I have made a DC motor circuit whose output is supposed to be controlled by the duty cycle of a PWM.The PWM sets the duty cycle of a transistor which sets the gain of the buck-converter.The program runs fine on Multisim until at around 40 ms where the program stops working and the armature current goes to some absurd value which obviously is not correct.What am I doing wrong?

 

I suggest running the simulation in a step-by-step mode to see what goes wrong..

 

I think its a bug.

 

You posted a zip file of a multi sim National Instruments file (ms14) which since I don't use anything NI since I retired I can't open or view. Unless someone else here is running multisim they can't view the file either.

Ron

 

THAT explains why I could not view the file.!!

 

You posted a zip file of a multi sim National Instruments file (ms14) which since I don't use anything NI since I retired I can't open or view. Unless someone else here is running multisim they can't view the file either.

Ron

Ok I thought many people were using Multisim here but I guess I should upload a schematic.

 

 

And the graph looks like this

and I dont know what it does the jump towards top at the end.

 

Look

 

Look
View attachment 366354

Could you seperate the 2 imags and enlarge them?What is wrong with my circuit?Is the polarity of I1 wrong?

 

Ok I thought many people were using Multisim here but I guess I should upload a schematic.

"Many People" is a very long way from "everybody", and certainly a whole lot of us do not have multi-sim, or even other simulators on our home systems.
AND, that most current posted circuit, with the transistor shunting the field coil supply of the motor, would not ever pass my design review.

 

Ok I thought many people were using Multisim here but I guess I should upload a schematic.

The most common Spice simulator used by posters on these sites is the free LTspice simulator from Analog Devices.

 

Could you seperate the 2 imags and enlarge them?What is wrong with my circuit?Is the polarity of I1 wrong?

Why use a 1 MHz signal frequency and 1 picosecond edges? A 1 kOhm resistor is not capable of making the transistor work in switching mode. That's because you took a transistor with a gain of 100. I changed the parameters of the control pulses and replaced the resistor with 100 ohms.

 

With a DC motor having a separatly excited field, and the motor running, a reduction of the field current will lead to an increase in armature current and an increase in speed. If there is not enough load to limit the speed , and the field weakens enough, the motor will speed up until it has a mechanical failure, or thearmature overcurrent protection device opens the feed circuit. That can be very exciting! A "Hit the floor" sort of moment.
In a simulator, it just stops working. Much less hazardous.

 

Why use a 1 MHz signal frequency and 1 picosecond edges? A 1 kOhm resistor is not capable of making the transistor work in switching mode. That's because you took a transistor with a gain of 100. I changed the parameters of the control pulses and replaced the resistor with 100 ohms.

Yes it works now.I reduced the value of the resistor to 100 Ohm and decreased the switching frequency x10 while keep the Duty cycle constant.Thanks.