Challenge

Week-7 Challenge: DC Motor Control

VVikas Goswami13 Dec 2021

Q-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

Matlab model for '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 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). 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 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%.

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 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. 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.

In motor,armature current (I) is given by:-

I=(V-E)/R.

Where, V is the voltage supplied, E is the back emf and R is the armature resistance.

Thus, I depends upon E and R when V is kept constant.When the motor starts its speed remains zero i. e E also remains zero(since,back emf is ∝ speed of the armature).

Thus,initial starting armature current :-

I=(V-0)/R= V/R . (as, E =0)

Since,the armature resistance of a motor is very small,generally less than one ohm,therefore the starting current would be very large.

This large current would damage the brushes, commutator, or windings. Also it may damage supply installation.

As the motor speed increases, the back emf increses and the difference (V-E) goes on decreasing. This results in the gradual decrease of armature current until the motor attains its stable rated speed. Under this condition the armature current reaches its desired rated value

The Four-Quadrant Chopper DC Drive (DC7) block

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 the complexity of the drive system

The Four-Quadrant Chopper DC Drive block uses these blocks from the Electric Drives/Fundamental Drive Blocks library:

Speed Controller (DC)

Regulation Switch

Current Controller (DC)

Chopper

H-bridge motor driver

The H-Bridge block output is a controlled voltage that depends on the input signal at the PWM port. If the input signal has a value greater than the Enable threshold voltage parameter value, the H-Bridge block output is on and has a value equal to the value of the Output voltage amplitude parameter.

If it has a value less than the Enable threshold voltage parameter value, the block maintains the load circuit using one of the following three Freewheeling mode options:

Via one semiconductor switch and one freewheeling diode
Via two freewheeling diodes
Via two semiconductor switches and one freewheeling diode

Comparison of Four-Quadrant Chopper DC-7 Drive with the BJT H-bridge model

Q-2) Develop a 2-quadrant chopper & explain the working of the same with the relevant results.

The Two-Quadrant Chopper block represents a two-quadrant controlled chopper for converting a fixed DC input to a variable DC output. The block contains two switching devices.

In this circuit, CH1 and CH2 represents two choppers which can be made ON / OFF. We shall consider two cases for explaining the operation.

Case-1: CH1 and CH2 both are switched ON simultaneously.

When both the choppers 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 i.e. i_o and v_o are positive in this case, hence the operation is in first quadrant.

It should be noted that Diode D1 and D2 are reversed biased in this case and hence they can be treated as an open switch.

Case-2: CH1 and CH2 both are switched OFF simultaneously.

When both the choppers 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 (carefully observe the circuit diagram). It shall be noted that, the direction of load current has not changed. The current is still flowing as shown by the direction of i_o in the diagram but the polarity of v_o has changed. Thus, i_o is positive but v_o is negative and hence operation of chopper is in second quadrant. The power flows from load to source.

Various Waveforms:

The various waveform of class-D or Type-D chopper. Figure below represents various waveforms during the time both the chopper are ON.

During the ON time of CH1 & CH2, output voltage v_o is positive and equal to v_s . When both the choppers are made OFF, the output voltage reverses and hence becomes negative .

It may be noticed that, the average value of output voltage v_o may either be positive or negative. It depends on T_ON and T_OFF time. If T_ON time is more than T_OFF time then positive part of v_o Vs. t graph will be more than the negative part and hence average value will be positive.

If T_OFF is more than the T_ON time, the negative part of v_o Vs. t graph will be more than the positive part and hence average value of output voltage will be negative. This is depicted in the waveform shown below.

Thus, the average output voltage of Class-D chopper may either be positive or negative depending upon whether T_ON is more or T_OFF.

Average output voltage V_o is given as below.

V_o = V_s [(T_ON – T_OFF) / T]

where T = T_ON+T_OFF

From the above formula, following points can be concluded and the same can be checked from quadrant operation of class-D chopper shown earlier in the figure:

In case T_ON>T_OFF i.e. duty cycle α>0.5, V_o is positive.
In case T_ON<T_OFF i.e. duty cycle α<0.5, V_o is negative.
In case of T_ON = T_OFF, α = 0 and hence, V_o = 0.

Now, refer the waveform of load current. It can be seen that; load current is always positive irrespective of CH1 & CH2 are ON or OFF. Thus, the average value of load current I_o will always be positive.

You may also refer waveform of source current is and chopper current i_CH1 or i_CH2. It may be noticed from source current i_s waveform that it is negative when both CH1 and CH2 are OFF. It may also be said that i_s is negative when diode D1 and D2 conducts. Negative source current means, current is flowing into the source

Q-3) Explain in a brief about operation of BLDC motor.

What is a Brushless DC motor (BLDC)?

A brushless DC motor (known as BLDC) is a permanent magnet synchronous electric motor which is driven by direct current (DC) electricity and it accomplishes electronically controlled commutation system (commutation is the process of producing rotational torque in the motor by changing phase currents through it at appropriate times) instead of a mechanically commutation system. BLDC 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 silicon controlled rectifiers 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 positioned around the stator. The rotor position feedback from the sensor helps to determine when to switch the armature current.

This electronic commutation 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.

Construction of BLDC Motor

BLDC motors can be constructed in different physical configurations. Depending on the stator windings, these can be configured as single-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 similarities of three phase induction motor as well as conventional DC motor. This motor has stator and rotor parts as like all other motors.

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, 48 V or less voltage BLDC motors are preferred. For industrial applications and automation systems 100 V or higher rating motors are used.

Rotor

BLDC motor incorporates a permanent 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 a low flux density for a given volume. Rare earth alloy magnets are commonly used for new designs. Some of these alloys are Samarium Cobalt (SmCo), Neodymium (Nd), and Ferrite and 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 sensor 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. Before energizing a particular stator winding, acknowledgment 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 it. The exact commutation sequence to the stator winding can be determined based on the combination of these three sensor’s response.

Working Principle and Operation of BLDC Motor

BLDC motor works on the principle similar to that of a conventional DC motor, i.e., the Lorentz force law which states that whenever a current carrying conductor placed in a magnetic field it experiences a force. As a consequence of reaction force, the magnet will experience an equal and opposite force. In case BLDC motor, the current carrying conductor is stationary while the permanent magnet moves.

When the stator coils are electrically switched by a supply source, it becomes electromagnet and starts producing the uniform field in the air gap. Though the source of supply is DC, switching makes to generate an AC voltage waveform with trapezoidal shape. Due to the force of interaction between electromagnet stator and permanent magnet rotor, the rotor continues to rotate.

Consider the figure below in which motor stator is excited based on different switching states. With the switching of windings as High and Low signals, corresponding winding energized as North and South poles. The permanent magnet rotor with North and South poles align with stator poles causing motor to rotate.

Observe that motor produces torque because of the development of attraction forces (when North-South or South-North alignment) and repulsion forces (when North-North or South-South alignment). By this way motor moves in a clockwise direction.

Here, one might get a question that how we know which stator coil should be energized and when to do. This is because; the motor continuous rotation depends on the switching sequence around the coils. As discussed above that Hall sensors give shaft position feedback to the electronic controller unit.

Based on this signal from sensor, the controller decides particular coils to energize. Hall-effect sensors generate Low and High level signals whenever rotor poles pass near to it. These signals determine the position of the shaft.

Brushless DC Motor Drive

As described above that the electronic controller circuit energizes appropriate motor winding by turning transistor or other solid state switches to rotate the motor continuously. The figure below shows the simple BLDC motor drive circuit which consists of MOSFET bridge (also called as inverter bridge), electronic controller, hall effect sensor and BLDC motor.

Here, Hall-effect sensors are used for position and speed feedback. The electronic controller can be a microcontroller unit or microprocessor or DSP processor or FPGA unit or any other controller. This controller receives these signals, processes them and sends the control signals to the MOSFET driver circuit.

In addition to the switching for a rated speed of the motor, additional electronic circuitry changes the motor speed based on required application. These speed control units are generally implemented with PID controller to have precise control. It is also possible to produce four-quadrant operation from the motor whilst maintaining good efficiency throughout the speed variations using modern drives.

Advantages of BLDC Motor

BLDC motor has several advantages over conventional DC motors and some of these are

It has no mechanical commutator and associated problems
High efficiency 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
Quite operation (or low noise) due to absence of brushes

Disadvantages of Brushless Motor

These motors are costly
Electronic controller required control this motor is expensive
Not much availability of many integrated electronic control solutions, especially for tiny BLDC motors
Requires complex drive circuitry
Need of additional sensors

Applications of Brushless DC Motors (BLDC)

Brushless DC Motors (BLDC) are used 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. Some specific applications of BLDC motors are

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

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