Blowers and Motors
A forced-warm-air furnace should be equipped with a variable- speed electric motor designed for continuous duty. The motor should be provided with overcurrent protection in accordance with the National Electrical Code (ANSI C1-1971).
Blower motors are available as either direct-drive or belt-driven units. On direct-drive units, the blower wheel is mounted directly on the motor shaft and runs at motor speed. The blower wheel of a belt-driven unit is operated by a V-belt connecting a fixed pulley mounted on the blower shaft with a variable-pitch drive pulley mounted on the motor shaft.
The blower motors used in residential furnaces are single-phase electric motors. Depending on the method used for starting, they may be classified as:
1. Shaded pole
2. Permanent split capacitor
3. Split-phase
4. Capacitor-start motors
A direct-drive blower generally uses a shaded pole (SHP) or permanent capacitor (PSC) motor. Belt-driven blower motors are usually split-phase (SPH) types up to the 1⁄3-hp size. Larger motors are capacitor-start, split-phase motors.
Blower motors are fractional horsepower motors commonly available in the following sizes: 1⁄12, 1⁄10, 1⁄6, 1⁄4, 1⁄3, 1⁄2, and 3⁄4 hp. A fractional horsepower motor should be protected with temperatureor current-sensitive devices to prevent motor winding temper- atures from exceeding those allowed in the specifications.
Provisions should be made for the periodic lubrication of the blower and motor. Use only the type and grade of lubricant specified in the operating instructions. Do not overlubricate. Too much oil can be just as bad as too little. A direct-drive blower should be capable of operating within the range of air temperature at the static pressures for which the furnace is designed and at the voltage specified on the motor nameplate.
Motors using belt drives must be supplied with adjustable pulleys. The only exception is a condenser motor used on a forced- warm-air furnace with a cooling unit. Always provide the means for adjusting a belt-driven motor in an easily accessible location.
Most forced-warm-air furnaces are shipped with the blower motors factory-adjusted to drive the blower wheel at a correct speed for heating. This is a slower speed than the one used for air conditioning, but it is relatively easy to alter the air delivery to meet the requirements of the installation (see Air Conditioning in this chapter).