Week-7 Challenge: DC Motor Control

1.Aim:

To design a matlab model ‘Speed control of a DC motor using BJT H-bridge’.

Matlab Model:

Model Explanation:

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.

When the pulse width is 75, the following output is obtained.

Results:

For a duty cycle of 50%, the following output is obtained.

Result:

b) Comment on the armature current shoot-up from the scope results:

The Pulse generator is given with the duty cycle value of 30 for analysing Armature current shoot-up.

It has been observed from the above output that reduce in duty cycle, leads to decrease in the Armature current.

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

Description:

The 200 HP DC motor is separately excited with a constant 150 V DC field voltage source. The armature voltage is provided by an IGBT converter controlled by two PI regulators. The converter is fed by a 515 V DC bus obtained by rectification of a 380 V AC 50 Hz voltage source. In order to limit the DC bus voltage during dynamic braking mode, a braking chopper has been added between the diode rectifier and the DC7 block.

The first regulator is a speed regulator, followed by a current regulator. The speed regulator outputs the armature current reference (in p.u.) used by the current controller in order to obtain the electromagnetic torque needed to reach the desired speed. The speed reference change rate follows acceleration and deceleration ramps in order to avoid sudden reference changes that could cause armature over-current and destabilize the system. The current regulator controls the armature current by computing the appropriate duty ratios of the 5 kHz pulses of the four IGBT devices (Pulse Width Modulation). For proper system behaviour, the instantaneous pulse values of IGBT devices 1 and 4 are opposite to those of IGBT devices 2 and 3. This generates the average armature voltage needed to obtain the desired armature current. In order to limit the amplitude of the current oscillations, a smoothing inductance is placed in series with the armature circuit.

Matlab Model:

Simulation:

Before starting the simulation, set the initial bus voltage to 515 V via the GUI block ('Initial States Setting' button and 'Cbus' variable).

Start the simulation. You can observe the motor armature voltage and current, the four IGBT pulses and the motor speed on the scope. The current and speed references are also shown.

The motor is coupled to a linear load, which means that the mechanical torque of the load is proportional to the speed.

The speed reference is set at 500 rpm at t = 0 s. Observe that the motor speed follows the reference ramp accurately (+400 rpm/s) and reaches steady state around t = 1.3 s.

The armature current follows the current reference very well, with fast response time and small ripples. Notice that the current ripple frequency is 5 kHz.

At t = 2 s, speed reference drops to -1184 rpm. The current reference decreases to reduce the electromagnetic torque and causes the motor to decelerate with the help of the load torque.

At t = 2.2 s, the current reverses in order to produce a braking electromagnetic torque (dynamic braking mode). This causes the DC bus voltage to increase.

At t = 3.25 s, the motor reaches 0 rpm and the load torque reverses and becomes negative. The negative current now produces an accelerating electromagnetic torque to allow the motor to follow the negative speed ramp (-400 rpm/s). At t = 6.3 s, the speed reaches -1184 rpm and stabilizes around its reference.

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

FOUR-QUADRANT	H-BRIDGE
Regenerative braking is possible	Regenerative braking is not possible but dynamic braking can be achieved
To change direction of motor, current signals are interrupted with one another	To change direction of motor, only the polarity of the current is changed
The current values are chopped and hence are not continuous	The current values are continuous
Uses PI controller for both speed and current control	It does not have a PI controller
Can operate in all four modes	Can operate only in two modes
Operates at high power levels	Operates at moderate power levels

2.

Chopper:

A chopper is a static device that converts fixed DC input voltage to a variable DC output voltage. It is basically a high speed ON/OFF semiconductor switch. It may be thought of as DC equivalent of an AC transformer since they behave in an identical manner.

Chopper is fed through a constant DC voltage source and its output is variable DC voltage. The average value of output DC voltage may be less than or higher than the input DC voltage source. A simple diagram defining the chopper is shown below.

A chopper is a DC equivalent to an AC transformer having continuously variable turn ratio. Like a transformer, a it can be used to step-up or step-down the fixed DC input voltage. On this basis, there are two types of chopper: Step-up and Step-down Chopper. A chopper whose average value of DC output voltage is more than the fixed DC input voltage is called Step-up Chopper. While, a chopper whose average value of DC output voltage is less than the DC input voltage is called Step-down chopper.

Working / Operation of Class-D Chopper:

To understand the operation / working principle of Type-D chopper, let us first have a look at its circuit diagram shown below.

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 fourth quadrant. The power flows from load to source.

The first and fourth quadrant operation of class-D / Type-D chopper is shown by the hatched area in the figure below.

Quadrant operation of Class-D chopper

Various Waveforms:

Now, we will discuss 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 vo is positive and equal to vs (Case-1). When both the choppers are made OFF, the output voltage reverses and hence becomes negative (Case-2).

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

If TOFF is more than the TON time, the negative part of v_o V_s. 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 TON is more or TOFF.

Average output voltage Vo is given as below.

V_o = V_s [(TON – TOFF) / T]

where T = TON+TOFF

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 TON>TOFF i.e. duty cycle α>0.5, V_o is positive.

In case TONo is negative.

In case of TON = TOFF, 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 Io will always be positive.

You may also refer waveform of source current is and chopper current iCH1 or iCH2. It may be noticed from source current is waveform that it is negative when both CH1 and CH2 are OFF. It may also be said that is is negative when diode D1 and D2 conducts. Negative source current means, current is flowing into the source as discussed in Case-2.

2-quadrant chopper Simulink Model:

The 200 HP DC motor is separately excited with a constant 150 V DC field voltage source. The armature voltage is provided by an IGBT buck-boost converter controlled by two PI regulators. The buck-boost converter is fed by a 630 V DC bus obtained by rectification of a 460 V AC 60 Hz voltage source. In order to limit the DC bus voltage during dynamic braking mode, a braking chopper has been added between the diode rectifier and the DC6 block.The first regulator is a speed regulator, followed by a current regulator. The speed regulator outputs the armature current reference (in p.u.) used by the current controller in order to obtain the electromagnetic torque needed to reach the desired speed. The speed reference change rate follows acceleration and deceleration ramps in order to avoid sudden reference changes that could cause armature over-current and destabilize the system. The current regulator controls the armature current by computing the appropriate duty ratios of the 5 kHz pulses of the two IGBT devices (Pulse Width Modulation).For proper system behaviour, the two IGBT devices have opposite instantaneous pulse values. This generates the average armature voltage needed to obtain the desired armature current. In order to limit the amplitude of the current oscillations, a smoothing inductance is placed in series with the armature circuit.

• Before starting the simulation, set the initial bus voltage to 630 V via the GUI block ('Initial States Setting' button and 'Cbus' variable).
• Start the simulation. You can observe the motor armature voltage and current, the two IGBT pulses and the motor speed on the scope. The current and speed references are also shown.
• The speed reference is set at 400 rpm at t = 0 s. Initial load torque is 814 N.m.
• Observe that the motor speed follows the reference ramp accurately (+250 rpm/s) and reaches steady state around t = 2 s. The armature current follows the current reference very well, with fast response time and small ripples. Notice that the current ripple frequency is 5 kHz.
• At t = 2.1 s, the load torque passes from 814 N.m to 100 N.m. The motor speed recovers fast and is back at 400 rpm at t = 2.75 s. The current reference lowers to about 40 A to generate a smaller electromagnetic torque, the load torque being reduced. As observed before, the armature current follows its reference perfectly.
• At t = 2.75 s, the speed reference jumps down to 100 rpm. In order for the motor to decelerate following the negative speed ramp, the armature current reverses down to -160 A to generate a braking electromagnetic torque (dynamic braking mode). This causes the DC bus voltage to increase. The braking chopper limits the voltage value.
• At t = 3.4 s, the motor speed reaches 100 rpm and the current reverses back to 40 A.
• At t = 4 s, the speed stabilizes around its reference.

3.Brushless DC motor:

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

Primary efficiency is a most important 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.

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.

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.

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

Limitations of Brushless DC motor

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

Applications 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.