Protection of electric motors up to 1 kV (asynchronous, synchronous and direct current)

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

Protection of electric motors up to 1 kV (asynchronous, synchronous and direct current)

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

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5.3.55. For AC motors, protection against multi-phase short circuits (see 5.3.56) must be provided, in networks with a solidly grounded neutral - also from single-phase short circuits, and in the cases provided for in 5.3.57 and 5.3.58 - in addition, protection against currents overload and undervoltage protection. On synchronous electric motors (if it is impossible to retract into synchronism with full load), protection against asynchronous operation must be additionally provided according to 5.3.59.

Short circuit protection must be provided for DC motors. If necessary, additional protection against overload and against excessive increase in speed can be installed.

5.3.56. To protect electric motors from short circuits, fuses or circuit breakers must be used.

The rated currents of the fuse links of the fuses and the releases of the circuit breakers must be selected in such a way as to ensure reliable disconnection of the short circuit at the terminals of the electric motor (see 1.7.79. , starting currents, self-starting currents, etc.) were not switched off by this protection. To this end, for electric motors of mechanisms with light starting conditions, the ratio of the starting current of the electric motor to the rated current of the fuse-link should be no more than 3.1.8, and for electric motors of mechanisms with difficult starting conditions (long acceleration, frequent starts, etc.) this ratio should be equal to 2,5-2,0.

For electric motors of critical mechanisms, for the purpose of especially reliable detuning of fuses from current surges, it is allowed to take this ratio equal to 1,6, regardless of the conditions for starting the electric motor, if the multiplicity of the short-circuit current at the motor terminals is not less than that specified in 3.1.8.

It is allowed to implement protection against short circuits by one common device for a group of electric motors, provided that this protection ensures the thermal stability of starting devices and overload protection devices used in the circuit of each electric motor of this group.

At power plants, circuit breakers should be used to protect against short circuits for auxiliary electric motors associated with the main technological process. In case of insufficient sensitivity of electromagnetic releases of automatic switches in the system of auxiliary needs of power plants, remote current relays with action on the independent release of the switch can be used.

To reliably ensure the selectivity of protection in the power supply network for the auxiliary needs of power plants, it is recommended to use electromagnetic cut-off releases as protection of electric motors from short circuits.

5.3.57. Protection of electric motors against overload should be installed in cases where it is possible to overload the mechanism for technological reasons, and also when, under especially difficult starting or self-starting conditions, it is necessary to limit the duration of the start at low voltage. The protection must be time-delayed and may be provided by a thermal relay or other devices.

The overload protection must act to trip, signal or unload the machine if unloading is possible.

Overload protection is not required for intermittent duty motors.

5.3.58. Undervoltage protection must be installed in the following cases:

for DC motors that do not allow direct connection to the network;
for electric motors of mechanisms, the self-starting of which after a shutdown is unacceptable due to the conditions of the technological process or for safety reasons;
for some other electric motors in accordance with the conditions given in 5.3.52.

For critical electric motors that require self-starting, if they are switched on using contactors and starters with a holding winding, mechanical or electrical time delay devices must be used in the control circuit to ensure that the electric motor is switched on when the voltage is restored within a specified time. For such motors, if it is permissible under the conditions of the technological process and safety conditions, it is also possible to use switches instead of control buttons so that the holding winding circuit remains closed in addition to the auxiliary contacts of the starter and this ensures automatic reverse switching on restoration of voltage, regardless of the time of power interruption.

5.3.59. For synchronous motors, protection against asynchronous operation should normally be provided by a stator overcurrent protection.

5.3.60. Short circuit protection in AC and DC motors should be provided for:

1) in electrical installations with a grounded neutral - in all phases or poles;

2) in electrical installations with isolated neutral:

when protected by fuses - in all phases or poles;
when protected by automatic switches - at least in two phases or one pole, while within the same electrical installation protection should be carried out in the same phases or poles.

Protection of AC motors against overloads must be carried out:

in two phases when protecting electric motors from short circuit with fuses;
in one phase when protecting electric motors from short circuit by automatic switches.

Protection of DC motors against overloads must be carried out in one pole.

5.3.61. Motor protection devices must meet the requirements of Ch. 3.1. All types of protection of electric motors against short circuit, overload, minimum voltage are allowed to be carried out by the corresponding releases built into one device.

5.3.62. Special types of protection against operation on two phases can be used as an exception on electric motors that do not have overload protection, for which there is an increased probability of losing one phase, leading to the failure of the electric motor with serious consequences.

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

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