Automatic lifting bracket work Current

Time：2021-03-26 View：

Rated Current refers to the current when the electrical equipment operates according to the rated power under the rated voltage. It can also be defined as the current that electrical equipment can work continuously for a long time under rated environmental conditions (ambient temperature, sunshine, altitude, installation conditions, etc.). The current of the electrical appliance during normal operation should not exceed its rated current.

 

Definition
The rated current of electrical equipment refers to the allowable current when the electrical equipment works continuously for a long time under the rated environmental conditions (ambient temperature, sunshine, altitude, installation conditions, etc.). When the electrical appliance is working, the current should not exceed its rated current.

 

Nature
Rated current is the current flowing through the winding terminal calculated by dividing the rated capacity of the winding by the rated voltage of the winding and the corresponding coefficient (single phase is 1, three phase is √ 3). Therefore, the rated current of transformer is the rated current of each winding, which refers to the line current. However, the single-phase transformer which forms a three-phase Group, for example, if the winding is triangular connection, the rated current of the winding is represented by line current as molecule and √ 3 as denominator, for example, line current is 500A, then the rated current of the winding is (500/√ 3)A.
When the transformer is running under the rated capacity, the current of the winding is the rated current. According to the national standard GB/T151641-1994 "oil-immersed power transformer load guidelines", the transformer can be overloaded. When the rated capacity of three phases does not exceed 100 MVA (single phase does not exceed 33.3 MVA), it can withstand occasional overload with a load rate (load current/rated current) of not more than 1.5, and can withstand occasional overload with a load rate of not more than 1.3 when the capacity is larger.
The casing should also have corresponding overload capacity, and the winding hot spot temperature and the top oil temperature should not exceed 140℃ and 105℃ respectively.

Calculation method and application of special circumstances
The rated current is generally calculated by the following formula:
Rated current = rated power/rated voltage (I = P/U)
Rated current I =
The rated current of three-phase motor refers to the line current of incoming line of the motor power supply. For a star-connected motor, the line current is equal to the phase current. For a triangle-connected motor, the line current is equal to the phase current of 3 times the root number.
Therefore, the calculation formula of rated current:
P -- rated power of motor;
U -- Motor line voltage;
Μ -- full load efficiency of motor;
cos φ --- power factor when the motor is fully loaded.

Rated current selection
(I) selection of rated current of ordinary reactor
(1) the reactor has almost no load capacity, so the reactor of the main transformer or outlet circuit should be selected according to the maximum work current of the circuit, instead of the normal continuous work current.
(2) for the reactor of the sectional circuit of the bus of the power plant, the maximum generator should be cut off according to the accident on the bus, which may be selected through the current of reactance simplicity. Generally, 50% ~ 80% of the rated current of this generator is taken.
(3) the reactance of substation busbar sectional circuit should meet the requirements of users' first-class load and most second-class load.
(Ii) selection of rated current of split reactor
(1) when the generator or main transformer circuit of the field power plant is generally selected according to 70% of the generator or main transformer rated current.
(2) when used in the main transformer circuit of the substation, it should be selected according to the maximum load current passing through the arm with large load current. When there is no load data, it is generally selected at 70% of the primary transformer rated current.

Rated current and starting current
When the motor outputs rated power, the current flowing through the power supply circuit is called rated current, and the unit is A (ampere). When the motor is just connected to the power supply, the current through staor winding is called starting current. When the current is just connected, the staor winding generates a rotating magnetic field. Because the rotor has not yet rotated, the relative motion speed between the stator magnetic field and the rotor is the largest, and the rotor winding cuts staor winding of the rotating magnetic field at the maximum speed, therefore, the maximum induced current is generated in the rotor, and the induced current passes through the inductive action of the air gap magnetic field to increase the current of staor winding. Therefore, the starting current of the motor can reach 6-9 times of the rated current.
Larger starting current will cause the voltage drop of the circuit to increase, affect the use of the same circuit electrical appliances, prolong the starting time of the electrical appliances, and cause the temperature of the motor to rise. Therefore, it is hoped that the smaller the starting current is, the better. In the test, the blocking current is usually used instead of the starting current. Locked-rotor current is the effective value of steady-state current input from the power supply circuit measured when the rotor is blocked under rated voltage and rated frequency.