DC Motor Speed Control

[Redbeard]

So, on browsing the Interwebs I came across this sort of gadget for speed control on DC electric motors and that got me wondering.

https://s.click.aliexpress.com/e/c8S31Ikg

There seems to be all sorts of issues involved in getting speed control on "normal" AC motors. So what's the issue with using one of these devices with a DC motor to upgrade (?) my Reeves drive M900? I'm sure it's not that simple, otherwise everyone would do it.

Wouldn't they?

Cheers
Redbeard

Sent from my SM-N950F using Tapatalk

[BobL]

You are right, if it was that simple everyone would use it and at that price I doubt whether its worth 1/2 a row of beans.

There are several ways to variable speed drive a DC motor.

The best (control and performance) way is with a Pulse Width Modulated DC Power supply and given the low price I doubt the Aliexpress unit is for one of those.

The other way is using what is effectively a light switch dimmer and a rectifier.
These two components can be purchased for <$20 for something that can handle 5X the power (2400W) compared to the Aliexpress unit is.
This method has problems and is not without risk - see http://www.woodworkforums.com/f271/dc-motor-performance-223851
The resulting performance on a DC motor is also very poor and it is why these are not commonly used.

Unless you feel confident in setting up a PWM controlled DC motor then using a 3Phase motor and a VFD is usually a better way to go.

[Redbeard]

2ji

Sent from my SM-N950F using Tapatalk

[DaveVman]

If I can over simplify things so that it is easy to understand for the electrically challenged.
In basic terms a DC motor speed can be controlled by varying the voltage. Varying voltage is a simple thing to do. Think of the volume knob on your radio to realise how common it is to vary DC voltage.

In basic terms the speed of an AC motor is determined by the frequency that it is switching phases. For a single phase AC that refers to the frequency that it is switching from positive to negative and back again. This frequency is created at the power station. In Aus and NZ it is 50 times per second. In USA it is 60 times a second.

It is complex to change this AC frequency. At least it is complex compared to changing DC voltages.

The other issue that was pointed out by another poster is that although it is easy to vary DC voltage, this is almost always done in a relatively inefficient manner. To over simplify things again. We reduce the DC voltage by creating resistance. But resistance also means some losses.

Like everything electrical there is more to it than I am making out but I'm a great believer in simplifying electrical theory so that people get the basic idea. Demystifing it basically.

I hope I've achieved that with my explanation.



Sent from my SM-G935F using Tapatalk

[Redbeard]

Personally, I prefer the "electricity runs on smoke, if the smoke gets out then the electricity stops working and you can't put the smoke back in" type of explanations...

Having said that, Bob and Dave have done fairly well. As expected, dodgy gear gives dodgy results.

I think I'll stick with the Reeves drive, I've got that working fairly well now. Save my pennies for a Vicmarc lathe (I like shopping local).

Thanks fellers
Redbeard



Sent from my SM-N950F using Tapatalk

[BobL]

In basic terms the speed of an AC motor is determined by the frequency that it is switching phases. For a single phase AC that refers to the frequency that it is switching from positive to negative and back again. This frequency is created at the power station. In Aus and NZ it is 50 times per second. In USA it is 60 times a second.
It depends on the type of AC motor. The motor you are referring to is called a synchronous motor because the speed synchronises with the frequency of the supply.
Many AC woodworking motors, especially for corded power tools and small machines are not synchronous but universal some of which can also run on DC. The speed of these has nothing to do with teh supply frequency. Changing the voltage on these will change the speed, see http://www.woodworkforums.com/f271/cheap-speed-control-universal-motors-224700 but this method can result in limited speed ranges and underpowered motors.


It is complex to change this AC frequency. At least it is complex compared to changing DC voltages.
Interestingly it's not that hard. The first thing that is done is to convert the AC to DC and then the DC is chopped into the required AC frequency required to produce the desired speed of th motor. This is how a VFD works.

[Paul39]

Redbeard,

If your Reeves drive is working well, stick with that. If it is cranky give it a clean and lube. There is a long thread about Reeves Drives here.

A proper variable speed conversion, buying new motor and controller will cost between $250 & $500 for parts. Look for AC & DC motors and variable speed drives.

[DaveVman]

Personally, I prefer the "electricity runs on smoke, if the smoke gets out then the electricity stops working and you can't put the smoke back in" type of explanations...


Gold!
I love it!

Of course, always remember that a lot worse can get out than smoke. :oo:

[Redbeard]

Paul, having completely rebuilt the Reeves drive only two months ago it's running quite sweetly, it was more of a matter of curiosity. I got to a point of frustration with the rebuild at one stage and I started to consider alternatives.

Dave, the stuff that bites is some form of black magic and I don't play with that stuff. I do 12 volt on my boat but that's about as fancy as I get with electrickery.

Sent from my SM-N950F using Tapatalk