The two most common types of electric motors are alternating current (ac) inductionand direct current (dc) commutated shunt- or series-wound.All electric motor configurations except one type (reluctance brushless) convert electrical energy to mechanical energy from the magnetic flux linkage of their two magnetic circuits. One of these circuits is in the stator and the other is in the bearing-mounted rotor. This flux linkage between the two magnetic circuits produces a moment of force at the rotor radius that results in a torque on the motor shaft causing shaft rotation. The speed of the rotation times the torque equals the output power at the motor shaft. This is, of course, the power used to drive a pump. The basic difference between these two types of electric motors has to do with their electrical power source. The first type has been historically powered by 60 cycle (Hz) alternating voltages direct from the public utility power grid (50 Hz in most of Europe and some other parts of the world). For this type of motor, the speed is determined by the number of magnetic poles designed in the motor and the alternating sinusoidal frequency of the voltage from the power grid. Methods have been developed to alter the speed of an ac motor with a fixed number of poles and a fixed line frequency (it is also possible to wind such a motor with several poles for other speeds). However, the squirrel-cage induction motor remains without question the most common type of motor used to drive pumps. The reasons for this are worldwide availability, excellent reliability, excellent performance characteristics, and ease of replacement. Because of its adaptability to variable speed using ac inverters and vector drives, the ac induction motor will continue to be the number one prime mover for all types of pumps as the pump industry continues to adopt variable speed motor/drives. The other principle motor type used to drive pumps is the dc motor. The dc electrical machine also contains two magnet fields, one in each of the stator and rotor assemblies of the motor. Dc current creates the rotor field provided through commutator brushes. These brushes have a finite life and are the principal maintenance/failure point in this type of machine. However, in some applications, particularly those where only dc (battery) power is available, these machines are common. The dc motor, powered by the voltage from storage batteries or from a dc generator, can be speed-adjusted by varying the voltage with a power supply. In a limited number of instances dc motors are used where adjustable speed is required. Its speed relationship to the voltage is linear and very useful for some pump applications such as constant displacement types, which require speed adjustment to set flow. [See Section 9.2.2.] Most dc motor applications are in the smaller sizes such as automotive and off road equipment. The overall use of dc motors for pump drives is predicted to decline. A third type of pump prime mover is the permanent magnet machine, sometimes called a brushless dc motor. Unlike dc motors permanent magnet motors have no brushes; unlike the induction machine there is no squirrel cage with induced electric current either; the field is generated by permanent magnets, offering a more reliable prime mover for a broad range of applications, with superior performance and long-life benefits. Permanent magnet motors require a motor drive (inverter, variable speed or frequency drive) to operate; very simple so-called “sensoreless” drives can be used for pumping applications. There are other types of electric motors considered for driving pumps. The reasons for this include the dramatic recent advances in power electronics and microprocessors, advances in motor materials such as permanent magnets, and advances in the pumps themselves. Besides the active interest in adjustable speed pumps, another reason for the interest in some newer motor types for pump applications is because of recent U.S. government regulations enacted to improve energy conservation by implementing motor efficiency mandates along with a time schedule. For example, the U.S. Energy Policy and Conservation Actof September 10,1992 (EPACT) stipulates that all covered electric motor products must meet the efficiency levels per NEMA MG1 1993. The requirement covers all electric motors from 1- 200 horsepower (1-150 kW) manufactured after October 24, 1997 that operate from 230/460 VAC power at 60 Hz line frequency. The U.S. Department of Energy approved test method to meet the new efficiency levels is per IEEE-112B. This regulation was originally intended for ac induction motors. For those pump applications that require the motor torque to increase as the square of speed, these new, more efficient motors will run at higher speeds because their slip is less. This could cause an appreciable increase in overload within the motor. Overload could also result in other mechanical parts of the system. There are other ramifications resulting from these new requirements that must be carefully analyzed when selecting an electric motor, starter, or inverter. The resulting analysis might very well cause the selection to be some other electric motor type, such as a permanent magnet brushless dc, permanent magnet ac, synchronous (sine wave driven version of the brushless dc), or even a switched reluctance brushless dc motor. Each of these types must be powered with an inverter and a controller. However, the result can most likely offer adjustable speed with a very high efficiency over a wide speed range. This use of an inverter is required for each of those other types of motors mentioned. With the availability of the new vector controlled ac inverters (Section 9.2.2), the high-efficiency ac induction motor can also be applied for variable speed. This will eliminate the need for a soft starter frequently required with line fed motors. The elimination of the starter helps somewhat to offset the additional cost of the inverter.
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