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Discussion on the Application of Motor Soft Starter

Author: Wei Junlin     Publish Time: 2022-08-26      Origin: SHANXI ARCHITECTURE Magazine Vol.39 No.2


With the continuous advancement of power electronic technology, motor soft starters are favored by all walks of life for their excellent functions, and their applications are becoming more and more extensive, gradually replacing star-delta starters and auto-decompression starters on old equipment.

The basic principle of the motor soft starter is to use a bidirectional thyristor in series in the main circuit of the motor, adopt the principle of phase shift control, and use a microprocessor to change the degree of conduction of the thyristor, and the output voltage rises according to a pre-designed curve, thereby reducing the starting current of the motor value.

At present, there are a wide variety of domestic motor soft starters, and their functions are also increasing. Now on how to choose correctly, I will share some opinions for your reference.

The Selection of Motor Soft Starter

First decide if a step-down start-up is required. For example, some people even use a soft starter for a motor of about 20 kW in the distribution circuit of the large-capacity low-voltage system that is closer to the substation.

This is actually harmful and unhelpful, not only increasing the investment, but also bringing trouble to the maintenance. Some people think that there are more microelectronic products now, and the quality requirements for voltage are strict. If the voltage drop or soft start is not used, the fluctuation of the grid voltage will affect the normal operation of the above equipment. This is not the case in reality, and equipment with strict requirements on voltage quality usually has its own voltage regulator.

In order to correctly select the motor soft starter, it is necessary to analyze the nature of the load and the various functions of the motor soft starter. There are light, medium and heavy loads. Generally, heavy-duty equipment such as long belt conveyors, crushers, roller compactors, grinders and mixers. When this type of equipment suddenly loses power during operation, there is still surplus material in the machine, and it becomes a heavy-load start when it is restarted.

It should be pointed out that some people often regard fans and pumps as light loads. In reality, this is not the case. Because the flow is proportional to the speed, the wind pressure (head) is also proportional to the speed, and the shaft power required by the fan and pump is proportional to the cube of the speed. Therefore, as the speed of the fan and the water pump increases, the resistance increases, which prolongs the start-up time, and causes the thermal relay to act frequently during the start-up process, which is a heavy-duty start-up device. For heavy-load starting equipment, the motor must output a large torque. Therefore, when selecting a soft starter, a soft starter for heavy-load starting should be used. If a common type is used, the capacity should be increased by one level.

At present, the soft starter has moved towards intelligence. In addition to satisfying the starting function, there are also protections such as overload, undervoltage, phase imbalance, reverse phase and negative sequence. Some panels are equipped with touch screen, display screen, logic menu, communication module, etc., making on-site adjustment simple and easy. In addition, there are self-test function, fault display and alarm function. These components and functions are completed by each module. When designing, selecting and ordering, certain functions should be selected according to the actual application, otherwise it will cause waste.

Selection of other Electrical Appliances in the Soft Start Circuit

In the manufacture of electrical complete cabinets, sometimes the design company's soft starter system diagram has the problem of improper selection of components. For example, the total protection of the soft start circuit uses a molded case circuit breaker, which is not reasonable. When using a circuit breaker to protect the thyristor in the soft starter, the action time is too slow to meet the protection requirements of electronic components. It is better to use a knife switch plus a fast fuse solution or a knife fuse switch with a fast fuse. There are also people who use the bloated practice of isolating switch → molded case circuit breaker → fast fuse → contactor → thermal relay → soft starter. The reason is that the isolating switch is used for the maintenance of the molded case circuit breaker, the molded case circuit breaker is used to protect the circuit and the motor, and the fast fuse protects the soft starter. This not only requires a large investment, but also occupies the installation space of the control cabinet, which is not desirable.

The soft starter has overload protection, whether it can be used as the overload protection of the motor to cancel the thermal relay in the loop depends on the specific situation. If a soft starter with large capacity is selected and its rated current is larger than the rated current of the motor or a circuit with a bypass contactor, a thermal relay must be installed in the circuit to protect the motor from overload alone. Only when the rated current of the motor is consistent with the current of the soft starter and it is running without a bypass contactor, it is not necessary to install a thermal relay to protect the motor from overload.

Whether to add a bypass contactor in the circuit is a problem to be solved in the design. At present, many models of soft starters have built-in bypass contactors, and most people think that adding a bypass contactor is desirable. Although the thyristor produced at present is very reliable, adding a bypass contactor not only increases the investment, but also increases the installation space, but the harmonic current generated by the thyristor will pollute the power grid. If a bypass contactor is used, only harmonic pollution will be generated to the power grid during startup. Due to the bypass operation, the service life of the soft starter will be increased and the failure rate of the soft starter will be reduced. When the soft starter is connected to the power grid for a long time, all the thyristors are turned on during normal operation, but harmonic pollution will still be generated, because the quality of the soft starter is different and sometimes it is not a complete sine wave when it is turned on.

Figure-XiChi CMC-MX Soft Starter With Built-in Bypass Contactor

bypass contactor soft starter supplier-XiCHiElectric

Since the bypass contactor is connected after starting, and the soft starter performs a soft stop of the motor when it is cut off, the bypass contactor works under the AC-1 working category. AC-1 working category, the rated current is basically equal to the rated heating current of the contactor, and the current capacity is much larger than that under the AC-3 working category. In this way, the rated current of the bypass contactor in the AC-3 state is much lower than the rated current of the motor.

For example, a 75kW motor has a rated current of 142A. According to the manufacturer's recommendation, its bypass contactor can meet the requirements with a rated current of 75A. Some people wonder how a contactor with a rated current of 75A can pass 142A when it is working normally, because the above 75A is the contactor working under AC-3 state. Due to the soft start and soft stop, the power consumption capacity of the contactor under the AC-1 operating category is almost twice that of the AC-3 category.

Many design companies often choose the rated current of the bypass contactor under the AC-3 category to be the same as the rated current of the motor, or larger than the rated current of the motor, which is really a waste. (Note: The rated current of the contactor usually refers to the current under the AC-3 working category).

AC is the English abbreviation of alternating-current, and the -number indicates the type of contact.

-1 means non-inductive or slightly inductive load, resistance furnace;

-2 indicates the starting and breaking of the wound induction motor;

-3 indicates the start of the cage induction motor and the interruption during operation;

-4 indicates the start of the squirrel induction motor, reverse braking or reverse operation, and electric.

One Soft Starter Starts Multiple Motors

Sometimes a soft starter is used to start multiple motors in order to save investment. Special attention should be paid to the fact that the starting interval of the soft starter should not be less than 5 minutes. When the driving equipment is multiple fire equipment motors, the starting time is too long to meet the fire fighting time requirements, so it cannot be used. However, it is worth considering the one-for-two scheme of one working and one standby equipment with the same capacity. To adopt the one-to-many solution, it is only necessary to install a fast fuse in the soft-start circuit. In normal operation, the contactor in the main circuit of each motor only acts as a bypass contactor, and the working category is AC-1, so its rated current is almost equal to 1/2 of the rated current of the motor. This saves investment and is more secure.

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