Week-7 Challenge: DC Motor Control

Q1.A. Explain a MATLAB demo model named ‘Speed control of a DC motor using BJT H-bridge’.

ans-

1) To control speed of motor H briged dc chopper is designed.

2)Four IGBT switches are used for inverting the DC voltage 0 to 240 v supply.The current flows from collector(C) to emitter(E). IGBT act as BJT in power switching device.

3) The pulses are controlled by gate according to armature voltage of motor given by generater.

4)This image shoes the voltage value given from initial to final during the step time.According to the pulses on and off the voltage supplied to the IGBT's.

5)Here four switches are used Q1,Q2,Q3,Q4. Pulses P1,P2,P3,P4 are conected with corresponding switches.

6)In first state, Q1 and Q4 are closed by giving 1V to IGBT'S,current from battery passes through Q1 and Q4.Diode D2,D3 are used as free wheeling diode in this state.In this case Forward motoring and forward breaking are held.

7)In Second state,Q2 and Q3 are closed by giving 1V to IGBT's,current from battery passes through Q2 and Q3.Diode D1,D4 are used as free wheeling diode in this stateIn this case reverse motoring and reverse breaking are held.The current direction change its direction in this state.

8)here freewheeling diode is for safety puepose.

9)In motor operation, armature terminals are connected with a 240V along the inverter circuit and field windings are connected with 300 V DC supply to act as a electro magnet for the motor.

10) magnetic fields generated, variable current carring conductors in rotor armature produces forces according to fleming left hand rule principle. Thus the motor starts to rotate.

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11) From this graph speed of motor increase 0 to 900 rpm upto 0.5 sec. then speed reduces 0 rpm(forward dynamic breaking action take place.) Then speed increases 0 to 900 rpm at upto 1 sec.( reverse motoring action takes place)

12) torque increases 0 to 0.5 forward motoring ction takes place. then torque reduces 0 Nm in forward dyanmic breaking mode.Then torque increases in reverse direction in reverse motoring action upto 1 sec.Again torque reduce to zero.

Q1 B)-Comment on the armature current shoot-up from the scope results.

Here initial current shoot up due to sudden increase in voltage which inurns regulated by duty cycle.

Sudden shoot up canceled in starting condition but slight shootup after 0.5 sec in reverse motoring condition.

It conclude that the reduce shootup armature current we have to change duty cycle gradually from 0 to 100% that inturns voltage increase gradually speed aslo change gradually.

Q1 C- Refer to the help section of ‘The Four-Quadrant Chopper DC Drive (DC7) block’. Compare it with the H-bridge model.-

ans-

INPUTS-Armature voltage ,speed referance ,motor torque referance

OUTPUT-Motor,convertor,controller,motor angular speed

1) Here pulses are given from PI current controller, that controles the pulses by comparing armature current measurement and current referance.

2)Mainly speed controlled by current controller.

3) In this 4th quadrant operation performed

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)the 4 quadrant chopper have high switching frequency

  while h bridge model have low switching frequency

3) 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

4) In h bridge model, the direction of rotation of motor is changed by changing the polarity of current.

5)The armature current in 4 quadrant chopper have a little spike at transition from one to other switch because of high swithing frequency .

6)H bridge model have high spike of current.

7) The 4 quadrant chopper is generally used in industrial application while h bridge model is used in robotics.

Q3)Explain in a brief about operation of BLDC motor.

ans-

1)Brushless DC electric motors also known as electronically commutated motors .A brushless DC motor

2)Electricity and it accomplishes electronically controlled commutation system instead of a mechanically commutation system. BLDC motors are also referred as trapezoidal permanent magnet motors.

3)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.

4)Hall sensors or rotary encoders are most commonly used to sense the position of the rotor and are positioned around the stator.

5)The electronic commutation arrangement eliminates the commutator arrangement and brushes in a DC motor and hence more reliable and less noisy operation is achieved.

6)The efficiency of BLDC motors is typically 85 to 90 percent, whereas as brushed type DC motors are 75 to 80 percent efficient.

7) There are wide varieties of BLDC motors available ranging from small power range to fractional horsepower, integral horsepower and large power ranges.

8)In this motor, the current-carrying conductors or armature winding are located on the stator. They use electrical commutation to convert electrical energy into mechanical energy.

9)The main design difference between a brushed and brushless motors is the replacement of mechanical commutator with an electric switch circuit.

10)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

11)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.

12)The underlying principles for the working of a BLDC motor are the same as for a brushed DC motor.

13)Most of the motors are having brushes in their rotor as it transfers current into the rotor circuit.But here differently rotor is placed with permanent magnet.

14)The stator coil is wounded with coils,they are stationary but producing variable DC voltages for rotating the permanent magnet as changing the magnetic field of stator poles continuously.

15)A BLDC motor with three coils on the stator will have six electrical wires (two to each coil) extending from these coils. In most implementations three of these wires will be connected internally, with the three.

ADVANTAGES

• Higher speed range and lower electric noise generation.
• It has no mechanical commutator and associated problems
• Long life as no inspection and maintenance is required for commutator system
• Less electromagnetic interference
• Low noise due to absence of brushes
• Less overall maintenance due to absence of brushes

DISADVANTAGES

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

APPLICATIONS:

• 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.

Q2Develop a 2-quadrant chopper using simulink & explain the working of the same with the relevant results.

ans-

1)Class - C chopper is a Two quadrant Chopper. Its functions in the 1st and 2nd quadrants.

following components used:

1. Power GUI
2. Voltage measurement Block
3. Current measurement block
4. DC voltage source
5. MOSFET
6. Pulse generator
7. Diode
8. Series RLC band
9. Logical Operator

The speed control loop output thereference armature current of the machine

The main advantages of this drive, compared with other DC drives, are its implementation simplicity and the that it can operate in two quadrants.

.In addition due to the use of a high switching frequency DC-DC converter, a lower armature current ripple is obtained.

However, for all two-quadrant DC drives reversible and regenerative operations, which are required in most DC drives cannot be obtained.