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The working principle, process, fault analysis, and wiring mode of the online soft start cabinet for the motor are fully solved

Nov 06, 2020

The working principle, process, fault analysis, and wiring mode of the online soft start cabinet for the motor are fully solved


Working principle of the online soft start cabinet for motors:


The online soft starter cabinet is a novel motor control device integrating motor soft start, soft stop, light load energy saving and multiple protection functions. It is called Soft Starter abroad. The online soft start cabinet uses three-phase parallel thyristors as voltage regulators, which are connected between the power supply and the motor stator. Such a circuit is a three-phase fully controlled bridge rectifier circuit.


When using the online soft starter to start the motor, the output voltage of the thyristor gradually increases, and the motor gradually accelerates until the thyristor is fully turned on, and the motor works on the mechanical characteristics of the rated voltage to achieve smooth starting, reduce the starting current, and avoid starting over-current tripping .


When the motor reaches the rated number of revolutions, the start-up process ends, and the online soft start cabinet automatically replaces the completed thyristor with a bypass contactor to provide rated voltage for the normal operation of the motor to reduce the heat loss of the thyristor and extend the online soft start The service life of the cabinet improves its working efficiency and prevents the power grid from harmonic pollution.


The online soft start cabinet also provides a soft stop function. The soft stop is the opposite of the soft start process. The voltage gradually decreases and the number of revolutions gradually drops to zero to avoid torque impact caused by free stop.

Installation process of the motor online soft start cabinet:


1. On-line soft start cabinet installation -> main circuit terminal wiring -> control circuit wiring -> start parameter setting -> simulation test -> trial operation with load;


2. The installation location of the online soft-start cabinet should be installed in a location with good dust-proof and heat dissipation conditions; the minimum distance between the space and the surrounding environment should be considered according to the instructions during installation;


3. The main circuit terminal connection can be divided into internal connection and external connection. During construction, connect the motor and the online soft start cabinet according to the design drawing;


4. Generally, the control circuit of the power distribution panel has been connected intact. Check the control circuit with a multimeter against the drawings; note that the insulation resistance of the control circuit cannot be tested with an insulation shaker to avoid damage to the electronic components in the online soft start cabinet;


5. The online soft start cabinet cannot be used for the no-load simulation action test, because the no-load has no current and voltage output, and the relay protection will malfunction; a light load test should be carried out to check whether the parameter settings of the online soft start cabinet meet the design requirements; This recommends an economical and convenient method at the project site: three 200W incandescent bulbs can be used to simulate the three-phase winding of the motor as a light load for the test;


6. At the trial operation stage, pay attention to whether there is any deviation between the actual measured value and the set value, analyze the cause of the deviation and continuously adjust until it meets the requirements; and make corresponding inspection records and operation reports;


Motor online soft start cabinet selection:


Based on the above analysis of the principle and advantages and disadvantages of the online soft start cabinet, the built-in bypass type should have the strongest applicability, the most advantages, and the least disadvantages.


In actual work, the functions and technical parameters of the specific product brand and the testability of the product should also be considered, and the selection should be made according to the actual use. It is necessary to distinguish between frequent start and infrequent start. For the online soft start cabinet, under normal circumstances, if the start interval does not exceed 2 minutes and does not exceed 30 times per hour, it can be set as infrequent start. Less than this number should be considered as frequent startup.


Fans and pumps generally start infrequently. Mechanical transmission has frequent or infrequent starting. For example, belt conveyors and ball mills can be considered as infrequent starting. If it is a starter or a motor equipped with a large mechanical equipment that requires reversible functions, it is mostly a frequent start.  


The selection of the online soft-start cabinet in frequently working places should be based on the starting current of the motor, because the thyristor current selected by the manufacturer of the online soft-start cabinet is 2.5 times the rated current of the motor.


Limit the maximum motor starting current to 4.5 times the rated current, and make full use of the short-term overload capacity of the SCR under infrequent operation. Therefore, under the condition of frequent starting, the capacity of the online soft start cabinet should be increased, according to the frequency The difference is 1.2-1.5 times. At the same time, due to the frequent operation of the SCR, in order to eliminate the large amount of heat emitted by the SCR, the on-line soft start cabinet must be equipped with mechanical air cooling. For the mechanical air-cooled soft starter, it is better to put a soft starter in a switch cabinet, and the switch cabinet should also be equipped with mechanical ventilation.

Main wiring types and common faults of the motor online soft start cabinet:


Main wiring method of the motor online soft start cabinet:


1. Online:


All the controllers of the online soft start cabinet have motor overload protection. When the online soft start cabinet is running online, the controller of the online soft start cabinet can protect the motor from overload. Do not install a thermal overload relay. Since the current harmonic current after passing through the thyristor is very large, an electronic thermal overload relay cannot be installed, otherwise the malfunction of the thermal relay will make the system unable to work normally. Because SCRs are more expensive and difficult to replace, in order to protect the SCRs, fast fuses should be used to prevent short-circuits at the bottom of the online soft start cabinet from burning the SCRs. Figure 4A refers to the upper part of the soft starter in frequently used places. No contactor is added to the port. Figure 4B refers to a place that is not frequently used. Disconnect the power supply of the online soft start cabinet after stopping.


2. Bypass type:


On-line soft-start cabinet for bypass operation cannot operate without the bypass contactor, so there are two main wiring schemes. The function of the contactor at the top of the online soft-start cabinet is the same as in the online operation mode, so I will not repeat it here. It is emphasized that the thermal relay should be installed at the lower port of the bypass contactor. It is best not to pass the starting current, especially the electronic thermal relay, because the current harmonics after passing through the online soft start cabinet can interfere with the electronic heat. The relay malfunctioned and stopped the motor. In addition, because it is not necessary to install a fast fuse for the short-term work of the SCR, there is no additional fast fuse in the main wiring scheme.


3. Built-in bypass type:


Its main wiring is roughly the same as the in-line type, and the only advantage is that it is not necessary to install a fast fuse because of the short-term operation of the SCR.  The overload protection of the motor is realized by the controller of the online soft start cabinet. It not only exceeds the electronic thermal relay in function and performance, but also does not malfunction due to the harmonic current of the main circuit and external interference.


Common faults of the motor online soft start cabinet:


The faults of the online soft start cabinet are roughly divided into the following categories:


1. The motor cannot get up:


There are roughly two reasons why the motor cannot get up:


One is that one of the six SCRs is unreliable or non-conductive. At this time, half-wave DC is passed through the one-phase circuit, and the DC passing through the two-phase windings of the motor has a braking effect on the motor. If it can't get up, the motor and SCR will be burnt in serious cases.


The second is that the starting parameters or starting curve are not suitable and the motor cannot start, which is a common fault. The former will occur during use, but the probability is lower than the failure rate of the contactor. The latter mostly occurs during the first commissioning and will not appear after commissioning. Most manufacturers do not have this phenomenon. The startup program has good performance and the applicability of factory settings is strong. Only a few manufacturers’ products require the manufacturers to debug themselves.


2. SCR burned:


Thyristor breakdown or explosion, this type of failure is not divided into domestic and foreign brands, and is easy for manufacturers, but the failure rate is lower than that of contactors, and the main problem occurs in the installation process of pie-type thyristors.


3. Controller burnout:


Compared with the online soft start cabinet, the controller burnout fault is the most serious. The repair rate of some manufacturers caused by such failures has exceeded 30%. Such problems are rare for imported or joint venture manufacturers. Mainly the three parts of the controller's power supply, trigger circuit and input circuit are easy to burn.


4. Malfunction of the online soft start cabinet:


The motor stops due to interference from the soft starter when it is running and starts from time to time due to interference from the soft starter when it is stopped. The former is more common, and the latter has only two brands. Investigate its reasons, one is the product quality problem, and the other is related to the line layout. However, all imported or joint venture soft starters do not have the above phenomenon, and this problem is more common in domestic brands.


5. Poor contact of the internal connectors of the online soft start cabinet:


The selection of the internal plug-in parts of the online soft-start cabinet is not a problem. This is a problem that domestic manufacturers easily ignore, and failures often occur. Import or joint venture manufacturers do not make such mistakes.