Motors relative costs

Evans gives handy relative cost tables for motors based on voltages (Table 1), speeds (Table 2), and enclosures (Table 3). 

Relative Cost at Three Voltage Levels of Drip-Proof 1,200-rpm Motors ... 

Relative Cost at Three Speeds of Drip-Proof 2,300-Volt Motors... 

When the plant distribution voltage is above utilization levels, Itjr instance 23 kV or higher, economics will usually favor the 2,300- relative cost of alternate schemes sometimes favors 4.100 volts. This is true particularly with motors in the 4,000 hp and larger si/.es. and short circuit levels are above 150 MVA. 

V ri ble Frecjuency Drives. An important energy by-product of soHd-state electronics is the relatively low cost variable speed drive. These electronic devices adjust the frequency of current to control motor speed such that a pump can be controlled direcdy to deUver the right flow without the need for a control valve and its inherent pressure drop. Eigure 11 shows that at rated load the variable speed drive uses only about 70% as much power as a standard throttle control valve system, and at half load, it uses only about 25% as much power. 

Alternating-Current Squirrel-Cage Induction Motors These motors are by far the most common constant-speed drives. They are relatively simple in design and therefore both low in cost and highly reliable. Representative prices are shown in Fig. 29-1 for various speeds and horsepowers. 

Probably the most widely used capacity control for the centrifugal compressor is speed control. The capacity curve when used with speed control covers a wide range. While electric variable speed motors offer a continuation to the speed control practice, there are some other alternatives available. Suction throttling has been widely used and offers a r sonable control range for a relatively low cost. 

Similarly, an electric motor can use electricity that costs more than the motor during a year of continuous operation. Even if the motor is in perfect condition, it may be cost effective to replace it with a new motor that is a few percentage points more efficient at converting electricity into work. In many applications, however, an electric motor operates only a few hours per year. In such cases, the cost of the electricity is negligible relative to the cost of a new motor, so that even a large gain in energy efficiency is not worth the cost. 

Good range. Relatively good low speed efficiency. Automatic speed control easy Low cost. Can use squirrel cage induction motor. Automatic control easy... 

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