Week-7 Challenge: DC Motor Control

Aim:-

To study DC Motor Control.

Objective:-

1] 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.

2] Develop a 2-quadrant chopper using simulink & explain the working of the same with the relevant results.

3] Explain in a brief about operation of BLDC motor.

Answer [1]:-

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

Simulink Model:-

Fig.1:- Simulink model for Speed control of a DC motor using BJT H-bridge.

Model Description:-

• The Bipolar Junction Transistor (BJT) when usedfor power switching applications; operates as an IGBT. When it is conducting, 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
• The DCmotor uses the preset model of 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 is varied from 0 to 100%.
• The H-bridgeconsists 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 motorand diodes D2-D3 operate as free-wheeling diodes when Q1 and Q4 are switched off.
• When Q2 and Q3 are fired, anegative voltage is applied to the motor and diodes D1-D4 operate as free-wheeling diodes when Q2 and Q3 are switched off.
• We have used ‘Goto’ block. Input from pulse generator is split in 4 parts for gate of 4 IGBT’s. The Goto block is used to send a signal to ‘From’ block that have specified name. This block is used to give input to the gate of all IGBT’s. ‘From’ block receives signal from ‘Goto’ block.
• There are two scope attached to view the results.

Working of H-bridge:-

• An H-bridgeis 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.
• Most DC-to-AC converters (power inverters), most AC/AC converters, the DC-to-DC push–pull converter, isolated DC-to-DC converter most motor controllers and many other kinds of power electronics use H bridges.
• H-bridge circuit consists of 4 semiconductor device (IGBT/MOSFET), 4 freewheeling diodes, motor as load and positive & negative terminal potential.
• Here we cannot trigger the two switches on same leg together. Hence we need to make ON/OFF one switch from one leg (A side) and other from other leg (B side) at a same time in order to work smoothly.
• There are two modes of operation of motor i.e. in forward & reverse direction.

Fig.2:- H-bridge circuit.

Case I] Q1 & Q4 are fired:-

Fig.3:- Forward direction mode.

• When Q1 & Q4 switches are ON, the flow of current will be:- (+)Vbat – Q1 – Motor – Q4 – (-)Vbat.
• As current is flowing from source to load the motor gets energized & direction of rotation of motor is in forward direction as shown in fig.3.
• When switch Q1 & Q4 are turned OFF, D2 & D3 operates as freewheeling. We ensure that current is still flowing in circuit & motor is still rotating so that we get continuous output at motor side. So now we can switch ON Q2 & Q3.

Case II] Q2 & Q3 are fired:-

Fig.4:- Reverse direction mode.

• When Q2 & Q3 switches are ON, the flow of current will be:- (+)Vbat – Q3 – Motor – Q2 – (-)Vbat.
• Here direction of current when compared to first case is reversed. Hence rotation of motor is in reverse direction as shown in fig.4.
• When switch Q2 & Q3 are turned OFF, D1 & D4 operates as freewheeling.

Operation of switches of H-bridge:-

The H-bridge arrangement is generally used to reverse the polarity/direction of the motor, but can also be used to 'brake' the motor, where the motor comes to a sudden stop, as the motor's terminals are shorted, or to let the motor 'free run' to a stop, as the motor is effectively disconnected from the circuit. The following table summarizes operation:-

Q1	Q2	Q3	Q4	Result
1	0	0	1	Motor moves right
0	1	1	0	Motor moves left
0	0	0	0	Motor coasts
1	0	0	0
0	1	0	0
0	0	1	0
0	0	0	1
0	1	0	1	Motor brakes
1	0	1	0
x	x	1	1	Short circuit
1	1	x	x

Simulation Result:-

[I] Pulse generator input:-

Here we have given amplitude=1, total period=0.002sec, pulse width = 75% & delay=0.

[II] Scope result:-

Fig.5:- Plot for IGBT & diode current.

Here we can see that when diode is ON, switch (IGBT) is OFF and vice versa. Max current achieved is between 60-70A.

Fig.6:- Plot for speed, load torque & armature current.

The motor starts in the positive direction with a duty cycle of 75%. At t= 0.5 sec., the armature voltage is suddenly reversed and the motor runs in the negative direction. At starting, as speed increases, load torque increases & after 0.5sec speed decreases with decrease in load torque. Speed & torque is negative as direction of motor is reversed. Armature current is high at starting.

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

The armature current shoot-up is controlled by controlling input parameters. Since the model uses pulse generator as a input, it is possible to vary the armature current by varying the duty cycle i.e. by varying pulse width. For 100% duty cycle, armature current spontaneously vary at high value. As we lower the duty cycle, armature current also becomes stable.

[I] For pulse width=75%

Scope Result for armature current at PW=75%:-

Fig.7:- Plot for speed, load torque & armature current at 75% Pulse width.

[II] For pulse width=40%

Scope Result for armature current at PW=40%:-

Fig.8:- Plot for speed, load torque & armature current at 40% Pulse width.

• Speed and armature current is obtained by DC motor & load torque is calculated by load torque proportional to square of speed. At starting point there is sudden shoot-up in both fig 7& 8.
• Fig 7 shows 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.
• Similarly for fig 8, the motor starts in the positive direction with a duty cycle of 40%. At t= 0.5 sec., the armature voltage is suddenly reversed and the motor runs in the negative direction.
• Scope results shows that speed is increased in certain period of time and at that time armature current is positive whereas load torque also moves forward.
• But after 0.5sec, due to sudden shoot-up, speed gets decreased and at that time armature current is negative whereas torque also decreases.
• By varying pulse width from 75% to 40% of pulse width from pulse generator block, the armature current is reduced. This is because as PW decreases, voltage applied across dc motor decreases and hence armature current also decreases.
• Armature current is limited due to which it does not overcomes the threshold current which is required to develop sufficient torque inside motor.

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

Model of The Four-Quadrant Chopper DC Drive:-

Description:-

• The 200HP 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 DCbus 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.
• 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 behavior, 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 between Four-Quadrant Chopper DC Drive (DC7) with the H-bridge model:

Sr. Nos.	H-Bridge	Four-Quadrant Chopper DC drive DC7
1	Operates only in two modes (Forward & reverse)	Operates in all four modes (Forward & reverse motoring and forward & reverse regenerative braking)
2	Does not contain any PI regulators for speed control and current control.	Composed of two PI regulators for speed control and current control.
3	Less efficient.	More efficient.
4	Simple to use.	Increases complexity of drive system.
5	Not suitable for regenerative braking	Suitable for regenerative braking
6	Polarity of voltage applied to the load is interchanged during switching operation.	It’s switching operation interrupts one signal under the control of other
7	High armature current ripple is obtained.	Lower armature current ripple is obtained.
8	Low switching frequency.	High switching frequency.
9	Model uses continuous variable step solver.	Model uses discrete step-time solver.
10	Used in robotics & other application.	Mainly used in power control & signal applications.

Answer [2]:-

To develop a 2-quadrant chopper using simulink & explain the working of the same with the relevant results.

Working of Two quadrant chopper:-

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 & CH2 should not be turned ON simultaneously as combined action may cause a short circuit in supply lines. For regenerative braking and motoring these type of chopper configuration is used.

Due to freewheeling diode (FD) across load, we will get positive output voltage (Vo) always. When CH1 is ON, current flows from source to load. Hence inductor gets energized and stores energy. Here current and voltage are positive. Hence this mode operates in I quadrant. When CH1 is OFF, the FD starts conducting and output voltage (Vo) will be equal to Vs. Here direction of load current is reversed. The current (io) will be flowing towards the source. Hence load current will be negative when CH2 is ON but voltage remains positive. Hence this mode operates in II quadrant. Current completes its path through diode D2.

Simulink Model :-

Fig.9:- Simulink model for 2-quadrant chopper

• Working of two quadrant chopper is already explained above. Now build a simulink model for it in Matlab. For this take two ‘MOSFET’ block representing switches, two ‘diode’ block , ‘Series RLC branch’ block for inductor, two ‘DC voltage source’ block, “NOT’ logic operator block, ‘current measurement’ & ‘voltage measurement’ blocks. Also connect scope and powergui block.

• Pulse Generator:-

Here we amplitude=5, period= 0.02sec, PW=50%.

• MOSFET:-

We have used two MOSFETs here, one operates in forward motoring and one in reverse motoring. All default values are kept as it is.

• Diode:-

Here also all default values are kept as it is. We use two diodes here, out of which one acts as freewheeling diode. Diode 1 is connected parallel to MOSFET 1 & Diode 2 (FD) is connected parallel to MOSFEET 1.

• Series RLC Branch:-

Thus is used as a load in the model. Branch type is choose as inductor “L”. This load is connected with 24V DC voltage source.

• Voltage source:-

We have connected one voltage source at input side as “48V” & another voltage source of “24V” at load side.

• Not operator is used so that two MOSFETs wont get closed at same time. This avoids short circuit between them.

Scope Results:-

Fig.10:- Pulse generator input waveform.

This shows amplitude of input signal is '5' and period is '0.02'. This signal is given as input to the gate terminal of two MOSFET's.

Fig.11:- Load voltage output waveform.

Voltage increases or decreases depending on the Switch that is ON. But due to presence of freewheeling diode (FD) across load, we will get positive output voltage (Vo) always. We have given input voltage as 48V, max load voltage attend is 48V.

Fig.12:- Load current output waveform.

Load current will be positive during forward motoring mode and negative during regenerative braking mode. Maximum current achieved is 150A. From waveform we can say that current increases or decreases linearly.

Answer [3]:-

Operation of BLDC motor:-

• 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 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.
• As there is no commutator, the current direction of conductor on the stator is controlled electronically. Rotor consists PM where as stator consists of nos. of windings. Current through these windings produces magnetic field and force.
• 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.
• 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.

• 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.

• 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.

• BLDC motors are most commonly employed for actuating drives, machine tools, electric propulsion, robotics, computer peripherals and also for electrical power generation.

Conclusion:-

• We learnt Speed control of a DC motor using BJT H-bridge and how the armature current shoot-up from the scope results.
• Also compared The Four-Quadrant Chopper DC Drive (DC7) block model with the H-bridge model.
• Developed a 2-quadrant chopper using simulink.
• Understood the operation of BLDC motor.