audioguru

The relays are probably for the motor to run forwards with one relay and run in reverse with the other relay.
The PWM circuit switches on and off the power to the motor much faster than a relay can to control the duty-cycle of the power to control the speed of the motor.

__________________
Uncle $crooge

AirWalker83

sorry, i still not quite understand about the PWM circuit and the relay?
is it PWM cannot use in relay?
Can sulution to control speed using relay?
Can u explain somemore?
thanks....

dknguyen

Relays can only go on or off. They cannot PWM. Relays wear out every time they switch. So if a relay can only handle 100,000 switches and you are switching them at 20kHz PWM...it wears out very very fast.

Transistors can switch on and off very fast.

So you might use relays to control direction since you aren't changing direction thousands of times per second, but then the current would have to pass through a transistor being switched with PWM to control the speed.

Sceadwian

PWM is almost never used for motor speed control when you're using a relay, the relay has to switch too fast and it will reduce it's lifetime by a factor of 10-100 makeing the relay burn out unuseably fast.

__________________

Nigel Goodwin

Not strictly true, in fact it's very commonly done - you use the relay (DPDT) to reverse direction, and a single transistor or FET to provide the PWM - a lot cheaper and simpler (and more reliable) than an H-Bridge.

__________________
PIC programmer software, and PIC Tutorials at:
http://www.winpicprog.co.uk

cadstarsucks

True. We do it on a couple of units... though most units that do reverse direction are for relatively low power actuators as opposed to the 3HP main motors.

D.

boardinsnowman

Hello,
I am doing an electronics project and I am completely stumped. I am trying to design a PWM speed controller for a 24v 300-500w DC motor. I've found a few circuit designs online but since my understanding is limited I'm afraid I'll build it wrong or pick the wrong one. So, could anyone point me in the right direction in designing the proper circuit for what it must do.

Ideally, this is what I want it to do--
-regulate the speed from 0-100%
-have a regenerative brake
-handle up to 50 amps (just in case)
-not make noise -although after reading this thread though it seems I'll have to stick to around 3khz
-be able to input 24-48v and still output 24v--I don't know if this is possible, but please let me know.

Some of what I've found-
http://www.4qdtec.com/pwm-01.html --pretty much what I'm looking for, scroll down to "A slightly more sophisticated controller" - but this one only goes from 0-90%
http://www.aaroncake.net/circuits/motorcon.asp --less like what im looking for
http://www.4qd.co.uk/fea/regen.html --how regenerative braking works
http://www.educypedia.be/electronics/powermotor.htm --lots of info

Any help at all would be greatly appreciated for I'm at quite an impasse.

Thanks again,
-Ed
howtoreached@gmail.com

Again, I'm a beginner so hopefully the specs I want are even possible....

diaeldeen

goooooooood

Refusalspam

I guess I'm very late to this discussion, but there are some points I'd like to bring up in case anyone is still reading. I'm not an expert, I've

1) FET swapping in a circuit diagram
I don't think you can just swap out one FET for another one in a PWM schematic. For N-channel MOSFETs you need the gate to source voltage to be around 8v to get max current (ie lowest resistance). Most of the time the supply voltage is the highest voltage and it will be tied to the source pin of the FET. Typically, you will want to use an N-channel MOSFET because of the lower resistance compared to the P-channel.

2) Reversed biased Diodes across the motor terminal
They is for both efficiency and protection as somebody said. This configuration is called a 'free-wheeling' diode and the diode must be able to switch faster than the pwm frequency. Its true that most FETs have a zener diode that will allow the kickback to be shorted to ground, but you would be wasting that power and heating up your FET even more.

3) Ideal PWM frequency
This tends to be a function of the motor speed. You only want to use as low of a frequency that will still give you smooth motor control at the highest rpm the motor is likely to go at under load. The reason for this is simple, no matter how fast a FET can switch, its still not instantaneous. During this time when the FET is half-on/half-off its resistance and current are fairly high meaning power dissipation is much higher than normal. The faster the pwm cycle the more time you spend heating up your FET (and diode). If you don't believe me, take a queue from competition level RC speed controllers like this one: http://www.teamnovak.com/products/esc/gtx/index.html
The "drive frequency" can be adjusted from 1-11Khz. If higher was better then why bother with making it adjustable?

4) Lack of current limiting for "stall currents"
Most motors will consume a lot more current when stalled (rotor is locked). This is because the inductance of commutating coils in the motor keeps the current down when running, but a stalled motor will stop commutating leaving only the resistance in the motor windings to control the current. I get about .1 Ohm across the motor terminals of my cordless drill motor. At 10v that would be a 100 amp stall current! This doesn't just burn out your motor, but also the FET and possibly the diode.

h.d

what we need to apply PWM controlling for 20A 2hp dc-motor using the PIC microcontroller?

indulis

I know this thread is old, but for those that are interested, the circuit could be used to to make a SMPS. You just need to add an error amp and reference and you have a voltage mode PWM.

audioguru

Hi Indulis,
Welcome to this forum.

__________________
Uncle $crooge

bunghole

what are the values of q2 and q3 in the schematic at the start of this thread, or do i have to make them something? and is there something that can takr the place of the bs170?

__________________
Every little bit counts! It works with money, and it works with electronics!

RAKS

I'm currently looking at PWM controller circuits because I need to design a DC-DC converter which I've never done as part of a home project before. I'll take a look at your circuit - thanks.

__________________
Blatent advertising deleted! - moderator.

Hero999

True but which is better and cheaper depends on the application, for a low powered motor, it's probably cheaper to use a full transistor h-bridge than a relay.

__________________
I also post at the following sites:
http://www.stop-microsoft.org http://www.heated-debates.com
Screen name: Aloone_Jonez