In case of belt conveyors
Right Answer is:
Double Squirrel cage motors with direct-on-line starters are used
Generally, a belt-type conveyor cannot be started in the same way as the most electric motor driven loads because the belt constitutes a large, unwieldy mass resting along its length on a large number of rollers, all of which tend to resist starting. In this particular system, a Double squirrel cage induction motor drives the input element of the coupling, and the output element of the coupling drives the conveyor. Hence, the motor can be brought up to full speed where it can develop full load torque before it is required to start the load.
A Double Squirrel cage motor is powerful enough to drive a loaded conveyor belt in the normal way but if the inertia is large and the power required considerable the motor may well be distressed when starting from rest. The starting current of a motor can be as high as 600% full load current, and if stalled there is no cooling air for its fan. This effect can be reduced by direct online starting.
In DOL starting method the motor will draw a very high inrush current for a very short time. As the motor accelerates, the current will begin to drop, but will not drop significantly until the motor is at a high speed, typically about 80 – 85% of synchronous speed.
The torque developed by the motor exceeds the load torque at all speeds during the start cycle, the motor will reach full speed. If the torque delivered by the motor is less than the torque of the load at any speed during the start cycle, the motor will stop accelerating.