Simulation

The motor starts in the positive direction with a duty cycle of 75% (mean DC voltage of 180V). 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 block has two values as a display of the output. One is the current output for the IGBTand the diode current and the other is the scope for the values of the DC motor. The values of the DC motor include-

1. Speed (in RPM)

2. Armature Current (in AMPS)

3. Load Torque (in N-m)

The output values are as follows:-

1. IGBT Current

2. Diode Current

The values of the SCOPE are as follows-

Reverse Motoring:-

When S2 and S3 are fired a negative voltage is applied to the motor which rotates in antilock-wise rotation the torque produced & Armature current are negative value and diodes D1 and D4 operates as free wheeling diodes when S2 and S3 are switched off.

When S1 and S4 are fired a positive voltage is applied to the motor which rotates in clock wise rotation the torque produced. Armature current are positive value thus power is positive and diodes D2 and D3 operates as free wheeling diodes when S1 and S4 are switched off.

ANSWER:- 2

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

The above model consist of 3-phase source AC-DC converter includes rectifier and braking chopper, 4-Quadrant chopper DC motor drive and Quadrant selector.

A chopper is a static device that converts fixed DC into a variable DC output voltage directly. Essentialy a chopper is a electronic switch that is used to iterrupt one signal under the control of another. Due to this current and voltage spike occurs. To avoide this snubber RC circuit used. The chopper is widely used in motor control and in regenerative braking. A snibber is an electrical device that prevents voltage spike due to sudden change in current. These voltage spikes or transient can damage the circuit and cause arcing and sparks. One type of electrical snubber is the RC snubber which is composed of a resisterin parallel with a capacitor. Transient are usually caused by switches in the circuit.

4-Quadrant Chopper Circuit:-

Four Quadrant Operation of DC Motor

Four Quadrant Operation of any drives or DC Motor means that the machine operates in four quadrants. They are Forward Braking, Forward motoring, Reverse motoring and Reverse braking. A motor operates in two modes – Motoring and Braking. A motor drive capable of operating in both directions of rotation and of producing both motoring and regeneration is called a Four Quadrant variable speed drive.

In motoring mode, the machine works as a motor and converts the electrical energy into mechanical energy, supporting its motion. In braking mode, the machine works as a generator, and converts mechanical energy into electrical energy and as a result, it opposes the motion. The Motor can work in both, forward and reverse directions, i.e., in motoring and braking operations.

The product of angular speed and torque is equal to the power developed by a motor. For multi-quadrant operation of drives the following conventions about the signs of torque and speed are used. When the Motor is rotated in the forward direction the speed of the motor is considered positive. The drives which operate only in one direction, forward speed will be their normal speed.

In loads involving up and down motions, the speed of the motor which causes upward motion is considered to be in forward motion. For reversible drives, forward speed is chosen arbitrarily. The rotation in the opposite direction gives reverse speed which is denoted by a negative sign.

The rate of change of speed positively in the forward direction or the torque which provides acceleration is known as Positive motor torque. In the case of retardation, the motor torque is considered negative. Load torque is opposite to the positive motor torque in the direction.

The figure below shows the four quadrant operation of drives.

In the I quadrant power developed is positive and the machine is working as a motor supplying mechanical energy. The I (first) quadrant operation is called Forward Motoring. II (second) quadrant operation is known as Braking. In this quadrant the direction of rotation is positive, and the torque is negative, and thus, the machine operates as a generator developing a negative torque, which opposes the motion.

The kinetic energy of the rotating parts is available as electrical energy which may be supplied back to the mains. In dynamic braking dissipated the energy is dissipated in the resistance. The III (third) quadrant operation is known as the reverse motoring. The motor works, in the reverse direction. Both the speed and the torque have negative values while the power is positive.

In the IV (fourth) quadrant, the torque is positive, and the speed is negative. This quadrant corresponds to braking in the reverse motoring mode.

Applications of Four Quadrant Operation

• Compressor, pump and fan type load requires operation in the I quadrant only. As their operation is unidirectional, they are called one quadrant drive systems.
• Transportation drives require operation in both directions.
• If regeneration is necessary, application in all four quadrants may be required. If not, then the operation is restricted to quadrants I and III, and thus dynamic braking or mechanical braking may be required.
• In hoist drives, a four-quadrant operation is needed.

The four quadrant operation and its relationship to speed, torque and power output are summarized below in the table.

Comparison 4-Quadrant DC7 Chopper Block Between DC Motor BJT H Bridge Block-

4-Quadrant DC7 Chopper Block-

1. Regenerative braking is possible.

2.The current values are chopped not continuous hence simulation takes less time.

3. It is widelt used in Automobile to control DC motor speed and direction.

4. To change to direction pf motor current signals are interrupted with one another.

DC Motor BJT H Bridge Block-

1. Regenerative braking is not possible but dynamic braking can be achieved .

2. The current values are continuous.

3. It is widely used in Robotics and Electro-Mechanical device to control DC motor speed and direction.

4. To change the direction of motor only the polarityof the current is changed.

Overall in AUTOMOBILE sector 4-Quadrant Chopper is prefered due to its high efficiency and ability to do regenerative braking which charges the battery whenever brakes are applied.

ANSWER:- 3

Make a suitable EV model using DC7 block and make result report.

The EV model created from the DC7 Block is as follows-

In this model the speed converter block is replaced by a drive block. The drive cycle used is FTP75drive cycle. For the simulation we also change the sampling time. There were many comparison that were made with the original block of the DC7 Motor.

The regular DC7 motor drive with a speed converter has the following scope values-

The Scope values of the EV drive with the DC7 motor block is as follows-

As we can see clearly that the values from the drive have changed once the drive cycle was added. The drive cycle has stabalized a bit more since we simulated on the drive cycle. The duty cycle has also changed and reduced to about 12% the drive.

When we observe the armature voltage fluctuations the voltage fluctuation has decreased on the drive cycle. The maximum armature voltage in the EV vehicle is 380V at a phase frequency of 50HZ.

When we look at the armature current it is almost constant at 10 Amps but at certain cases where it shoots up. This conditions is due to the fact that the motor runs in reverse motoring chracterstics and thus this behaviour in the characterstics is observed.

The motor speed characterstics are also plotted in the scope for the EV. These characterstics show that the motor speed is almost constant amongst the starting and then when the reverse motor plot is reached the motor speed increases drastically as well.