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Figure 10-12 is a schematic diagram for another across-the-line motor starter connected to a three-phase AC motor and powered from a three-phase, 440-V AC supply However, the stop and start pushbuttons are in a 110-V AC circuit stepped down from 440 V AC by a transformer, and the main power switch is ahead of the starter The 110 V from the transformer also powers other external control circuits
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Autotransformer starters have two autotransformers connected in open delta to provide reduced-voltage starting Figure 10-13 is a schematic for an automatic opentransition autotransformer starter It has a five-pole starting contactor S and a three-pole running contactor R Three taps are included, giving 50, 65, and 80 per-
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Figure 10-12 Schematic for an across-the-line motor starter powered by threephase, 440 V AC but controlled by 110 V AC
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Figure 10-13 starter
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cent of full line voltage Motor current is proportional to the voltage on the motor terminals Line voltage varies as the square of the impressed voltage, and is therefore lower than with resistor-type starters Torque also varies as the square of the impressed voltage A 50 percent voltage tap will provide 25 percent starting torque Connections should be made to the lowest tap that will give the required starting torque There is also a closed-transition autotransformer starter that disconnects the motor from the power line during the transfer period, a function that the open-transition starter does not perform Motor acceleration is nonlinear with both of these starters because the torque developed by the motor remains essentially constant during each indexed step There are also manually operated autotransformer starters controlled by a lever that extends outside the starter enclosure These are equipped with low-voltage release magnets
PRIMARY-RESISTOR STARTERS
The primary-resistor starter connects the motor to the line through a series resistor The voltage drop across the resistor reduces the voltage at the motor terminals As the motor accelerates, the current drawn from the power line declines, and the voltage drop across the resistor is lowered This increases the motor voltage at the terminals Motor torque increases constantly as motor speed increases After a definite time lapse, a timer operated by the main contactor energizes the accelerating contactor, which short-circuits the starting resistor and permits full voltage to be applied to the motor Compared with the autotransformer starter, the primary-resistor starter draws more power from the line on starting, but it provides smoother acceleration because there is no transfer period during which the motor can lose speed
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ELECTRIC MOTORS AND STARTERS
Figure 10-14
Schematic of a solid-state starter for a three-phase AC motor
SEMICONDUCTOR AC STARTERS
Semiconductor AC starters include paralleled back-to-back phase-controlled silicon controlled rectifiers (SCRs) in series with two or three of the conductors to the motor, as shown in Fig 10-14 The SCRs are controlled during the starting period to maintain about 300 percent line and motor current by gradually increasing the motor voltage from its initial value Starting is smooth because the current and starting torque can be adjusted easily A solid-state starter is usually specified where the line current is critical and where repetitive motor starting limits the life of electromagnetic contactors
DC Motor Principles
Direct current (DC) motors operating from DC convert electrical energy into mechanical energy as rotary torque, just as AC motors convert electrical energy into torque The fundamental physical principle on which every electric motor operates is that a conductor carrying a current within a magnetic field experiences a force If a conductor carrying DC current is placed at right angles to a magnetic field formed by either an electromagnet or a permanent magnet, it will experience a force perpendicular to the field and to itself This force is proportional to magnetic flux density, the current in the wire, and the length of the wire conductor
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