Monitoring voltage and current ?

[iugmoh]

after I connect the circuit, I notice that I always measure the source voltage not the motor voltage because I connect the wire from 150VDC to voltage divider then to ground, so I think I need another voltage divider after motor windings and make a difference OPAMP to mease the voltage on motor windings ( before and after motor windings) , any one have comments ?

[Nicmus]

Hi iugmoh,

You are right that you are reading the input voltage not the voltage on the motor (maybe I missed this requirement) but here is the possible solution:
Your IRF840 has a 0.85 Ohms ON resistence and your current sensor 0.2 Ohms (total 1.05 Ohms). Under maximum current allowed, 3 Amps, your maximum voltage drop will be 3.15V so the difference from the input voltage will be on your motor. You can compensate for that in your PIC program as one fixed voltage drop (read it under normal running conditions if such thing exists) or a variable one by using your current reading and apply it to the known fixed resistance involved (1.05 Ohms) if you want to get fancy.
This will change with the load, input voltage or how worn the motor is.

I’m pretty sure that we can better help you make decisions and select the best approach if you define your requirements in more detail.
Here are just a few questions for you:

• What will be an over voltage or over current situation? Define it.
• How are you going to address the inrush currents at start up?
• What is your target resolution and accuracy on voltage and current readings?
• Do you want to continuously have and monitor/display these readings or just be able to set trigger points at max/min conditions?
• Is your motor running under constant or variable load?
• What is your power source and its specs?
• Are there other requirements that you have in mind and didn’t mention yet?
• Is your project cost sensitive?
• Is this a one time deal or it will become a production item?
• Is this a hobby style product or used in a sensitive application (medical field or so)?

Please keep in mind that the answers you get from the forum are as good as your supplied information on what you trying to accomplish.

Regards,

Nick

[timmers]

Hi,

All the comments are valid, but in my experience the voltage accross the motor, even with smoothing caps in the potential divider, will allways be near HT. The only way to measure the voltage ACCROSS the motor is to have two dividers, one on each terminal of the motor then motor voltage is the difference. But bear in mind the voltage accross a PWM driven motor is pretty much the supply voltage plus the back emf effects.

I suspect you are wanting to monitor the motor drive, and to take meaningful readings I would dispense with the voltage reading and use the current monitoring. To isolate the circuit from the PIC, use an opto coupler.

Tim.

[Nicmus]

I have to fully agree with timmers on monitoring only the current. With a little experimenting you can define your normal operating limits and take whatever action when things go wrong.

I’m a little unclear on the idea of monitoring voltages on both sides of the motor. If I understand it correctly you will need to add one more voltage divider on the lower side of the motor. That means your resistors should be almost the same as the other divider to accommodate the full voltage when Q1 is OFF. In this case when Q1 turns ON you will only read a 1/30 portion of the voltage drop on Q1 and R1 which is even less than your voltage drop on R1 only and which will force you to use one more Op-Amp.

I see no problem with a possible less than 1V error by approximating the voltage drop on Q1+R1 but this is one of the questions for iugmoh.

Regards,

Nick

[iugmoh]

oky In brief Iam working on thesis which title is "Parameters Identification of Permenant Magnet DC Motor" , in this thesis I built the mosfet driver to control the speed of motor , the motor is series wind motor but I convert it to separately excited motor by applying constant voltage to field winding , Motor specification is 110VDC , 3A , 200 watts,1500rpm , the motor aslo will be used as an educational kit which I will add all terminals to control motor and monitor voltages and currents and its speed so the student can use it with PIC or DAQ.

In my work I will make some simulations on voltage and current of motor and it's speed I will use DAQ first for flexibility with computer then after that I will use microcontroller PIC.