Week-7 Challenge: DC Motor Control

In motors, the winding inductance is fairly low -- not enough to limit the current safely by itself. Only once the rotor is spinning and generating back-EMF is the current limited.The inductor equation V = L(di/dt) shows that inductors will resist instantaneous changes in current. so adding inductor in series reduced the sudden change in current caused smooth startup and smooth reversal due to slow charge of inductor by pwm.

Inductor in series to converter

in this condition dc power is following through inductor and there the voltage is constant, such that it wont allow any inrush current and no smooth startup.

Alternative solution:

Is to use soft pwm technique in which heavy coding required

Is to use inrush current limiters

Is to use snubber across gating terminals of igbt which will give rise to soft turn on and limits inrush.

Note: change in pwm or external modifications may limit inrush but it will effect the braking performance, if requirement is sudden reversal then the system should be able to supply inrush to motor inorder to achieve performance.

DC7 model : The Four quadrant chopper bridge control of motor

Direct current (d.c) motors umpteen times are used in most adjustable speed drives and position controlled applications. They usually draw a very high armature current during starting which is usually higher than its rated value due to negligible back emf. This current value is often lethal to the commutator which conventionally should not have a current value greater than twice the rated value. To curb this hazardous effect of excess armature current, the d.c motor with separately excited d.c machine operation is regulated through a d.c armature voltage supply using a four quadrant duty cycle controlled chopper with variable d.c voltage output.

The main difference between h bridge and four quadrant operation is feedback based PWM control

four quadrant operation has regenerative properities and amperage control to achive soft latching over set speed.

while H bridge doesnt have amperage control over desired speed but both have chopper phenomena.

Four quadrant operation is efficient and it is costly due to increased complexity over pwm and inductor current control.

EV model using DC7

Here the DC7 model is modified similar to EV vehicle actions

First the speed reference is replaced by drive cycle in which the control is given in rpm with respect to time.

as the motor control reference input is in rpm

And we replaced the torque characterstics with vehicle torque equation Trr=24.7+0.0051w2 in a subsytem by linear torque block

In this way DC7 model is behaved little similar to EV by control over drive cycle with repect to vehicle torque characterstics.