JPH11206001A - Protector for motor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To accurately and swiftly detect delays in starting of a motor. SOLUTION: An induction motor IM is provided with a three-phase power supply via an electromagnetic contactor VCS. A sequence control circuit is provided with a power supply via a transformer PT for obtaining a control supply voltage. A phase U and a phase W of the three-phase power supply UVW provided to the induction motor 1M are provided a with a current transformer CT, respectively. The secondary-side outputs of the current transformers CT are supplied to a 2E-relay 2ERY which detects overloading and disconnection in the induction motor IM. The 2E-relay 2ERY is provided with a start delay detection relay SRY on its output side. The start delay detection relay SRY monitors current every second, for example, and compares the present current value with the previous value. If the current value has not decreased, that is, the current value is constant regardless of the lapse of time, the start delay detection relay SRY sends out a contact signal. The contact SRYb of the sequence circuit is turned off by the contact signal, to stop the starting of the induction motor IM.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a motor protection device at the time of starting the motor.

[0002]

2. Description of the Related Art Normally, a thermal relay, a timer, and the like are used to protect an electric motor from an overcurrent or the like.
When the current does not decay within the allowable overcurrent-time characteristic of the motor, as shown in FIG. 2, the thermal relay starts congesting the motor (when the motor is started, the starting stops and the starting current becomes constant). This is not the detection of the start congestion itself. In addition, thermal relays have poor accuracy, so the motor will run for longer than the normal starting time,
It is designed so that there is no problem even if it is restrained.

The amount of heat generated during start-up of the electric motor is about 1/2 to 1/3 of the amount of heat generated when the motor is restricted for the start-up time. For this reason, usually, the electric motor is configured such that no trouble occurs even if the electric motor is restrained for a time corresponding to the starting time.

[0004] In the case of protecting the motor by a timer, it is determined whether or not there is congestion in the starting operation based on whether or not the starting is completed within the starting time at the minimum voltage.

[0005]

As described above, the protection of the electric motor by the thermal relay is configured so as not to cause any trouble even if the electric motor is restrained for a time corresponding to the starting time. It must have a heat capacity two to three times that of the case where the power supply is immediately disconnected from the power supply and protected.

Further, in the case of protecting the motor by a timer, the motor must have a thermal characteristic having an allowable restraining time longer than the starting time at the minimum voltage even at the maximum voltage.
A heavy inertial load such as a blower makes the configuration extremely difficult.

Further, the allowable restraint time of the motor changes due to the fluctuation of the power supply voltage. However, the above-mentioned relay cannot automatically correct the fluctuation due to the fluctuation of the power supply voltage. Therefore, it is necessary that the electric motor sufficiently withstand the start traffic until the relay operates reliably.

The present invention has been made in view of the above circumstances, and it is an object of the present invention to provide a motor protection device capable of accurately and quickly detecting a start-up congestion of a motor.

[0009]

SUMMARY OF THE INVENTION In order to achieve the above object, the present invention provides an electromagnetic contactor, an induction motor to which three-phase power is supplied via the electromagnetic contactor, and an overdrive of the induction motor. A first relay for detecting a load or disconnection, and a second relay provided on the input side or the output side of the first relay and operating when the starting current of the induction motor has not decreased from the previous value. When the second relay is activated,
The electromagnetic contactor is opened.

[0010]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a circuit diagram illustrating an embodiment of the present invention. In FIG. 1, IM denotes an induction motor, VC
S is an electromagnetic contactor, and PT is a transformer for obtaining the control power supply voltage. Current transformers CT are provided for the U-phase and the W-phase of the three-phase power supply UVW supplied to the induction motor IM, respectively.

The output of the secondary side of the current transformer CT is an induction motor IM.
Is supplied to a 2E relay 2ERY for detecting an overload or a disconnection of the relay. 2E relay 2ERY output start congestion detection relay
SRY is provided. This start traffic jam detection relay SRY is, for example,
The current is monitored every second and a contact signal is sent if the current has not decreased from the previous value, that is, if the current has a constant value regardless of the lapse of time as shown in FIG. The contact signal turns off a contact SRYb of a sequence circuit described later. A is an ammeter.

As shown in FIG. 2, the starting current of the motor is
The current decreases as the rotation speed increases. However, if a start congestion occurs on the way, the current does not change (decrease) at that time. Accordingly, when the start congestion detection relay SRY detects whether or not the current has stopped decreasing before the start is completed, the occurrence of the start congestion of the induction motor IM can be immediately detected.

The sequence circuit is a control circuit for operating the induction motor IM. When the operation switch SW is pressed, the exciting coil VCSX of the electromagnetic contactor VCS is energized, and the electromagnetic contactor VCS is turned on. The excitation coil VCSX is held by itself. The starting current of the induction motor IM that has started operation decreases with time with the characteristics shown in FIG. However, when there is traffic congestion during the start, the current does not decrease and becomes constant.

Then, the starting congestion detection relay SRY detects this constant current and turns off its contact SRYb, so that the excitation coil VCSX in the sequence circuit is deenergized and the electromagnetic contactor
Release VCS. Thereby, the starting of the induction motor IM is stopped.

The stop switch SW in the sequence circuit
Is a contact for stopping the operation, and 2ERYb is a contact that turns off after a certain time limit when the 2E relay detects an overload or the like.

[0016]

As described above, according to the present invention,
Since the starting congestion of the motor can be immediately detected, it is possible to employ a motor having a small allowable restraint time characteristic. There is an advantage that special formation is not required. Since the starting congestion detection relay compares only the increase and decrease of the current, the absolute value does not matter, so there is an advantage that it is not necessary to set the current value and the time characteristic each time. The load tolerance can be reduced. Furthermore, even in the case of starting congestion due to factors on the load side, it can be detected immediately,
It is possible to start measures while the trouble in the load equipment is minor.

[Brief description of the drawings]

FIG. 1 is a circuit configuration diagram illustrating an embodiment of the present invention.

FIG. 2 shows the starting current of the electric motor and the time when the starting congestion occurs.
FIG.

[Explanation of symbols]

 SRY… Start congestion detection relay VCS… Electromagnetic contactor IM… Induction motor 2ERY… Relay CT… Current transformer

Claims (1)

[Claims] 1. An electromagnetic contactor, an induction motor to which three-phase power is supplied via the electromagnetic contactor, a first relay for detecting an overload or disconnection of the induction motor, and an input of the first relay And a second relay that is provided on the output side or the output side, and that operates when the starting current of the induction motor has not decreased from the previous value. When the second relay operates, the electromagnetic contactor is opened. A protective device for an electric motor, characterized in that: