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Challenge

Week-7 Challenge: DC Motor Control

NNeeraj Mishra30 Dec 2022
MATLAB

AIM:

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. (Refer to article - Multiquadrant operation of motor )

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

1A)

An H-bridge is a simple circuit that lets us to control a DC motor to go backward or forward.

In general an H-bridge is a rather simple circuit, containing four switching element, with the load at the center, in an H-like configuration:

The switching elements (Q1..Q4) are usually bi-polar or FET transistors, in some high-voltage applications IGBTs. Integrated solutions also exist but whether the switching elements are integrated with their control circuits or not is not relevant for the most part for this discussion. The diodes (D1..D4) are called catch diodes and are usually of a Schottky type.

The top-end of the bridge is connected to a power supply (battery for example) and the bottom-end is grounded.

In general all four switching elements can be turned on and off independently, though there are some obvious restrictions.

Though the load can in be anything we want, by far the most pervasive application if H-bridges is with a brushed DC or bipolar stepper motor (steppers need two H-bridges per motor) load. In the following we will concentrate on applications as a brushed DC motor driver.

Static Operation

The basic operating mode of an H-bridge is fairly simple: if Q1 and Q4 are turned on, the left lead of the motor will be connected to the power supply, while the right lead is connected to ground. Current starts flowing through the motor which energizes the motor in (let’s say) the forward direction and the motor shaft starts spinning.

If Q2 and Q3 are turned on, the reverse will happen, the motor gets energized in the reverse direction, and the shaft will start spinning backwards.

In a bridge, you should never ever close both Q1 and Q2 (or Q3 and Q4) at the same time. If you did that, you just have created a really low-resistance path between power and GND, effectively short-circuiting your power supply. This condition is called ‘shoot-through’ and is an almost guaranteed way to quickly destroy your bridge, or something else in your circuit.

MATLAB demo model named ‘Speed control of a DC motor using BJT H-bridge is shown below,

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).

MODEL DISCRIPTION:

1) dc motor used is model is 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%.

2)The H-bridge consists of four BJT/Diode pairs (BJT simulated by IGBT models)

3)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.

4)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

thus from above graph we can see that, for 0 to 0.5 sec , the current flows through diode D3 and from 0.5 sec to 1 sec , the current flows through Q3.

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.

the waveform for speed, armature current and load torque is shown in above graphs.

from armature current graph , we can see that ,at starting there is sudden spike increase in current at 0 sec also and at 0.5 sec also.

We can see that , as the voltage gets changed in the circuit , the speed of motor also changes.

In this transition of positive to negative direction of rotation we observe a high surge in current, this is due to the transient switching occurring in the circuit.

As there is already current flowing in the circuit & on changing of switches after 0.5 sec, the current in the circuit adds to the changed direction of current and we observe a surge.

1B)

from armature current graph , we can see that ,at starting there is sudden spike increase in current at 0 sec also and at 0.5 sec also means at transition from positive to negative direction of rotation we observe a high surge in current, this is due to the transient switching occurring in the circuit.

As there is already current flowing in the circuit & on changing of switches after 0.5 sec, the current in the circuit adds to the changed direction of current and we observe a surge in current.

the surge in the current can be reduced by reducing the duty cycle.

in above results , the duty cycle selected as 75%.

by changing the duty cycle from 75% to 50% , the current surge can be reduced tremendously.

thus we can find that , the armature curent spike is reduced by changing the duty cycle from 75% to 50%.

1C)

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 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, compared with other DC drives, is that it can operate in all four quadrants (forward motoring, reverse regeneration, reverse motoring, and forward regeneration). 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 thecomplexity of the drive system.

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

WORKING OF FOUR QUADRANT CHOPPER:

The circuit of a four quadrant chopper or class-E chopper basically consists of four semiconductor switches CH1 to CH4 and four diodes D1 to D4. The four diodes are connected in anti-parallel. The circuit diagram of this type of chopper is shown below.

In the above circuit diagram, the choppers are numbered CH1 to CH4. For first quadrant operation CH1 is made ON, for second quadrant operation CH2 is made ON and so on. To better understand the working of four quadrant chopper, we will discuss its operation separately for each quadrant.

First Quadrant Operation:

For first quadrant operation, CH4 is kept ON, CH3 is kept OFF and CH1 is operated. When both CH1 & CH4 are ON simultaneously, the load gets directly connected to the source and hence the output voltage becomes equal to the source voltage. This essentially means that vo = vs. It may be noted that the load current flows from source to load as shown by the direction of io.

When CH1 is switched OFF, the load current free wheels through CH4 and D2. During this period, the load voltage and current remains positive.

Thus, both the output voltage vs and load current io are positive and hence, the operation of chopper is in first quadrant. It may be noted that, Class-E chopper operates as a step-down chopper in this case.

Second Quadrant Operation:

To obtain second quadrant operation, CH2 is operated while keeping the CH1, CH3 & CH4 OFF. When CH2 is ON, the DC source in the load drives current through CH2, D4, E and L. Inductor L stores energy during the On period of CH2.

When CH2 is turned OFF, current is fed back to the source through D1, D4. It should be noted at this point that (E+Ldi/dt) is more than the source voltage Vs. As load voltage Vo is positive and Io is negative, it is second quadrant operation of chopper. Since, the current is fed back to the source, this simply means that load is transferring power to the source. Kindly read step up chopper for detailed analysis and better understanding.

For second quadrant operation, load must contain emf E as shown in the circuit diagram. In second quadrant, configuration operates as a step-up chopper.

Third Quadrant Operation:

To obtain third quadrant operation, both the load voltage and load current should be negative. The current and voltage are assumed positive if their direction matches with what shown in the circuit diagram. If the direction is opposite to what shown in the circuit diagram, it is considered negative. One important thing to notice is that the polarity of emf E in load must be reversed to have third quadrant operation. Circuit diagram of Class-E chopper for third quadrant operation is shown below.

Polarity of Load emf E reversed

For third quadrant operation, CH1 is kept off, CH2 is kept ON and CH3 is operated. When CH3 is ON, load gets connected to source and hence load voltage is equal to source voltage. But carefully observe that the polarity of load voltage vo is opposite to what shown in the circuit diagram. Hence, vo is assumed negative. Let us now see what is the status of load current io. It may be seen that io is flowing in the direction opposite to shown in the circuit diagram and hence negative.

Now, when CH3 is turned OFF, the negative load current free wheels through the CH2 and D4. In this manner, vo and io both are negative. Hence, the chopper operates in third quadrant.

Fourth Quadrant Operation:

To obtain fourth quadrant operation, CH4 is operated while keeping CH1, CH2 and CH3 OFF. The polarity of load emf E needs to be reversed in this case too like third quadrant operation.

When CH4 is turned ON, positive current flows through CH4, D2, L and E. Inductance L stores energy during the time CH4 is ON. When CH4 is made OFF, current is fed back to the source through diodes D2, D3. Here load voltage is negative but the load current is always positive. This leads to chopper operation in fourth quadrant. Here, power is fed back to the source from load and chopper acts as a step-up chopper.

The operation of a four quadrant chopper or Class-E chopper is summarized in the figure below.

COMPARISON OF 4 QUADRANT CHOPPER AND BJT H BRIDGE:

1) the 4 quadrant chopper works in all four modes like

  • forward motoring
  • forward breaking
  • reverse motoring
  • reverse breaking

but the BJT h bridge model works on only two modes

  • forward motoring
  • reverse motoring

2)to change the direction of rotation of motor from forward to reverse and vice versa , the current signals get transited from one to other switch while in h bridge model, the direction of rotation of motor is changed by changing the polarity of current.

3)the 4 quadrant chopper have high switching frequency

while h bridge model have low switching frequency

4)the armature current in 4 quadrant chopper have a little spike at transition from one to other switch because of high swithing frequency while h bridge model have high spike of current.

5) the 4 quadrant chopper is generally used in industrial application while h bridge model is used in robotics.

2)

CIRCUIT DIAGRAM FOR TWO QUADRANT CHOPPER:

two quadrant chopper is also called as type c chopper. Type C chopper is obtained by connecting type –A and type –B choppers in parallel. We will always get a positive output voltage V0 as the freewheeling diode FD is present across the load. When the chopper is on 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 and it will be positive regardless the chopper is on or the FD conducts. The load current will be negative if the chopper is or the diode D2 conducts. We can say the chopper and FD operate together as type-A chopper in first quadrant. In the second quadrant, the chopper and D2 will operate together as type –B chopper.

The average voltage will be always positive but the average load current might be positive or negative. The power flow may be life the first quadrant operation ie from source to load or from load to source like the second quadrant operation. The two choppers should not be turned on simultaneously as the combined action my cause a short circuit in supply lines. For regenerative braking and motoring these type of chopper configuration is used.

TWO QUADRANT CHOPPER MODEL USING SIMULINK:

let us take two mosfet blocks and diodes blocks and connect it as shown in circuit diagram.

the dc voltage source is connected as shown.

let the voltage of dc voltage source be 24 volts.

let us consider an series RLC branch block and select inductor L . thus block works as a load.

to consider LE load ie motor load , and dc voltage source of 12 volt is also connected in series with inductor.

The pulse generator block is used to create gate pulse.

the not gate is used in the model so that the two mosfet will not get closed at the same time otherwise short circuit will occur .due to use of not gate , when one mosfet is ON the other mosfet will get off and when other mosfet is ON the first mosfet will get off .

The amplitude of pulse generator is kept 10 so that we can clearly see the pulses at scope and let period be 0.02 seconds and let pulse weidth be 50%

SCOPE RESULT:

the voltage waveform is not going less than zero . the minimum value of voltage is zero and maximum value is always in the positive direction.

however in current waveform , the current increases and decreases. the current may be positive as well as negative.

and the pulse amplitude is 10 thus from pulse waveform we can see that the pulse amplitude is 10.

3) BLDC motor operations.

BLDC motor named as brushless dc motor having high effenciency than other types of motors.This motor is also known to be synchronous motor.

Construction:

Most of the motors are having brushes in their rotor as it transfers current into the rotor circuit.But here differently rotor is placed with permanent magnet.

The stator coil is wounded with coils,they are stationary but producing variable DC voltages for rotating the permanent magnet as changing the magnetic field of stator poles continuously.

The controlling of motor is by changing the magnitude and direction of current flowing through the stator windings.

A BLDC motor with three coils on the stator will have six electrical wires (two to each coil) extending from these coils. In most implementations three of these wires will be connected internally, with the three

remaining wires extending from the motor body. Wiring in the BLDC motor case is more complicated than simply connecting the power cell’s positive and negative terminals. we will look more closely at how

these motors work in the second session of this series.

Operation of BLDC motor:

We can consider three winding trminal are comming out as U,V,W and their another ends are connected in single point as shown in below figure.The angle shift between two coils makes 120 degree.

The current moves from U to W makes magnetizing the coils attracts the rotor consisting of permanent magnet makes moment as shown above to align with the resultant arrow(larger arrow).

North pole of magnet towards arrow mark of resultant arrow.

Rotation is maintained by continually switching the flux so that the permanent magnet is constantly chasing the rotating magnetic field induced by the coils. In other words, energizing of U, V, and W must be

continually switched so that the resultant flux keeps moving, producing a rotating field that continually pulls on the rotor magnet.

The above movement are acheived by continuous change in current direction over two poles of stator at single time.But this is slightly not continuous in their rotation.

This makes motor vibration and noice will be produced.

So, to attain smooth continuous rotation of rotor we have to control the current flow through all the three poles in same time.

As shown below we can acheive continuous change in current flow in all stator poles at same time in 120 degree phase shift.

As we have seen, we drive BLDC motors by continually changing the directionality of the flux produced by the coils. The permanent magnets on the rotor continually chase the shifting rotating magnetic field,

causing the rotor to turn.

This is the working of BLDC motor.

Advantages of BLDC motor:

  • One big advantage is efficiency, as these motors can control continuously at maximum rotational force (torque).
  • Even small BLDC motors can deliver considerable power.
  • BLDC motors can be controlled, using feedback mechanisms, to delivery precisely the desired torque and rotation speed.
  • Reduce energy consumption and heat generation.
  • BLDC motors also offer high durability and low electric noise generation.
  • The sparks will not be produced as brushed motor between brushes and commutators.

Control of BLDC motor:

By using the inverter circuitry to also adjust the voltage into each coil, we can control the magnitude of the current.

A typical way to adjust the voltage is with pulse width modulation (PWM). We alter the voltage by lengthening or reducing the pulse ON time ( “Duty cycle”: the ON time

expressed as a ratio of the ON+OFF switching interval). Increasing the duty cycle has the same effect as raising the voltage,reducing the duty cycle has the same effect as lowering the current.

PWM can be implemented using MPUs equipped with dedicated PWM hardware. While 120-degree conductiong control only requires two-phase voltage control.

sinusoidal control uses three-phase voltage control and is considerably more complicated as appropriate inverter circuitry is therefore essential for driving BLDC motors.

ADVANTAGES OF BLDC MOTOR

  • Less overall maintenance due to absence of brushes
  • Reduced size with far superior thermal characteristics
  • Higher speed range and lower electric noise generation.
  • It has no mechanical commutator and associated problems
  • High efficiency and high output power to size ratio 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
  • Low noise due to absence of brushes


    DISADVANTAGES OF BLDC MOTOR

  • These motors are costly
  • Electronic controller required control this motor is expensive
  • Requires complex drive circuitry
  • Need of additional sensors


    APPLICATION OF BLDC MOTOR

Brushless DC motors (BLDC) use 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.

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