Equations electric motors

Power, Energy, and Drives. Centrifuges accomplish their function by subjecting fluids and soHds to centrifugal fields produced by rotation. Electric motors are the drive device most frequently used however, hydrauHc motors, internal combustion engines, and steam or air turbines are also used. One power equation appHes to all types of centrifuges and drive devices. 

Within these basic principles there are many types of electric motors. Each has its own individual operating characteristics peculiarly suited to specific drive applications. Equations (29-1) through (29-9), presented in Table 29-1, describe the general operating characteristics of alternating-current motors. When several types are suitable, selection is based on initial installed cost and operating costs (including maintenance and consideration of rehability). 

The degree of hybridization is a useful parameter characterizing parallel hybrid vehicles. The value of this parameter is defined as the total power of the electric motor divided by the sum of the total power of electric motor and IC engine, according to the following equation ... 

A centrifugal pump is usually driven by an electric motor whose cost is added to the pump cost from Eq. (16.15). The size parameter for the motor is its power consumption, Pq, which is determined from the theoretical horsepower of the pump, Pj, its fractional efficiency, -qp, and the fractional efficiency of the electric motor, by the equation ... 

From the electric motor cost correlations of Corripio et al. (1982a), indexed to mid-2000 (CE = 394), the f.o.b. purchase cost of an electric motor operating at 3,600 rpm, with an open, drip-proof enclosure (referred to here as the base cost, Cg), is plotted in Figure 16.4 as a function of the horsepower consumption, The cost curve is given by the equation ... 

The brake horsepower for a fan may be computed in any of three ways, depending on whether the total change in head is mostly dynamic, static, or a mixture of the two. The corresponding nominal fan efficiency, iqf, is 40% for mostly a dynamic change, 60% for mostly a static change, and 70% for a mixture of the two. The power consumption is given by the following equation, which is similar to Eq. (16.16) and where the electric motor efficiency, can be taken as 90% ... 

When the required shaft horsepower for power input to an item of process equipment is less than 100 Hp, an electric motor is usually the selected drive. For higher horsepower input, consideration is given to combustion gas turbines, steam turbines, and internal combustion gas engines. However, except for remote, mobile, or special situations, steam turbines are the most common alternative to electric motors. Furthermore, steam turbines are considerably more efficient, 50-80%, than gas turbines or engines, which have efficiencies of only 30-40%. Equations for f.o.b. purchase costs of steam and gas turbine drives are included in Table 16.32 as a function of shaft horsepower. 

However, this power loss is just the power delivered to the rotor of the compressor. This power comes from an electric motor, which has an efficiency of less than 1, and there are also losses in the connecting shaft and the bearings of the compressor rotor. If we express the combined efficiency of the motor and drive system as r m, then Ihe electrical power used will be greater than the compressor power by a factor of I/rjm, and so there will be a loss of electrical power given by the equation... 

The available motor speeds (RPM, revolution per minute) for a standard alternating current A,C.) electric motor are based on following equation for 60-cycle current ... 

From equation 60 one can obtain a theoretical power requirement of about 900 kWh/SWU for uranium isotope separation assuming a reasonable operating temperature. A comparison of this number with the specific power requirements of the United States (2433 kWh/SWU) or Eurodif plants (2538 kWh/SWU) indicates that real gaseous diffusion plants have an efficiency of about 37%. This represents not only the barrier efficiency, the value of which has not been reported, but also electrical distribution losses, motor and compressor efficiencies, and frictional losses in the process gas flow. 

This expression, except for the mechanical design, is totally independent of the type of start and the electrical design of the motor. Electrically also, this is demonstrated in I he subsequent example. The expression, however, does not hold good for an ON-LOAD start. On load, the accelerating torque diminishes substantially with the type of load and the method of start, as can be seen from Figure 2.14, and so diminishes the denominator of equation (2.5), raising the time of start. 

In addition to electrical braking, a mechanical brake, as discussed in Section 6.20.1(A) may also be essential if the motor is required to be stopped completely because, at any value of excitation current, the motor will never reach a standstill condition. The heat of braking up to the standstill condition (N = 0) is roughly equal to one start and is expressed by equation (6.9). 

Thus from Equation 2-215 we see that for a given dynamo geometry, the developed torque only depends on the interaction between two magnetic fields and their orientation with respect to each other. One or both of the magnetic fields may be induced by a current. If one of the fields is the field of a magnet, then it may be either in the rotor or the stator. If the rotation results from the imposition of mechanical power on the rotor, the device is called a generator. If the rotation is caused by the flow of current, the device is called a motor, i.e., converts electric power to mechanical power. 

In equation 7.12, P is the impeller power, that is, the energy per unit time dissipated within the liquid. Clearly, the electrical power required to drive the motor will be greater than P on account of transmission losses in the gear box, motor, bearings, and so on. 

We ought to be able to answer this question with Robert Mayer s equation—also called the Second Law of Thermodynamics (see Chap. 27), which states that motor amperage (or electrical work) is proportional to... 

As with most electrical power equipment the thermal characteristic is based on an Pt law. The equation for the thermal image as given by 1EC60255 Part 8 when the motor is cold is,... 

---