RU2216839C2 - Electric motor overload protective device - Google Patents

Abstract

FIELD: electrical engineering. SUBSTANCE: phase-failure and overload relay protective device for three- phase motors has two current transformers and two thermal relays. Current transformers are saturated at starting currents and maximum reduction on secondary current through these transformers occurring at low ratios of overload current are compensated for by slight correction of secondary turns. EFFECT: enhanced reliability of motor phase-failure and overload protection. 1 cl, 1 dwg

Description

 Scope: protection of electric motors.

 The essence of the invention: the inclusion of thermal relays through measuring current transformers, saturated at inrush currents, and a small decrease in the secondary current in these transformers at small multiples of the overload current is compensated by a small correction of the secondary turns. The protection circuit contains two saturable measuring current transformers TLA1 and TLA2 with correction of the secondary turns connected to the thermal relays KST1 and KST2, respectively (see drawing).

 The invention relates to electrical engineering, and in particular to devices for the relay protection of three-phase electric motors from two-phase operation and overload.

 A device for protecting electric motors from overload and phase failure, based on the use of thermal relay types TRN-10A, TRN-10, TRN-25, TRN-40, RTT, TRP-25, TRP-60 and others / 1 /. These relays are connected directly to the load power circuit or through linear measuring current transformers (TK-20), starting with a power of 90 kW.

 The disadvantages of this device are low reliability due to the mismatch of the heating time constant of the thermocouple to optimal / 2 / due to the use of massive thermocouples and the imposition of a large time constant by current collectors to the measuring thermal element / 3 /, as well as non-selective disconnection of the load with a large current multiplicity in the thermal relay during starts. In these cases, PUEs allow to increase the setpoint of operation, which leads to a failure of protection at low current multiples and damage to electric motors.

 Known thermal protection devices that do not need to be rebuilt from inrush currents, however, they have a high temperature inertia and, compared with protection on thermal relays, are difficult to implement, install and maintain.

 The closest in technical essence is the device / 4 / for protection of three-phase electric motors, containing measuring current transformers and thermal relays. Moreover, in the known device thermal relays are connected in the same way as in the claimed invention, through measuring current transformers, saturated at inrush currents. However, in this device, a decrease in the secondary current in the thermal relay is observed at small multiples of the overload current of the protected motor. In absolute terms, this decrease is a small amount, but it leads to a significant decrease in the sensitivity of the protection and even a failure in operation when the phase conductor breaks.

 The essence of the proposed device lies in the fact that the measuring current transformers are saturated at inrush currents, and a small decrease in the secondary current in these transformers at small multiples of the overload current is compensated by a small correction of the secondary turns.

 The drawing shows a diagram of the proposed device. The motor protection circuit contains two saturable measuring current transformers TLA1 and TLA2, the secondary circuits of which are connected to the thermal relays KST1 and KST2, respectively.

 The contacts of the thermal relays control the coil of the KM 1.1 magnetic starter. The magnetic starter is connected to the mains via the SF1 disconnector.

 In case of overloads of the protected circuit due to the technological process or phase failure, the overload current through at least one current transformer TLA1 or TLA2 is supplied to the thermal relay. At small overload multiplicities or a break in one phase, the primary current is transformed without distortion due to a winding correction up to a factor of about two. With heavy starts, the primary circuit current increases to 5-7 rated motor currents. In this case, the measuring current transformers are saturated, and the secondary current of low multiplicity is supplied to the thermal relays. The response time of the relay increases. In this case, a heavy start of the engine occurs without shutting off the protection. Thus, in the proposed protection, the successful start of the engine is carried out not due to the coarsening of the thermal relay, but due to the saturation of the measuring current transformers.

 This protection ensures the successful starting of motors with a start time of up to 40-50 s while maintaining the same sensitivity at low overload ratios as when using serial linear current measuring transformers.

 The technical and economic efficiency of the device is formed due to a significant simplification of the design, the absence of coarsening of the protection at low overloads and the reduction of damage associated with damage to the insulation of the engine.

 In addition to the indicated advantages, the use of saturable measuring current transformers with round correction allows to prevent the combustion of thermocouples at short circuit currents.

Literature
1. Andreev V. A. Relay protection, automation and telemechanics in power supply systems. - M .: Higher school, 1985

 2. Industrial energy, 10, 1998. Zinchenko V.F., Kimketov M.D. - Optimization of the functioning of thermal protection of electric motors.

 3. Industrial power engineering, 1, 1999. Zinchenko V.F., Kimketov M.D. "Zinchenko L.V., Kimketov M.M. - Effect of down conductors on the functioning of thermal protection relays for electric motors.

 4. USSR patent 407414, 1973, H 02 H 5/04.

Claims (1)

 A device for protecting the motor against overload, containing measuring current transformers and thermal relays, while the thermal relays are connected through measuring current transformers, characterized in that the measuring current transformers are saturable at starting currents that increase up to 5-7 times the rated currents, and the secondary current is reduced in these transformers at low overload ratios it is compensated by the correction of the secondary turns.