Electronica

AND8008/D Solid State Control Solutions for Three Phase 1 HP Motor
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APPLICATION NOTE
INTRODUCTION In all kinds of manufacturing, it is very common to have equipment that has three phase motors for doing different work functions on the production lines. These motor functions can be extruders, fans, transport belts, mixers, pumps, air compressors, etc. Therefore, it is necessaryto have equipment for controlling the start and stop of the motors and in some cases for reversing them. Actually, one of the most common solutions for performing this control functions is by using three phase magnetic starters. It consists of a block with three main mechanical contacts which provide the power to the three main terminals of the motor once its coil is energized. However, themagnetic starter has a lot of disadvantages and the most common appear when they are driving high current levels that can cause arcing and sparks on their contacts each time they are activated or de−activated. Because of these kind of effects the contacts of the magnetic starters get very significantly damaged causing problems in their functionality. With time it can cause bad and inefficient operationof the motors. This is why, thyristor should be considered as a low cost alternative and indeed a powerful device for motor control applications. Thyristors can take many forms but they have certain features in common. All of them are solid state switches that act as open circuits capable of withstanding the rated voltage until triggered. When they are triggered, thyristors become low impedancecurrent paths and remain in that condition (i.e. conduction) until the current either stops or drops below a minimum value called the holding level. Once a thyristor has been triggered, the trigger current can be removed without turning off the device. DEFINITIONS Three phase induction motor. A three phase induction motor consists of a stator winding and a rotor of one of the two following types: onetype is a squirrel−cage rotor with a winding consisting of conducting bars embedded in slots in the rotor iron and short circuited at each end by conducting end rings. The other type is a wound rotor with a winding similar to and having the same number of poles as the stator winding, with the terminals of the winding being connected to the slip rings or collector rings on the left end of theshaft. Carbon brushes bearing on these rings make the rotor terminals available at points external to the motor so that additional resistance can be inserted in the rotor circuit if desired. Three phase voltages of stator frequency are induced in the rotor, and the accompanying currents are determined by the voltage magnitude and rotor impedance. Because they are induced by the rotating stator field,these rotor currents inherently produce a rotor field with the same number of poles as the stator and rotating at the same speed with respect to the stationary rotor. Rotor and stator fields are thus stationary with respect to each other in space, and a starting torque is produced. If this torque is sufficient to overcome the opposition to rotation created by the shaft load the motor will come upto its operating speed. The operating speed can never equal the synchronous speed of the stator field. The following figure shows a three phase 1HP motor controlled through a conventional magnetic starter which has an over−load relay for protecting the motor against over−load phenomena.

© Semiconductor Components Industries, LLC, 2008

August, 2008 − Rev. 1

1

Publication Order Number:AND8008/D

AND8008/D
Power Schematic
220 Vrms 60 Hz L1 Start A Stop L2 L3

A

A

A

A

NC

OL

OL

OL

3 Phase Motor 1 H.P.

When the start button is pushed on, the coil of the magnetic starter (A) is energized, thereby, the mechanical switch contacts close allowing current−flow through the motor which starts it to operate. If the stop button is pushed, the coil (A) will be de−energized causing the...