Week-7 Challenge: DC Motor Control

QUESTION 1:

1).

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

Run MATLAB demo ‘Speed control of a DC motor using BJT H-bridge’. Modify the model such that the armature current doesn’t shoot up when the motor changes direction from forward to reverse.

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 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.

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 scope results for various pulse width percentage of 75,50 and 35 are attached below and the current scope consists of IGBT current and DIODE current.

1)

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

1)Pulse Width is 75%

Scope after running the default model to examine the 'shoot up' in armature current when the poles are reversed.

The motor starts in a positive direction with a duty cycle of 75% . At t= 0.5 sec., the armature voltage is suddenly reversed and the motor runs in a negative direction.

'Scope' shows motor speed, armature current, and load torque and 'Currents' shows currents flowing in BJT Q3 and diode D3.

2)Pulse Width is 50%

After reducing the pulse width to 50% the armature current shoot up is reduced and to get even more smoother we have to reduce the pulse width.

Because of high current shoot up the rpm are reduced and load torque also reduced after 0.5 sec

3)Pulse Width is 40%

After reducing the pulse width to 40% we can able to see the armature current is much more smoother than the above plots.

After changing the pulse width the rpm and torque are much better.

so, the armature shoot up is controlled by adjusting the pulse width.

• From 1^st plot we can see that the current plot has initially very high value and then becomes steady and after that shows same path in the -ve direction.
• In these three plots the value of this current shoot up is decreasing and the 3^rd plot has almost smooth current flow.
• To get steadier and smoother current floe we need to decrease and control the duty cycle.
• This shoot up happens because of the switching in the circuit that cause heat losses during switching. To avoid these losses, we use HEAT SINK material to dissipate the heat. From this the motor can change its direction of rotation easily and very smoothly.

1).

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

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.

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.

QUESTION 2:

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

Class C chopper is a two quadrant chopper. It operates only in the first and second quadrant. Here, the output voltage remains positive whereas output current can be positive or negative.

During each half cycle of input voltage, one switch (here Mosfet) and one diode conducts. No two switches (mosfet here) are on at the same time, in order to avoid the short circuit.

Here the pulse generator input can be adjusted with a duty cylce. Say for example, Input voltage for the pulse generator is 10V and duty cycle is 50%, Input DC voltage source is 24V and load back emf is 12V. Inductor value is 1e-3 whereas the diode forward voltage drop is 0.8V. Under these conditions, the output obtained is shown below.

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

INTRODUCTION:

A motor converts supplied electrical energy into mechanical energy. Various types of motors are in common use. Among these, brushless DC motors (BLDC) feature high efficiency and excellent controllability, and are widely used in many applications. The BLDC motor has power-saving advantages relative to other motor types.

Motors differ according to their power type (AC or DC) and their method for generating rotation. Below, we look briefly at the features and uses of each type.

BRUSHLESS DC MOTOR:

A BLDC Motor.

Since the rotor is a permanent magnet, it needs no current, eliminating the need for brushes and commutator. Current to the fixed coils is controlled from the outside.

Types of Brushless DC Motors

Basically, BLDC are of two types, one is outer rotor motor and other is inner rotor motor. The basic difference between the two is only in designing, their working principles are same.

Inner Rotor Design:

In an inner rotor design, the rotor is located in the centre of the motor and the stator winding surround the rotor. As the rotor is located in the core, rotor magnets do not insulate heat inside and heat get dissipated easily. Due to this reason, inner rotor designed motor produces a large amount of torque and validly used.

Outer Rotor Design:

In outer rotor design, the rotor surrounds the winding which is located in the core of the motor. The magnets in the rotor trap the heat of the motor inside and do not allow to dissipate from the motor. Such type of designed motor operates at lower rated current and has low cogging torque.

Applications for BLDC Motors:

We’ve seen that BLDC motors offer high efficiency and controllability, and that they have a long operating life. Because of their efficiency and longevity, they are widely used in devices that run continuously. They have long been used in washing machines, air conditioners, and other consumer electronics and more recently, they are appearing in fans, where their high efficiency has contributed to a significant reduction in power consumption.

They are also being used to drive vacuum machines. In one case, a change in the control program resulted in a large jump in rotational speed an example of the superlative controllability offered by these motors.

BLDC motors are also being used to spin hard disc drives, where their durability keeps the drives operating dependably over the long term, while their power efficiency contributes to energy reduction in an area where this is becoming increasingly important.

The advantages of a BLDC motor:

1. Brushless motors are more efficient as its velocity is determined by the frequency at which current is supplied, not the voltage.
2. As brushes are absent, the mechanical energy loss due to friction is less which enhanced efficiency.
3. BLDC motor can operate at high-speed under any condition.
4. There is no sparking and much less noise during operation.
5. More electromagnets could be used on the stator for more precise control.
6. BLDC motors accelerate and decelerate easily as they are having low rotor inertia.
7. It is high performance motor that provides large torque per cubic inch over a vast sped rang.
8. BLDC motors do not have brushes which make it more reliable, high life expectancies, and maintenance free operation.
9. There is no ionizing sparks from the commutator, and electromagnetic interference is also get reduced.
10. Such motors cooled by conduction and no air flow are required for inside cooling.

The disadvantages of a BLDC motor:

1. BLDC motor cost more than a brushed DC motor.
2. The limited high power could be supplied to BLDC motor, otherwise, too much heat weakens the magnets and the insulation of winding may get damaged.

USAGES:

1.They will probably be widely used to drive service robots small robots that deliver services in fields other than manufacturing.

2. BLDC motors may also be replacing simple brushed dc motors in golf carts and mobility carts. In addition to their better efficiency, BLDC motors can also deliver more precise control which in turn can further extend battery life.

3. BLDC motors are also ideal for drones. Their ability to deliver precision control makes them especially suited for multirotor drones, where the drone’s attitude is controlled by precisely controlling the rotational speed of each rotor.