PWM Pulse Control

[Tragus]

This may be an obvious question, but from my understanding PWM on a VFD controlled DC motor can run up to 10KHZ. What I was wondering was, what actually allows the power transistor to switch at such a rapid rate? I imagine it would have to be something reliable.

Thanks

 

[DRWeig]

Your big power transistors can probably switch even faster if needed. They're transistors, after all. So I am assuming that they are not the subject of your question.

The bus speed on my PC is over 3 GHz, and I can set it to whatever I want with software. Your VFD just has bigger components to deal with the higher currents and resultant heat, and lower frequency. The circuit that fires your power transistors is designed for your particular drive. There's a microprocessor in there that is probably turning one of its output pins on and off at the rate needed, then an amplifier, then the base of the big transistors.

OK, wait now for the more experienced people to embarrass me before assuming I'm right.

Best to you,

Goober Dave

Haven't see the forum policies? Do so now: Forum Policies

 

[Tragus]

I understand what your saying, but I was actually wondering what actually happens internally to the microprocessor/amplifier to allow it to switch so rapidly?

Being totally solid state, Is it something similar to a 555 timer, which uses the rate to charge and discharge a capacitor to control the pulsing? Would it even be possible to achieve such a high pulse rate with a capacitor/555?

Am I thinking too far into this? lol

Thanks

 

[Skogsgurra]

Oh no! You haven't even begun thinking.
The 555 timer is an analogue device and, as such, not capable of very high frequencies.
A microprcessor is an almost purely digital device. The clock frequency is determined by a crystal (or a resonator) that oscillates at 1 - 20 MHz. That frequency is then multiplied in a PLL (phase locked loop) and taken up to the frequency the micro processor uses for its internal clock, which typically is something like 100 MHz in small MCUs and a lot higher in processors used in PCs.
You are at the beginning of a long learning experience. Find some basic introductory texts and jump onto it! It is fun and rewarding. But be prepared to forget all that you think you know about the subject. Keeping to those analogue "analogies" will only hinder you.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[JadeYork]

DC motors do not run on VFD's; they run on direct current. The voltage and speed can be adjusted and vary. You may be thinking of an AC motor if it runs on VFD?

Jade York,
www.repairzone.com

 

[cswilson]

Tragus - By all means, do follow Skogs' advice and start studying from the very basics. It's a fascinating subject at all levels. I've been working with these circuits for 30 years now, and I'm still learning lots, and I have plenty more to learn. (I just did an analysis of the nanometer-level oscillations that the pulsing causes in frictionless precision systems - lots of surprises there!)

I will explain a basic concept to get you started. These days, most PWM signals are generated purely digitally, as Skogs stated. In most, there is an up/down counter circuit incrementing at a high frequency -- our latest design uses 300 MHz, which is higher than most, but 100 MHz is pretty common these days. The limiting number on the counter before reversal sets the frequency of the waveform. If you are counting at 100 MHz, and have the counter turn around at +/-10,000, you will have a 10 kHz waveform. Each increment of the counter, it is compared to a digital command number generated by the software control algorithm. Depending on whether the counter or the command value is greater, the output is high or low. This output is the PWM command signal. The percentage on-time is proportional to the numerical command value.

Using PWM for AC motor control adds an additional level of complexity. As JadeYork points out, the term VFD is not used for DC motor PWM drivers, because there is no "variable frequency" in this DC motor control. The variable frequency in AC motor control is the frequency of the voltage and current waveforms in the phases of the motor, usually in the range of tens of Hertz. So there are three key frequencies in a PWM VFD: the phase voltage/current frequencies in Hertz, the PWM (and control algorithm) frequencies in kilohertz, and the counter frequencies in megahertz.

Curt Wilson
Delta Tau Data Systems