Wound-rotor motor

Control of wound-rotor motors, as discussed, can be effected by adjusting the external secondaiy (rotor) resistance either in steps or continuously by liquid rheostat (this method is seldom used). Commonly when secondaiy resistance is varied to adjust speed or torque or to control acceleration, multiple resistance steps are used. These steps may be switched manually (typically a drum switch) or electrically by contac tor. 

In addition to secondarv resistance control, other devices such as reactors and thyristors (solid-state controllable rectifiers) are used to control wound-rotor motors. Fixed secondary reactors combined with resistors can provide veiy constant accelerating torque with a minimum number of accelerating steps. The change in slip frequency with speed continually changes the effective reac tance and hence the value of resistance associated with the reactor. The secondaiy reactors, resistors, and contacts can be varied in design to provide the proper accelerating speed-torque curve for the protection of belt conveyors and similar loads. 

Saturable reactors, which are adjustable by a small dc signal, have also been used for both primary (stator) and secondary (rotor) control. In the primary they control motor voltage and therefore torque. In combination with fixed secondary resistors and feedback from a tachometer, this system can be used for precise speed and torque control of cranes, hoists, etc. Even reversing can be accomplished by using two saturable reactors in each of two (of three) phases. Other combinations of fixed or saturable reac tors in the primaiy and/or secondaiy, all combined with secondary resistors, provide a wide range of capabiUties and flexibihty for the wound-rotor motor. 

Direct-current motors are adjustable in speed over a wide range. Further, efficiency is high over the entire speed range, unlike wound-rotor motors, in which efficiency is roughly proportional to speed. This flexibility is attained at the expense of additional complexity and cost. 

Secondary Control of Wound-Rotor Motors Wound-rotor motors may be effectively reduced-voltage-started or have their speed controlled by using external secondaiy resistance. The addition of resistance into the secondaiy circuit of a wound-rotor motor reduces the starting current and affects the speed under load conditions. 

Slip recovery system (to control wound rotor motors)... 

The inverter may be a current source inverter, rather than a voltage source inverter (.Section 6.9.4) since it will be the rotor current that is required to be vtiried (equation (1.7)) to control the speed of a wound rotor motor, and this can be independently varied through the control of the rotor current. The speed and torque of the motor can be smoothly and steplessly controlled by this method, without any power loss. Figures 6.47 and 6.48 illustrate a typical slip recovery system and its control scheme, respectively. 

On a wound rotor motor, the following additional checks are also necessary after removing the slip-ring covers, prior to testing ... 

It is therefore necessary to take precautions during the test to avoid a excessive temperature rise and consequent damage to the windings. For wound rotor motors, speed-torque and speed-current tests may be taken between synchronous speed and the speed at which the maximum torque occurs. 

Today s standard motor enclosure for indoor applications is the open, drip-proof enclosure for induction and high-speed synchronous motors. For large motors, open, drip-proof construction is available up to about 20,000 hp and is used for squirrel-cage, synchronous, and wound-rotor motors. 

Single-Phase Wound-Rotor Motors. Single wound-rotor motors are defined and classified as follows ... 

Secondary Voltage of Wound-Rotor Motors. The secondary voltage of wound-rotor motors is the open-circuit voltage at standstill, measured across the slip rings, with rated voltage applied on the primary winding. 

In general, the typical electric motor applications in the oil and gas industry arc polyphase motors (either squirrel-cage or wound-rotor motors). 

Wound Rotor. Characteristics of wound-rotor motors are such that the slip depends almost entirely upon the load on the motor. The speed returns practically to maximum vs hen the load is removed. This characteristic limits the use of these motors on applications where reduced speeds at light loads are described. 

Wound-rotor motors are suitable for constant-speed applications that require frequent starting or reversing under load or where starting duty is severe and exceptionally high starting torque is required. 

Typical torque-speed characteristics of wound-rotor motors are shown in the curves in Figure 3-11 for full voltage and for reduced voltage that are obtained with different values of secondary resistance. 

Wound-Rotor Motor Drives. The wound-rotor induction motor, using adjustable rotor-circuit resistance, is rarely used. However, two versions that use solid-state auxiliary equipment are finding limited application. 

Wound Rotor Motors - A type of motor that has a rotor with electrical windings connected through slip rings to the external power circuit. An external resistance controller in the rotor circuit allows the performance of the motor to be tailored to the needs of the system and to be changed with relative ease to accommodate system changes or to vary the speed of the motor. 

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