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Session 2533

Design of a Reversible Soft-Starter for an Induction Motor

M. Rabiee Eastern Kentucky University

Abstract: This paper describes a student project that studies the design of a Reversible Soft-Starter for an Alternating Current (AC) Induction Motor. Alternating Current (AC) converters are used as Motor Starters / Reversing for large AC induction motors. Starting a large motor that utilizes an AC Converter will eliminate the need to draw a large amount of current during the transient period, thereby insuring that starting a large motor will not disturb adjacent power lines. This student project designed a 50 Horse Power (HP) variable-speed motor starter. First, the equivalent circuit parameters of a 50-HP induction motor were used to calculate and size the Silicon Controlled Rectifiers (SCRs) utilized within the converter. The converter has ten (10) main SCRs. The students with the author’s guide then used the thermal equivalent circuit to solve the heat transfer problem, and to size the correct heat sinks for the SCRs. In the third section of the paper, we will explain the effect of current and voltage variations (dv/dt, and di/dt) on the SCRs. Then, we will design the “RC Snubber” circuits to protect the SCRs against the voltage (dv/dt) variation. Finally, we will devise a control routine in order to provide the appropriate gate current firing of the SCRs. The specifications state that pushing the “STARTER” will automatically bring the command voltage (V K ) up as a ramp function, it will take fifteen (15) seconds for the command signal to reach 15 V value. This signal is subtracted from a constant 15 V DC voltage. We have called the resulting signal the Error Signal, VE . The error signal is used to calculate the Gate to Cathode voltage value of each SCR. When the gate to cathode voltage appears across one forward biased SCR, it will start to conduct, and there will be low voltage on the terminal of the motor. The terminal voltage increases as the command signal (V K ) increases until the motor reaches its rated speed. At that point, the motor has a constant Root Mean Square (RMS) voltage value across its terminals. For reversing, we push another button labeled “REVERSE”. Once the reverse push button is pressed, the command signal decreases until it is zero when the motor has stopped. Now the system will wait for one (1) second to insure that no current is passing through the coils. Since the frequency is 60 Hz, a one (1) second delay is a sufficient delay time. The controller will change the phase sequence and automatically push the motor starter. The motor will have a reversed direction of rotation. In this paper, we will also explain the way system controls SCR gate currents. There are several textbooks available which can be used for reviewing and learning the design steps [1, 2, 3].

Design specifications: The induction motor parameters for the design are listed below.

Source Line Voltage, V L = 480V

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Rabiee, M. (1999, June), Design Of A Reversible Soft Starter For An Induction Motor Paper presented at 1999 Annual Conference, Charlotte, North Carolina. 10.18260/1-2--7554