DC MOTOR CONTROL

1.) SPEED CONTROL OF A DC MOTOR USING H BRIDGE :

An H-bridge is a simple circuit that lets you control a DC motor to go backward or forward.

A DC motor spins either backward or forward, depending on how you connect the plus and the minus.

If you close switch 1 and 4, you have plus connected to the left side of the motor and minus to the other side. And the motor will start spinning in one direction.

If you instead close switch 2 and 3, you have plus connected to the right side and minus to the left side. And the motor spins in the opposite direction.

instead of switches S1,2,3,4 we use the following configuration:

A side-effect of how a motor works are that the motor will also generate electrical energy. When you disable the transistors to stop running the motor, this energy needs to be released in some way.

by adding diodes in the reverse direction for the transistors, you give a path for the current to take to release this energy. Without them, you risk that the voltage rises and damages your transistors.

now opening H bridge in MATLAB by typing power_Hbridge in command window we get the following Simulink model:

Explanation of the model:

it consists of 4 IGBT/Diode. at a time only two transistors are switched on simultaneously:

Q1 and Q4 or Q2 and Q3.

when Q1 and Q4 are fired, a positive voltage is applied to the motor and D2 and D3 diode acts as a freewheeling diode when Q1 and Q4 are switched off. (FORWARD MOTORING)

similar things happen when Q2 and Q3 are fired, a negative voltage is applied to the motor. (REVERSE MOTORING)

the pulse generator is connected to give an output to the forms of motor i.e. which IGBT will be ON i.e. communication channel.

this whole circuit is connected to a 240 V DC supply and the motor is powered by 300V supply.

the output of motor bus includes speed and armature current while load torque is calculated accordingly. (T_L is directly proportional to the square of the speed)

on executing the model we get :

the red part tells about the peak in the current when the mode is shifted from forward to reverse evident from the omega graph.

the parameters of the pulse generator :

which tells the dc=75% i.e. average voltage of 180V and frequency=500Hz.

by changing pulse ratio or Time period and dc we will try to reduce this current by trial and error method.

we get the following value:

by reducing the amount of voltage entering we can control the armature current value.

2.)

The power circuit diagram for a four-quadrant chopper is shown in the figure. It consists of four semiconductor switches CH1 to CH4 and four diodes D1 to D4 in antiparallel. Working of this chopper in the four quadrants is explained as under:

First quadrant: For the first-quadrant operation, CH4 is kept pon, Ch3 Is kept off and CH1 is operated. With CH1, CH4 on, load voltage v_o = V_s (source voltage) and load current i_o begins to flow. Here both v_o and i_o are positive giving first quadrant operation. When CH1 is turned off positive current freewheels through CH4, D2. In this manner, both V_o and I_o can be controlled in the first quadrant.

Second quadrant: Here CH2 is operated and CH1, CH3, and CH4 are kept off. With CH2 on, reverse (or negative) current flows through L, CH2, D4 and E. Inductance L stores energy during the time CH2 is on. When CH2 is turned off, the current is fed back to source through diodes D1, D4. Note that here (E + L di/dt) is more than the source voltage V_s. As load voltage V_o is positive and I_o is negative, it is the second quadrant operation of the chopper. Also. Power is fed back from load to source.

Third quadrant: For third-quadrant operation, CH1 is kept off, CH2 is kept on and CH3 is operated. The polarity of load emf E must be reversed for this quadrant working. With CH3 on, the load gets connected to source V_s so that both v_o, i_o are negative leading to the third quadrant operation. When CH3 is turned off, negative current freewheels through CH2, D4. In this manner, v_o and i_o can be controlled in the third quadrant.

Fourth quadrant: Here CH4 is operated and other devices are kept off. Load emf E must have its polarity reversed to that shown in the figure for operation in the fourth quadrant. With CH4 on, positive current flows through CH4, D2, L and E. Inductance L stores energy during the time CH4 is on. When CH4 is turned off, current is fed back to source through diodes D2, D3. Here load voltage is negative, but load current is positive leading to the chopper operation in the fourth quadrant. Also, power is fed back from load to source.

in the same way MATLAB model 4 quadrant chopper works.

SIMULINK MODEL:

now opening 4 quadrant chopper in MATLAB by typing dc7_example in command window we get the following Simulink model:

in this AC voltage is given which is then converted to DC by using an AC-to-DC converter. SP takes the value of speed in rpm for which the motor tries to follow the drive or load speed.

The 200 HP DC motor is separately excited with a constant 150V DC field voltage source. The armature voltage is provided by an IGBT converter controlled by two PI regulators as shown.

The first regulator is a speed regulator, followed by a current regulator. The speed regulator outputs the armature current reference used by the current controller in order to obtain the electromagnetic torque needed to reach the desired speed.

COMPARISON BETWEEN H BRIDGE AND 4 QUADRANT CHOPPER

To control the motor:

in H bridge we need to change the polarity to make the motor move forward or backward.

in 4 quadrant chopper, we need to interrupt signals to another

Regenerative braking:

in H bridge it is not possible.

in 4 quadrants regenerative braking is possible.

simulation:

in H bridge simulation is continuous while in a 4 quadrant chopper it is discrete. hence the H bridge takes more time.

overall a 4 quadrant or type E chopper has more efficiency than H bridge.

3.) to change the DC7 to incorporate the characteristics of an EV

the following changes were made :

1.)the AC voltage source and AC to DC converter was replaced by a DC source of 400V.

2.)the SP input is now the RPM desired to cover a drive cycle.

https://docs.google.com/spreadsheets/d/1tm-dSz0pndSZo1l8YE_xL822hI-PPXQU7WpNGwn9W8o/edit?usp=sharing

3.)the motor characteristics were made by using basic operations.

T=24.7+0.0051*w_m

subsystem

after doing all this sampling time was fixed and then executed which gave the following results:

it is evident that motor speed tries to match the target speed.