controll ing dc motor need help

[norms09]

i am an undergrad student and im having my thesis project entitled "voltage and speed contol of induction generator" i have a coupled machine a dc motor and an induction motor. the dc motor acts as the pimover of my induction motor to turn it into a generator by speeding up the dc motor and operating the IM speed above its rated speed. now the pobelm is when i connected a load in the generator side the voltage drops and also the speed when certain number of loads were connected the generator stops generating voltage. i am thinking of controlling the dc motor so i can have a constant voltage produced by the generator. is there any possible things to do it? i am using a control system in my pc so that it will automatically command a certain ckt connected to the PM to change its speed or voltage. any help or suggestions? by the way i am using microcontollers in detecting the voltage change in the generator side. my pime mover has an 220V ac supply and 180V dc rating, which is i think difficult to control since base from my searches in the net most of the control circuits can only control dc motos up to 12V...

any help o suggestion will be deeply appriciated. thank you very much...

 

[audioguru]

DC motor speed controllers use Pulse-Width-Modulation. The width of the pulses controls the speed. Your pc might be able to produce the PWM.

Then the PWM drives one or two high voltage Mosfets that pulse the motor.
The PWM is at a high frequency so the momentum of the motor smooths the pulses.

 

[norms09]

DC motor speed controllers use Pulse-Width-Modulation. The width of the pulses controls the speed. Your pc might be able to produce the PWM.

Then the PWM drives one or two high voltage Mosfets that pulse the motor.
The PWM is at a high frequency so the momentum of the motor smooths the pulses.

can it contol that dc motor which has a rating of 180V aoutput?

 

[audioguru]

Mosfets are available with many hundreds of volts rating.

 

[Ravichandar]

dc controler crcuit

 

[stevez]

Some thoughts, comments, questions;

A. When you say "pimover" or pime mover did you mean prime mover?

B. Is the desired output of your system AC or DC?

C. If your intent was to generate AC then adjusting the speed will change the frequency - and if you were planning on powering other AC motors their rotational speed might also be linked to frequency. If you are rectifying AC to DC before using then this doesn't matter all that much.

D. Regarding the induction motor used as a generator - where will the magnetic field come from - as I assume the windings that connect to the wires are the output? I know with a synchonous motor (big industrial stuff) there is a field winding. If there is such a field winding you might adjust the power to that and fix the rotational speed. I am not a motor expert - these are just the things that come to my mind.

 

[audioguru]

In the article on the web about using an ordinary AC motor as a generator, they said that the motor has a small remnant magnetic field from the last time it was used and the small field is enough for the motor to become a generator then it makes a strong magnetic field.

 

[stevez]

I do recall seeing something about that. Not to add to the confusion but "ordinary" might be ambiguous. I might visualize an "ordinary" industrial 3 phase motor while others might consider a single phase capacitor start motor as "ordinary". Ok if the thought applies to either but I don't really know.

 

[audioguru]

An "ordinary" AC motor is the big induction motor that drives the fan in a furnace.
I think its starting capacitor was disconnected for it to be an alternator.
I couldn't find the article on the web.

 

[HSM1986]

hi norms :i may have a solution for u , u can use the PLC device as the main controller for u that reads a specific switch or sensor and give u an output of 12 v and the u can use that 12 v o/p with a relay that fed by a 180v power supply . ok
i hope that i helped u a litte... and wish u the best of luck

 

[rjvh]

hi norms :i may have a solution for u , u can use the PLC device as the main controller for u that reads a specific switch or sensor and give u an output of 12 v and the u can use that 12 v o/p with a relay that fed by a 180v power supply . ok
i hope that i helped u a litte... and wish u the best of luck

I am not saying it can not but it's rather an expensive solution if you don't have annything else to control for he plc

just take the sensor signal and switch it with help of a opamp Comparator circuit wil have the same result

Robert-Jan

 

[Diver300]

The classic way to control a DC motor that is powered from AC is to use a bridge rectifier made with two diodes and two thyristors, with a freewheel diode. The firing angle of the thyristors controls the average output voltage and the speed of the motor.

This is usually used with speed feedback.

This PDF explains it quite well. It is dated 1992 and it is scanned so it it over 12 MB.

https://www.electro-tech-online.com/custompdfs/2008/07/drives-td000_-en-p.pdf

 

[mneary]

Is norms09 still reading this old thread?

 

[Hero999]

An "ordinary" AC motor is the big induction motor that drives the fan in a furnace.
I think its starting capacitor was disconnected for it to be an alternator.
I couldn't find the article on the web.

I could see that working with a synchronous motor if some of its output was rectified and fed to the rotor field. As you start spinning rotor the small magnitic field would produce a small voltage which would increase the rotor current and field (proving it's > the voltage drop of the rectifer) which would generate more voltage. Except you might want you limit the rotor current as I could see it cooking itself due to the positive feedback between voltage and rotor current.

I can't see it working with an induction motor because the field is too small and any current drawn would surely make the field weaker not stronger.

 

[audioguru]

The article used a lawnmower engine driving a big furnace AC induction motor.
It said that residual magnetism in the motor made it begin to generate then its own current built up the magnetism. It said if it doesn't begin to generate then connect the motor to AC or DC for a moment to give it some residual magnetism.

 

[Mr.Eng13]

Hey guys....it's my first reply in this sit.....as a comment on this topic I have a question,
I have a project...can I use H-bridge (l298N) with pic 18f4550, for controlling the position of a dc motor....the position and using pwm ...all I have seen is a speed control
can the h bridge+pwm be used for posistion control

many thanks

 

[Mikebits]

You need a feedback mechanism to tell your circuit where the motor position is. A stepper motor might be a better choice.

 

[Mr.Eng13]

I do have a feed back for poistion....still to know if I can use pwm+hbridge+pid controller through pic for the position control

 

[Mikebits]

Your motor needs to drive a gear or some such that is linked to feedback device. At motor position zero your feedback needs to be at a known state. Feedback such as a potentiometer. You will need to calibrate this. You can then turn motor x degrees and interpolate in between values of resistance versus motor position. Write you code accordingly.

Or you can just buy a pre made servo.

DK is pretty good at this stuff. maybe DK has a better response for you.

 

[Mr.Eng13]

Mikebits...thank you very much for your help....i have a feed back coming from two photocells....connected to a solar panel will rotate ...the problem is can I use the principle of PID controller in a pic 18f4550...using the pwm and the H bridge (l298N) or should I follow the (digital to analog converter and the op-apms..and the current amplifire circuits to build this system)...regards