Selection of AC contactor for controlling electric heating equipment

This type of equipment includes resistance furnaces, temperature adjustment equipment, etc. The wire-wound resistance elements used in the electric heating element load can reach 1.4 times the rated current. If the power supply voltage increase is considered, the current will increase. The current fluctuation range of this kind of load is very small, it belongs to AC-1 according to the use category, and the operation is infrequent. When selecting a contactor, it is only necessary to make the rated operating current Ith of the contactor equal to or greater than 1.2 times the operating current of the electric heating equipment.
3.2 Selection of contactors for controlling lighting equipment
There are many types of lighting equipment, and different types of lighting equipment have different starting current and starting time. The use category of this type of load is AC-5a or AC-5b. If the start-up time is very short, the heating current Ith can be selected to be equal to 1.1 times the operating current of the lighting equipment. The start-up time is longer and the power factor is lower, and its heating current Ith can be selected to be larger than the operating current of the lighting equipment. Table 2 shows the selection principles of contactors for different lighting equipment.
Selection principles of contactors for different lighting equipment
Serial number Lighting equipment name Starting power supply Power factor Starting time Contactor selection principle
1 incandescent lamp 15Ie1Ith≥1.1Ie
2 Mixed lighting 1.3Ie≈13Ith≥1.1×1.3Ie
3 Fluorescent lamp ≈2.1Ie0.4～0.6Ith≥1.1Ie
4High-pressure mercury lamp≈1.4Ie0.4～0.63～5Ith≥1.1×1.4Ie
5 metal halide lamp 1.4Ie0.4～0.55～10Ith≥1.1×2Ie
6 Lamps with power printing number compensation 20Ie0.5～0.65～10 are selected according to the starting current of the compensation capacitor
3.3 Selection of contactors for controlling electric welding transformers
When the low-voltage transformer load is connected, the transformer will have a short-term steep high current due to the short-circuit of the electrodes on the secondary side, and a large current will appear on the primary side, which can reach 15 to 20 times the rated current. related to core characteristics.When the electric welding machine frequently generates sudden strong current, the switch on the primary side of the transformer
>Under huge stress and current, the contactor must be selected according to the short-circuit current and welding frequency of the primary side when the electrodes are short-circuited under the rated power of the transformer, that is, the switching current is greater than the primary-side current when the secondary side is short-circuited. The usage category of such loads is AC-6a.
3.4 Selection of motor contactor
Motor contactors can choose AC-2 to 4 according to the use of the motor and the type of the motor. For the starting current at 6 times the rated current and the breaking current at the rated current, AC-3 can be used. For example, fans, pumps, etc., can use the look-up table The method and the selected curve method are selected according to the sample and manual, and no further calculation is required.
The winding current and breaking current of the wound motor are both 2.5 times the rated current. Generally, when starting, a resistor is connected in series with the rotor to limit the starting current and increase the starting torque. The use category is AC-2, and a rotary contactor can be selected.
When the motor is jogging, running in reverse and braking, the connected current is 6Ie, and the use category is AC-4, which is much harsher than AC-3. Motor power can be calculated from the currents listed under Utilization Category AC-4. The formula is as follows:
Pe=3UeIeCOS￠η,
Ue: motor rated current, Ie: motor rated voltage, COS￠: power factor, η: motor efficiency.
If the life of the contact is allowed to be short, the AC-4 current can be appropriately increased, and it can be changed to AC-3 at a very low on-off frequency.
According to the requirements of motor protection coordination, the current below the locked-rotor current should be connected and broken by the control device. The locked-rotor current of most Y series motors is ≤7Ie, so the opening and closing locked-rotor current should be considered when selecting a contactor. The specification stipulates that when the motor is running under AC-3 and the rated current of the contactor is not greater than 630A, the contactor should be able to withstand 8 times the rated current for at least 10 seconds.
For general equipment motors, the working current is less than the rated current, although the starting current reaches 4 to 7 times the rated current, but the time is short, and the damage to the contacts of the contactor is not large. This factor has been considered in the design of the contactor, and it is generally selected The contact capacity should be greater than 1.25 times the rated capacity of the motor. For motors working under special circumstances, it should be considered according to the actual working conditions. For example, the electric hoist belongs to the impact load, the heavy load starts and stops frequently, reverse connection braking, etc., so the calculation of the working current should be multiplied by the corresponding multiple, because the heavy load starts and stops frequently, choose 4 times the rated current of the motor, usually reverse connection under heavy load The braking current is twice the starting current, so 8 times the rated current should be selected for this working condition.