Motor Controller Manages Pump Surges

When a sudden change of flow is introduced into centrifugal pumping systems, fluid surges or pressure transients can occur. These line surges, better known as fluid hammer, can result from opening or closing valves or from starting and stopping pump motors.

When a sudden change of flow is introduced into centrifugal pumping systems, fluid surges or pressure transients can occur. These line surges, better known as fluid hammer, can result from opening or closing valves or from starting and stopping pump motors.

Mechanical surge reduction techniques exist, but they tend to be costly and complex. In certain applications, solid-state reduced voltage starters have proven to help reduce starting surges. In other cases, a more effective solution can be implemented with normal check valves and specialized motor controllers that have a pump control feature or algorithm.

This article reviews the application considerations and requirements for applying these specialized motor controllers and compares this method with others for starting and stopping pump motors.

Since hammering is caused by rapid changes in flow, hammering from starting pumps can be minimized by controlling the acceleration of the pump motor. To understand how fluid flow is affected during pump startup, it is necessary to review the methods of motor starting. These include: across the line or direct on line (closing a contactor or starter and applying full voltage to the motor); solid-state reduced voltage starting; and specialized pump control starting.

Direct On Line Starting

Since centrifugal pumps are typically direct-coupled to the shaft of the motor, the torque of the motor is transmitted to the pump and fluid. Starting the motor direct on line (DOL) applies full torque to the pump and fluid. With DOL starting, the motor torque output significantly exceeds the torque requirements of the pump system from zero up to the normal operating point (see chart on p. 18). This torque is typically 180 percent of full load torque.

Direct On Line Starting

The difference between the torque produced by the motor and that required by the load (called accelerating torque) causes the pump to ramp up to speed very quickly. (The typical starting time for pumps with DOL is less than 1/4 second). The rapid acceleration of the pump and fluid from zero to full speed results in surges as the system seeks equilibrium. These pressure surges can cause pipes to burst or move, and lead to support and/or pipe failure. Also, in many applications, energy costs are a concern with DOL starting due to the excessive starting torque applied to the system.

Reduce Voltage Starting

In many pumping applications, solid-state starters are employed using the “soft” start mode to provide stepless reduced voltage starting. Because the voltage applied to the motor increases with respect to time, torque also increases with time. Compared to DOL starting, at the beginning, the accelerating torque is greatly reduced. This is due to the solid-state starters ability to start at a lower value of initial voltage, and “ramp” to full voltage over a user-selected time period.

Reduce Voltage Starting

Nevertheless, at the end of the “ramp,” there is a large differential between the motor torque and the torque required by the pump system. This large acceleration torque can result in the pump “snapping” up to speed. As before, this sudden change in speed generates a corresponding burst of flow at the end of the start cycle and results in hammering.

Specialized Motor Controllers

With a specialized motor controller with pump control, the speed torque curve of the motor is managed to closely follow the speed torque characteristics of the pump system. Such pump control provides an acceleration torque which is as close to constant as possible.

Specialized Motor Controllers

This is accomplished by using motor feedback in conjunction with a microprocessor, without the need for tachometers, flow or pressure transducers, or other types of external feedback into the controller. As the motor accelerates, line voltage and line current readings are used to continuously adjust the voltage applied to the motor. This form of motor management or continuous control is what modifies the basic motor characteristic to enable near constant acceleration torque.

Specialized Motor Controllers

Specialized motor controllers eliminate sudden changes in torque, which translates into a smooth acceleration of the motor, minimizing system surges. The pump control reduces the sudden changes and extends the time to produce a 100 percent flow, thus minimizing hammering.

Solving The Stopping Problem

Hammering also is found when the motor and pump stop too quickly or when check valves slam shut. Like starting, this is due to the rapid change in rate of flow. Managing the stopping of the motor and pump can be just as important as managing the starting.

Solving The Stopping Problem

With DOL starters, the dynamics of the pump system determine how quickly the pump stops. Since the motor is either on or off, the system head quickly overcomes the motor/pump inertia, and the pump will come to a rapid stop. The fluid must come to a complete halt as well. Since it does this rapidly, the result is pressure surges on the pipes and valves.

Solving The Stopping Problem

Solid-state reduced voltage starters contain an optional soft start or extended stop function as a solution to hammering problems. When a “soft stop” is initiated, the voltage ramps from full voltage to zero over a user-selected period. The reduction in voltage results in a reduction in torque. Although the pump begins a gradual slowdown, a point is quickly reached where the reduced motor torque caused by the reduced voltage is less than the load torque and the motor stalls. In some instances, the soft-stop function cannot prevent sudden changes in flow. The effect, though not as severe, is the same as slamming a valve closed.

Solving The Stopping Problem

The specialized motor controller manages the deceleration of the pump motor with a method similar to how it is accelerated. After the stop command is given, the controller reduces the motor’s speed to prevent any sudden changes in torque, thereby minimizing surges. The controller continues to reduce the torque of the pump motor and follows the typical speed torque of a centrifugal pump. This type of motor deceleration curve results in minimal surges or hammering.

Solving The Stopping Problem

For example, a municipal pumping station was using a solid-state starter with soft stop to control a pump motor. The soft stop was controlling the motor in an open loop fashion by reducing the voltage to the motor. Because there was not enough torque provided to the motor to drive the load, the motor’s stall point was quickly reached. During the stop mode, severe surges were causing pipe vibration and breakage. To combat the problem, a specialized motor controller with pump control algorithm was installed. The pump control feature removed the surges by managing the speed of the motor during starting and stopping. The microprocessor inside the controller analyzed the motor variables and generated control commands to reduce the surges in the system.

Solving The Stopping Problem

Specialized motor controllers also control the voltage applied to the motor, even when the motor is up to speed. For example, the Allen-Bradley SMC Dialog Plus controller features an energy saver function for motors that run unloaded or lightly loaded for long periods of time. The controller can determine when the motor is lightly loaded, detects when a load is reapplied and increases the voltage to prevent stalling.

Conclusion

DOL starting is an acceptable method of starting certain pumps, while solid-state reduced voltage starting does effectively reduce surges in some applications. For many other pump systems, however, a specialized motor controller with pump control algorithm is needed to reduce surges and fluid hammer caused by motor starting and stopping. This advanced motor technology provides an alternative to modulating control systems or simply adding additional pipe restraints.

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