Is an emergency stop arrangement needed between the motor and the machine?

It is required on most machines whose moving parts can injure people; however, this arrangement belongs not to the motor but to the machine's control and safety circuit. The emergency stop command cuts the motor's power through a contactor, a safety relay or the drive's safe stop input. Which level is required on which machine is determined by machinery safety legislation and a risk assessment.

Does the machine stop immediately when power is cut?

Not always. A standard induction motor has no brake; when power is cut, the shaft coasts freely until the inertia of the rotating masses is spent. A light fan may stop in a few seconds, whereas a crusher with a large pulley, a press with a flywheel or a saw keeps turning for much longer. This stopping time must be taken into account when designing the emergency stop.

Ways to shorten the stop

• Brake motor: The shaft is held by a mechanical brake; DRG's standard range does not include brake motors.
• Braking with a drive: A frequency inverter can perform controlled deceleration; with large inertias a braking resistor may be needed.
• Brake on the machine: On some machines the brake is placed at another point of the drive train instead of on the motor.
• Guard interlock: An arrangement that prevents the guard from opening until motion has completely ceased.

Positioning the emergency stop

The button must be easily reachable from every point where the operator works and must stay latched when pressed. On long conveying lines, pull-wire switches are used along the line. Releasing the button must not restart the machine; restarting must require a separate command. The design and implementation of the circuit depend on machinery safety rules, the motor nameplate and the judgement of qualified electrical personnel.

As an example of heavy applications where long stopping times are pronounced, see the stone crushing plant motors page, and view models by power and speed in the shop listing.