How does a soft start drive multiple motors

- Jul 24, 2019-

How does a single soft starter control multiple pump motors?

This paper introduces a control method that uses a single soft starter to drive multiple pump motors, realizes the control method of soft start soft stop, explains the selection of the main equipment, and gives the system electrical schematic and the program flow frame diagram. The engineering application proves that the method is simple and reliable, and effectively controls the phenomenon of open power failure flow and water hammer.

A company has built a new flue gas desulfurization dust and waste liquid treatment project to deal with waste liquid exhaust gas, in order to meet the sewage standards. The key equipment of the project is 3 37kW washing slurry circulation pumps, 3 37kW washing slurry liquid transfer pumps, 3 30kW waste liquid delivery pumps and 3 30kW sewage pumps.

Whether the 12 pumps can be reasonably, effectively and reliably controlled has become the key to the success of the whole management project. The traditional self-coupled buck start-up method, the starting voltage is not adjustable, the impact current is large, the motor torque characteristics are poor and the daily maintenance workload is large. In view of the situation in which the motor power is the same, the quantity is more and the control is concentrated, the programmable controller (PLC) is used as the main control component, the intelligent digital motor soft starter is the main execution device, the soft starter is used to drive multiple pump motors, simplify the structure and control the cost. The pump control function of the soft starter effectively reduces the system water hammer phenomenon caused by the liquid flow impact When starting and stopping.

Working principle and electrical schematic of the system

1) How it works

The basic principle of the motor soft starter is shown in Figure 1. Soft starters generally use three pairs of inversely parallel high-power thyristors to link to the setter circuit of the AC motor, and its trigger angle is controlled by the microprocessor.

At start-up, the output voltage of the soft starter rises rapidly to the set initial voltage (the voltage corresponding to the initial torque), and then gradually rises according to the set start time, and the motor accelerates gradually to achieve the soft start of the motor. When the output voltage of the soft starter reaches the rated voltage of the motor, the motor reaches the rated speed and starts the start-up. During shutdown, the output voltage of the soft starter is gradually reduced depending on the set downtime and the motor is gradually stopped. to achieve a soft stop of the motor.


Figure 1 How the Soft Starter Works

2) System electrical schematic


Figure 2 A soft starter drives three motor control schematics

The system control principle is illustrated by the use of 1 soft starter to drive 3 washing slurry liquid circulation pump motors to achieve soft start and soft stop, as shown in Figure 2. The scrubber fluid circulation pump motor M1, M2, M3 models jo2-51-6, 37kW, 70A, 970rpm, with 1 37 kW soft starter (model STRB-371T3P) as the controller unit.

On the basis of a soft starter with a motor, add a contactor (KM2, KM4, KM6) to each motor's main circuit as a bypass running contactor for the soft starter. Soft start motor M1, first absorb the main circuit contactor KM1, the soft starter began to start the motor M1, the motor with the set start time gradually accelerate, until the motor M1 accelerated to the rated speed, the soft starter issued a start-up completion signal, bypass running contactor KM2 suction, while delaying the main circuit contactor KM1 disconnect, Cut the soft starter, motor M1 soft start complete. At this point, the soft starter can continue to start the motor M2 or M3.

After the motor M1, M2, M3 start-up is complete, the soft starter is off-call. Motor M1, M2, M3 to implement the time-sharing has been started, the start of each motor interval in about 2 minutes. Its specific motion process and delay are easily completed by the programmable controller (PLC).

When the motor is started, the system supplies power to the motor by running the contactor sits by bypass. At this point the soft starter is removed and loses its protective effect on the motor.

Therefore, a motor overload protection element (FR1, FR2, FR3) must be installed. Soft stop motor M1, first absorb the main circuit contactor KM1, the bypass running contactor KM2 disconnect, motor M1 switch to be controlled by the soft starter, soft starter soft stop function gradually reduce the output voltage, motor M1 with the set stop time gradually stopped, and finally the main circuit contactor KM1 disconnect, Motor M1 soft stop completes, soft starter exits, accepts the next stop operation.