Energy Use in Aquaculture Pond Aeration Part 2/2

Electric motors

Synchronous induction, alternating current motors of\r\ndifferent types and sizes are used to power aerators. Smaller electric motors (1\r\nto 3 hp) often are single-phase and operate on 110-120 volts (V) or 208-230 V,\r\nwhile larger motors usually are three-phase and operate on 208-230 V or even\r\n460 V. Motors usually are rated at full load to require a specific current\r\nmeasured in amperes (Table 3); the amperes required double between 115 V and\r\n230 V motors and between 230 V or 460 V motors.

Boyd, aeration Pt. 1, Table 3

\r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n

\r\n Motor power rating (hp) \r\n	\r\n Single phase (115 V) \r\n	\r\n Single phase (230 V) \r\n	\r\n Three phase (230 V) \r\n	\r\n Three phase (460 V) \r\n
\r\n 1 \r\n	\r\n 16 \r\n	\r\n 8 \r\n	\r\n 3.6 \r\n	\r\n 1.8 \r\n
\r\n 2 \r\n	\r\n 24 \r\n	\r\n 12 \r\n	\r\n 6.8 \r\n	\r\n 3.4 \r\n

Table 3. Full-load amperes for single-phase and small,\r\nthree-phase electric motors
\r\nSource: http://mechreps.com/PDF/MRI_Formulas_Conversions.pdf

The efficiency of electric motors is calculated as the\r\nenergy output by the motor shaft (brake power) divided by the energy input to\r\nthe motor (wire power) as follows:

kW = Brake hp × 0.746 ÷ Motor efficiency

The typical efficiencies of small electric motors are\r\nprovided in Table 4. Motors should be operated at around 75 percent full load\r\nat which service life and efficiency are greater than at full load. A 10-hp,\r\n230 V, three-phase motor operated at a brake power of 7.5 hp would use around\r\n6.32 kW or 6.32 kW·hr of electricity per hour.

Boyd, aeration Pt. 1, Table 4

\r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n

\r\n Electric motors \r\n	\r\n Efficiency 1 (%) \r\n	\r\n Small (<25 hp) stationary internal combustion engine \r\n	\r\n Efficiency (%) \r\n
\r\n 1-4 hp \r\n	\r\n 78.8 \r\n	\r\n Ethanol (E100) \r\n	\r\n 20-25 \r\n
\r\n 5-9 hp \r\n	\r\n 84.0 \r\n	\r\n Liquid propane gas (LPG) \r\n	\r\n 25-30 \r\n
\r\n 10-19 hp \r\n	\r\n 85.5 \r\n	\r\n Gasoline \r\n	\r\n 20-30 \r\n
\r\n 20-49 hp \r\n	\r\n 88.5 \r\n	\r\n Diesel \r\n	\r\n 28-32 \r\n

Table 4. Efficiencies of small, electric motors and small,\r\nstationary internal combustion engines.
\r\nSource: https://www.engineering toolbox.com/electrical-motor-efficiency-d_655.html

The amperes of electricity used by a motor is roughly in\r\ndirect proportion to load between 50 and 100 percent full load (Fig. 1). The\r\namperes drawn by an aerator motor can be easily measured with an ammeter during\r\nmotor operation and the approximate load estimated. There is a substantial\r\nno-load current by electric motors, and they waste electricity when\r\nsubstantially under-loaded. The amount of power required to operate paddlewheel\r\naerators increases with greater paddlewheel depth at any rotation speed.\r\nAttention to the depth of paddle tip submergence is important because it can\r\nchange as the floats for aerators shift during operation. This results in\r\ngreater or lesser submergence of paddles which can lead to overloading or\r\nunder-loading of motors.

Fig. 1: Load versus fuel use by internal combustion engines\r\nand current use by electric motors (prepared with information from\r\nhttp://dieselserviceandsupply.com/Diesel_Fuel_Consumption.aspx and from\r\nhttp://energy.gov/sites/prod/files/2014/04/f15/10097517.pdf).

The rotation speed of electric motors varies with the number\r\nof magnetic poles in the motor (Table 5). Motor slip under-load reduces output\r\nshaft speed by 3 to 5 percent in 1 to 5 hp motors and by around 2 to 3 percent\r\nin motors of 7.5 to 20 hp. Motors for paddlewheel aerators usually have a shaft\r\nspeed of about 1,735 ppm. By contrast, paddlewheels, depending upon their\r\ndiameter, transfer oxygen most efficiently at 80 to 120 rpm. Motors that turn\r\nat such low speeds are more expensive, and speed reducers are placed in the\r\ndrive train of aerators to allow higher speed motors to be used.

Boyd, aeration Pt. 1, Table 5

\r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n

\r\n Magnetic poles \r\n	\r\n Electrical frequency, 50 Hz \r\n	\r\n Electrical frequency, 60 Hz \r\n
\r\n 2 \r\n	\r\n 3,000 \r\n	\r\n 3,600 \r\n
\r\n 4 \r\n	\r\n 1,500 \r\n	\r\n 1,800 \r\n

Table 5. Electric motor rotation speed as related to a number\r\nof magnetic poles of motor and electrical frequency.
\r\nSource: https://www.engineeringtoolbox.com/synchronous-motor-frequency-speed-d_649.html

There also is considerable use of propeller-aspirator-pump\r\naerators in shrimp ponds. Most of these aerators have two-pole motors that\r\nrotate at about 3,600 rpm (Table 5). This type of aerator does not transfer as\r\nmuch oxygen per unit of energy if it is supplied with a four-pole motor.\r\nVertical pump aerators for aquaculture typically are supplied with a four-pole\r\nmotor.

When electric motors are started up, an inrush of electric\r\ncurrent is necessary to start the rotor and accelerate the shaft to full speed.\r\nFor single-phase motors of 1 to 10 hp, the inrush current is about three times\r\nthe full load motor current requirement in amperes. For three-phase motors, the\r\ninrush current is about twice the full load current. Large motors usually are\r\nstarted with a motor controller (often called a motor starter) with fuses that\r\nwithstand the large ampere input at startup.

\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n

Next Part of The Article: https://www.minapoli.com/info/energy-use-in-aquaculture-pond-aeration

Source: Global Aquaculture Alliance