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Identifying Motors

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  Identifying Motors   Electric motors with no identification (no nameplate or lead tags) must often be maintained and repaired. Follow these steps to determine an unknown motor’s characteristics, based on the NEMA Standard method of motor identification. First, sketch the coils to form a wye. Identify one outside coil end with the number one (1), and then draw a decreasing spiral and number each coil end in sequence as shown in Figure 6-3. Using a DMM, ohmmeter, or continuity tester, the individual circuits can then be identified as follows: Step 1. Connect one probe of the tester to any lead, and check for continuity to each of the other eight leads. A reading from only one other lead indicates one of the two-wire circuits. A reading to two other leads indicates the three-wire circuit that makes up the internal wye connection. Step 2. Continue checking and isolating leads until all four circuits have been located Tag the wires of the three lead circuits T-7, T-8,...
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Troubleshooting Luminaires (Lighting Fixtures)

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  Troubleshooting Luminaires (Lighting Fixtures)   Luminaire. A complete lighting unit consisting of a lamp or lamps together with the parts designed to distribute the light, to position and protect the lamps and ballast (where applicable), and to connect the lamps to the power supply. A typical commercial, industrial, or institutional building contains hundreds or even thousands of luminaires. For this reason, troubleshooting luminaires is an important part of the typical maintenance electrician’s work. This chapter covers the three most common types of lighting used in commercial, industrial, and institutional applications: o    Fluorescent luminaires o    Incandescent luminaires o    High-intensity discharge (HID) luminaires   Troubleshooting Fluorescent Luminaires Fluorescent lamps are electrical discharge lighting sources. Current flows in an arc through a glass tube filled with mercury vapor between contacts called cat...
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Troubleshooting Split-Phase Motors

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  Troubleshooting Split-Phase Motors   If a split-phase motor fails to start, the trouble may be due to one or more of the following faults: o    Tight or “frozen” bearings o    Worn bearings, allowing the rotor to drag on the stator o    Bent rotor shaft o    One or both bearings out of alignment o    Open circuit in either starting or running windings o    Defective centrifugal switch o    Improper connections in either winding o    Grounds in either winding or both o    Shorts between the two windings   Tight or worn bearings Tight or worn bearings may be due to the lubricating system failing, or when new bearings are installed, they may run hot if the shaft is not kept well oiled. If the bearings are worn to such an extent that they allow the rotor to drag on the stator, this will usually prevent the rotor from starting. The inside of the stator lamina...
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Troubleshooting Relays and Contactors

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  Troubleshooting Relays and Contactors   Relay is an electromagnetic or solid-state device used in control circuits of magnetic motor starters, heaters, solenoids, timers, and other devices. They are frequently used for remote control applications. Relays are manufactured in a number of different configurations, in both mechanical and solid-state designs. Figure 8-1 shows a type of relay often used to control small, single-phase motors and other light loads suc h as heaters or pilot lights. Contactors are electromagnetic devices similar in construction and operation to relays, but designed to handle much higher currents (Figure 8-2) involved in applications such as switching large banks of stadium lights on and off. Figure 8-3 describes troubleshooting procedures for relays and contactors. 8-1 Single-pole, single-throw (SPST) relay rated 30 A, 600 V.   8-2 NEMA size 1 contactor rated 10HP, 575 V.     8-3 Contactor...
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Troubleshooting Motor Bearings

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  Troubleshooting Motor Bearings   Alternating-current motors account for a high percentage of electrical repair work. A high proportion of failures are caused by faulty bearings. Sleeve and ball bearing failure can occur in both newer and older motors; but sealed motor bearings are much less prone to failure. Types of Bearings There are many types of motor bearings, with ball bearings being the most common. There are several different types of ball bearings used in motors: - Open - Single shielded - Double shielded - Sealed - Double row and other special types Open bearings, as the name implies, are open construction and must be installed in a sealed housing. These bearings are less apt to cause churning of grease, and for this reason are used mostly on large motors. Single-shielded bearings have a shield on one side to exclude grease from the motor windings. Double-shielded bearings have a shield on both sides of the bearing. This type of bearing i...
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Troubleshooting Electric Motors

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  Troubleshooting Electric Motors   Electric motors operate on the principle of electromagnetic induction. An electric motor has a stationary magnet, or stator, with windings connected to the supply conductors, and a rotating magnet. There is no electrical connection between the stator and rotor. The magnetic field produced in the stator windings induces a voltage in the rotor. When an electric motor malfunctions, the stator (stationary) windings are often defective, and must be repaired or replaced. Stator problems are usually caused by one or more of the following: Ø   Worn bearings Ø   Moisture Ø   Overloading Ø   Poor insulation Ø   Single-phase operation of a three-phase motor Troubleshooting Motors To detect defects in electric motors, the windings are normally tested for ground faults, opens, shorts, and reverses. The exact method of performing these tests depends on the type of motor being serviced. However, regardless of th...
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