Publish Time: 2022-08-25 Origin: Site
In the expansion project of the production line and supporting facilities of a steel plant in Fujian, Xichi CMC-MX soft starter was used for the start and stop control of the water pump unit. Since the soft starter was put into production, the equipment has been in good use, and the comprehensive motor protection function has effectively improved the service life of the pump and the reliability of power supply, ensuring the continuity of production.
Figure - CMC-MX soft starter installed in GCK low-voltage withdrawable switchgear
CMC-MX soft starter is suitable for starting and protecting general squirrel-cage AC asynchronous motors. Adapted motor power from 7.5kW to 280kW. It can smoothly start/stop the motor without steps, avoid mechanical and electrical shocks caused by traditional methods such as direct start, star/delta start, auto-coupling step-down start, etc., and can effectively reduce the starting current and power distribution capacity to avoid investment in capacity expansion. The MX model integrates the current transformer and bypass contactor inside, and users do not need to connect externally.
1. Low-voltage soft-start cabinet electrical wiring and component arrangement
The electrical components in the cabinet are arranged in order, so that the main wiring is short and does not cross, which is convenient for the connection of copper bars. The soft starter is installed on the right side of the contactor and is not affected by the heating of other components. The distance between the soft starter and the side wall of the control cabinet and other components is required to be ≥100mm, which is convenient for the soft starter to dissipate heat.
2. soft starter selection
In addition to technology, performance, and price comparison, specific conditions such as the power grid capacity of the equipment site, the severity of the equipment start-up load, and the frequency of start-up should also be considered. For equipment with lighter starting loads such as pumps, soft starters with simple functions, lower prices and easy operation can be selected. This type of equipment selects the specified soft start of the same capacity according to the rated power of the motor.
3. Isolator and fuse options
For the isolation electrical appliances in the soft-start cabinet, isolation switches or molded case circuit breakers with isolation function can be selected. The low-power soft-start cabinet should choose a knife-fuse switch with a combination of isolating switch and fuse. It not only plays the role of isolation and protection, but also reduces the project cost. The rated current of the isolating switch is greater than the rated current of the motor to meet the operating requirements.
Due to the limited surge Joule integral (I2t) value of the thyristor in the soft starter, choosing a circuit breaker as a short-circuit protection device cannot effectively protect the thyristor element. It is recommended to use a fast fuse as a short-circuit protection device. The fast fuse can choose aR or NGT type semiconductor protection fuse. The selection of fast fuse generally does not need to do breaking capacity verification, because the rated breaking capacity of aR fuse is 50kA, and the breaking capacity of NGT fast fuse is 120kA, which can meet the needs of general power distribution projects. In addition, the fuse also has a current limiting function, and the protection of the thyristor is more reliable than that of the circuit breaker.
The selection principle of the rated current of the fast fuse is that the equipment will not be blown when starting, and must be reliably blown when the minimum short-circuit current occurs at the installation location of the equipment. The specific selection can be based on the load nature of the equipment and the starting current of the motor, and the fuse time-current characteristic curve, I2t value and I2t value of the thyristor provided by the fuse manufacturer can be calculated and selected. In the absence of the above information, the following empirical formula can also be used for calculation and selection:
Ifn≥(1.8～2.0)*Ie(A)
Ifn - fast fuse rated current (A)
Ie - motor rated current (A)
4. Selection of Bypass Contactors
At the end of the soft start, the motor has been running on the rated voltage, so the AC contactor can meet the requirements according to the rated current of the motor. It should be noted that when the cabinet is wired, the soft starter and the contactor should be connected in the same phase, and do not connect the wrong phase sequence.
XiCHi CMC-MX soft starter has built-in bypass contactor, no need for external connection.
5. Selection of overload protection devices
The overload protection device of the soft start device should choose a thermal overload relay with overload protection, phase failure protection and temperature compensation functions. When selecting, the working current of the motor should be within the setting current range of the thermal element. For equipment that is easy to be overloaded during operation, the rated current value of the motor should be close to the lower limit of the setting current range of the thermal element.
6. Transformer load capacity and protection setting value verification
In addition to paying attention to the above requirements when selecting a soft-start device, it is also necessary to pay attention to the load capacity of the transformer that supplies power to the equipment. If the transformer is close to full load beforehand, the soft-start equipment should be carefully selected. Especially when adding equipment with relatively large power, it is necessary to check the load capacity of the verification transformer and the setting value of the protection.
After adding the soft-start device, the setting value Ir2 of the short-circuit short-time delay release of the circuit breaker on the secondary side of the transformer is:
Ir2≥1.1(IL+1.35*K*Ie)A
In the formula: IL - the load current A of the transformer during normal operation
K - the ratio of the starting current of the newly added soft-start device to the rated current of the motor
Ie-Add the rated current A of the motor.
The calculated Ir2 should be less than the current setting value of the actual short-circuit short-time delay release of the transformer secondary side circuit breaker; otherwise, when the newly added equipment starts, the transformer secondary side circuit breaker will be disconnected and tripped.
7. Tuning and Commissioning
After the soft start control cabinet is installed, it should be carefully checked. Check whether the wiring is correct and whether the connection is reliable according to the design drawings. Because the thyristor and other electronic devices are not allowed to do the insulation resistance test, only the high resistance gear of the digital multimeter can be used to check the insulation of the thyristor in the soft starter.
After checking and confirming that it is correct, connect a low-power motor to the output end of the soft starter and set the technical parameters such as starting mode, initial voltage, starting time and stopping time. Before connecting the motor to the soft starter, the insulation state of the motor and cable must be checked with an insulation resistance tester. Only when the insulation resistance complies with the relevant regulations, it is allowed to connect the motor to the output terminal of the soft starter for starting test. Before the first soft start, manually crank the machine to check whether the machine is "jammed". Then, after the equipment start-up test and the start-up parameters are adjusted, it is put into production and operation.