Week-7 Challenge: DC Motor Control

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.

Answer:

1(a) An H-Bridge is a simple circuit that lets us to control a DC motor go forward and backward.

In general H-Bridge is a rather simple circuit, containing four switching element, with the load at the centre, in an H-like configuration.

       

The switching elements (Q1,Q4) are usually bi-polar or FET transitors, 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 sample) 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 motors (steppers need two H-bridges per motor) load. In the following we concentrate on applications as a brushed DC motor driver.

Static Operation

The basic operation 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 enerzises the motor in (let's say) the forward direction and 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 starts spinning backwards.

In a bridge you should never close both Q1 and Q3 (or Q2 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 guranteed 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 Transister (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 (1/10).

MODEL DESCRIPTION

1) DC motor used is model is preset model (5HP 24V 1750rpm). 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 consist 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 operates 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 diodesD1-D4 operates as free wheeling diodes when Q2 and Q3 are switched off.

SIMULATION RESULTS 

 

  

thus from the 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 180 V). At t = 0.5sec, the armature voltage is suddenly reversed and the motor runs in the negative direction.

'Scope' shows motor speed, armature current and load torque and 'Current' shows current 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 and also at 0.5 sec.

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 occuring in the circuit.

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

1(B) 

          

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

As there is already current floeing 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 the 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 current spike is reduced by changing the duty cycle from 75% to 50%.

1(C)

     

The four Quadrant Chhoper DC Drive (DC7) block represents a fpor quadrant DC supplied Chopper (or DC-DC PWM Converter) drive for DC motors. This drives 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 motoringf 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 the complexity 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 regurators. 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 Chhoper

The circuit of a four quadrant chopper or class E chopper basically consists of four semiconductor switches CH1 to CH4 and four diodes Di 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 umbered CH1 and 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 and 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 V_o = V_s. It may be noted that the load current flows from source to load as shown by the direction of i_o.

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 V_s and load current i_o are positive and hence, the operation os 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 V_s. As load voltage V_o is positive and lo 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.

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:

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 was 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 observing carefully that the polarity of load voltage v_o is opposite to what shown in the circuit diagram. Hence v_o is assumed negative. Let us now see what is the status of load current i_o. It may be seen that i_o 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 v_o and i_o 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. Inductor 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 fourth 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 braking
• Reverse motoring
• Reverse braking

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 switching 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)

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 v_o as the freewheeling diode FD is present across the load. When the chopper is on the freewheeling diode starts conducting and the output voltage v_o will be equal to v_s. the direction of the load current i_o will be reversed. The current i_o 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 like the first quadrant operation i.e 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 may cause a short circuit in supply lines. For regenerative braking and motoring these type of chopper configuration is used.

Let us take two mosfet blocks and diode blocks and connect it as shown in circuit diagram.

The DC voltage source is connect as shown.

Let the voltage of DC voltage source be 24 volts.

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

To consider LE load i.e motor load, and DC voltage source of 12 volt is also connected in series with the 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 will occur. Due to use of NOT gate, when one mosfet is ON the other mosfet will be OFF and when other mosfet will be ON first will go OFF.

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

 

SCOPE RESULTS

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 incleases and decreases. The current may be positive as well as negative and the pulse amplitude is 10, thus from waveform we can see that the pulse amplitude is 10.

3) BLDC MOTOR

Brushless DC electric motors also known as electronically commutated motors. A brushless DC motor (known as BLDC) is a permanent magnet synchronous electric motor which is driven by direct current (DC) electrically and it accomplishes electronically controlled communication 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 motors are also referred as trapezoidal permanent magnet motors.

Unlike conventional brushed type DC motor, wherein the brushes make the mechanical contact with commutator on the rotor so as to form an electric path between a DC electric source and rotor armature windings, BLDC motor employs electrical commutation with permanent magnet rotor and a stator with a sequence of coils. In this motor permanent magnet (or field poles) rotates and current carrying conductors are fixed.

The armature coils are switched electronically by transistors or SCR at the correct rotor position in such a way that armature field is in space quadrature with the rotor field poles. Hence the force acting on the rotor causes it to rotate. Hall sensors or rotary encoders are most commonly used to sense the position of the rotor and are positined around the stator. The rotor position feedback from the sensor helps to determine when to switch the armature current.

This electronic commutattion arrangement eliminates the commutator arrangement and brushes in a DC motor and hence more reliable and less noisy operation is achieved. Due to the absence of brushes BLDC motors are capable to run at high speeds. The efficiency of BLDC motors is typically 85 to 90 percent, whereas as brushed type DC motors are 75 to 80 percent efficient. There are wide varieties of BLDC motors available ranging from small power range to fractional horsepower, integral horsepower and large power ranges.

In this motor, the permanent magnets attach to the rotor. The current carrying conductors or armature winding are located on the stator. They use electrical commutation to convert electrical energy into mechanical energy.

The main design difference between a brushed and brushless motors is the replacement of mechanical commutator with an electric switch circuit. A BLDC Motor is a type of synchronous motor in the sense that the magnetic field generated by the stator and the rotor revolve at the same frequency.

Brushless motor does not have any current carrying commutators. The field inside a brushless motor is switched through an amplifier which is triggered by the commutating device like an optical encoder.

BLDC motors can be constructed in different physical configurations. Depending on the stator windings, these can be configured as singlr phase, two phase or three phase motors. However three phase BLDC motors with permanent magnet rotor are most commonly used.

 

The construction of this motor has many simlarities of three phase induction motor as well as conventional DC motor. This motor has stator and rotor parts as like as other motors.

 

The layout of a DC brushless motor can vary depending on whether it is in "out runner" style or "inrunner" style.

• Outrunner - The field magnet is a drum rotor which rotates around the stator. This style is preferred for applications that require high torque and where high rpm isn't a requirement.
• Inrunner - The stator is a fixed drum in which the field magnet rotates. This motor is known for producing less torque than the out-runner style, but is capable of spinning at very high rpm.

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 or delta. However 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, 48V or less voltage BLDC motors are preferred.

  

Rotor

BLDC motor incorporates a prmanent 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.

Ferrite magnets are inexpensive, however they have low flux density for a given volume. Rare earth alloy magnets are commonly used for new degins. Some of these alloys are Samarium Cobalt (SmCo), Neodymium (Nd) and Ferrite & Boron (NdFeB). 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 Sensors

Hall Sensors 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. Befor energizing a particular stator winding, acknowlegement 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 in. The exact commutation sequence to the stator winding can be determined based on the combination of these three sensor's response.

Working of BLDC Motors:

The underlying principles for the working of a BLDC motor are the same as for a brushed DC motor i.e., internal shaft position feedback. In case of brushed DC motor, feedback is implemented using a mechanical commutator and brushes. Within a BLDC motor, it is achieved using multiple feedback sensors. The most commonly used sensors are hall sensors and optical encoders. Hall sensors work on the hall effect principle that when a current carrying conductor is exposed to the magnetic field, charge carriers experience a force based on the voltage developed across the two sides of the conductor.

If the direction of the magnetic field is reversed, the voltage developed will reverse as well. For Hall effect sensors used in BLDC motors, whenever rotor magnetic poles (N or S) pass near the hall sensor, they generate a HIGH ot LOW level signal, which can be used to determine the position of the shaft.

In a commutation system - one that is based on the position of the motor identified using feedback sensors - two of the 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 causes the RED winding to become the NORTH poe 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 alingment) and attraction forces (BLUE winding - NORTH - SOUTH alignment), which moves the rotor in the clockwise direction.

              

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 preseve 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.

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

ADVANTAGES OF BLDC MOTOR

• Less overall maintenance due to absence of brushes.
• Reduced size with far superior thermal charecteristics.
• Higher speed range and lower electric noise generation.
• It has no mechanical commutator and associated problems.
• High efficiency and high output poer 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 foe 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 to control this motor is expensive.
• Requires complex drives circuitry.
• Need of additional sensors.

APPLICATION OF BLDC MOTOR

Brushless DC Motor (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.

• Computer Hard Drives and DVD/CD players.
• Electric Vehicles, Hybrid Vehicles and Electric Bicycles.
• Industrial Robots, CNC machine tools and simple belt driven systems.
• Washing Machines, Compressors and Dryers.
• Fans, Pumps and Blowers.

CONCLUSION:

1) MATLAB demo model named " Speed Control of a DC motor using BJT H-bridge" is studied and the armature current shoot-up is studied and the "The Four-Quadrant Chopper DC Drive (DC7)" block is also studied and compared it with the H-Bridge model.

2) 2-Quadrant Chopper using Simulink Model is made and the various results obtained is studied.

3) Operation of BLDC motor is studied.