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Three Stage Induction Motors Executive Essay

Three stage induction motors are one of the most widely used commercial machines. That is mainly due to their convenience, high stability and robustness. Although three period induction motors have the same physical stator as a synchronous machine, they actually have different rotor constructions. Furthermore, three phase induction motors are split into two organizations: squirrel-cage and wound rotor. The magnitude of the flux in the quadrature axis in the stator diminishes, so the flux will be zero, but the speed is increased above the synchronous swiftness. The rotor current originates from rotor voltage, so the resultant backward rotating for the flux part provides the magnetomotive force (mmf) therefore the flux is positive in the quadrature axis. So, the speed of which the magnetic field rotates can be established. If the engine is connected to rated occurrence, the swiftness of the magnetic field is named the synchronous acceleration (Ns).

The following table shows the partnership between volume of poles and synchronous acceleration when the frequency equals 50HZ.

Pole pairs

1

2

3

4

6

Number of poles

2

4

6

8

12

Synchronous speed

3000

1500

1000

750

500

Table (3. 1) marriage between number of poles and synchronous

The romantic relationship between range of poles and synchronous quickness is inversely proportional.

3. 2 Equal circuit of any induction motor.

The per period equivalent circuit is vital for induction motors and may be used to provide a lot of understanding and prediction of performance of the induction motor unit in a stable state. The induction motor unit needs to provide for procedure on the induction voltage and current in the rotor circuit from the stator circuit (transformer action); this is because the voltage and current in the rotor circuit of any induction motor is basically a transformer operation. So the equal circuit of the induction electric motor is similar to the equivalent circuit of the transformer. (Within the transformer, principal like the stator in the induction motor, extra in the transformer like the rotor in the induction motor.

In an equal circuit per period of your induction motor, as shown in figure (3. 1), Rs and Rr will be the stator and rotor winding level of resistance per stage. Ls and Lr are stator and rotor leakage inductance per period. However, the purpose of test and evaluation is usually to be able to refer the rotor resistance and reactance to the stator circuit and in that way to operate a vehicle an equal circuit described the stator.

Figure (3. 1) induction motor unit per-phase similar circuits.

(3. 1)

(3. 2)

(3. 3)

(3. 4)

(3. 5)

(3. 6)

(3. 7)

(3. 8)

(3. 9)

(3. 10)

(3. 11)

(3. 12)

(3. 13)

In this circuit between a and b apply Thevenin

Figure (3. 3) Simplification of similar circuits of any induction motor

Open Circuit voltage" at ab

(3. 14)

"Brief Circuit resource"

(3. 15)

(3. 16)

(3. 17)

(3. 18)

Maximum occurs:

(3. 19)

(3. 20)

.

3. 3 Review on control of Induction motors.

Induction motors with squirrel-cage motors are used in industry because the good thing about these kinds of motors is their relatively low cost and simple building. Induction motors always just work at a nearly continuous speed. However, power electronic converts; it can work to alter the speed of the induction motor unit. The induction motor drives can be divided into groups based on their applications: (a) "Adjustable-speed drive. One important application of this drive is within process control by managing the velocity of followers, compressors, pumps". (b) "Servo drive: through advanced control, induction motors can be used as servo drives in computer peripherals, machine tools, and robotics" [reference point].

I wish to give a quick explanation of two methods that are used in the control of induction motors and I will go into greater detail about these procedures later. Vector control is a method of control of induction motors so the stator current is managed in the field rotating reference point using PWM inverter [Guide]. The rotor flux and stator flux linkages are symbolized by »ar (t) and »as(t) depends on the viewpoint of the rotor m because the mutual inductance between the stator windings and rotor windings position is connected. However, "the main reason for the q and d axis examination in machines like the induction machines is to control them properly, for example: vector control [reference point], the method of vector control of induction motor drives produces better vibrant performance than scalar control [research]. The following stop diagram shows the immediate torque control system of an induction electric motor.

Figure (3. 4) shows direct torque control of induction motors.

This alternative kind of control of an induction motor unit is simple and basic in conditions of structure. It contains a switch stand, hysteresis controllers, flux estimator and torque. It is much simpler to signify in a block diagram compared to the stop diagram representing the vector control system because of the lack of coordinate transformation between the synchronous body and stationary frame and and yes it does not desire a pulse width. Direct torque control drives are handled by the method of the close loop system without utilizing a current regulation loop and are also related to make use of of a stationary d-q reference structure as well as getting the d-axis aligned with the stator q axis. Additionally, "the flux and torque are controlled by the stator voltage space vector described in this guide frame"[reference]. Scalar control is another method of control of induction motors and is also also the first approach to control prior to the vector control method. The good thing about this method is easy control and ease of use. The motor drive is defined by three factors: (a) rate of recurrence (b) voltage (c) parameters of the electric motor and its power supply [research]. The scalar variable is firmly one symbolized by magnitude exclusively. This method uses either close loop or open loop control and any feedback loop such as that for swiftness. This use of scalar quantities provides basic characteristics of adequate steady state behavior, but poorly managed transient response.

3. 4 Description of the Induction engine drive

Since a motor drive plays a major part in the control system, it is necessary to have some background information about any of it. In an average induction electric motor drive, power gadgets are used to operate AC motors at frequencies apart from the supply one. It includes two main sections, a controller to set the operating regularity and a three-phase inverter to create the mandatory sinusoidal three-phase system from a DC bus voltage.

Therefore, an induction motor requires a variable-frequency three-phase source for adjustable speed operation by by using a ability converter system comprising a rectifier linked for an inverter through a DC link. Another body shows a block diagram of the power circuit of a typical variable-frequency induction engine drive.

Figure (3. 5) shows Variable-Frequency Induction Motor unit Drive

The rectifier turns the energy grid AC voltage into a fixed DC voltage. An LC filter to provide a soft DC voltage, which is then applied to the inverter source, filters out the harmonics.

3. 5 Induction Electric motor Control Methods

3. 5. 1 Vector control

The vector control of induction motors has been trusted for high performance drives. There were many studies developed and shown which allow an overview of vector control (research). Many proposals for the theory of electric machines discuss using space vector control to represent sinusoidal circulation in the air distance and they also discuss types of control of ac drives including induction electric motor drives, permanent-magnet ac drives and turned reluctance drives (research).

Induction motor drives have performance control as the same high performance four-quadrant DC drive. In 1960 field-oriented control (FOC) was found in the area of induction motors, however in the past decades, induction motors have been controlled by using scalar control methods like the voltage/hertz. However, this fashion can be an old way used before vector control in the area of induction machines, but it was a fairly easy method of handling an induction electric motor (Guide).

3. 5. 1. 2 Concept of vector control

The idea of vector control of AC drives is related to an area phaser which provides a means of representing three period factors in a machine, voltage, current and flux (Reference). Both flux and torque (DTC) are basically controlled by methods of closed loop; so can be methods of control of an induction motor, using finished loop without current loop, similar to the regular vector control drives (Guide). So the stator current will be utilizing transformation to the d q synchronize system and immediate axis with the rotor flux space factor; therefore, the stator d q - axis current is controlled dependently and the d q-axis for rotor flux will be zero.

(3. 21)

The relationship between the stator current, rotor flux and electromagnetic are shown by this formula:

(3. 22)

Where

Is rotor flux linkage, Rr, Lr, Lr are rotor resistance, and Lm magnetizing inductance. (Reference point).

However, the space vector in the three phase inverter will produce eight outcome areas [1 0 0] transition states, top in switch stage is symbolized by and b- is shut and c are open up. The eight space vector symbolized by V0 [ 0 0 0] and V7 [ 1 1 1 ] are null and carrying on six are of identical magnitude and arranged 600 part in space diagram as shown in body. (Reference point).

Figure (3. 6) shows witching voltage space vectors.

3. 5. 1. 3 Control Characteristics

Vector control has allowed the energetic performance of AC drives so that they better DC drives; the flux and the torque can be controlled independently by using vector control producing components of the resource current. The terminal voltage cannot be directly monitored, but can be using the dc link voltage and moving over function of the inverter offering the motor. Presently, the drive dynamic is largely used in combination with the inverter control of the stator current of the induction machines; this in turn is determined by the supply voltage and inductance of the device. The main features of the immediate torque control (DTC) are direct control of the torque and flux, and indirect main control of voltages and currents. This sort of control has lots of advantages: sinusoidal stator current and reduced torque oscillations; excellent torque dynamics; and the key advantage of (DTC) immediate torque control, lack of coordinate transformations which related to vector control implementations. However, in this type of control there are some cons: possible problems during starting and low quickness procedure and also during change in torque command line; it also requires flux and torque estimators (Reference point).

The vector control theory provides unbiased control between torque and flux; torque is manipulated by the q-axis component of current if the flux is constant and oriented over the d-axis of the referred structure. The referred structure can be rotor flux-oriented control, stator flux-oriented control or air space flux-oriented control. Thus, the phase viewpoint and the modulus of the current or current vector need to be controlled. Shape 2. 1(a), [Research], shows the rotor viewpoint ±r with respect to the stator. Because the vector control is to be integrated in the rotor flux oriented reference structure, the induction machine is set in that guide frame by rotating the variable as appropriate. Shape (3. 7) demonstrates the rotor flux reference point structure rotates at velocity (angle ±e) with regards to the stator guide and the d-q axes are set to the rotor flux space phasor. This ends in decoupling of the flux and torque which can be separately handled by stator direct-axis current ids and quadrature-axis current iqs. [Guide]

(B) Vector rotation Number (3. 7) (A) Rotor angle

3. 5. 2 Direct and Indirect Rotor Flux-Oriented Control

The vector control buttons can be split into two categories, indirect and direct, for the indirect (slip frequency manipulated) and the immediate (field focused). The characteristics of these two controllers have been regarded as the same, but there are a few differences between these procedures. However, the immediate control type is today's control theory and also offers high performance which is more well-known than indirect control (Reference point).

In this two methods above are believed of voltage and current of any stator. The first method, indirect, relates to stator current control and the next method relates to stator voltage control. In both cases the system inputs are torque and flux reference point which is required current principles for isd and isq. Field focused control to induction engine operation in a synchronously rotating d-q reference from that is aligned with one of the motor flux. So, control of the torque and flux is decoupled including the d- axis element of the stator current and rotor flux magnitude and the q-axis element control, the end result torque, where in fact the ids stator current of d-axis component and is also the rotor flux magnitude demand, so can get in equation as

(3. 23)

Where, Lm = magnetizing inductance.

For the q-axis element of the stator current iqs, the torque demand as

(T* em) so can be determined as the formula:

(3. 24)

The d-axis of the synchronously spinning reference frame to be aligned with the rotor flux, the slip relation.

(3. 25)

Direct rotor flux-oriented control (RFOC) has a control loop for flux where the measurement is conducted using flux sensing coils (or Hall-effect devices) or by the flux model. In indirect rotor flux-oriented control (RFOC), the rotor flux position is not measured but is predicted from the equivalent circuit model. One of the techniques for estimating the rotor flux viewpoint (±e) is based on the slip connection, which requires measurement of the rotor swiftness () and slide consistency (). The slide frequency will depend on rotor time continuous (r) and estimated rotor flux amplitude. This indicates that indirect methods are much easier to implement since they do not need a flux model, but are less exact. Nevertheless, if the model were perfect, the performance obtained would be indistinguishable to escort torque control. In [6], the romantic relationships between immediate and indirect approaches have been examined. It proves that they have the same control but have differences in coordinate of status variable, the rotor flux and stator currents. A fresh indirect vector control with an observer has been presented which has the same feature as the conventional direct torque control.

Figure (3. 8) immediate vector control

Figure (3. 9) indirect vector control

3. 5. 3 Scalar control method

Scalar Control manner is related to AC machines and can use voltage fed-inverters. In addition, the scalar control relates to control of the magnitude of any variable only, also using of applications for frequent voltage/hertz supply at the motor unit terminals are given constant air distance flux, so it could be that the stator voltage is fell. (Reference point) The scalar control methods are believed just for study state behaviour, but have badly managed transients. (Reference point)

This method applies to use within either close loop or available loop, and in any reviews loop such as that of speed. The scalar control method was fundamentally developed for research state procedure. However, it is a method also used in variable acceleration applications. (Reference)

In this curve, the voltage and occurrence are applied on the stator, therefore, when the resource regularity is constant, the velocity will be constant, however the torque can be changing as the square of the applied voltage. I'll give more details about scalar control in another section.

3. 5. 4 Evaluation of vector control (VC) and scalar control (SC).

Induction machines are widely used in various sectors as prime workhorses to create rotational motions and makes. However, the squirrel-cage type is a straightforward and rugged electric machine with low priced and minimal maintenance; this is reason that the squirrel-cage types are hottest in industrial electronic motors [research]. In regards to to the scalar control methods for an induction machine, only the electric motor model is considered for steady express and the scalar control methods are manipulated established of the induction electric motor, but this technique won't give good performance transients for an induction motor and it is also poor in terms of dynamics; however the vector control methods considered above are valid for transient conditions and the vector control gives a energetic performance far more advanced than that of scalar control [research]. The scalar control method to control an induction motor unit is easy to do and easy to program, but the vector control relates to the differing magnitude and stage positioning of the vector quantities of the motor. Additionally, the scalar control relates to the voltage per hearts v/f control which is usually used for low priced drives where high energetic performance is not really a key need. The applications include supporters, blowers and pumps where in fact the applied load is known. In this technique, the form is simple, the control does not require any sensors and the control algorithm can be applied in a relatively low performance microprocessor. Vector control relates to a numerical model which deals with voltage, current, flux torque and the motor parameters. We are able to control the instantaneous stator currents, control the magnitude and position is.

The pursuing diagram shows a feedback control system for calculating currents.

When the controller has an easy response then is vector can be enforced on the stator swiftly.

Advantages and drawbacks of vector control and scalar control [guide]

Method

Advantages

Disadvantages

SC

Simplest approach to obtaining variable quickness.

Low cost and easy to put into practice solution.

Is trusted.

Is much less complicated as other control methods.

Poor transient performance and poor vibrant.

It cannot control torque straight.

The transit response such as control is not fast.

VC

For those d component and q element are two decouple components can be separately controlled by transferring through distinct PI controller.

This control method has an outstanding torque and speed curve.

It has excellent dynamic performance.

Sensitive parameter variance use PI current regulators that lower transient performance.

Is more complicated than some other control method.

Cannot control torque before transformation is performed.

Table (3. 2) benefits and drawbacks of scalar control and vector control

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