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NORA Oilheat Technicians Manual

Chapter 10—Motors 10-5 Start switch The start switch of a motor is wired internally in series with the start winding. There are two parts to the start mechanism— the stationary switch, with its contacts—and a governor. The governor holds the switch contacts together when the motor is not running and releases the pressure to open the contacts when the motor is up to speed. This action must occur quickly; if not, the start winding can be damaged. The starting switch is a centrifugal switch. Its contacts are weighted and a spring holds them closed. As the motor shaft begins to turn, centrifugal force pushes the weights against the spring, opening the contacts of the switch. Figure 10-2 is a drawing of the centrifugal start switch. Locked rotor amperage This is the amount of amperage that can be measured for the brief instant when the motor first starts. A motor will draw substantially more amperage on the start than it will while running. After starting, the motor should only draw the amount of amperage listed on the motor nameplate. The locked rotor amperage (not usually listed on the motor nameplate) is the amount of current present if the motor fails to start after current is delivered to it. A motor may not turn because of bad bearings, bound up fuel pump, or any other reason for the rotor to be “locked in” place. Motor wiring Figure 10-3 shows the internal schematic of the split phase fractional horsepower motor pictured in Figure 10-1. It is used in most residential burners. Note the location of the start switch. Apart from the size and voltage shown on the motor nameplate, there is also a frame number. This refers to the type of cradle or mounting the motor has. Capacitor start motors We use capacitors to build up an electric charge and store it until it is needed. A capacitor is made of two conducting plates separated by an insulator. A capacitor start motor has a starting capacitor inserted in series with the start switch and starting winding, creating a circuit which is capable of a much greater phase shift (and so, a Figure 10-3: Split phase motor, internal wiring Figure 10-2: Start switch Chapter 10 Motors Starting Switch (Centrifugal) Overload Switch Running Winding Starting Winding 120 Volts AC Line


NORA Oilheat Technicians Manual
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