Week-7 Challenge: DC Motor Control

#solution 1

Speed control of a DC motor using BJT H-Bridge –

The Bipolar Junction Transistor (BJT) when used for power switching applications and operates as an IGBT. When it is conducting (BJT operating in the saturated region), a forward voltage V_f 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 Pulse Generator is used to generate pulses which are fed to the four IGBTs to create Pulse Width Modulation. Here in this model, the simulation is such that the motor starts in the positive direction with a duty cycle of 75% (mean DC voltage of 180V). 'Scope' shows motor speed, armature current and load torque and 'Currents' shows currents flowing in BJT Q3 and diode D3.

Initial the Block Parameters for Pulse Generator are as follows;

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At t = 0.5 sec., the armature voltage is suddenly reversed and the motor runs in the negative direction. So in the first half cycle, we can see the motor attains a speed of 1000 rpm a load torque of 6 N.m whereas the second half cycle the same values in the negative direction. Here we can see that the armature current of DC motor does get shoot up. To avoid this, we will have to reduce the pulse width from the pulse generator.

changes in pulse genrator

Here we can observe that the Armature current is almost 0 A, that means the armature current doesn’t shoot up at 40% pulse width. Also the motor speed and torque is reduced to 300 rpm and 0.6 N.m respectively due to the reduction in Pulse width. This indicates that, as we reduce the pulse width, the motor speed and torque reduces and can attain zero at a certain value.

Comparing the Four-Quadrant Chopper DC Drive (DC7) block with BJT H-bridge –

• In Four Quadrant Chopper DC Drive Block, the speed reference input is used to change the direction of the motor rotation whereas in BJT H-bridge model, the polarity is reversed to change the direction of motor rotation.
• In BJT H-bridge model, the motor can be controlled by changing the pulse width of the pulse generator whereas in Four Quadrant Chopper DC Drive Block, the speed reference values can be changed to control the motor.
• The Four Quadrant Chopper DC Drive operates in four modes wiz. forward motoring, forward regenerative braking, reversing motoring and reverse regenerative braking mode whereas BJT H-Bridge model operates only on two modes wiz. forward and reverse.
• PI (Proportional Integral) Controllers are used for speed and current control in DC7 block and it is absent in BJT H-bridge model.
• The Four Quadrant Chopper DC Drive Block uses ramps to avoid current shoot up and smooth working stability.
• The disturbances and current shoot up is high in H-Bridge model because of switching losses.
• This is why the overall performance and stability is good in Four Quadrant Chopper DC Drive comparing BJT H-Bridge model.

#solution 2

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.

Unlike conventional brushed type DC motor, wherein the brushes make the mechanical contact with commutator on the rotor so as to form an electric path between a DC electric source and rotor armature windings, BLDC motor employs electrical commutation with permanent magnet rotor and a stator with a sequence of coils. In this motor, permanent magnet (or field poles) rotates and current carrying conductors are fixed.

Working Principle and Operation of BLDC Motor

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

Advantages of BLDC Motor

• It has no mechanical commutator and associated problems
• High efficiency due to the use of permanent magnet rotor
• High speed of operation even in loaded and unloaded conditions due to the absence of brushes that limits the speed
• Smaller motor geometry and lighter in weight than both brushed type DC and induction AC motors
• Long life as no inspection and maintenance is required for commutator system
• Higher dynamic response due to low inertia and carrying windings in the stator
• Less electromagnetic interference
• Quite operation (or low noise) due to absence of brushes

Disadvantages of Brushless Motor

• These motors are costly
• Electronic controller required control this motor is expensive
• Not much availability of many integrated electronic control solutions, especially for tiny BLDC motors
• Requires complex drive circuitry
• Need of additional sensors

# solution 2

n this type of Chopper, the average voltage will be always positive but the average load current might be positive or negative. The two Switches CH1 and CH2 should not be turned on simultaneously as the combined action may cause a short circuit in supply lines. For regenerative braking and motoring these type of chopper configuration is used.

We will always get a positive output voltage V₀  as the freewheeling diode FD is present across the load. When the chopper CH1  is on the current flows from source to load. The inductor stores energy. When Ch1 turn OFF  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. The load current will be negative when the CH2 is on. The current completes its path through diode D2.

      simulink model