In fact utilities often limit size of motor that can be stated "across the line" to protect their distribution system.

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, current drawn by the motor will be as high as 6 to 8 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, effect other electrical equipment and possibly overload distribution transformers. The larger the motor, the greater the effect. In fact, utilities often limit 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 reduced voltage techniques.

Autotransformer motor starters utilize a transformer with several voltage taps (usually 50, 65, and 80% of full voltage), multiple contactors, and a timer to switch from a reduced voltage tap to full voltage after a few seconds. Autotransformer starting delivers highest starting torque per amp of line current, thus providing reduced inrush current with minimum sacrifice of starting torque.

There are two types of autotransformer starters, open and closed transition. Open transition starters momentarily interrupt power during the starting cycle and are not recommended for two reasons. First, a current spike, which could exceed the across the line starting current, would likely occur. Secondly, a torque spike could damage the pump coupling or the shaft. Only closed transition autotransformer starters should be used on pump motors.

Franklin Electric makes the following recommendation for using autotransformer starters with its motors: If the pump cable length is less than 50% of the maximum allowable, the 65% or 80% taps can be used. When the pump cable length is more than 50% of the allowable, only the 80% 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% voltage drop at the motor. This 5% drop will reduce starting current by 20% and starting torque by 36% compared to having the rated voltage at the motor, which may be enough reduction in starting current on some applications to preclude need for a reduced voltage starter.

Wye-Delta motor starters are used in conjunction with a specially-wound motor having leads from each set of windings brought 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 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 impedance of the motor, reducing current and torque to 33% of normal.

In the "Delta" configuration, 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, most Wye Delta starters are "open transition" types. There are some closed transition Wye Delta starters available on special order but circuitry required to make them "closed transition" makes them prohibitively costly.

Part-winding starters also require 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 twelve leads. One set of windings are the start windings, and the other set are the run windings. The starter, typically a closed transition starter, starts the motor on the start windings, and after a preset time, typically 2 to 3 seconds, connects the other set of windings in parallel with the start windings. A part-winding starter will reduce starting current draw to approximately 65% of normal locked rotor amps, and torque to 45% of normal motor torque. A part-winding starter uses two contactors and a timer.

Solid state reduced voltage starters (Soft Starts) utilize solid state devices called Silicon Controlled Rectifiers (SCR's) to increase and decrease motor voltage according to user-defined parameters. They can be used with standard induction motors. The inrush current can be reduced to less than 50% of full voltage start amps (locked rotor amps), and starting torque can be controlled to closely replicate starting torque requirements of the pump, reducing mechanical stress on the system. Soft starts have become very reliable and the cost is coming down, so they are an attractive alternative to old tried and true electro-mechanical reduced voltage starters.

Table 1 shows the relationship between line current, motor current, and motor torque for 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 voltage panel. If your motor is wound for Wye-Delta or Part Winding starts, you can go that way. If you have a conventionally wound motor, your choices are Auto-Transformer or Solid State Soft Start. The choice is yours.

Next month we will finish this series on three-phase systems by discussing the electronic protection devices available for three-phase pumps and motors. Till then...