Driving a 12V DC motor in PWM mode 1

[felixc]

Hi,
I want to drive a DC motor with a bridge in PWM mode, a little Maxon with a coil inductance of 0.4mH. I'll use an integrated H-bridge driver like the National LMD18200.
How can I calculate the ideal drive frequency for the PWM? Would I need some snubber circuitry?
Felixc

 

[leonar40]

I have some of those little Maxon motors. When I ordered the motors I noticed that they have a little drive circuit available. If you are not already locked into your PWM design, then you could just use their off the shelf driver and save yourself some design work.

 

[felixc]

I am going to use the A-max series. Maxon sell modules for multiple coil series. IF they're like the motors, the drives are surely not cheap. With PWM drive, a motor speed control can be made quite cheaply when you already have a microcontroller with all the necessary horsepower and outputs. PWM motor drive electronics add just $20. to the cost so it is worth trying if the quantities are interesting. So far it seems to work, but I wanted some advice from people who had experience with PWM motor drive in order to accelerate the tuning phase.
Thanks.
Felix

 

[logbook]

I wouldn’t even bother to try to calculate the frequency. The issues are electrical noise and efficiency. The noise and efficiency will both tend to drop off with increasing frequency. ‘Guess’ at somewhere between 20kHz and 100kHz and make it. You only have to change a single capacitor to change the frequency and you will then be able to see if it has any effect at all. I have gone for 20kHz minimum because it is better not to have the motor audibly buzzing at the chopping frequency.

 

[felixc]

I'll get a first proto pcb soon to start optmizing. The reference design given by a consultant dates from the early 90s, works at a low frequency (an ear-piercing 8KHz) and makes use of a big series inductor to smoothen the PWM into a DC current. I don't think that the inductor is necessary. I just want something more efficient and less bulky. The PWM frequency will be driven by a microcontroller so playing with the frequency will be easy. But the inductance of the motor winding may have a role to play in power efficiency. The driver chip has a current feedback, so I'll find out!
Thanks guys.
Felixc.

 

[maged541946]

Hi, Please note that motord are large mechanical objects with cosidrable inductance. The small ones cah tolerate high frequencies as 8KHz ( the term high and low normally referes to application) while larger motors bearly go over 600 Hz, mostly run at 30-100Hz, Remember they are DC motors, and need time to build up current through inductance, this increased current is opposed bt the back emf generated by the rotating armature.
increasint switching frequency will not allow this current to build through the armature to give drive force.
On the other hand, too low switcing frequecy will only cause the movement to be not smooth, you will notice that and increase it to a smooth level. remember it you make zero frequency, the motor runs( at max speed of cource) but at too much high frequency, will cause the armature inductance to appear open circuit to the high frequency.

 

[maged541946]

I forgot to add, if you want to test for proper frequency, try a simple LM555 with a pot for PWM or better two 555's one sets freq the second sets the pulse width, it is low cost, easy to breadboard, after you get your data, you do not lose much, the micro controoler may need re-programming to change frequency or set proper method for varing both pulse width and frequency..

 

[felixc]

Thanks maged.