N2 Electrical Trade Theory|The Easy Way! 67
4.9 Starting of induction motors 4.9.1 Purpose and function of a motor starter
Te purpose of a motor starter is to start a motor manually or automatically from a position close to the motor or remotely (some distance away). Te function of a motor starter, other than a direct-on-line starter, is to reduce the current drawn by the motor while the rotor accelerates.
▲ 6.16.5.1.4 Any manually operated device used to control a motor shall be readily accessible to the person who operates it.
▲ 6.16.5.1.5 Each motor shall be supplied by a manually operated disconnector or any other manually operated disconnecting arrangement such as a withdrawable circuit-breaker, a removable link, a fuse or by the removal of a plug from a socket- outlet, which provides at least the same isolating distance, for the sake of safety, as a disconnector that is; (a) readily accessible and mounted on or next to the motor, or (b) visible from the motor, or (c) lockable in the open position, or (d) housed in a lockable enclosure other than a distribution board.
Alternatively, the motor may be controlled by any manually operated disconnecting arrangement that provides at least the same isolating distance, for safety, as a disconnecting device.
Motor starters
▲ 6.16.5.3 A no-volt coil must be included in all motor starters other than direct-on- line motor starters to ensure that the circuit will open if the supply voltage fails or falls to a value that would cause the motor to stop. When unexpected restarting of a motor, such as in the case of the supply voltage being unexpectedly restored, could cause injury to the operator of the motor, the starter shall have a means of preventing the motor from restarting, irrespective of the type of starter used.
Undervoltage protection is usually obtained by means of a no-volt coil. A separate relay coil may be used but in the case of contactors, the contactor coil usually serves this purpose.
4.9.2 Direct-on-line starting
A direct-on-line starter is used to start a motor easily, quickly and cheaply while supplying a high starting torque. Direct-on-line starting is when a motor is started by switching it directly across the supply. Direct-on-line starting is applied when the motor and starting torque are relatively small.
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