Week-7 Challenge: DC Motor Control
Question#1(a):
The below example shows the simulation of an H-bridge used to generate a chopped voltage and to control the speed of a DC motor.
Description:
- 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.
Simulation & Results:
- 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.
Question#1(b):
The armature current shoot-up in the motor is due to the suddent rise in input voltage which is driven by duty cycle from 0 to 75%, ie., 0.75*240 = 180V. The Armature current shootup is shown below:
at 50% of duty cycle:
it is abserved that the starting armature current shoot up is reduced by some extent as shown below:
at 30% of duty cycle: it is abserved that there is no starting armature current shoot up.
Conclusion:
Hence it is clearly seen that inorder to reduce the shootup armature current in the motor , the duty cycle have to be changed gradually changed from 0 to 100%, that results to gradual rise in voltage and speed of the motor.
Question# 1(c):
Four-Quadrant CHopper DC drive DC7 block model:
In this model Four-Quardrant Chopper DC Drive Block takes Armature Voltage, speed reference, motor torque as inputs and Motor,convertor,controller,motor angular speed as outputs.
Model and its operation:
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.
Comparison:
| Speed control of a DC motor using BJT H-bridge | Speed control of a DC motor using Four-Quadrant Chopper DC Drive (DC7) block |
| 4 BJT (Q1,Q2,Q3,Q4) switches and 4 freewheeling Diodes are used | 4 IGBT Switches are used |
| Pulse signal is generated using PWM generator with some constant logic input into bus so that Q1,Q2 and Q3,Q4 pairs cannot swiched on at the same moment, inorder to avoid shortcircuit. | Pulse signal are generated using the PI current controller which takes armature current and current reference as input. |
| Speed of the motor is controlled by varying the duty cycle of the armature voltage. | Speed of the motor is controlled by varying current pulses from PI current controller |
| the armature current shoot-up is produced when the sudden change in duty cycle is occured and smoothing inductance is absent | Smoothing inductance is used so that the increase or decrease in the armature current does not shootup which makes the speed control of DC motor smooth. |
| Dynamic Braking is occured due to the change in polarity of the armature voltage | Regenerative Braking is occured |
| The Operation is Continious | The operation is discrete |
Conclusion:
Hence Four-Quadrant Chopper is preferred for the speed control of EV for its advantages as stated above.
Question#2:
2-quadrant chopper using simulink
Working:
In 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. Hence, Pulses (P1,P2) for the two IGBT switches are given by using pulse generator and NOT logical operation for alternate switch ON/OFF.
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 Vs. 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.
Scope Results:
Duty cycle = 50%
Conclusion:
In first quadrant, Current flows in positive direction and torque is also positive, hence forward motoring action take place when swich IGBT 1 is on and IGBT 2 is off untill t=0.5.
In second Quardrant, Current flows in negative direction and torque is also negative, hence regenerative breaking mode take place when swich IGBT 2 is on and IGBT 1 is off from T=0.5 to 1.
This cycle repeats till the simulation time.
Question#3:
Brushless DC electric motors also known as electronically commutated motors (ECMs, EC motors).
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.
Construction of Brushless DC motor:
Stator:
Stator of a BLDC motor made up of stacked steel laminations to carry the windings. These windings are placed in slots which are axially cut along the inner periphery of the stator. These windings can be arranged in either star or delta. However, most BLDC motors have three phase star connected stator.
Each winding is constructed with numerous interconnected coils, where one or more coils are placed in each slot. In order to form an even number of poles, each of these windings is distributed over the stator periphery.
The stator must be chosen with the correct rating of the voltage depending on the power supply capability. For robotics, automotive and small actuating applications, 48 V or less voltage BLDC motors are preferred.
Rotor Construction:
BLDC motor incorporates a permanent magnet in the rotor. The number of poles in the rotor can vary from 2 to 8 pole pairs with alternate south and north poles depending on the application requirement. In order to achieve maximum torque in the motor, the flux density of the material should be high. A proper magnetic material for the rotor is needed to produce required magnetic field density.
Ferrite magnets are inexpensive, however they have a low flux density for a given volume. Rare earth alloy magnets are commonly used for new designs. Some of these alloys are Samarium Cobalt (SmCo), Neodymium (Nd), and Ferrite and Boron (NdFeB). The rotor can be constructed with different core configurations such as the circular core with permanent magnet on the periphery, circular core with rectangular magnets, etc.
Hall Sensors:
Hall sensor provides the information to synchronize stator armature excitation with rotor position. Since the commutation of BLDC motor is controlled electronically, the stator windings should be energized in sequence in order to rotate the motor. Before energizing a particular stator winding, acknowledgment of rotor position is necessary. So the Hall Effect sensor embedded in stator senses the rotor position.
Most BLDC motors incorporate three Hall sensors which are embedded into the stator. Each sensor generates Low and High signals whenever the rotor poles pass near to it. The exact commutation sequence to the stator winding can be determined based on the combination of these three sensor’s response.
Working Principle 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.
Operation of BLDC Motor or Brushless DC Motor:
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.BLDC motor operation
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 of BLDC Motor:
BLDC motor has several advantages over conventional DC motors and some of these are
- 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
Applications of Brushless DC Motors (BLDC)
Brushless DC Motors (BLDC) are used for a wide variety of application requirements such as varying loads, constant loads and positioning applications in the fields of industrial control, automotive, aviation, automation systems, health care equipments, etc. Some specific applications of BLDC motors are
- Computer hard drives and DVD/CD players
- Electric vehicles, hybrid vehicles, and electric bicycles
- Industrial robots, CNC machine tools, and simple belt driven systems
- Washing machines, compressors and dryers
- Fans, pumps and blowers