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Reduced Voltage Starters — Electromechanical
General
Auto Transformer Starter
Operation
At starting, the auto transformer reduces motor terminal voltage to 50, 65, or 80 percent of line voltage. For a timed inter- val, a segment of the transformer coil is connected in series (as an inductor) with the motor. Then a contactor automatically connects the motor across the line and shorts out the inductor.
Wye Delta Starter
Operation
Initially the starter connects the motor on a Wye connection, reducing voltage across the winding to approximately
58 percent. Starting torque and current is about 33 percent of across the line. After a time delay the motor is connected Delta.
Part Winding Starter
Operation
Part winding starters apply starting current in timed steps to part winding motors. Two step controllers apply voltage through one starter to one motor winding followed by the second starter which connects voltage to the second winding. The timer may be adjusted to automatically close the second controller at the most favorable time to minimize voltage fluctuations.
Auto Transformer Starter | Wye Delta Starter |
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Typical Speed Torque Curves
15
AC Controls
Table 15.35 Comparison of Various Electromechanical Methods of Reduced Voltage Motor Starting
| Reduced Voltage Starting |
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| Autotransformera | Part Winding |
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| 50% | 65% | 80% |
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Characteristics | Tap | Tap | Tap | 2 Step | Wye(Star) Delta | |
Starting current drawn from line as % of |
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that which would be drawn upon full | 25% | 42% | 64% | 64%c | 33% | |
voltage starting b |
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Starting torque developed as % of that | Increased slightly with | 45%c | 33% | |||
which would be developed on full | ||||||
speed |
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voltage starting |
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Smoothness of acceleration d | First in order of |
| Third in order of | Second in order of | ||
| smoothness |
| smoothness | smoothness | ||
Allowable accelerating times (typical) | 30 seconds |
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| Based on NEMA |
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| Limited by | Limited by | |||
| medium duty |
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| motor design | motor design | |||
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aClosed Transition.
bFull voltage start usually draws between 500% and 700% of full load.
cApproximate values only. Exact values may be obtained from motor manufacturer. d Solid state starters provide the smoothest starting characteristics.
306 | Siemens Electrical Products and Systems | CSI Section 16160 | |
Specification Guide | |||
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