http://64.233.161.104/search?q=cache:mdmYGwyz0yEJ:www.sea.si...

--------------------------------------------------
Note added at 8 mins (2004-08-27 16:11:26 GMT)
--------------------------------------------------

A combination starter is a single enclosure containing the motor starter and the fuses or circuit breaker.
--------------------------------------------------------------------------------
Page 8
7Advantages of TIASTAR™ is the trade name for the Siemens motor controlSiemens TIASTAR MCCs center. Some of the advantages of using TIASTAR are:• Faster and easier installation and wiring• Centralized motor control• Generally less total space is required• Neat, attractive appearance• Simplicity in adding special components such as service entrance switches, load centers, and transformers• Ease of future modifications, such as increasing the size of the starters, adding additional starters, or adding additional vertical sections.
--------------------------------------------------------------------------------
Page 9
8Power SuppliesThe major source of electrical power used by motor control centers is an AC generator located at a power-generating facility. AC generators operate on the theory of electromagnetic induction. This simply means that when conductors are moved through a magnetic field, a voltage is induced into the conductors. A basic generator consists of a magnetic field, an armature, slip rings, brushes, and some type of resistive load. An armature is any number of conductive wires (conductors) wound in loops which rotate through the magnetic field. For simplicity, one loop is shown.If the rotation of the AC generator were tracked through a complete revolution of 360°, it could be seen that during the first quarter of a revolution voltage would increase until it reached a maximum positive value at 90°. Voltage would decrease during the second quarter of revolution until it reached zero at 180°. During the third quarter of a revolution voltage would increase in the opposite direction until it reached a maximum negative value at 270°. During the last quarter of a revolution voltage would decrease until it reached zero at 360°. This is one complete cycle or one complete alternation between positive and negative.
--------------------------------------------------------------------------------
Page 10
9If the armature of the AC generator were to rotate 3600 times per minute (RPM) we would get 60 cycles of voltage per second, or 60 hertz. Most alternators have more than two poles and one loop of wire. Alternators can have two or three pairs of electromagnetic poles, allowing the AC generator to generate voltage at 60 Hz at slower speeds.Three-Phase Voltage In most large commercial and industrial motor applications three-phase voltage is used. In a three-phase system the generator produces three voltages. Each voltage phase rises and falls at the same frequency (60 Hz in the U.S., 50 Hz in many other countries); however, the phases are offset by 120° from each other.
--------------------------------------------------------------------------------
Page 11
10The motor control center receives this power through complex distribution systems which include power distribution lines, transformers, substations, and switchboards. Transformers used with three-phase power require three interconnected coils in both the primary and the secondary. These transformers can be connected in either a wye or a delta configuration. The type of transformer and the actual voltage depend on the requirements and capability of the power company and the needs of the customer. The following illustration shows the secondary of a wye- and delta-connected transformer.Motor Rotation Three-phase voltage is used throughout large commercial and industrial facilities to run AC motors. An AC motor is made up of a stationary member, called a stator, and a rotating member, called a rotor. Three-phase AC power is applied to the stator through the power connections.
--------------------------------------------------------------------------------
Page 12
11The direction a three-phase AC motor rotates depends on the phase sequence of the incoming power supply. In the following example, L1 (A) is connected to motor lead T1, L2 (B) is connected to motor lead T2, and L3 (C) is connected to motor lead T3. When power is applied through the “F” contacts the motor will turn in a clockwise, or forward direction. However, by reversing any two of the three power supply leads, the motor will run in the opposite direction. In this example L1 (A) is connected to motor lead T3, L2 (B) is connected to motor lead T2, and L3 (C) is connected to motor lead T1(L1 and L3 have been reversed). When power is applied through the “R” contacts the motor will run in the counterclockwise, or reverse direction.
--------------------------------------------------------------------------------
Page 13
12Many applications are designed for forward and reverse operation. An overhead crane, for example, might use the forward direction to raise the crane and reverse direction to lower the crane. It should be noted that it is not possible to reverse the direction of rotation on some applications. The consequences of running a motor in the reverse direction on an application designed to run only in the forward direction can be disastrous, resulting in equipment damage and possibly injury or loss of life. Review 11. Which of the following is an advantage of using a motor control center?a. Faster and easier installationb. Simplicity in adding special componentsc. Ease of future modificationsd. All the above2. ____________ is the trade name for the motor control center manufactured by Siemens.3. In most large commercial and industrial motor applications ____________ -phase voltage is used.4. Motor rotation of a three-phase AC motor can be reversed by reversing any ____________ of the three power-supply leads.

--------------------------------------------------
Note added at 10 mins (2004-08-27 16:12:38 GMT)
--------------------------------------------------

SORRY. I MEANT ONLY TO PASTE IN THE COMPLETE SENTENCE. \"A combination starter is a single enclosure containing the motor starter and the fuses or circuit breaker\".