Tech Topics: Soft Start Controls for Three-Phase Motors
December 10, 2012
The simplest method of starting a three-phase pump motor is by closing a contactor and allowing the motor to start at full voltage, or “across-the-line,” as it is called. However, when a pump motor is started at its full-rated voltage, the current drawn by the motor will be as high as six to eight times its normal full-load running amps for a short period of time while the motor accelerates to speed. Referred to as a “locked rotor condition,” this surge can cause a momentary voltage drop in the motor circuit that can dim lights, affect other electrical equipment and possibly overload distribution transformers. Because the effect increases with the size of the motor, electrical utilities will often limit the size of a motor that can be started “across the line” to protect their distribution system.
Additionally, starting a large pump motor at full voltage may cause water hammer in the piping system or may damage the pump due to high torque. For these reasons, it may be advantageous to start a pump motor slowly, using one of the following soft start techniques. Although soft starters are available for both single- and three-phase motors, this article will focus on three-phase starters.
The first starter type is the autotransformer-these motor starters use a transformer with several voltage taps (usually 50 percent, 65 percent and 80 percent of full voltage), multiple contactors and a timer to switch from one of the reduced voltage taps to full voltage several seconds after starting. Autotransformer starting delivers the highest starting torque per amp of line current, providing reduced inrush current and minimal torque loss. Because these starters are inherently “closed-transition,” they provide a relatively smooth transition from reduced-voltage to full-voltage mode.
Franklin Electric makes the following recommendation for using autotransformer starters with its motors: “If the pump cable length is less than 50 percent of the maximum allowable, either the 65 percent or 80 percent taps can be used. When the pump cable length is more than 50 percent of the allowable, only the 80 percent tap should be used.” This restriction is necessary due to an inherent voltage drop in the cable. Franklin’s maximum cable length charts are based on a 5 percent voltage drop at the motor. By itself, this 5 percent drop will reduce the starting current 20 percent below what it would be with the rated voltage and will reduce the starting torque 36 percent below what it would be. This reduction in starting current on some applications might be enough to preclude the need for a reduced voltage starter.
Second is Wye-Delta-these motor starters are used in conjunction with a specially wound motor. The motor has leads from each winding that continue to the outside of the motor. In other words, Wye-Delta motors have only one set of windings, like a standard three-phase motor, but each end of each winding has a connection wire on the outside of the motor. These six wires can then be hooked in one of two ways. In the “Wye” configuration, one leg of each winding is brought to a common point, and the three legs of the three-phase power are hooked to the other end of each winding. This configuration increases the impedance of the motor, reducing the current and torque to 33 percent of normal torque.
In the “Delta” configuration, the windings are wired in the normal way producing full torque and current draw. The transition from “Wye” to “Delta” is made using three contactors and a timer. During this transition, the motor is taken offline for an instant to avoid short-circuiting the contactors. Consequently, Wye-Delta starters are typically “Open Transition” types. There are some closed transition Wye-Delta starters available on special order, but the circuitry required to make them “closed transition” makes them cost-prohibitive.
The third type is part-winding-these starters also require the use of specially wound motors, but, unlike the Wye-Delta motors that have only one set of windings with six leads, part-winding motors have two sets of windings and six or 12 leads. One set of windings are the start windings, and the other set are the run windings. The starter, inherently a closed transition starter, starts the motor on the start windings, and, after a preset time interval-typically two to three seconds-connects the other set of windings in parallel with the start windings. A part-winding starter will reduce the starting current draw to approximately 65 percent of normal locked rotor amps, and the torque to 45 percent of normal motor torque. A part-winding starter uses two contactors, two overload relays and a timer.
The final starter type is solid state-these starters utilize solid state devices called Silicon Controlled Rectifiers (SCRs) to decrease the motor voltage according to user defined parameters and can be used with standard induction motors. In the case of water-cooled, submersible motors, the soft starter has to be programmed to ramp up the motor to speed within the time period specified by the motor manufacturer (usually within three seconds). The in-rush current can be reduced to less than 50 percent of full voltage start amps (locked rotor amps), and the starting torque can be controlled to closely replicate the starting torque requirements of the pump, reducing mechanical stress on the system. Soft starts have become very reliable, and the cost is falling to the point that they are an attractive alternative to electro-mechanical reduced voltage starters. Also, variable frequency drives (VFDs) have soft-start capabilities and have become more affordable.
To summarize, if the utility can provide enough power to your motor for a full-voltage (across the line) start, most people go that way because it is cheaper. Sometimes, however, the utility does not have enough capacity to accommodate the in-rush starting current of a large motor and may ask you to provide a reduced current starter. If you have a conventionally wound motor, your choices are auto-transformer or solid-state soft start. If you need a new motor, you can buy one that is wound for Wye-Delta or part-winding starts, depending on your preference.
Next month we will begin a three-part series on variable frequency drives. Till then … ND