MULTIQUADRANT OPERATION OF MOTOR

 

AIM : To study about the multi-quadrant operation of motor.

 

OBJECTIVES : a) To explain the simulation of speed control of a DC motor using BJT H-Bridge.

                       b) To compare the full bridge model with four quadrant chopper of a DC7 drive.

                       c) To develop of a Two quadrant chopper using Simulink.

                       d) To explain the operation of BLDC motor.

 

THEORY :

The progress in power electronics has been primarily due to the advances in power semiconductor devices. The inventions in converter topologies, pulse width modulation (PWM) techniques, control and estimation techniques, digital signal processors, application specific integrated circuits (ASICs), control hardware and software, etc also have contributed to this advancement.

Power semiconductor devices can be classified into three categories according to their degree of controllability. The categories are :

• Uncontrolled turn-on and off devices (e.g. diode).
• Controlled turn –on and uncontrolled turn-off (e.g. SCR).
• Controlled turn-on and off characteristics (e.g. BJT, MOSFET, GTO, IGBT).

The on and off states of diodes are controlled by power circuit. Thyristors are turned-on by a control signal and are turned-off by the power circuit whereas the controllable switches are turned-on and off by controlled signals. The power MOSFET is used primarily due to its ability to high-frequency switching mode power supply. The BJT appeared and then fell into obsolescence due to the advent of IGBT at the higher end and the power MOSFET at the lower end.

                                                   

                                                                                                                              fig : 1.1

BJT, MOSFET and IGBT can withstand unipolar voltage whereas thyristors and GTO can withstand bipolar voltages. Also BJT, MOSFET and IGBT requires continuous signal for keeping them in turn-on state but SCR and GTO requires pulse-gate signal for turning them ON and once these devices are ON, gate-pulse is removed.  

Pulse Width Modulation (PWM) is a modulation technique that generates variable-width pulses to represent the amplitude of an analog input signal. A PWM signal has two phases, the ‘on-time’ and the ‘off-time’.

                                                                     

It is a periodic signal with a constant frequency. The operating parameters of the bridge is changed by varying the ratio between the on-time and off-time. The various drive modes differ in how the switches are set during the on-time and the off-time.

                                                                        

                                                                                                                           fig : 1.2

PWM is used in for controlling the motor speed. From the PWM timing diagram above we can see that by changing the pulse width we could change the average voltage receipt by the DC motor. The wider the pulse width, the higher the average voltage receipt by the DC motor. The shorter the pulse width, the lower the average voltage receipt by the DC motor. Therefore by varying the pulse width we could vary the DC motor speed. The ratio between the pulse width and the total length of the pulse is called the duty cycle.

The powergui block has been used to solve circuit using one of these methods :

• Continuous, which uses a variable-step solver from simulink.
• Discretization of the electrical system for a solution at fixed time step.
• Phasor solution.

The powergui block also opens tools for steady-state and simulation results analysis and for advanced parameter design. It simulate any Simulink model containing Simscape Electrical Specialized Power System blocks. It stores the equivalent Simulink circuit that represents the state-space equations of the model. Multiple powergui block can be used in a system that contains two or more independent electrical circuits which is to be simulated with different powergui solvers.   

The combination of DC Motor and Power converter is known as DC drive. The complete knowledge of DC drive is needed in order to achieve acceleration and braking.

The electromagnetic forces or torques developed in the driving motor tend to propagate motion of the drive system. This motion may be uniform if the linear velocity or the angular velocity is constant, or non-uniform, as it occurs while starting, braking or changing the load on the drive. In case of uniform motion the torque developed by the driving motor is to overcome any resisting torque offered by the driven equipment as well as the torque due to friction. In other words, only static resisting torques, commonly called as load torques, are to be counterbalanced if the motion were uniform.

In general, in a drive system, the motor driving the load may operate in different regimes – not only as motor but also as a generator and as a brake. Further, in many applications the motor may be required to run in both directions. Therefore, it is preferable to use speed-torque characteristics in all four quadrants rather than to confine the speed-torque characteristics to the first quadrant alone.

 

SPEED CONTROL OF A DC MOTOR USING BJT H-BRIDGE

A Wound field DC machine is provided to the field connections so that the machine can be used as a separately excited, shunt connected or a series connected DC machine. The BJT (Bipolar Junction Transistor) is a current-controlled electronic device mainly employed for amplification and switching purpose, operates as an IGBT. The IGBT/Diode is implemented in parallel with a series RC snubber circuit. In ON-state the IGBT/Diode model has an internal resistance Ron and inductance Lon whereas in OFF-state it has an infinite impedence.

                                                   

                                                                                                                            fig : 1.3

The currents flowing through the IGBT Q3 and Diode D3 has been represented through the scope.The above graph shows that IGBT Q3 is in OFF state when Diode D3 is in ON state and vice-versa.

                                                

                                                                                                                            fig : 1.4

Here, the IGBT Q3 and Diode D3 can handle the maximum current for upto 66.9 A and 38.42 A respectively. The diodes represented in the H-Bridge are called ”fly back diodes”. The diodes have been added in reverse direction of the IGBT so that a path is provided for the current to dissipate when the motor switches from on to off. Since the shaft of the motor is itself spinning, stopping the motor without those diodes in place will cause a large spike in current that could potentially burn out the IGBTs of the H-Bridge. Therefore, it act as a protecting device for the IGBT when there is a risk from voltage rise.

Working principal of a H-bridge model     

In general an H-bridge is a rather simple circuit, containing four switching element, with the load at the centre, in an H like configuration. The simulation of an H-Bridge is used to generate a chopped voltage and to control the speed of a DC motor.

                                                                       

                                                                                                                           fig : 1.5

The IGBTs of the H-bridge acts as switches which can be used to control the speed of the motor, as well as the rotational direction.

• If a pulse width modulation (PWM) signal is applied to input 1 and logic high input 4 then the IGBTs switches will create a path from supply voltage to ground through the motor. This would make the motor rotate forward.
• Likewise, if PWM signal is applied to input 3 and logic high to input 2, the same would occur, with the exception that the rotational direction would be reversed.
• By varying the duty cycle of the pulse width modulated signal the motor is essentially switched ON and OFF at a certain rate to control the speed correspondingly.
• In a bridge, both Q1 and Q2 (or Q3 and Q4) are never closed at the same time because it creates a very low-resistance path between power and GND, effectively short-circuiting the power supply.

When it is conducting (BJT operating in the saturation region), a forward voltage Vf is developed between collector and emitter (in the range of 1V). 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. 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 the speed). The armature mean voltage can be carried from 0 to 240 V when the duty cycle (specified in the Pulse Generator block) is varied from 0 to 100%.

ARMATURE CURRENT SHOOT-UP

The current starts flowing through the motor, the motor gets energized and it starts rotating.

                                                                 

                                                                                                                          fig : 1.6

• The armature current reaches upto 38.43 A at 0.022sec during starting. It overshoots and undershoots upto 13.7018% and 90.917% respectively.
• The motor starts in the positive direction with a duty cycle of 75% (mean DC voltage of 180V). This rotation is in forward direction. After that there is always a time when the switch is turned off using PWM technique.
• At t = 0.5sec, the armature voltage is suddenly reversed resulting into the reversal of current and the motor runs in the negative direction.
• There is a transition of armature current from upper trigger level of 7.5 A to lower trigger level of 65 A during the reverse motoring condition. The fall time for the given transition is 1.586 ms.

                                                                 

                                                                                                                      fig : 1.7

• Now, by varying the pulse width from 75% to 50% duty cycle from pulse generator block, the fall time of the armature current is reduced to 745.321 μs.
• The armature current is limited due to which it does not overcomes the threshold current which is required to develop sufficient torque inside the motor.

The scope shows the motor speed, armature current and load torque. In both the graphs, the torque at the load is proportional to the square of the speed.

 

DC7-FOUR QUADRANT CHOPPER DC DRIVE

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 armature current is derived. This duty cycle is then compared with a sawtooth carrier signal to obtain the required PWM signals for the chopper. 

                                                                      

                                                                                                                         fig : 1.8

 

The Four quadrant chopper DC Drive block uses these blocks from the electric drives blocks library :

• Speed Controller (DC).
• Regulation switch.
• Current Controller (DC).
• Chopper.

The machine is separately excited with a constant DC field voltage source. There is thus no field voltage control. The field current is set to steady-state value when 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.

The model is discrete. Good simulation results have been obtained with a  1-μs time step. In order to simulate the a digital controller device, the control system has two different sampling times :

• The speed controller sampling time.

                                                                            

                                                                                                                                fig : 1.9(a)

• The current controller sampling time.

                                                                            

                                                                                                                               fig : 1.9(b)

The speed controller sampling time has to be a multiple of the current sampling time. The latter sampling time has to be a multiple of the simulation time step.

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 50Hz 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. 

                                

                                                                                                                                fig : 1.10

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 5kHz 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 these 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 oscillation, a smoothing inductance is placed in series with the armature circuit.

• The initial bus voltage is set to 515 V via the GUI block.
• After starting the simulation, the motor armature voltage and current, the four IGBT pulses and the motor speed on the scope is observed.
• 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.

                                       

                                                                                                                                   fig : 1.11

• Now, observe 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, with fast response time and small ripples.
• At t = 2 s the 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.

 

COMPARISON OF DC7 BLOCK WITH H-BRIDGE

 

S.No	H-Bridge Model	DC7 Block
1.	It can operate only in two modes (Forward and Reverse).	It can operate in all four modes (Forward Motoring, Forward regeneration, Reverse Motoring and Reverse regeneration).
2.	The polarity of the voltage applied to the load is interchanged during switching operation.	It’s switching operation interrupts one signal under the control of other.
3.	It does not contain any PI regulators for speed control and current control.	It is composed of two PI regulators for speed control and current control.
4.	It is not suitable for Regenerative Braking.	It is suitable for Regenerative Braking mode.
5.	A high armature current ripple is obtained.	A lower armature current ripple is obtained.
6.	The model uses continuous variable-step solver.	The model uses a discrete step-time solver.
7.	It is simple to use.	It increases the complexity of the drive system.
8.	It is less efficient.	It is more efficient.
9.	It is used in Robotics and other application.	It is mainly used in power control and signal application.

 

TWO – QUADRANT CHOPPER

The Two-Quadrant chopper represents a two-quadrant controlled chopper for converting a fixed DC input to variable DC output. There are two model variants :

• Type A (or Class C) chopper : This block choice is the default. The figure shows the equivalent circuit and the operation for the first and second-quadrat model.

                                                                                                       

                                                                                                                              fig : 1.12(a)

In Class C Chopper, the average Voltage will be always positive but the average load current is either positive or negative. The two switches S1 and S2 should not be turned ON simultaneously as it may result into short-circuit in the supply lines.

                                                                                                 

                                                                                                                               fig : 1.12(b)

For first quadrant operation, S1 and D1 performs the function and if the average load current I_o is high enough, S2 and D2 do not conduct, even though S2 receives a gating signal. For second quadrant operation, S2 and D2 perform the functions and if the average load current I_o has a sufficiently large negative value, S1 and D1 do not conduct, even though S1 receives a gating signal.

                                                                                        Class C operation of DC motor explained with MATLAB model

                       

                                                                                                                                fig : 1.13

Model Description

The Class C chopper model was developed in Simulink with IGBT switches and diodes. A DC machine was connected as load. The 5 HP DC motor is separately excited with a constant 300 V DC field voltage source. The pulse generator gives switching pulses to the two switching elements. The armature voltage is provided by an IGBT converter. The converter is fed by a 240 V 500 Hz source voltage. The switches are triggered with 50% duty cycles. At the output side, the speed output in rad/s is converted to its equivalent speed in RPM and the torque of the motor was measured as in closed-loop control.

                                       

                                                                                                  fig : 1.13(a) Load Voltage during whole operation

                                         

                                                                                                 fig : 1.13(b) Load Current during whole operation

                                        

                                         

                                                                                  fig : 1.13(c) Load Voltage and Load Current during Motoring operation

                                         

                                           

                                                                                      fig : 1.13(d) Load Voltage and Load Current during Braking operation

                                

                                                                                                    fig : 1.13(e) Speed and Torque characteristics of the motor

First and Second Quadrant Operation :

When S1 is triggered and it starts to conduct. The load current I_o is positive and the load receives power from the supply. Therefore, the output voltage e_o = E_dc when S1 or diode D2 conducts. After S1 is turned OFF and inductance L forces the load current to flow through diode D1 till the value of Ldi/dt becomes equal to the back emf (E_b) of the load and the load current I_o becomes zero. Therefore, diode D1 conducts as shown in the fig : . At this point, if the gate signal to S2 is available the back emf (E_b) of the motor forces current in the opposite direction through L and S2. This continues until S2 is turned- OFF and S1 is turned-ON. Now, when S2 is turned-OFF, the energy of the inductance forces the current through diode D2 to the supply. The input current becomes negative. During this period, S1 cannot conduct due to reverse bias but comes into conduction when the input current reduces to zero, provided the gate signal is available to S1 and both the load and input current becomes positive.
Hence, it becomes clear that load voltage e_o = 0 if chopper S2 or diode D1 conducts, e_o = E_dc if chopper S1 or diode D2 conducts. Therefore, average load voltage E_o is positive. However load current I_o have both positive and negative directions. It is positive if S1 is ON or D1 conducts and negative if S2 is ON or D2 conducts. Since average load voltage E_o is positive and average load current I_o is reversible, power flow is reversible. Thus, for regenerative braking and motoring this type of Chopper configuration is used.

• Type B (or Class D) chopper : The figure show the equivalent circuit and the operation for the first and fourth-quadrant model.

                                                                                                    

                                                                                                                          fig : 1.14(a)

In Class D chopper, it permits a change in the polarity of the terminal voltage keeping the direction of the current positive. The two switches S1 and S2 are turned ON simultaneously for continous conduction.

                                                                                              

                                                                                                                          fig : 1.14(b)

For first quadrant operation, S1 and S2 performs the functions and the load current increases, with the rate of rise depending upon the back emf and torque of the machine. For fourth quadrant operation, diodes D1 and D2 are conducting and the load current decreases with negative value of the voltage.

                                                                                Class D operation of DC motor explained with MATLAB model

                                     

                                                                                                                            fig : 1.15

Model description

The Class D chopper model was developed with MOSFET switches and diodes. The DC motor uses the preset model (5 HP 240 V 1750 rpm). The two switches are triggered with a pulse generator having 75% duty cycle. The switches are fed with a 240 V 500 Hz supply. At the output side, the speed rad/s is converted to its equivalent speed in RPM and the torque of the motor was measured as in the closed loop.  

                             

                                                                                               fig : 1.15(a) Load Voltage during whole operation

                             

                                                                                                fig : 1.15(b) Load Current during whole operation

                                 

                                                                                          fig : 1.15(c) Speed and Torque characteristics of the motor

First and Fourth Quadrant Operation :

When both S1 and S2 are switched ON, the load is directly connected to source and hence the output voltage V_o will become equal to the source voltage V_s. The current flows from source to load in this case. Thus, both the current and output voltage are positive in this case. However, when both the S1 and S2 are made OFF simultaneously, the current through the load doesn’t suddenly drops to zero due to inductive nature of load. However it decays gradually and hence a huge amount of voltage is induced in the inductor in the reverse direction (opposite to the direction of V_o). This makes diode D1 and D2 forward biased. Thus, D1 and D2 starts conducting and connects the load to source again. But this time, the current flows from load to source due to change in polarity of V_o.

 

BLDC MOTOR

A brushless DC motor has an electronic commutator, therefore its inherent characteristics is such that the motor has a power converter or electronic commutator. The commutator provides pulses of current to the motor windings which control the speed and torque of the synchronous motor. The power converter have the arrangement of the switches which helps in the excitation of the different phases turn by turn. The current in phase 1, phase 2 and phase 3 have to be maintained synchronously in such a way that motor rotates in a clockwise direction. Thus most of the brushless DC motor are 3 phase type and have an AC current. Although its internal working requires an AC voltage but its behavior in terms of output speed-torque are more like  a DC motor.

The brushless DC motor works on the same principle as that of a brushed DC motor i.e, internal shaft position feedback. In brushed DC motor, feedback is implemented using a mechanical commutator and brushes but in BLDC motor it is achieved using multiple feedback sensors.

https://www.pantechsolutions.net/wp-content/uploads/2020/05/BLDC-MOTOR-CONTROL.gif

                                                                                           

                                                                                                                           fig : 1.16

The brushless DC motor has only two basic parts : rotor and the stator. The rotor is the rotating part and has rotor magnets whereas stator is the stationary part and contains stator windings. In BLDC permanent magnets are attached in 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 sensors such as hall sensors and optical encoders.

BLDC Hall Sensors

Hall Effect sensor provides the portion of information needed to synchronize the motor excitation with rotor position in order to produce constant torque. It detects the change in magnetic field. The rotor magnets are used as triggers to the Hall Sensor. A signal conditioning circuit integrated within the Hall switch provides a compatible pulse with sharp edges. Three Hall Sensors placed 120° apart, are mounted on the stator frame. The Hall Sensor digital signals are used to sense the rotor.  

OPERATION OF BLDC MOTOR

A permanent Magnet AC motor, which has  a trapezoidal back emf, is referred to as brushless DC motor (BLDC). The BLDC drive system is based on the feedback of rotor system at fixed points for commutation of the phase currents.

The BLDC motor requires quasi-rectangle shaped currents fed into the machine. Alternatively, the voltage may be applied to the motor every 120°, with current limit to hold the current within motor capabilities. Because the phase currents are excited in synchronism with the constant part of the back emf, constant torque is generated. The electromagnetic torque of the BLDC motor is related to the product of phase, back emf and current. The back emf in each phase are trapezoidal in shape and are displaced by 120 electrical degree with respect to each other in three phase machine. A rectangle current pulse is injected into each phase so that current coincides with the back emf waveform; hence the motor develops an almost constant torque.

In a commutation system where the position of the motor is identified using feedback sensors – two of three electrical windings are energized at a time as shown in figure 4.

In figure 4 (A), the GREEN winding labeled ‘’001” is energized as the NORTH pole and the BLUE winding labeled as “010” is energized as the SOUTH pole. Because of this excitation, the SOUTH pole of the rotor aligns with the GREEN winding and the NORTH pole aligns with the RED winding labeled “100”. In order to move the rotor, the “RED” and “BLUE” windings are energized in the direction shown in figure 4(B). This cause the RED winding to become the NORTH pole and the BLUE winding to become the SOUTH pole. This shifting of the magnetic field in the stator produces torque because of the development of repulsion (RED winding – NORTH – NORTH alignment) and attraction forces (BLUE winding – NORTH – SOUTH alignment), which moves the rotor in the clockwise direction.

                                                                  

                                                                                                                      fig : 1.17

This torque is at its maximum when the rotor starts to move, but it reduces as the two fields align to each other. Thus, to preserve the torque or to build up the rotation, the magnetic field generated by stator should keep switching. To catch up with the field generated by the stator, the rotor will keep rotating. Since, the magnetic field of the stator and rotor both rotate at the same frequency, they come under the category of synchronous motor.

The switching of the stator to build up the rotation is known as commutation. For 3-phase windings, there are 6 steps in the commutation; 6 unique combinations in which motor windings will be energized.

 

CONCLUSION : The speed control of DC motor using BJT H-Bridge is simulated and its results are obtained successfully. The chopped voltage is generated using PWM technique and speed is controlled accordingly. At t = 0.5sec armature current shoots up due to which motor runs in negative direction. Further, the operation of H-Bridge and DC7 block is compared with each other. The two-quadrant operation of dc drive included the Class C and Class D choppers having IGBTs and MOSFETs as their switching devices. Controlling the pulses across these switches we can control the speed of the drive. Lastly, the operation of BLDC motors with electronic commutator (Hall Sensors) is explained such that it runs in synchronization.