Two speed motors

Variable Air Flow Fans. Variable air flow fans are needed ia the process iadustry for steam or vapor condensing or other temperature critical duties. These also produce significant power saviags. Variable air flow is accompHshed by (/) variable speed motors (most commonly variable frequency drives (VFDs) (2) variable pitch fan hubs (J) two-speed motors (4) selectively turning off fans ia multiple fan iastaHations or (5) variable exit louvers or dampers. Of these methods, VFDs and variable pitch fans are the most efficient. Variable louvers, which throttle the airflow, are the least efficient. The various means of controlling air flow are summarized ia Table 3. 

Two-speed motors are typically used on noncondensing services where the process is not sensitive to temperature but mostly seasonal or variable throughput of fluids in the air cooler requires some degree of air flow control. This is a simple, rather inexpensive means to control air flow when volume air flow is not critical. Typical motor ratings are 1800/900 rpm, although 1800/1200 rpm types are available. 

Air-Flow Control Process operating reqmrements and weather conditions are considered in determining the method of controlling air flow. The most common methods include simple on-off control, on-off step control (in the case of multiple-driver units), two-speed-motor control, variable-speed drivers, controllable fan pitch, manually or automatically adjustable louvers, and air recirculation. 

When the mean annual temperature is 16.7°C (30°F) lower than the design dry-bulb temperature and when both fans in a bay have automatically controllable pitch of fan blades, annual power required has been found to be 22, 36, and 54 percent respectively of that needed at the design condition for three process services [Frank L. Rubin, Tower Requirements Are Lower for Air-Cooled Heat Exchangers with AV Fans, Oil Gas J., 165-167 (Oct. 11, 1982)]. Alternatively, when fans have two-speed motors, these dehver one-half of the design flow of air at half speed and use only one-eighth of the power of the full-speed condition. 

Typical pilot-plant top-suspended baskets are 305 mm (12 in) diameter by 127 mm (5 in) deep. Commercial machines are available in sizes from 508-mm (20-in) diameter by 305-mm (l2-in) depth to 1524-mm (60-in) diameter by 1016-mm (40-in) depth and develop up to 1800g in the smaller and intermediate sizes. Except in the sugar apphcation, operation with a two-speed motor (half speed for loading and full speed for purging) is typical. Hydraulic drives with variable-speed capability are commonly used in the chemical industiy To maximize the number of cycles per hour, a combination of electrical and mechanical braking is employed to minimize the deceleration period, which is a transition period of no value to the process. 

The effect of cold weather on the freezing of tube-side fluids and increasing horsepower due to increased air density can not be overlooked. Usual practice is to reduce fan output by using a two-speed motor, louvers on variable-pitch fans, or drivers. ... 

When a two-speed motor is reduced to half-speed, the air capacity will be cut 50%, and the required horsepower consumption will drop to Vs of full speed power. 

The fans on such condensers may he fitted with two-speed motors or other speed control. It should he home in mind that, if one fan of a pair stops, the noise level will fall hy 3 dB, hut if hoth fans drop to half speed, the noise drops hy 15 dB. This method is of special use in residential areas where the greater noise level will he tolerated in the daytime when condensing air is warmest, hut a lower fan speed can he used at night. 

For one- or two-cell cooling towers the water flow is normally maintained, and to limit the fall in water temperature the air flowrate is controlled through the tower. The simplest method is an on/off control of the air fan by means of a thermostat with a sensing element in the water leaving the tower. Another relatively simple method of control is to use a two-speed motor together with a two-step thermostat to have high-speed or low-speed fan rpm, depending on the water temperature. 

Select standard size motor. A motor that is loaded to 85 percent by a 79.1-hp impeller will require a minimum size of (79.1 hp)/0.85 = 93.1 hp, which means a 100-hp (75-kW) motor. This motor and impeller assembly is correctly sized for conditions with the design gas flow. However, because of the gassed power factor, that is, P/P0 = 0.38, should the gas supply be lost for any reason, the impeller power would increase to 78/0.38 = 205 hp and seriously overload the motor. To avoid this problem, some method (typically electrical control) prevents motor operation without the gas supply. When the gas supply is off, the control either stops the agitator motor or, in the case of a two-speed motor, goes to a lower speed. 

The angular momentum is read using a dynamometer and is indicated by an analog a-meter in the range 0-100 mV. The device is equipped by a 12-stage mechanical gearbox and a two-speed motor. This broad variability enables a sufficiently smooth adjustment... 

Reduced Agitator Cost. It is not necessary to buy a two-speed motor where the slow speed (low horsepower) is used for mixing during batch sterilization and high speed only during aeration. 

High-Speed Dispersers. The high-speed disperser consists of a disk that rotates at high speed in the center of a vertical tank (Figure 8). The disk, mounted on a shaft, rotates at a speed of 4500-5500 ft/min. Although there are stationary shaft dispersers, most high-speed dispersers are designed so that the shaft can go up or down the shaft in some cases can be moved sideways. The disk is driven either by a variable-speed or a two-speed motor. 

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