Week-7 Challenge: DC Motor Control

Aim: A. Explain a MATLAB demo model named ‘Speed control of a DC motor using BJT H-bridge’.

B. Comment on the armature current shoot-up from the scope results.

C. Refer to the help section of ‘The Four-Quadrant Chopper DC Drive (DC7) block’. Compare it with the H-bridge model.

Model:

To open this block in simulink we need to search "Speed control of a DC motor using BJT H-bridge". Open the mathwork site and we will see this model there. Now to open this in our simulink we need to click on copy command and once it is done we will paste that in command window and press enter. Then this model will be opened in our simulink.

The Bipolar Junction Transistor (BJT) when used for power switching applications, operates as an IGBT. When it is conducting (BJT operating in the saturated region), a forward voltage Vf is developed between collector and emitter (in the range of 1 V). Therefore, the IGBT block can be used to model the BJT device.

The IGBT block does not simulate the gate current controlling the BJT or IGBT. The switch is controlled by a Simulink® signal (1/0). The DC motor uses the preset model (5 HP 24V 1750 rpm). It simulates a fan type load (where Load torque is proportional to square of speed). The armature mean voltage can be varied from 0 to 240 V when the duty cycle (specified in the Pulse Generator block) is varied from 0 to 100%.

The H-bridge consists of four BJT/Diode pairs (BJT simulated by IGBT models). Two transistors are switched simultaneously: Q1 and Q4 or Q2 and Q3. When Q1 and Q4 are fired, a positive voltage is applied to the motor and diodes D2-D3 operate as free-wheeling diodes when Q1 and Q4 are switched off. When Q2 and Q3 are fired, a negative voltage is applied to the motor and diodes D1-D4 operate as free-wheeling diodes when Q2 and Q3 are switched off.

OUTPUT FOR 75% DUTY CYCLE:

After running the model with the default values and with a duty cycle of 75%. We see that the armature current shows a high value and at mid comes steady and and time of 0.5 sec goes to reverse direction i.e we see SHOOT UP when we reverse poles.

OUTPUT FOR 50% DUTY CYCLE:

To change the value of duty cycle we need to open pulse generator and there we see pulse width which represent duty cycle. Change the value which was earlier 75 to 50. Then press on apply and then ok.

Here as we have decrease our duty cycle to 50% we can see that the SHOOT UP has decreased alot from the previous case.

OUTPUT FOR 25% DUTY CYCLE:

In all the above graph we can see that the as we decrease duty cycle value the shoot up is reducing and in this last graph where duty cycle is 25% the current graph is almost plane.

The main reason of shoot up is switching which causes heat loss in circuit. To avoid this losses we use HEAT SINK material to decipate the heat.

The Four-Quadrant Chopper DC Drive (DC7) block:

Chopper is a DC DC converter it will convert fixed DC into a variable DC.

The below shows the DC7 four-quadrant chopper DC drive during speed regulation.

Description:

The 200 HP DC motor is separately excited with a constant 150 V DC field voltage source. The armature voltage is provided by an IGBT converter controlled by two PI regulators. The converter is fed by a 515 V DC bus obtained by rectification of a 380 V AC 50 Hz voltage source. In order to limit the DC bus voltage during dynamic braking mode, a braking chopper has been added between the diode rectifier and the DC7 block.

The first regulator is a speed regulator, followed by a current regulator. The speed regulator outputs the armature current reference (in p.u.) used by the current controller in order to obtain the electromagnetic torque needed to reach the desired speed. The speed reference change rate follows acceleration and deceleration ramps in order to avoid sudden reference changes that could cause armature over-current and destabilize the system. The current regulator controls the armature current by computing the appropriate duty ratios of the 5 kHz pulses of the four IGBT devices (Pulse Width Modulation). For proper system behaviour, the instantaneous pulse values of IGBT devices 1 and 4 are opposite to those of IGBT devices 2 and 3. This generates the average armature voltage needed to obtain the desired armature current. In order to limit the amplitude of the current oscillations, a smoothing inductance is placed in series with the armature circuit.

H-Bridge:

An H-bridge is an electronic circuit that switches the polarity of a voltage applied to a load. These circuits are often used in robotics and other applications to allow DC motors to run forwards or backwards.

Here we can see that we have used four switches i.e S1,S2,S3,S4 to control the direction of motor. If switch number S1 and S4 Are closed we will see one one type of motion and if we connect S2 and S3 switch we will see opposite motion of the motor.

Circuit Diagram of H-bridge:

Comparision:

Aim: Develop a 2-quadrant chopper using simulink & explain the working of the same with the relevant results. (Refer to article - Multiquadrant operation of motor )

Answer: 2-quadrant chopper are of two types first one in which the current moves in first and fourth quadrant.

The output of this type of chopper will be in 1 and 2 quadrant as shown below.

The second type is where the output current is in first and forth quadrant.

I have moddeled the first type of 2 quadrant chopper in which current moves in first and second quadrant.

Here is the output graph of current in which see that current is moving positive and negative.

Aim: Explain in a brief about operation of BLDC motor.

Theory: A brushless DC motor (known as BLDC) is a permanent magnet synchronous electric motor which is driven by direct current (DC) electricity and it accomplishes electronically controlled commutation system (commutation is the process of producing rotational torque in the motor by changing phase currents through it at appropriate times) instead of a mechanically commutation system. BLDC motors are also referred as trapezoidal permanent magnet motors.

Working: BLDC motor works on the principle similar to that of a conventional DC motor, i.e., the Lorentz force law which states that whenever a current carrying conductor placed in a magnetic field it experiences a force. As a consequence of reaction force, the magnet will experience an equal and opposite force. In case BLDC motor, the current carrying conductor is stationary while the permanent magnet moves.

When the stator coils are electrically switched by a supply source, it becomes electromagnet and starts producing the uniform field in the air gap. Though the source of supply is DC, switching makes to generate an AC voltage waveform with trapezoidal shape. Due to the force of interaction between electromagnet stator and permanent magnet rotor, the rotor continues to rotate.

Consider the figure below in which motor stator is excited based on different switching states. With the switching of windings as High and Low signals, corresponding winding energized as North and South poles. The permanent magnet rotor with North and South poles align with stator poles causing motor to rotate.

Observe that motor produces torque because of the development of attraction forces (when North-South or South-North alignment) and repulsion forces (when North-North or South-South alignment). By this way motor moves in a clockwise direction

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Here, one might get a question that how we know which stator coil should be energized and when to do. This is because; the motor continuous rotation depends on the switching sequence around the coils. As discussed above that Hall sensors give shaft position feedback to the electronic controller unit.

Based on this signal from sensor, the controller decides particular coils to energize. Hall-effect sensors generate Low and High level signals whenever rotor poles pass near to it. These signals determine the position of the shaft.

Advantages:

1. High efficiency due to the use of permanent magnet rotor.
2. Operates at high speed due to absence of brushes that limits speed.
3. Smaller and and lighter in weight.
4. Quite operation due to absence of brushes.

Disadvantages:

1. Electronic controller required control this motor is expensive
2. Need of additional sensors