SU68498A1 - Method of thermal protection of electric drives - Google Patents

Description

In modern automated electric drives operating in a wide variety of modes, in particular, in the modes of short-term and repeated-short-term work, the thermal protection of the engine must ensure, on the one hand, maximum use of the engine, and on the other, to protect it from unacceptable overheating. Thermal relays with bimetallic elements (thermal relays) widely used for this purpose do not always satisfy the requirements of the most advantageous operation of the engines, which is explained by the discrepancy between the thermal characteristics of the thermal switches and the motors they protect. When choosing a thermostat, it is usually limited to elementary considerations on the correspondence of the rated currents of the relay and the motor and, in essence, do not take into account the operating mode of the latter at all. Thus, the operation of the thermal switch in a number of cases largely approaches protection with fuses, while the task of the thermal switch is to operate, approaching the thermal copy of the protected motor. Since a well-defined, sufficiently good results can be achieved using a conventional thermal switch, but with a special selection of the value of the rated current of the relay depending on the rated current of the motor. such a calculation will not reach the goal. the electric motor will be the best if the values of the constant time of the relay and the electric motor are equal (tr and Tiv). However, this equality is difficult to achieve due to constructive considerations and almost always the ratio Tr. In order to improve motor protection, some workarounds for changing the characteristics of a relay are used in order to improve the protection of the motor, for example, those in which the relay is activated according to the load characteristic of the engine. If the schedule is intermittent load It is obvious that the quality of protection In varying known conditions, this is done either by elucidating the magnitude of the operation current (i.e. by adjusting the relay) or by changing the current flowing through the relay.

The first is carried out with the help of special designs of relays, in which the magnitude of the contact stroke, necessary for closing (opening) the contactor control circuit, varies depending on the current ratio in the motor circuit.

Changing the amount of current flowing through it through the relay, depending on the current ratio in the electric motor, is performed in various ways, for example, by bypassing the thermal relay with variable resistances.

It is also possible to set up several stages of the current in the relay by applying a number of shunt resistors, which are switched on when certain current ratios of the engine are reached with the help of additional maximum relays.

The object of the present invention is a method of thermal protection of electric drives using a device in the form of several thermal relays. The method is based on the automatic coordination of the thermal characteristics of the engine and the protective device. According to the invention, the set point of the protective device is changed stepwise according to the method of obtaining the characteristics of the protection of the electric drive using the envelopes of this characteristic thermal characteristics of the individual relays entering the device.

The proposed device for implementing this method of thermal protection from G; It uses several thermal switches with different settings, the working contacts of which are included in the control circuit of the line contactor through the contacts of maximum relays with different settings in such a way as to ensure the activation of the corresponding relay in a certain load range.

The invention is explained in FIG. 1, which presents the thermal performance of the engine and

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protective device. FIG. Figure 2 shows a sweep of the switching circuit of the relay contacts for the case of using five relays, and FIG. Z and ZH is another device variant (schel and characteristics).

The diagrams accept the following notation:

TPj ... TPj - characteristics or contacts of the thermal relay: PAl- .... PM -contacts of maximum relays; D - thermal characteristic of the engine; L / - line contactor;

O and L - stop and start buttons.

For a stepwise change in the magnitude of the current multiplicity in the relay, two or more relays are selected (depending on the engine power) with different ratios of their rated currents to the rated current of the engine, and one of the relays has a rated current equal to the rated current of the engine.

The relays are selected so that certain parts of their characteristics closely enough coincide with those or other sections of the thermal characteristics of the engine, as shown in FIG. 1. The switching circuit of the thermal switch in a certain load range should automatically provide the entry into effect of the corresponding relay. Such a task can be accomplished, for example, using the circuit shown in FIG. 2, where n - 7 maximum relays with different setpoint currents are used for the adventure of the n thermal switch. The PM relays provide the required order of switching the working contacts of one or another thermal relay into the circuit of the linear contactor.

In most cases, a circuit with two thermal switches and one maximum relay should provide satisfactory results.

In the proposed device, it is possible to use two thermal relays, one of which is directly connected to the motor circuit, and the second through a serial-type transformer, saturating at a certain value of the load current.

The essence of this modification is explained in FIG. 3, which shows the switchings of the thermal switch (a) and the thermal characteristics of the relay and the motor (b) for the case of two relays.

The use of one or another device modification is determined by the engine power and exploitation conditions.

Subject invention

Claims (3)

1. Method of thermal protection of electric drives using a device in the form of several thermal relays, based on automatic matching of thermal characteristics of an engine and a protective device, characterized in that the setpoint of the latter is changed stepwise according to the method of obtaining the character of 1stick of protection of the electric drive using the envelopes of this characteristic of the thermal characteristics of the individual relays entering the device. 2. A device for carrying out the method of clause 1, starting with the use of several thermal switches with different set points and whose working contacts are included in the control circuit of the linear contactor through the contacts of the max. Relays with different settings such in order to ensure that the corresponding relay enters into action in a certain load range. 3. In the device of claim 2, there are: the development of two thermal switches, one of which is directly connected to the motor circuit, and the second through a serial-type transformer that saturates at a certain value of the load current. pin , FIG. 2 TPf apper