TYPES OF MOTORS
Alternating-Current Motors SQUIRREL-CAGE INDUCTION MOTOR By far the most common motor used to drive pumps is the squirrel-cage induction motor (Figure 1). This motor consists of a conventional stator wound with a specific number of poles and phases, and a rotor that has either cast bars or brazed bars imbedded in it. The squirrel-cage induction motor operates at a speed below synchronous speed by a specific slip or revolutions per minute. The synchronous speed is defined as 
FIGURE 1 Exploded view of a typical squirrel-cage induction motor (Courtesy of Leeson Electric Corp.) 
When the stator winding of a squirrel-cage induction motor is connected to a suitable source of power, a magnetic flux is generated in the air gap between the stator and rotor of the motor. This flux revolves around the perimeter of the air gap and induces a voltage in the rotor bars. Because the rotor bars are short-circuited to each other at their ends (end rings), a current circulates in the rotor bars. This current and the air-gap flux interact, causing the motor to produce a torque.
The squirrel-cage induction motor exhibits a characteristic speed-torque relationship that is determined by the resistance of the rotor bars. Thus, the desired speed-torque characteristics are obtained by selecting a metal of suitable resistance when designing the rotor bars. The slot shape and size for the bars in the rotor can be selected to achieve a certain rotor resistance. Figure 2 suggests several typical speed-torque characteristics that have been standardized by NEMA (National Electrical Manufacturers Association), covering motor frames 143T through 449T Motors larger than 449T may not have these same values, but generally have the same characteristic curves. Also, single-phase motors may not exhibit these characteristics and are defined specifically by NEMA with different values. Most pumps are driven by NEMA B characteristic motors when operated from three-phase power sources. WOUND ROTOR INDUCTION MOTOR The wound-rotor induction motor is in every respect similar to the squirrel-cage version except that the rotor is wound with insulated wire turns and this winding is terminated at a set of slip rings on the rotor shaft. Connections are made 
FIGURE 2 These curves are characteristic of NEMA frame size squirrel-cage induction motors through size 449T (typically through 300 hp-224 kW)
to the slip rings through brushes and in turn to an external resistor, which can be adjusted in ohmic value to cause the motor speed-torque characteristics to be changed. These types of motors have been used in some pump applications in the past, but due to the availability of inverter-fed ac squirrel-cage induction motor drives, they are no longer very practical.
Figure 3 demonstrates the speed-torque characteristics of a wound-rotor induction motor for several resistor values. It will be noticed that increasing the external resistance of the control will cause the peak torque of the motor to be developed at lower speeds until the peak torque occurs at zero speed. Increasing the resistance beyond this value will cause the motor to have a limited torque as, for example, curves 4, 5, and 6. This motor can be used where torque control is required or where variable speed is necessary. In the variable-speed application, the rotor resistance is adjusted to produce a motor torque that matches the load torque at the specific speed desired. This system is not as useful and cost-effective as an inverter-fed ac induction motor due to the recent developments of power electronics and microprocessors.
SYNCHRONOUS MOTOR The synchronous motor is also similar to the squirrel-cage induction motor except that it operates at synchronous speed and its rotor is constructed with definite salient poles on which a field coil is wound and connected to a source of direct current for excitation. The most common synchronous motor is constructed with slip rings on the rotor shaft to connect the dc excitation to the field coils. FIGURE 3 Typical speed-torque characteristics of a wound-rotor induction motor (Westinghouse Electric) There are various means of providing the dc power to the slip rings: - Static excitation The power to be connected to the slip ring brushes on the motor shaft is obtained from a transformer and rectifier package external to the motor.
- Direct-connected exciter This arrangement has a dc generator directly connected to the synchronous motor shaft (Figure 4). The dc power from this generator is connected to the brushes of the synchronous motor slip rings.
- Motor-generated exciter The dc power for exciting the synchronous motor is generated by means of a remote motor-generator set operating from normal ac power, and the dc voltage from this motor-generator set is connected to the brushes of slip rings of the synchronous motor.
Another form of synchronous motor is known as the brushless synchronous motor (Figure 5). As the name implies, this motor has its rotating field excited without the use of slip rings for connecting the external direct current to the motor field. The construction 
FIGURE 4 Synchronous motor with direct-connected exciter (Electric Machinery Manufacturing) 
FIGURE 5 Brushless synchronous motor with ac generator mounted on the shaft with rectifier and control devices (Electric Machinery Manufacturing) of this motor incorporates a shaft-connected ac generator. The field of the ac generator is physically stationary and connected to a source of dc voltage. The rotor of this ac generator is connected through a solid-state controlled rectifier mounted on the synchronous motor rotor and in turn connected to the synchronous motor field. This arrangement facilitates a connection between external excitation power and the rotating field of the synchronous motor through the air gap of the shaft-connected ac generator. The brushless synchronous motor has many advantages over the conventional slip-ring synchronous motor. Among these are the elimination of brushes and slip rings, which are high-maintenance items; the elimination of sparking devices, which are not permissible in certain atmospheres; and the use of static devices for field control, which are more reliable than conventional electromagnetic controls. Synchronous motors are used for pump applications requiring larger horsepower ratings at lower speed conditions, as illustrated in Figure 6. Also, they are used on applications where a high power factor or a power-factor-correction capability is desired. Of less importance is the characteristic of the synchronous motor that it will always operate at synchronous speed (does not have a slip) regardless of load. Synchronous motors are started on their damper windings (the same as squirrel-cage induction motors), and when they have accelerated to within 5% of synchronous speed, the field is applied and the motor accelerates to synchronous speed (Figure 7). Typical characteristic curves are shown in Figure 8. Direct-Current Motors Dc motors are only occasionally used to drive pumps. Most of the current applications for dc pump drives are in some form of automobile or off-the-road equipment due to the dc power from storage batteries. There are some other situations that might call for dc motor pump drives such as shipboard duty, railway applications, aircraft, mining installations, and some other emergency battery operations. There are three types of dc motors available (Figure 9): shunt, series, and compound-connected. Larger horsepower ratings of shunt-wound dc motors are frequently qualified as “stabilized shuntwound” motors and incorporate a series field similar to that of a compound wound motor. This is necessary to adjust the regulation of the shunt motor so as not to exhibit a rising speed-torque characteristic. It is important to be aware of the speed at which a dc motor will operate on pump applications because of pump performance guarantees. The windings in the stators of the wound types are connected to the armature windings through brushes and a commutator three different ways to achieve different performance 
FIGURE 6 Below 514 rpm, or powers greater than approximately 1 hp/rpm (0.746 kW/rpm), synchronous motors are a better selection than squirrel-cage induction motors because higher cost can generally be offset by higher power factor and efficiency (from Powerspecial report, “Motors,” June 1969)
characteristics (see Figure 3 for details). With the use of pulse width modulation (PWM) or silicon-controlled rectifiers (SCR), variable dc power supplies the armature voltage, which can be controlled along with a separate control for the field current (see Section 9.2.2). Using this control scheme, the dc motor can be used to control speed over a very wide range in a smooth manner. For example, constant voltage can be produced from zero rpm up to base design speed of the motor by armature control and constant power above base speed using flux weakening of the armature field. When operating from a constant voltage, dc motors are available to provide up to a 4 to 1 speed range with an adjustable dc field power supply. With this separate control scheme for both field and armature, a 100 to 1 speed range is easily achievable. Figure 9 summarizes the various
FIGURE 7 Method of starting synchronous motor. (A) Typical field control (brush type) energizes the dc field to pull the rotor into synchronism as it comes up to speed. Field relay senses change in induced frequency as motor speeds up. (B) Damper winding is similar to a squirrel-cage rotor winding. It produces most of the starting torque, and the synchronous motor starts with essentially induction-motor characteristics (from Powerspecial report, “Motors,” June 1969).
FIGURE 8 Characteristics of synchronous motors depend on rotor design. Torque and current relations are influenced by synchronous speed. High-resistance cage produces high starting torque but low pull-in torque (from Powerspecial report, “Motors,” June 1969).
field connection schemes for dual wound dc machines with mechanical commutation systems. Figure 10 illustrates typical characteristics of the dc motors discussed. Figure 11 shows the equivalent circuit of a permanent magnet type dc motor. | 
