Challenge

Project 2 - Modeling of 3 phase Induction Motor Drive

NNamrata Rao15 Feb 2023

OBJECTIVE OF THE PROJECT: 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.

Motor Parameters:

Stator Resistance = 0.5 Ohms
Rotor Resistance = 0.2 Ohms
Stator Leakage Inductance = 20 mH
Rotor Leakage Inductance = 10 mH
Magnetizing Inductance = 200 mH
Number of Poles = 4

Design Parameters:

Initial Speed = 0 RPM
Flux Target = 2.5 webers
DC Voltage = 500 V;
Fundamental Frequency = 60 Hz
Speed Step Time = 0.01 secs
Determine fundamental speed based on fundamental frequency
Determine the switching sequence for direct torque control
Moment of Inertia = 0.1 Kgm^2
Load Torque Initial = 0 Nm
Final Load Torque = 50 Nm
Torque Step Time = 0.03 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

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, and 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.

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, if this is done 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.

Although most of the control and design part is at the inverter section.

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.

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

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:

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

Motor Parameters: (Create an Average Model)

Stator Resistance = 0.5 Ohms
Rotor Resistance = 0.2 Ohms
Stator Leakage Inductance = 20 mH
Rotor Leakage Inductance = 10 mH
Magnetizing Inductance = 200 mH
Number of Poles = 4
Torque to Speed

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 Controller

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:

Flux and Torque Calculation — The stator flux linkage is estimated by integrating the stator voltages, and torque is calculated based on the estimated flux and the motor currents.
Speed Regulator — The regulator compares the actual motor speed with the speed reference and generates the torque reference.
Hysteresis Control — The calculated flux magnitude and torque are compared with the reference values. When the resulting flux or torque error crosses either the positive or negative hysteresis band value, a control signal is activated in order to correct the error.
Optimal Switching — Pulses to the motor inverter are produced based on the control signals generated by the hysteresis control and the stator flux linkage position.

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:

V3 is selected when the electromagnetic torque should be increased and the flux should stay unchanged. Selecting the V3 voltage vector speeds up the flux and thus applies an acceleration torque to the rotor while slightly decreasing the flux magnitude.
V2 is selected when the electromagnetic torque should be increased and the flux should be increased. Selecting the V2 voltage vector slightly speeds up the flux and increases its magnitude.
V6 is selected when the electromagnetic torque should be reduced and the flux should be increased. Selecting the V6 voltage vector slows down the flux and thus applies a deceleration torque to the rotor while increasing the flux magnitude.
V5 is selected when the electromagnetic torque should be reduced and the flux should stay unchanged. Selecting V5 the voltage vector applies a deceleration torque to the rotor and slightly decreases the flux magnitude. Note that voltage vectors V1 and V4 are not selected in sector 0. Using these two vectors would have too much negative impact on the desired flux value. Finally, to keep the torque and the flux unchanged, the null voltage vectors V0 or V7 are selected.

When the flux linkage vector moves to sector 1, the selected voltage vectors become V4 for case 1, V3 for case 2, V1 for case 3, and V6 for case 4, and vectors V2 and V5 are not used. This 60 degree shift in the voltage vectors happens each time the flux linkage vector enters a new sector.

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.

CONCLUSION:

For any IM drives, direct torque control is one of the best controllers proposed so far. It allows decoupled
control of motor stator flux and electromagnetic torque. From the analysis it is proved that, this strategy of
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. A new strategy
is introduced further to get quick response by introducing one “modified flux error block” and proved by
MATLAB/SIMULINK model.