Week-7 Challenge: DC Motor Control

Q1.A.

Ans: -

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 present model (5 HP 24V 1750 rpm). It simulates a fan type load (where Load torque is proportional to square 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.

Duty cycle input: -

The motor start in positive direction with duty cycle of 75% means the DC voltage of 180v

OUTPUT: -

Motor start with positive direction and then after 0.5sec armature voltage is suddenly reverse and the motor run in reverse direction motor speed, because of transient operation of motor armature current and load torque in scope shows current flowing in BJT Q3 and diode D3

Q1.B.

Ans: -

The armature current shootup when motor changes direction from forward to reverse, because of transient operation of motor this has to be overcome. For this duty cycle which is 75% (Pulse Width) now is reduced and observe the changes in the results. Here the speed of the motor is reduced to 442.4rpm in forward motoring with 6.493A current.

Q1.C.

Ans: -

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.

Output: -

You can observe the motor armature voltage and current, the four IGBT pulses and the motor speed on the scope. The current and speed references are also shown. The motor is coupled to a linear load, which means that the mechanical torque of the load is proportional to the speed.

The speed reference is set at 500 rpm at t = 0 s. Observe that the motor speed follows the reference ramp accurately (+400 rpm/s) and reaches steady state around t = 1.3 s.

The armature current follows the current reference very well, with fast response time and small ripples. Notice that the current ripple frequency is 5 kHz.

At t = 2 s, speed reference drops to -1184 rpm. The current reference decreases to reduce the electromagnetic torque and causes the motor to decelerate with the help of the load torque.

At t = 2.2 s, the current reverses in order to produce a braking electromagnetic torque (dynamic braking mode). This causes the DC bus voltage to increase.

At t = 3.25 s, the motor reaches 0 rpm and the load torque reverses and becomes negative. The negative current now produces an accelerating electromagnetic torque to allow the motor to follow the negative speed ramp (-400 rpm/s). At t = 6.3 s, the speed reaches -1184 rpm and stabilizes around its reference.

Q2.

Ans: -

As we know that, a Class-A and Class-B chopper operates in the first and second chopper respectively. Therefore, if we connect both these types of choppers in parallel then it is possible to have chopper operation in first as well as second quadrant. That combination is called 2 quadrant copper.

In this copper chopper CH1, free-wheeling diode (FD) and load are forming Class-A Chopper whereas chopper CH2, D2 and Load are forming Class-B chopper. Both these choppers are connected in parallel. To obtain first quadrant operation we should switch ON chopper CH1 and to get a second quadrant operation we should switch ON chopper CH2.

Case-1: When CH1 is switched ON / OFF

When chopper CH1 is switched ON, source Vs directly gets connected to the load and hence, load voltage Vo is equal to source voltage. The direction of load current is from source to load as shown in the circuit diagram which is assumed positive.

When CH1 is switched OFF, the free-wheeling diode FD comes into the circuit as it gets forward biased and hence shorts the load. Therefore, the output voltage Vo becomes zero. However, the io continues to die down through the FD and L in the same direction as shown in circuit diagram. Thus, the average output voltage Vo and current Io are positive and hence operation of chopper is in first quadrant. In fact, this is the Class-B mode of operation.

Case-2: When CH2 is switched ON / OFF

When chopper CH2 is switched ON, load DC source E drives current through CH2 and load. The direction of this current io will be opposite to that shown in circuit diagram and hence is assumed negative. Output voltage Vo is zero during this time. When CH2 is made OFF, diode D2 gets forward biased and hence the current into the source from the load. The output voltage is Vs in this time as the load is connected to the source through D2 during OFF time of chopper CH2. Thus, the load current is always negative i.e. operation of chopper is within second quadrant. In fact, this is the Class-B mode of operation.

From the above two cases, we can conclude the following points:

• The output voltage Vo is zero when chopper CH2 is ON or free-wheeling diode FD conducts.
• The output voltage Vo is equal to source voltage Vs when chopper CH1 is ON or diode D2 conducts.
• The load current flows in the direction shown in circuit diagram i.e.io is positive when CH1 is ON or FD conducts.
• The load current flows opposite to the direction shown in circuit diagram i.e.io is negative when CH2 is ON or D2 conducts.

The average load voltage is always positive but the average load current may be positive or negative. Therefore, power flow may be from source to load (first quadrant operation) or load to source (second quadrant operation). The operating region of Class-C or Type-C chopper is shown below by hatched area.

Caution:

Choppers CH1 and CH2 should not be ON simultaneously as this would lead to direct short circuit on the supply lines.

Application:

used for motoring and regenerative breaking of DC Motors.

Simulink model: -

We created this 2-quadrant model with help of blocks like mosfet, diode, logic operator, pulse generator, dc voltage source, series RLC branch, voltage measurement, current measurement etc. as we discuss above its connected and following results are optain.

The output voltage Vo is equal to source voltage Vs when chopper CH1 is ON or diode D2 conducts.

The load current flows in the direction shown in circuit diagram i.e.io is positive when CH1 is ON or FD conducts.

Q3.

Ans: -

BLDC motors have many similarities to AC induction motors and brushed DC motors in terms of construction and working principles respectively. Like all other motors, BLDC motors also have a rotor and a stator. BLDC motors can be constructed in different physical configurations. Depending on the stator windings, these can be configured as single-phase, two-phase, or three-phase motors. However, three-phase BLDC motors with permanent magnet rotor are most commonly used.

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 (Y) or Delta (∆). The major difference between the two patterns is that the Y pattern gives high torque at low RPM and the ∆ pattern gives low torque at low RPM. This is because in the ∆ configuration, half of the voltage is applied across the winding that is not driven, thus increasing losses and, in turn, efficiency and torque. 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: -

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. 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 effect sensor: -

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 responses.

Working Principle: -

BLDC motor works on a principle similar to that of a conventional DC motor, i.e., the Lorentz force law which states that whenever a current carrying conductor is 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 the case of the 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 generates 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 aligns 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

BLDC motor has several advantages over conventional DC motors and some of these are

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

Disadvantages of Brushless Motor

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