Types Of Controls

In order to use an electric motor to drive a pump, a means of starting and stopping the motor is required. The devices used for this purpose can be called controls. Only small— perhaps up to two or three hp (kW)—motors can be controlled with a simple on-off switch (starting from full rated line voltage). This is because when a motor is first started it produces no back EMF and will draw very high starting currents. If the load inertia is very high, the motor can overheat during the long acceleration time. In addition, the high starting current peaks can cause heavy dips in the line voltage. Therefore, for larger motors, these line voltage dips must be minimized by the use of starters. There are four basic ways to start ac induction motors:

1. Direct online starting

2. Low voltage starting

3. Rotor resistance starting

4. Low frequency starting

Other devices used to control motors such as soft starters, adjustable frequency controls, and vector drives are discussed along with the inverters and controls for other motor types such as dc motors, SR and PM brushless in Subsection 6.2.2.

Alternating-Current Motor Starters manual starters Manual motor starters are designed to provide positive overload protection and start and stop control of single-phase and polyphase motors up to about 3 HP (2.5 kW). A single manual toggle switch or a push button switch is used for "on," "off," and "tripped" states. These types of starters use full line voltage applied to the motor.

magnetic starters Magnetic motor starters cost more and are more reliable than manual starters. They are designed to control a motor by incorporating a magnetically operated contactor to apply power to the motor terminals rather than allowing full current to pass through the contacts.

An overload relay is incorporated to protect the motor from overloading. Magnetic starters are available for reversing and nonreversing service and are also made as non-combination or combination types. The non-combination starter combines only the motor contactor and overload relay. The combination starter combines these parts along with either a circuit breaker or a fused switch to provide shortcircuit protection.

reduced-voltage starters Reduced-voltage starters are available in several types and are basically magnetic starters with additional features to provide reduced voltage, which in turn provides for reduced motor-starting current or torque. These starters include the types discussed in the following paragraphs.

Primary-Resistor Starters Primary-resistor starters, sometimes known as cushion-type starters, will reduce the motor torque and starting inrush current to produce a smooth, cushioned acceleration with closed transition. Although not as efficient as other methods of reduced-voltage starting, primary-resistor starters are ideally suited to applications where reduction of starting torque is of prime consideration. A typical diagram for this type of starter is shown in Figure 20.

Autotransformer Starters Autotransformer starters are widely used reduced-voltage starters because of their efficiency and flexibility. All power taken from the line, except

FIGURE 20 Typical primary-resistor, reduced-voltage starter wiring diagram (Westinghouse Electric)

transformer losses, is transmitted to the motor to accelerate the load. Taps on the transformer allow adjustment of the starting torque and inrush to meet the requirements of most applications. The following characteristics are produced by the three voltage taps:

Tap, % Starting torque, % locked torque Line inrush, % locked current

50 25 28

65 42 45

80 64 67

A typical diagram for this type of starter is shown in Figure 21.

Part-winding starting provides convenient, economical, one-step acceleration at reduced current where the power company specifies a maximum or limits the increments of current drawn from the line. These starters can be used with standard dual-voltage motors on the lower voltage and with special part-winding motors designed for any voltage. When used with standard dual-voltage motors, it should be established that the torque produced by the first half-winding will accelerate the load sufficiently so as not to produce a second undesirable inrush when the second half-winding is connected to the line. Most motors will produce a starting torque equal to between one-half and two-thirds of NEMA standard values with half the winding energized and draw about two-thirds of normal line-current inrush. A typical diagram is shown in Figure 22.

Star-delta starters have been applied extensively to starting motors driving high-inertia loads with resulting long acceleration times. They are not, however, limited to this application. When 6 or 12 lead delta-connected motors are started star-connected, approximately 58% of full-line voltage is applied to each winding and the motor develops 33% of full-voltage starting torque and draws 33% of normal locked rotor current from the line.

FIGURE 21 Typical autotransformer, reduced voltage starter wiring diagram (Westinghouse Electric)
FIGURE 22 Typical part-winding starter wiring diagram (Westinghouse Electric)
Reduced Voltage Starter Wiring
FIGURE 23 Typical star-delta, reduced-voltage starter wiring diagram (Westinghouse Electric)

When the motor has accelerated, it is reconnected for normal delta operation. A typical diagram is shown in Figure 23.

Wound Rotor Motor Starters These magnetic motor starters are used for starting, accelerating, and controlling the speed of wound-rotor motors. The primary control includes overload protection and low-voltage protection or low-voltage release, depending on the type of pilot device. Disconnect switches, circuit breakers, and reversing can be added to the primary circuit when required. Reversing starters are not designed for plugging.

The secondary circuit contains the NEMA recommended number of accelerating or running contactors and resistors to allow approximately 150% of motor full-load torque on first point of acceleration. Additional accelerating points can be added for high-inertia loads or exceptionally smooth starts. Adjustable timing relays permit field adjustment. Standard starting duty NEMA 135 resistors allow 10 s starting out of every 80 s. A typical diagram is shown in Figure 24.

Synchronous-Motor Starter Synchronous, magnetic, full-voltage starters provide reliable automatic starting of synchronous motors. They can be used whenever full-voltage starting is permissible. Automatic synchronization is provided by field relay, which assures application of the field at the proper motor speed and at a favorable angular position of stator and rotor poles. As a result, line disturbance resulting from synchronization is reduced and effective motor pull-in torque is increased. A typical diagram is shown in Figure 25.

Brushless synchronous-motor starters require special consideration inasmuch as all brushless synchronous motors are not constructed in the same way. The usual starter incorporates a low-power adjustable dc excitation source to energize and control the output of an integral shaft-connected exciter.

In addition, a pull-out relay and a timing relay are incorporated to initiate synchronization and stop the motor in event of pull-out.

Direct-Current Motor Starters Direct-current motor starters are designed to apply normal voltage to the motor field and, by means of a resistor, reduce voltage to the armature.

Typical Helicopter Blades
FIGURE 24 Typical wound-rotor motor starter wiring diagram (Westinghouse Electric)

FIGURE 25 Typical synchronous-motor starter wiring diagram (Westinghouse Electric)

© field rheostat or permanent resistor when used

FIGURE 25 Typical synchronous-motor starter wiring diagram (Westinghouse Electric)

Timing relays and contractors progressively short out the resistor until full voltage is on the armature.

If the motor is equipped with a field rheostat for field-range speed adjustment, the starter will apply the preset field voltage as adjusted by the field rheostat after full voltage is applied to the armature.

These starters incorporate a field failure relay to deenergize in the event of field failure and overload relays to protect the motor against overspeed.

The use of dc drives or SCR voltage controllers includes the facilities for all required types of starting conditions for using dc motors for pump applications. Some of these issues are covered in Subsection 6.2.2.

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Survival Treasure

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