lab5 - MEEN 260 Texas A&M University Laboratory Manual...

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MEEN 260 Laboratory Manual Texas A&M University 1 E LECTROMECHANICAL M EASUREMENT --R ELAYS Created Oct. 31, 2001, A. Palazzolo Last updated Aug. 23, 2007, B. Rasmussen Purpose of the Experiment 1) Provide an opportunity to gain knowledge of an electromechanical device (relay). 2) Demonstrate how measurements can facilitate the learning process in (1). 3) Introduce the students to concepts, more fully developed in MEEN 364 such as steady state value, time constant, electromechanical system, measurement, etc. 4) Gain experience in force, current, resistance and flux density (B) measurements. Theory The Electromagnetic Relay Modern machinery and equipment contain many components that require simultaneous design of electrical and mechanical parts which function interdependently. Some examples include motors and generators that are studied in detail in ME 364. This interdependency is illustrated by the need of the motor (generator) to develop torque (voltage) which depends on current (shaft speed). The focus of this lab will be relays, switching devices that require minimal current to open and close circuits that may carry 100's of amps. This permits operation of high power equipment from remotely located low power switches that open or close the relay. The relay utilized in this lab is a MK4608 single pole-double throw switch, shown below.
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MEEN 260 Laboratory Manual Texas A&M University 2 Figure 1: MK4608 Relay Used in Lab This device may be operated in a normally open mode (open circuit when relay coil is de-energized), or in a normally closed mode (closed circuit when relay is de-energized). Figure 2 illustrates the operation of the relay. Figure 2: Schematic of Relay
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MEEN 260 Laboratory Manual Texas A&M University 3 The result of current i circulating through the n turns of coil D is a magnetic field with flux density B that circulates in the closed loop shown. This creates a downward force on arm E causing it to lower. Let x be the distance E moves downward along the centerline (G) of coil D (see Figure 3) under the influence of the magnetic field, and h be the initial gap between E and the top of the coil D measured along the centerline (G) of the coil. Thus the final gap between the arm E and the top of coil D is given by g = h – x (1) Figure 3: Diagram of the Equivalent System The contact bar H touches the ”normally closed” pole J of the switch when the coil is de- energized and the ”normally open” pole M of the switch when the coil is fully energized. The modeled displacement occurs at the center of the coil. The Equivalent Stiffness (K eq ) is the effective resistance to motion, x, from the spring located on the backside of the switch (See Figure 2). The figure below will clarify:
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MEEN 260 Laboratory Manual Texas A&M University 4 Figure 4: Relationship between x and deflection of spring The electrical load may be connected to the normally open (NO) or normally closed (NC) pins of the relay depending on how the logic controlling the relay's
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lab5 - MEEN 260 Texas A&M University Laboratory Manual...

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