Standard Motor Catalog

Section TR Technical Reference Guide


3. Autotransformer Starting In contrast to part winding start, autotransformer starting uses the complete motor winding but limits input voltage to reduce inrush current. The most commonly furnished taps on autotransformer starters are 50, 65, and 80% of full voltage and most also provide an adjustable timer for switching to full voltage after the motor has accelerated. No special motor winding is required.

The design of a motor’s windings and the pattern of connecting the leads from these windings will determine voltage ratios for dual-voltage motors and may also determine what starting options are available. Following is a brief description of some of the more popular starting methods: 1. Full Voltage Starting This method is the least expensive from a initial cost standpoint and is the most commonly used starting method on smaller motors. While it results in the highest inrush current values, connections and starter operation are greatly simplified. All standard motors are designed for full voltage (across-the-line) starting.

Table 17: Autotransformer Starter

% Full Load Torque

Voltage & Current at Motor

Supply Line Current

Motor Output Torque

Transformer Tap

80% Tap 65% Tap 50% Tap

80 65 50

64* 42* 25*

64 42 25

*Autotransformer magnetizing current not included. Magnetizing current is usually less than 25 percent of motor full-load current. 4. Wye Start/Delta Run This connection method allows a motor to be started at reduced load with reduced phase voltage and, therefore, with reduced inrush current. Wye Delta starters may be furnished with either open or closed transitions. Open transition, generally lower in cost, will produce higher transient current than a closed transition starter at the transition from wye to delta.

Figure 22: Full Voltage Starting/Direct-On-Line (DOL) Starting

2. Part Winding Starting (PWS) This method energizes only part of the total winding when the motor is started and is suitable for pump, fan and compressor loads. PWS will reduce inrush current, but the motor heating rate will increase considerably. There are no standard performance requirements for part winding start ing and, therefore, a motor started in this manner may fail to accelerate a high-inertia or constant torque load. PWS usually requires special winding connections, which must be specified at the time the motor is ordered.

Figure 24: Wye Start/Delta Run

Figure 23: Part Winding Start


Data subject to change without notice. 02/23 •

Made with FlippingBook - Online Brochure Maker