Echo Electric Motors

Reference

Locked Rotor Code Le ers & Reduced Voltage Star ng Methods - con nued

Star ng Methods Across the line star ng is used on a high percentage, probably over 95% of normal motor applica ons. Other star ng methods (reduced voltage) are used mainly to control inrush current and limit it to values that can be safely handled without excessive voltage dips and the accompanying light flicker. Occasionally, reduced voltage starters are used to reduce star ng torque for smoother accelera on of loads. Various methods of reduced voltage star ng have been developed. Table 2 shows the common reduced voltage starter types and the results that can be expected in terms of motor voltage, line current, and the output torque of the motor. Cau on should be used in applying reduced voltage starters on certain types of loads. For example, a centrifugal pump, which is very easy to start, can be operated with either wye-delta star ng or part winding star ng. These star ng methods produce 33% and 50% of rated motor star ng torque respec vely and can easily start centrifugal pumps. They could also be expected to start a compressor so long as it is unloaded. They could have difficulty star ng a loaded inclined conveyor or a posi ve displacement pump because of high star ng torques required on these types of loads. The best star ng method has to be one that achieves the desired result in inrush reduc on and yields adequate star ng torque to reliably start the load.

Table 2

% of Full Voltage Value*

Voltage at Motor

Line Current

Motor Output Torque

Star ng Method

Full Voltage

100

100

100

Autotransformer

80% Tap 65% Tap 50% Tap 80% Tap 65% Tap 50% Tap

64** 42** 25**

80 65 50 80 65 50 80

64 42 25 64 42 25 64 50 33

Primary Reactor

80 65 50 80 70 33

Primary Resistor Typical Ra ng

Part Winding High Speed Motor (1/2 - 1/2)

100 100

Wye Start - Delta Run

* Percent of "Across The Line Value". ** Autotransformer magne zing current not included. ** Magne zing current usually less than 25% motor full-load current.

Squirrel Cage Induc on Motors In all cases, reduced voltage starters will cost substan ally more than full voltage (across-the-line) starters. Standard motors can be used with autotransformer, primary reactor and primary resister type starters. In addi on, dual voltage motors can usually be u lized with part winding starters, but only at low voltage. Generally speaking, wye-delta motors and part winding motors for higher voltage (for example, 460 or 575 volts) must be made to order. This will raise cost of the motor over a standardly available motor suitable for use on other types of reduced voltage star ng.

©2017 ABB. Technical Papers, Cowern Papers 9AKK107303

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