160 per cent of the full load torque and the pull out torque is limited to 200 per cent to 250 per cent of the full load torque. The motors are also normally designed for IS per cent overload for 15 seconds and 20 per cent overload for 10 seconds taking into consideration the adverse loading conditions encountered in practice. Fan drives: The motors used are of the slip ring type with a speed variation, generally, between 1000 and 750 rpm. The cast iron grid resistance controller’s arc normally used for starting and controlling the speed of these drives. As the motors are located outdoor or in semi-outdoor locations totally enclosed motors of TEFC are employed.