Week-7 Challenge: DC Motor Control

Q1 A. Explain the MATLAB demo model named ‘Speed control of a DC motor using BJT H- bridge.

1. Comment on the armature current shoot-up from the scope results
2. Transistors have been used as amplification devices, where control of the base currents used to make the transistor conducive to a greater or lesser degree. Since the control and protective circuits were complicated and expensive, these transistors were not widely used in power electronics. These devices cannot withstand high loads and it has become difficult to protect with fuses.

Then, in power electronics, the NPN transistor is known as bipolar Junction transistor which is less cost-effective has come into use. This BJT has two power terminals called the collector(C) and emitter (E) and a third control terminal called the base (B)

                         BJT Symbol

BJT’s can be used in inverter bridges as they can be protected from high reverse voltages by connecting reverse diode in series and parallel. Forward conduction is blocked until a positive current is applied to the gate terminal and will conduct as long as the voltage is applied. During forward conduction, it also exhibits a forward voltage drop which causes losses in the power circuit. The BJT maybe turn off by applying a negative current to the gate.

Advantages:

1. It has good power handling capabilities
2. Low forward conduction voltage drop.

Disadvantages:

1. Relatively low switching times
2. The inferior safe operating area
3. Has complex current controlled gate drive requirements

Speed Control of a DC Motor Using BJT H-Bridge

 The Bipolar Junction Transistor (BJT) when used for power switching applications, operates as an IGBT. When conducting (BJT operating in the saturated region), a forward 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 present model (5HP 24V 1750rpm). It simulates a fan type load (where load torque is proportional to the square of speed). The armature means voltage can be varied from 0 to 250V 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 triggered, 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 triggered, a negative voltage is applied to the motor, and diodes D1 –D4 operate as free-wheeling diodes when Q2 and Q3 are switched off. Since the switches are connected in an ‘H’ shape, it is called an H-bridge converter.

A number of industrial applications take power from DC rather than AC voltage sources like subway cars, battery-operated vehicles. Here power is operated from a fixed DC voltage. However, to satisfy the requirements of speed control, the DC voltage must be adjusted with the speed of the motor. The traditional method of achieving voltage control was the cam controller which is, in effect, a resistor that can be varied in steps. For this, we needed force-commuted switches like transistor (Q1,Q2, Q3, Q4) which is capable of turning off without the aid of external emf from the supply or load.

The principle of operation of a chopper is quite simple i.e., in the operation of the four-quadrant step-down chopper both the motor voltage and current are reversible. The force commuted switches Q1, Q3 periodically connects the DC supply to the load and then disconnects it for a predetermined period. Motoring and braking in the forward direction are obtained by modulating transistors Q1 and Q3. For reversal, switch Q1 is turned off, switch Q2 is turns on and the polarity of the current is reversed by modulating switches Q2 and Q4. When the switch is disconnected the motor armature current finds a path through the free-wheeling diode. Hence the voltage across the motor alternates between the DC voltage and zero. The equivalent circuit of the four-quadrant chopper is a simple reversible voltage source proportional to the duty cycle of switch1 (forward direction) or switch 4(the reverse direction). Hence the duty cycle varies from -1 to 1.

The average voltage across the motor is Vd=d*Vdc where di the duty cycle of the chopper.

The average DC output voltage can be varied in two ways

1. Constant frequency modulation
2. Variable frequency modulation

In the case of constant frequency modulation, the chopping frequency and therefore the period T is kept constant. This type of modulation is called pulse width modulation. In variable frequency modulation, the pulse width is kept constant and the off-time and total time is kept varied. Since the chopper working at a continuous maximum frequency set by limitations of the switch produces the best overall efficiency.

  Since neither the voltage nor the current of the step-down chopper can reverse polarity, the equivalent circuit of the step-down chopper is a variable voltage source in series with an ideal diode.

So for the modulation of frequency, the pulse is generated with the predetermined values of the duty cycle and triggered to the BJT gate terminals through P1, P2, P3, P4 tags in order to vary the supply voltage. The duty cycle ON and Off is kept constant but pulse width can be varied to provide different voltages across the motor. Here the pulse width taken is 75 and amplitude is 1.

-digital data to the transistors in forward and reverse bias is provided to the switches by the pulse generator.

The scope result is given for the transistor Q4 and the diode connected in parallel during conduction mode.

From the graph, we can understand that the current supply is zero through the Q4 transistor when the motor rotates in forwarding direction and the diode is in conducting mode i.e. the armature current from the motor passes through the freewheeling diode D4. After a time period of 0.5 seconds, the Q4 switch turns ON starts conducting the motor due to regenerative braking, then the armature current supply to the motor is high initially and slowly decreases due to the losses. During this period the diode stops conducting and so the current is zero.

When the pulse generator is triggered all four switches. The duty cycle is set with a total time of 1 minute which is ON for 0.5 sec and the duty cycle is OFF for the next 0.5 seconds. The total time set for the duty cycle is one minute.  From the graph, we can explain duty cycle ON means the motor is rotating in a forwarding direction for 0.5 seconds with a maximum speed of 990 rpm and decreases the motor speed when the regenerative braking is applied during the OFF time of the duty cycle.

The armature current graph shows the during ON time duty cycle, the current is supplied by the switches to the motor is maximum initially and slowly decreases to zero, and when the polarity changes during OFF time duty cycle, a negative voltage is supplied to the motor which tends the motor to rotate in the reverse direction. So the graph shows negative values of current which means the current flows from load to source indicating regenerative braking.

The load torque graph shows the motor transmits maximum torque to the load during the ON time duty cycle and decreases when the negative pulse is triggered to the switch.  

       Compare on the armature current shoot-up from the scope results.

• Pulse width or voltage during duty cycle is changed from 75 to 65 and see the results. With the decrease in input voltage control, the current supply by the switches has decreased. There is a decrease in the motor speed, armature current, and load torque.

• Pulse width or voltage during duty cycle is changed from 65 to 55 and see the results. With the decrease in input voltage control, the current supply by the switches has decreased. There is a decrease in the motor speed, armature current, and load torque.

With the above observations, we can come to a conclusion that by changing the pulse width and are triggered to power converter switches as control signals we can control the speed of the motor and thereby load torque. So the BJT transistors can be used as amplification devices, where we can control the base currents so that the transistor can conduct according to the demand of power supply to run the motor.

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

Four-Quadrant Chopper DC Drive:

The Four-Quadrant Chopper DC Drive (DC7) block represents a four-quadrant, DC-supplied, chopper (or DC-PWM converter) drive for DC motors. This drive features closed-loop speed control with a 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 commanded armature current is derived. This duty cycle is then compared with a sawtooth carrier signal to obtain the required PWM signals for the chopper.

The main advantage of this drive is, compared with other Dc drives, is that it can operate in all four quadrants (forward motoring, reverse regeneration, reverse motoring, and forward generation). In addition, due to the use of high switching frequency DC-DC converters, a lower armature current ripple (compared with thyristor-based DC drives) is obtained. However, four switching devices are required, which increases the complexity of the drive system.

               Four quadrants DC Motor Drive                                                 H-Bridge

1. Input voltages are connected across IGBT1 and               1. Input and output voltages are connected

IGBT2 terminal and the output voltages are                         to the source.

connected to ground

2. It can operate in all four quadrants                                2.  It can operate in two quadrant

3. The four-quadrant operations are forward motoring,        3. The operating functions are forward motoring and                                                         Reverse regeneration, reverse motoring, and                     Reverse motoring.

forward Generation.

3. Both speed controller and current controller are              4.  The only pulse generated voltage control is                            

applied and used to predict the duty cycle                             regulate the current supply and motor output                                                                                                                                                   Speed data is taken and torque is calculated and

                                                                                         is used as an input to transmission.                                     

5. Smoothing inductance is used to reduce                     5. No inductance is used separately because the.

the current ripples and make the rotation of the motor.       the motor is a self-inductance motor. 

 Smooth.

Q2. Develop a 2-quadrant chopper using Simulink & explain the working of the same with the relevant results.

     A:   The two-quadrant DC chopper allows bidirectional current and power flow with a unidirectional voltage supply.

Bidirectional current switch                   Bidirectional power flow

    The schematic diagram of a 2-quadrant chopper is- chopper means it is a DC drive which decreases high voltages supply to low voltages demand-supply.

The motor current i0 is inductive current and, therefore, cannot change instantaneously. The transistor Q1 and diode D1 together form a bidirectional current switches S1 and S2. The one of the main reason for developing bidirectional switches is that the energy during braking of the vehicle do not convert into thermal losses in the mechanical brakes through friction instead it can be utilized to recharge the batteries. This mode of operation is called Regenerative mode and the direction of power is reversed during this mode where the energy flows from the wheels to the battery and does recharging the battery which leads to improving the efficiency of the vehicle. We use the IGBT switch as a bidirectional switch which has combined features of MOSFET and BJT, i.e., BJT has high power ratings with low conduction losses and MOSFET has high switching frequency. There needs to be bidirectional power flow, so the power converters also need to be bidirectional.

The 2-Quadrant operations are Quadrant I and Quadrant II, in Quadrant I operation, turning on Q1 allows current and power to flow from the battery to the motor. Motor terminal voltage v0 and current i0 are greater than or equal to zero, this is because the diode will be conducting in freewheeling mode. Q2 is required to remain continuously off in Quadrant I operation and, hence ib2 =0. When Q1 turns off, D2 turns on, because i0 is continuous. Quadrant I operation takes place during the acceleration and constant velocity cruising of a vehicle. The chopper operating modes toggle between switching states 1 and 2 in this quadrant.

Acceleration and constant speed cruising are controlled by Q1 in Quadrant I operation, while braking is controlled by Q2 in Quadrant II operation. By fixing the duty ratio, the required current at Q1 is obtained to achieve the required motor speed and torque output at the shaft. The gate drive signal for Q1 is a function of d, the input voltage to the motor is also dependant on d. that is how the slope of the acceleration pedal and brake pedal indicates the desired vehicle motion and the amount of braking. So the angle of the pedal is used to set the duty ratio d1 and d2.

     Circuit condition for (a) switch Q1 on and (b) switch Q1 off

Simulink model:

Simulink model developed but could not complete due to the below-mentioned reasons.

• Couldn’t able to connect the phase terminal of digital control to the circuit even with the converter.
• I tried connecting with the controlled voltage source to PDI control but an error showing

• Error : no square law function

• Gate driver is different and couldn’t able to connect to the chopper

Q3. Explain briefly the operation of the DC motor.

Brushless DC Motor:

          The brushless DC motor is an AC motor as the current passing through the motor is alternating current which is converted and supplied from a DC source. It is also called a self-synchronous motor, permanent magnet synchronous motor, and electronically commuted motor.

   The basic operations of the BLDC motor are the switches direct the direct from a DC source through a coil on the stator. The rotor consists of a permanent magnet. The current that flows in the direction that magnetizes the stators the rotor is turned clockwise. When the rotor passes between poles of the stator, then the stator current is switched off. Due to the momentum, the rotor rotates and the stator coil is re-energized, but the current and hence the magnetic field, are reversed. Then the rotor is pulled on the round in a clockwise direction. The process continues, with the current in the stator coil alternating. Using sensors the position of the rotor will be diagnosed and accordingly, the switching of the current is synchronized

Motor torque rate can be improved by increasing the coils from single to three as shown in the below figure

Coil B is energized to turn the motor clockwise and when the rotor is between the poles of coil B, coil C will be energized, and so on. The electronic circuit used to drive and control the coil currents is called an inverter. The main feature of the BLDC motor is that the torque will be reduced as the speed increases. This is because when the magnet keeps rotating, there generates a back EMF in the coil which reduces the current flowing in the coil. When current reduces the magnetic field also reduces which leads to a decrease in torque. These BLDC motors need a strong permanent magnet for the rotor.