Week-7 Challenge: DC Motor Control

A.

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.

• The basic operating mode of an H-bridge is fairly simple: if Q1 and Q4 are turned on, the left lead of the motor will be connected to the power supply, while the right lead is connected to ground. Current starts flowing through the motor which energizes the motor in (let’s say) the forward direction and the motor shaft starts spinning.

• If Q2 and Q3 are turned on, the reverse will happen, the motor gets energized in the reverse direction, and the shaft will start spinning backwards.

• In a bridge, you should never ever close both Q1 and Q2 (or Q3 and Q4) at the same time. If you did that, you just have created a really low-resistance path between power and GND, effectively short-circuiting your power supply. This condition is called ‘shoot-through’ and is an almost guaranteed way to quickly destroy your bridge, or something else in your circuit.

MATLAB demo model named ‘Speed control of a DC motor using BJT H-bridge is shown below

MODEL DISCRIPTION:

•  dc motor used is model is 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.

B. Simulation results

The graph shows IGBT current and Diode current

From above graph we can see that, for 0 to 0.5 sec , the current flows through diode D3 and from 0.5 sec to 1 sec , the current flows through  Q3.

The motor starts in the positive direction with a duty cycle of 75% (mean DC voltage of 180V). At t= 0.5 sec., the armature voltage is suddenly reversed and the motor runs in the negative direction.

'Scope' shows motor speed, armature current and load torque and 'Currents' shows currents flowing in BJT Q3 and diode D3.

The waveform for speed, armature current and load torque is shown in above graphs.

from armature current graph , we can see that ,at starting there is sudden spike increase in current at 0 sec also and at 0.5 sec also.

We can see that , as the voltage gets changed in the circuit , the speed of motor also changes.

In this transition of positive to negative direction of rotation we observe a high surge in current, this is due to the transient switching occurring in the circuit.

As there is already current flowing in the circuit & on changing of switches after 0.5 sec, the current in the circuit adds to the changed direction of current and we observe a surge.

RPM, armature current and load torque is given in the next graph

Result:-

At starting there is sudden spike increase in current at 0 sec also and at 0.5 sec also means at transition from positive to negative direction of rotation we observe a high surge in current, this is due to the transient switching occurring in the circuit.

As there is already current flowing in the circuit & on changing of switches after 0.5 sec, the current in the circuit adds to the changed direction of current and we observe a surge in current.

the surge in the current can be reduced by reducing the duty cycle.

in above results , the duty cycle selected as 75%.

by changing the duty cycle from 75% to 50% , the current surge can be reduced tremendously.

C.

• The Four-Quadrant Chopper DC Drive (DC7) block represents a four-quadrant, DC-supplied, chopper (or DC-DC PWM converter) drive for DC motors.
• This drive features closed-loop speed control with four-quadrant operation. The speed control loop outputs the reference armature current of the machine.
• Using a PI current controller, the chopper duty cycle corresponding to the commanded armature current is derived.
• This duty cycle is then compared with a sawtooth carrier signal to obtain the required PWM signals for the chopper.
• The main advantage of this drive, compared with other DC drives, is that it can operate in all four quadrants (forward motoring, reverse regeneration, reverse motoring, and forward regeneration).
• In addition, due to the use of high switching frequency DC-DC converters, a lower armature current ripple (compared with thyristor-based DC drives) is obtained. However, four switching devices are required, which increases the complexity of the drive system.
• The machine is separately excited with a constant DC field voltage source. There is thus no field voltage control. By default, the field current is set to its steady-state value when a simulation is started.
• The armature voltage is provided by an IGBT converter controlled by two PI regulators. The converter is fed by a constant DC voltage source. Armature current oscillations are reduced by a smoothing inductance connected in series with the armature circuit.

COMPARISON WITH H BRIDGE

	H BRIDGE	4 QUADRANT CHOPPER
1	Works in Open Loop system(Non-feedback mechanism).	Works in closed-loop system(feedback mechanism). The armature voltage is controlled by PI regulators
2	The motor is not separately excited	The motor is separately excited with a constant 150 V DC field voltage source
3	It cannot work in regenerative mode.	It can work on all 4 quadrants.
4	We need to change the polarity in order to make the motor move forward or backward	We need to interrupt the signals.
5	Simulation is continuous	Simulation is discrete
6	The simulation takes more time and is less efficient	The simulation takes less time and its more efficient

2.

The above model is a 2nd -quadrant chopper modeled using Simulink software. The main components are :

• A pulse generator
• Current measurement
• Voltage measurement
• Diode
• MOSFET
• Vcope
• NOT gate
• Battery 

The working of the same is given below:

• Type C chopper is obtained by connecting type –A  and type –B choppers in parallel.
• We will always get a positive output voltage V₀  as the freewheeling diode FD is present across the load.  
• When the chopper is on the freewheeling diode starts conducting and the output voltage v₀ will be equal to V_s.
• The direction of the load current i₀ will be reversed. The current i₀  will be flowing towards the source and it will be positive regardless of the chopper is on or the FD conducts. 
• The load current will be negative if the chopper is or the diode D2 conducts. 
• We can say the chopper and FD  operate together as a type-A chopper in the first quadrant.
• In the second quadrant, the chopper and D2 will operate together as type –B chopper.

Simulation model:-

Output:-

Results:-

• Voltage measured is Positive value it only touches minimum value of 0.
• Current is in the pulsuating.

3.

• Brushless DC motors are common in industrial applications across the world.
• At the most basic level, there are brushed and brushless motors and there are DC and AC motors.
• Brushless DC motors,do not contain brushes and use a DC current.
• These types of motors are highly efficient in producing a large amount of torque over a vast speed range. In brushless motors, permanent magnets rotate around a fixed armature and overcome the problem of connecting current to the armature. Commutation with electronics has a large scope of capabilities and flexibility. They are known for smooth operation and holding torque when stationary.
• A Brushless DC motor has only two basic parts: the rotor and the stator. The rotor is the rotating part and has rotor magnets whereas the stator is the stationary part and contains stator windings.
• In BLDC permanent magnets are attached to the rotor and move the electromagnets to the stator. The high-power transistors are used to activate electromagnets for the shaft turns. The controller performs power distribution by using a solid-state circuit.

Working Principle and Operation of BLDC Motor

1. BLDC motor works on a principle similar to that of a conventional DC motor, i.e., the Faraday's law which states that whenever a current-carrying conductor placed in a magnetic field it experiences a force.
2. As a consequence of reaction force, the magnet will experience an equal and opposite force. In the case BLDC motor, the current-carrying conductor is stationary while the permanent magnet moves.
3. When the stator coils are electrically switched by a supply source, it becomes an 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 a trapezoidal shape.
4. Due to the force of interaction between the electromagnet stator and permanent magnet rotor, the rotor continues to rotate.
5. Consider the figure below in which the 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.
6. The permanent magnet rotor with North and South poles align with stator poles causing the motor to rotate.
7. Observe that the 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). In this way motor moves in a clockwise direction.

Advantages of Brushless DC Motor

The advantages of a BLDC motor are:

1. Brushless motors are more efficient as their velocity is determined by the frequency at which current is supplied, not the voltage.
2. As brushes are absent, the mechanical energy loss due to friction is less which enhanced efficiency.
3. BLDC motor can operate at high speed under any condition.
4. There is no sparking and much less noise during operation.
5. More electromagnets could be used on the stator for more precise control.
6. BLDC motors accelerate and decelerate easily as they are having low rotor inertia.
7. It is a high-performance motor that provides large torque per cubic inch over a vast sped range.
8. BLDC motors do not have brushes which make them more reliable, high life expectancies, and maintenance-free operation.
9. There are no ionizing sparks from the commutator, and electromagnetic interference is also get reduced.
10. Such motors cooled by conduction and no airflow are required for inside cooling.

Disadvantages of Brushless DC Motors

1. BLDC motor cost more than a brushed DC motor.
2. The limited high power could be supplied to the BLDC motor, otherwise, too much heat weakens the magnets and the insulation of winding may get damaged.