Week-7 Challenge: DC Motor Control

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

ans.An H-bridge is a rather 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 be anything we want , by far the most pervasive application if H bridge 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 '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 (I/O).

Model Description

• dc motor used is model is preset model(5 hp 24v 1750 rpm). It simulates a fan type of 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 IGBT models)
• Two transisters 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 freewheeling 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.

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

The motor starts in the positive direction with duty cycle 75%(means dc voltage of 180 V). 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.

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 and 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.

B. Comment on the armature current shoot-up from the scope results.

ans.

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

As there is already current flowing in the circuit and 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 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.

Now the armature current plot is

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

C.Refer to the help section of ‘The Four-Quadrant Chopper DC Drive

(DC7) block’. Compare it with the H-bridge model.

ans.

Chopper drive

A chopper drive based speed control is widely preferred sa far as speed control is concerned. Choppers are mostly applied in electric cars, conversion of wind and solar energy, and DC motor regulators. A chopper uses the high speed to connect and disconnect from a source load. A fixed DC voltage is applied intermittently to the source load by continuously triggering the power switch ON/OFF. The period for which the power switch stays ON or OFF is referred to as the chopper's ON and OFF state times , respectively.

A dc-dc converter , also known as dc chopper, is a static device which is used to obtain a variable dc voltage from a constant dc voltage source. Choppers are widely used in trolley cars, battery-operated vehicles , traction motor control, control of large numbers of dc motors, etc. They are also used as dc voltage regulators.Choppers are of two types : (1) Step down choppers, and (2)Step-up choppers. In step-down choppers , the output voltage will be less than the input voltage, whereas in step up choppers output voltage will be more than the input voltage.

The 4 quadrant chopper is a chopper which can operate in all 4 quadrants. The power can flow either from source to load or load to source in this chopper. In the 1st quadrant , a class E chopper acts as a step down chopper whereas in the 2nd quadrant it behaves as a stepup chopper. This type of chopper is also known as a class E or type E chopper. The working principle and operation of a class E chopper is described below with the help of circuit diagram

1st 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 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 freewheels through CH4 and D2.During this period , the load voltage and current remain positive. Thus both the output voltage Vs and load current io are positive and hence, the operation of the chopper is in the 1st quadrant. It may be noted that class E chopper operates as astep down chopper in this case.

2nd Quadrant Operation

To obtain second quadrant operation , CH2 is operated while keeping the CH1, CH3 and 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, the 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 the second quadrant operation of the chopper. Since the current is fed back to the source , this simply means that load is transferring power to the source.

3rd Quadrant Operation

To obtain the 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 is shown in the circuit diagram, it is considered negative. One important thing to notice is that the polarity of emf E in the load must be reversed to have 3rd quadrant operation.

4th Quadrant Operation

To obtain 4th 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.:

The 4 quadrant chopper drive is shown below:

figure1

figure2

comparison of H-bridge model and Four quadrant chopper DC Drive(DC7)

Four Quadrant chopper DC Drive (DC7)

• In this type of drive there are all possible ways of switching like forward motoring, reverse motoring , forward motoring, forward regenerative braking, reverse braking switch.This cannot be operated by the signal generator.
• The chopper switchings were mostly used in the industrial applications and large and high power drawing applications.
• Maintainance for the chopper drives will be more as these are controlled by many switches individually.
• These will be used in the small machines uch as robotic applications.
• Efficiency is more as change in frequency is more here.
• Managing the change in signals is not as easy as in H-bridge.

H bridge model

• This can only be operated in forward motoring and reverse motoring
• This can be operated with the signal generated
• These are used in recent EV applications
• The maintainance for the H-bridge circuit is less than the chopper circuit as this is run by a signal modulation.
• Efficiency of H-Bridge is less than the chopper circuit.
• Ease of operation is better than the four_quadrant chopper DC drive.

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 )

ans.

circuit diagram of 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 Vo as the freewheeling diode FD is present across the load. When the chopper is on the freewheeling diode starts conducting and the output voltage Vo will be equal to Vs. The direction of the load current io will be reversed. The current io 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 is 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 2nd quadrant the chopper and D2 will operate together as type B chopper.

The average voltage will always positive bbut the average might be positive or negative. The power flow may be like the 1st quadrant ie from source to load or from load to source like the 2nd 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 and diodes blocks and connect it as shown in the circuit diagram.

the dc voltage source is connected as shown.

let the 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 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 width be 50 %

scope result

I have attached the slx file twoquadchopper below.

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. Explain in a brief about operation of BLDC motor.

ans. Brushless DC electric motors also known as electronically commutated motors (ECMs, EC motors).

Primary efficiency is a most importent feature for BLDC motors. Because the rotor is the sole bearer of the magnets and it doesn't require any power. i.e. no connections, no commutator and no brushes. In place of these, the motor employs control circuitry. To detect where the rotor is at certain times, BLDC motors employ along with controllers, rotary encoders or a Hall sensor.

Construction of Brushless DC motor

In this motor, the permanent magnets attach to the rotor. The current-carrying conductors or armature windings 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.

figure 3

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.

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.

• In runner – 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.

Working principle of brushless DC motor

BLDC motor works on the principle similar to that of a Brushed DC motor. 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 the BLDC motor, the current carrying conductor is stationary and the permanent magnet is moving.

figure 4

When the stator coils get a supply from 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.

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 which causes the motor to rotate.

Advantages of Brushless DC 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

Limitation of Brushless DC motor

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

Application of Brushless DC 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.

• 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.