Switching devices

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Section 5. Electric power plants

Electric motors and their switching devices. Switching devices

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Encyclopedia of radio electronics and electrical engineering / Rules for the installation of electrical installations (PUE)

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5.3.30. For a group of electric motors that serve to drive one machine or a number of machines that carry out a single technological process, as a rule, a common apparatus or a set of switching devices should be used if this is justified by the requirements of convenience or safety of operation. In other cases, each electric motor must have separate switching devices.

Switching devices in the circuits of electric motors must disconnect from the network at the same time all conductors that are energized. In the circuit of individual electric motors, it is allowed to have a device that disconnects not all conductors, if a device that disconnects all conductors is installed in the common circuit of a group of such electric motors.

5.3.31. If there is a remote or automatic control of the electric motor of any mechanism, an emergency shutdown device must be installed near the latter, excluding the possibility of remote or automatic start-up of the electric motor until the device is forced to return to its original position.

It is not required to install emergency shutdown devices for mechanisms:

a) located within sight from the place of management;

b) accessible only to qualified service personnel (for example, fans installed on rooftops, fans and pumps installed in separate rooms;

c) the design of which excludes the possibility of accidental contact with moving and rotating parts; near these mechanisms, posters should be posted warning of the possibility of remote or automatic launch;

d) having a local control apparatus with fixation of the shutdown command.

The feasibility of installing local control devices (starting, stopping) near remotely or automatically controlled mechanisms should be determined during design, depending on the requirements of technology, safety precautions and the organization of control of this installation.

5.3.32. Motor control circuits can be powered both from the main circuits and from other sources of electricity, if this is caused by technical necessity.

In order to avoid sudden starts of the electric motor when the voltage is restored in the main circuits, a blocking connection must be provided to ensure automatic shutdown of the main circuit in all cases of power failure in it, if self-starting is not provided.

5.3.33. On the housings of control devices and disconnecting devices, clear signs should be applied to easily recognize the on and off positions of the control handle of the device. In cases where the operator cannot determine by the state of the control apparatus whether the main circuit of the electric motor is on or off, it is recommended to provide a light alarm.

5.3.34. Switching devices must, without damage and abnormal wear, switch the highest currents of normal operating modes of the electric motor controlled by them (starting, braking, reversing, working). If reverses and braking do not take place in normal mode, but are possible with incorrect operations, then the switching devices in the main circuit must switch these operations without destruction.

5.3.35. Switching devices must be resistant to rated short-circuit currents (see Ch. 1.4).

5.3.36. Switching devices in terms of their electrical and mechanical parameters must correspond to the characteristics of the driven mechanism in all modes of its operation in this installation.

5.3.37. The use of plug-in contact connectors for controlling portable electric motors is allowed only if the electric motor power is not more than 1 kW.

Plug-in contact connectors used to connect mobile electric motors with a power of more than 1 kW must have a lock, in which disconnection and activation of the connection are possible only when the starting device is off in the main (power) circuit of the electric motor.

5.3.38. Turning on the windings of magnetic starters, contactors and circuit breakers in a network up to 1 kV with a grounded neutral can be carried out for phase-to-phase or phase-to-phase voltage.

When the windings of the above devices are switched on for phase voltage, it must be provided for the simultaneous shutdown of all three phases of the branch to the electric motor by an automatic switch, and when protected by fuses, special devices that act to turn off the starter or contactor when the fuses burn out in one or any two phases.

When the winding is switched on for phase voltage, its zero terminal must be securely connected to the zero working conductor of the supply line or to a separate insulated conductor connected to the zero point of the network.

5.3.39. Switching devices of electric motors fed according to the transformer-motor block scheme should, as a rule, be installed at the input from the network supplying the unit, without installing them at the input to the electric motor.

5.3.40. In the presence of remote or automatic control of mechanisms, a preliminary (before launch) alarm or sound notification of the upcoming launch should be provided. Such signaling and notification need not be provided for machinery in the vicinity of which the installation of an emergency shutdown device is not required (see 5.3.31).

5.3.41. Wires and cables that connect starting rheostats with phase rotors of asynchronous electric motors must be selected according to the continuous current for the following conditions:

operation with short-circuiting of the motor rings: at the starting static torque of the mechanism not exceeding 50% of the rated torque of the motor (easy start), - 35% of the rated current of the rotor, in other cases - 50% of the rated current of the rotor;

operation without short circuit of the motor rings - 100% of the rated current of the rotor.

5.3.42. Starting of asynchronous electric motors with a squirrel-cage rotor and synchronous electric motors should be carried out, as a rule, by direct connection to the network (direct start). If direct start is not possible, start through a reactor, transformer or autotransformer should be used. In special cases, it is allowed to use a start with a rise in the mains frequency from zero.

See other articles Section Rules for the installation of electrical installations (PUE).

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