Project 2 - Modeling of 3 phase Induction Motor Drive

1. Model 3 Phase Inverter using Simulink (model the parasitic as well) with a switching frequency of 10Khz using 3 phase Squirrel Cage Induction motor, using V/F method, open loop

• Design Parameters:

1. Power = 100 HP
2. DC Voltage = 800 V
3. Output Voltage = 460 VRMS – line to line
4. Initial Speed = 1500 RPM
5. Final Speed = 1000 RPM
6. Initial Load Torque = 10 Nm
7. Final Torque=20 Nm
8. Torque Step Time = 0.1 secs

Principle:
V/f control is a method to control a ratio between primary voltage (V) to be applied to the induction motor and
inverter output frequency (f) to be constant. This control enables to obtain satisfactory torque characteristics in a wide
frequency range by maintaining the magnitude of the rotating magnetic field vector independently from the inverter
output frequency when voltage drop due to a primary winding resistance is ignored.
However, as the frequency declines, the voltage drop due to the primary winding resistance becoming too large to be
ignored, and consequently, it becomes impossible to obtain sufficient torque. In such a case, applying a higher voltage
than the voltage calculated from the V/f ratio which was used for a high frequency makes it possible to secure enough
torque even at a low frequency.

V / f Control or Frequency Control:-

Whenever three phase supply is given to three phases induction motor rotating magnetic field is produced which rotates at a synchronous speed given by

Where,

• Ns is the Synchronous speed
• P is the numbers of poles
• f is the frequency

In three phase induction motor emf is induced by induction similar to that of a transformer which is given by

• Where K is the winding constant,

• T is the number of turns per phase,

• f is frequency.

• Now if we change frequency synchronous speed changes but with a decrease in frequency flux will increase and this change in the value of flux causes saturation of rotor and stator cores which will further cause an increase in the no-load current of the motor. So, it's important to maintain flux, φ constant and it is only possible if we change voltage. i.e if we decrease frequency flux increases but at the same time if we decrease voltage flux will also decrease causing no change in flux and hence it remains constant. So, here we are keeping the ratio of V/f constant. Hence its name is the V/ f method. For controlling the speed of three phase induction motor by the V/f method we have to supply variable voltage and frequency which is easily obtained by using a converter and inverter set.

As per requrired voltage is given, I have used the 800Vdc , for this circuit.

Line Current:-

Line Voltage:-

Rotor Current:-

Stator Current :-

Torque:-

Speed (RPM):-

Display showing speed in RPM

Conclusion:-

We already know that the speed of the induction motor depends on the V/f ratio. It is also possible to control the speed only by changing the frequency alone, but at this time our torque will decrrease, so air gap flux will saturate at some frequency. The saturation of air gap flux has to be avoided because it causes a lot of harmonics.

The main controlling part in the inverter section beacause the inverter controls and switching at a time both job done very well.

Using a multilevel inverter for speed control of the induction motor will be smooth, and flexible and also there will be remarkable limiting of transients when we change the V/f ratio from one value to another value. The main drawback of multilevel inverters is that they require a huge number of high-speed switching power electronic devices, flying capacitors and clamping diodes.

2. Use the 3 phase inductor motor theory to derive the transfer function of 3 phase Induction Motor.

Here is the transfer function for the 3 phase induction motor.

3. Model 3 phase induction motor with a 3 phase inverter using Direct Torque control using an Average Model, Implement both torque and flux control to achieve desired power.

Design Parameters:

1. Initial Speed = 0 RPM
2. Flux Target = 2.5 webers
3. DC Voltage = 500 V;
4. Fundamental Frequency = 60 Hz
5. Speed Step Time = 0.01 secs
6. Determine fundamental speed based upon fundamental frequency
7. Determine the switching sequence for direct torque control
8. Moment of Inertia = 0.1 Kgm^2
9. Load Torque Initial = 0 Nm
10. Final Load Torque = 50 Nm
11. Torque Step Time = 0.03 secs

Motor Parameters: (Create an Average Model)

1. Stator Resistance = 0.5 Ohms
2. Rotor Resistance = 0.2 Ohms
3. Stator Leakage Inductance = 20 mH
4. Rotor Leakage Inductance = 10 mH
5. Magnetizing Inductance = 200 mH
6. Number of Poles = 4
7. Torque to Speed

Direct Torque Control (DTC) is a vector motor control technique that implements motor speed control by directly controlling the flux and torque of the motor. Unlike field-oriented control (FOC) that controls d- and q-axis motor currents, the DTC algorithm estimates the torque and flux values from the motor position and currents. Then it uses PI controllers to control the motor torque and flux to eventually generate the optimum voltages that run the motor.

Motor Control Blockset™ uses the DTC space vector pulse-width modulation (DTC-SVPWM) variant to control a permanent magnet synchronous motor (PMSM). The technique uses space vector modulation (SVM) to produce the pulse-width modulation (PWM) duty cycles that are used by the inverter to generate the three-phase voltages that run the PMSM.

The DTC-SVPWM algorithm estimates the motor torque and flux feedback values using the current feedback (in the α-β reference frame) from the motor. The algorithm uses the motor speed feedback to compute the flux reference value. The speed PI controller (part of the outer control loop) uses the speed error input to compute the torque reference value. The flux and torque PI controllers (part of the inner control loop) use these flux and torque references and flux and torque feedback values to compute the d-axis and q-axis reference voltages. The algorithm uses the PWM Reference Generator block to generate PWM duty-cycles (using SVM) from these reference voltages.

To acquire high enactment, Direct Torque Control (DTC) is a technique used in AC drive systems. In this paper, based on the hysteresis controller, a very simple improved DTC scheme is proposed for induction motor drive. Owing to its simple assembly & effective enactment, DTC is used for AC as well as DC drive as compared to other controlling schemes. the paper approaches mitigation of the ripples in torque by varying the predictable 3-level torque-hysteresis controller used in DTC. The expansion of distinctive switching approach has been generated for chosen voltage-vector. Based on ripple content simulation results carried out in MATLAB/SIMULINK for torque-hysteresis controller to minimize the ripples.

Electrical Model

The electrical energy is supplied by a three-phase AC/DC diode rectifier connected to a 460 V, 60 Hz grid equivalent. The DC bus is connected to a three-phase, two-level converter. This converter generates the variable voltage and frequency required for variable-speed operation of the 150 HP induction motor. In addition, a braking chopper is connected to the DC bus in order to dissipate the kinetic energy of the motor during deceleration.

An inverter-fed induction motor drive can be controlled through various techniques depending on application, desired performance, and controller design complexity. Commonly used schemes are scalar control (V/Hz control or open loop flux control) or vector control (field-oriented control or direct torque control). This example uses a hysteresis-based direct torque control (DTC) technique.

Direct Torque Control Method:-

Direct torque control (DTC) is a technique that allows you to instantaneously control the motor magnetic flux and its electromagnetic torque in a decoupled way. Controlling the torque directly permits accurate static and dynamic speed regulation. The main components of the DTC subsystem are:

The figure below illustrates the strategy used to determine the best voltage vector when the flux linkage is located in sector 0.

The image shows four cases:

Simulation:-

Run the simulation and observe the waveforms on Scope2. Initially, the flux reference is set to 0.9 V.s.

At 0.1 s, the speed reference is set to 1500 RPM and the motor starts to accelerate. The motor speed precisely follows the speed reference, whose maximum rate of change is limited to 1200 rpm/s. The 1500 RPM set point is reached at 1.35 s.

At 1.5 s, a load torque of 500 N.m is applied to the motor. The DTC control operates to maintain the motor speed at 1500 RPM.

At 2 s, the load torque is reduced to 50 N.m and at 2.5 s, the speed reference is reduced to 500 RPM. Observe on Scope 1 that the braking chopper operation dissipates the kinetic energy produced by the motor in order to avoid overvoltage on the DC bus. Finally, at 3.5 s, the flux reference is increased from 0.9 to 1.0 V.s.

Input Voltage supply Parameter.

Result for the Voltage and current.

Result for speed and other parameter.

Result for the power output.

Conculsion:-

Here we have use the direct torque control method to determine the required power and control the torque. As per the above result we can see that I have achieved the desired result. IM control is simpler to implement than other vector control methods as it does not require pulse width
modulator and co-ordinate transformations. The estimation accuracy of stator flux is very much essential which mostly depends on stator resistance because an error in stator flux estimation will affect the behavior of both torque and flux control loops. The
torque and current ripple can be minimized by employing space vector modulation technique.