Soft-start Three-phase Motor Controls
The simplest method of starting a 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. Referred to as “locked rotor amps,” this can cause a momentary voltage drop in the motor circuit, which can dim lights, affect other electrical equipment and possibly overload distribution transformers. The larger the motor, the greater the effect. In fact, electrical utilities often will limit the size of 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 damage the pump due to high torque. For these reasons, it may be desirable to start a pump motor slowly using one of the following soft-start techniques.
AutotransformersAutotransformer motor starters use a transformer with several voltage taps (usually 50, 65 and 80 percent of full voltage), multiple contactors and a timer to switch from one of the reduced voltage taps to full voltage after a few seconds. Autotransformer starting delivers the highest starting torque per amp of line current, thus providing reduced inrush current with minimum sacrifice of starting torque. These starters are inherently closed-transition, thus 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 is because there is an inherent voltage drop in the cable, which must be accounted for. Franklin's maximum cable length charts are based on a 5-percent voltage drop at the motor. This 5-percent drop, by itself, will reduce the starting current by 20 percent and the starting torque by 36 percent, compared to having the rated voltage at the motor, which may be enough of a reduction in starting current on some applications to preclude the need for a reduced voltage starter.
Wye-delta Motor StartersWye-delta motor starters are used in conjunction with a specially wound motor having leads from each of the sets of windings brought to the outside of the motor (see Figure 1). 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 then can 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.
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 off-line for an instant to avoid short-circuiting the contactors, so most wye-delta starters are 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 prohibitively costly.
Part-winding and Solid-state StartersPart-winding 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, on the other hand, have two sets of windings and six or 12 leads. One set of windings is the start windings, and the other set is the run windings. The starter, inherently a closed transition starter, starts the motor on the start windings and after a preset time interval, typically 2 to 3 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.
Solid-state starters utilize solid-state devices called silicon-controlled rectifiers (SCRs) to decrease the motor voltage according to user-defined parameters. These starters 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 3 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 coming down to the point where they are an attractive alternative to electro-mechanical reduced voltage starters (see Figure 2).
Table 1 shows the relationship between line current, motor current and motor torque for the different types of starting methods.
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 autotransformer 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, and go that way. The choice is yours.
Next month, we will take a closer look at one of the reasons mentioned above for using a soft-start - water hammer. 'Til then ….