Week-7 Challenge: DC Motor Control

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

As shown in the figure, the simulation is done for a dc motor control where the load is a fan load.

DC MOTOR:

An electrical motor is an electro-mechanical device that converts electrical energy to mechanical energy. The dc motor shown here is a separately excited brushless DC motor. The field of the motor is energised by a separate source of 300v DC to create a constant magnetic field inside the motor. Similarly, the armature is energised from an AC source via a H-bridge circuit. As it is brushless motor it has electronic commutation system instead of a mechanical commutator.

H-Bridge:

The circuit is a full bridge circuit that converts dc to ac. The circuit is fed with a 240v dc supply. When The Q1 IGBT is on, full 240v supply is being applied to the motor. In this case the current flows from 240v supply through the IGBT to the motor to the lower IGBT (Q4) to the negative terminal of the 240v supply. To turn ON the switches, a gate signal is provided. The ON and OFF time of the switches decide the effective average voltage being applied to the motor. All the IGBTs are being provided with parallel diodes to protect the IGBTs in case of reverse current flow condition.

LOAD:

A fan is connected to the motor as a load. The governing equation of the fan is:

Where, T= torque in Nm, ꞷ= speed of the motor in rad/sec and K is a constant.

The load torque generated from the fan is given as a torque (T_m) input to the motor.

Gate Signal:

The gate signals (P1,P2,P3,P4) for all the four IGBTs are generated from the pulse generator circuit. The duty cycle of the pulses is being set to 75%. The sequence of tuning ON Q1, Q2, Q3, Q4 depends on the signal from the mux.

The pulse generator has a frequency of 500Hz (time period= 2*10^-3 s).

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

Initially when the motor starts running there is no back emf or very less back emf generated in the motor which leads to the increase in the armature current. Gradually as the motor starts rotating, a significant back emf is generated which controls the armature current. The same can be verified using the following equation.

=>

Initially , so .

Gradually, the total applied voltage decreases due to increase in back emf which results in decrease in armature current. Hence, initially there is a shoot up in current which dies down over the time.

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

Four Quadrant Chopper DC Drive H-Bridge Model

It converts DC to DC. It converts DC to AC.

It has four quadrant operation. It has two quadrant operation.

It is a closed loop circuit. It is open loop.

It requires a H-bridge model after this to feed to the motor. It may require a chopper before this to reach the voltage level.

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

Introduction to Chopper:

A chopper is a static device that converts fixed DC to variable DC. This variable DC is used in various applications like trolley buses, electric operated vehicles, battery charging, etc. chopper systems provide smooth control, high efficiency, fast response and regeneration.

The semiconductors used for designing a chopper cab be IGBT, Power MOSFET or even thyristors. These devices are used as a switch to connect and disconnect load from the supply. This process of On and Off helps us to regulate the average voltage output. Depending on the average voltage output of the chopper it is divided into two types: step up and step down.

In a step up chopper the effective output voltage is higher than that of the supply voltage. Whereas in case of a step down chopper the output voltage is less than the input.

TWO QUADRANT CHOPPER OPERATION:

In this case two choppers are connected in parallel along with the freewheeling diodes. When switch CH1 is on load current flows from source to load and during this time the inductor gets charged. When CH1 is off, the inductor discharges through FD. In both switch on and off condition the load voltage remains positive but the load current is positive when CH1/FD is on and it is negative when CH2/D2 is on. Hence this model is known as two quadrant operation.

When CH1 is on both voltage and current are positive and is known as motoring mode. In this case the power is positive. When CH2 is on voltage is positive but the load current is opposite to that of the previous case and the power becomes negative. This mode is known as regenerative mode. In the regenerative case, the load supplies voltage to the source hence helps in charging the battery in case of an electric vehicle. This situation arises when the vehicles go for downhill movement or a brake is being applied.

Fig 1. Chopper block diagram

Chopper waveform:

Chopper Simulink Model:

The model has 2 MOSFETs and two diodes. The load contains an inductor and a voltage source of 24v. The supply has a voltage source of 48v. The MOSFETs are given gate signals through a pulse generator. Upper MOSFET is directly being fed from the pulse generator whereas the lower MOSFET is given through a NOT gate. The arrangement is being done so as not to turn ON both the MOSFETs simultaneously which will lead to short circuit of the supply.

fig2. chopper output waveform

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

BLDC motor stands for Brushless Direct Current Motor. Motor is an electro mechanical device that converts electrical energy to mechanical energy. To do this conversion, the must have two parts: the stator (static in nature) and the rotor (rotating one which is connected to the load). Depending on nature of magnetic field these two part can either be armature or field.

In case of a DC motor the field is excited by a DC supply. This results in a constant magnetic field. Now-a-days the DC supply is removed and the field is created by permanent magnets. And for the armature the supply is AC in nature. BLDC motor is a special case of DC motor in which the field is created by permanent magnets and armature is supplied from an AC supply. Also, unlike DC motor the machine does not contain any brushes and commutator.

In a DC motor the work of the commutator is to convert AC-DC (act as mechanical Inverter). But the presence of brushes creates spark and unnecessary wear and tear of the commutator. To get rid of these issues, BLDC is directly being fed from an AC supply. Here instead of a mechanical commutator, an electronic inverter is being used to convert DC to AC.

WORKING PRINCIPLE:

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.

Advantages of BLDC:

• Noise less operation.
• High efficiency due to the use of permanent magnets.
• Smaller in geometry.
• High speed operation.
• Long self-life.
• Less electromagnetic interference.

Disadvantages of BLDC:

• Comparatively costly.
• Expensive electronic controller is required.
• Needs additional sensors.
• Complex control algorithm.