The increasing tendency in modern machine construction is to eliminate waste motion by dispensing with as much connecting mechanism as possible and instead bringing the source of power as near as possible to the machine spindle.
Automatic controllers are employed which incorporate overload and under voltage protection as well as provision for securing dynamic braking (which is generally required on machines of capacity more than 10 h.p.). With a.c. supply the squirrel cage motor can often be started straight on, and the control gear is simply a triple-pole switch with under voltage and overload releases.
This may comprise an electrically operated contactor with push button control, the equipment being conveniently mounted under the end of the lathe bed or alongside the machine. The control for d.c. motors comprises a drum-type starter, overload trips, electrically operated main switch providing under voltage feature and isolator.
Such a unit with a ratchet handle gives step-by- step operation and allows inching the motor by push-button while setting-up. Stop buttons can also be located at suitable points. Small capacity a.c. motors are generally direct on line starting type and large capacity motors utilise star-delta starter. The control circuits for both these types are shown in Fig. 33.14 and 33.15.
In sophisticated machines, a complete scheme of interlocking is arranged to prevent faulty sequence of control by the operator. The push-button station is mounted in a convenient operating position and the control panel may be either mounted on one of the uprights (in case of small machines) or located separately on the floor. Usually the main contactor is interlocked with the door of the switch gear compartment so that before the door can be opened all the control gear must be rendered dead.
Machine can be conveniently controlled by end switches in conjunction with adjustable dogs.
Recommended Practice for Electrically Controlled Machine Tools:
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The following points should be carefully observed:
(i) Control equipment should be mounted in suitable compartments and enclosures in order to ensure adequate protection against dust, against mechanical damage and protection against the ingress of chips, oils or coolants.
(ii) The accessibility of the installed electrical apparatus should be good. The apparatus should be mounted at least 200 mm above the floor level thereby assuring easy accessibility for maintenance, adjustment and checking purposes.
(iii) The operating devices such as switches, push buttons, etc. should be mounted within easy reach of the operator.
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(iv) Electrical safety measures should be respected e.g. providing sufficient protection, prevention of accidental contact with live parts etc.
(v) When the machine has more than one motor, independent short circuit protection by fuses for all sub- circuits must be provided. The overload protection should invariably be provided on all motors rated above 1 kW.
(vi) The wires of proper cross-section be selected for wiring the electrical equipment. Outside the control enclosure, stranded copper wires should be used.
(vii) As far as possible, totally enclosed motors should be used, and these should be so mounted that these can be easily replaced for maintenance, and other parts need not be dismantled for removing the motor. The location of the motor should also take care of its proper ventilation to avoid its overheating.
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(viii) Earthing of the whole machine tool, motors and all exposed metal parts of the machine tool and accessories should be provided in accordance with the relevant Indian Standard. The earth continuity conductor should be coloured green.
(ix) The electrical equipment (motors, contactors etc.) used should comply with the requirements laid down in the appropriate Indian Standards thereby assuring the interchange-ability and the technical quality.