Letras de codigo de motores

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codeissues
By James Stallcup Sr., NEC/OSHA Consultant

Motor Design Letters and Code Letters – How They Are Utilized
Motor circuits must be designed to provide protection for motor windings and components when motors are starting, running, and driving loads. Motor windings are protected by overcurrent protection devices that are selected according to the type used, based upon the amount ofstarting current required. Overcurrent protection devices are sized by percentages based on the type motor, starting method, and design or code letter. Starting methods should be selected based on the amount of current required to start and run the motor or the amount that is reduced by utilizing a particular starting method. This article, one of a series, will address a variety of motors andtheir many different characteristics and why it is sometimes desirable to choose one over the other, based on the requirements of the driven load or equipment. Types Of Motors Table 430.52 The following are five types of motors that must be considered when sizing OCPDs to allow motors to start and run: (1) Single-phase AC squirrel-cage, (2) Three-phase AC squirrel-cage, (3) Wound-rotor, (4)Synchronous, (5) DC. Single-Phase Squirrel-Cage Motors Squirrel-cage motors are how induction motors are known in the electrical industry. An induction motor operates on the same principles as the primary and secondary windings of a transformer. When power energizes the field windings they serve as the “transformer” primary by inducing voltage into the rotor, which serves as the secondary. Squirrel-cagemotors have two windings on the stator (the stationary windings): one is the
26 >> necdigest winter 2004 necdigest.org

run winding and the other is the starting winding. A motor with this additional starting winding on the stator is called a split-phase, single-phase, induction motor; it provides the ability to both start and run. The starting winding has a higher resistance than the runningwinding, which creates a phase displacement between the two. It is this phase displacement between the two windings that gives split-phase motors the power to start and run. The angular phase displacement is about 18 to 30 degrees, which provides enough starting torque (twist or force) to start the motor. The motor operates on the running winding after the rotor starts and has reached a speed ofabout 75 to 80 percent of the motor's synchronous speed. The starting winding is then disconnected by a centrifugal switch. (See Figure 1)

POWER SUPPLY

WINDING • RUNNING WINDING • STARTING SWITCH • CENTRIFUGAL

POWER SOURCE • 1Ø • 120 VOLT, OR • 240 VOLT

NEC TABLE 430.52
Figure 1. The above illustrates an example of a single-phase squirrel-cage motor listed in Table 430.52 of the NEC.Figure 1

Three-Phase Squirrel-Cage Motors Three-phase squirrel-cage motors have three separate windings per pole on the stator, which generate magnetic fields that are 120 degrees out-of-phase with each other. Three-phase motors do not require an additional starting winding. A three-phase induction motor will always have a peak phase of current. This is due to the

codeissuesalternating current reversing its direction of flow. In other words, when alternating current of one phase reverses its direction of flow, a peak current will be developed and as current reverses direction again, a second phase will peak, etc. Three-phase motors provide a smooth and continuous source of power once they are started and driving the load. (See Figure 2)
480 V POWER SUPPLY CONDUCTORS •430.22(A) CONTROLLER • 430.81

SECONDARY CONDUCTORS • 430.23(A) DRUM CONTROLLER

DISCONNECTING MEANS • 430.102 RESISTOR BANK • 430.23(C)

POWER SOURCE • 3Ø

NOTE: THE MOTORS BELOW CAN BE EITHER NEMA DESIGN B, C, OR D. DISCONNECTING MEANS WINDINGS • 3Ø • WYE MOTOR
RESISTOR BANK CONDUCTORS USED AT CONTINUOUS DUTY • 430.23(B)

NEC TABLE 430.52
Figure 3. The above illustrates an example of a...
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