Need tiny bit of assistance.

[Ian Rogers]

Remember my welding turn table! Well its working well.. I bought a small 1A 12v motor and put it on a PC ATX PSU and worked very well, BUT! there is always a but! Using a pwm controller ( pic12f1840 and a hexfet ) I get the motor down to 1 turn per 10 seconds.. I know slow.. But that is still too fast and the torque becomes non existant.. So... I got hold of a a much better motor / gearbox setup 10x the torque.. The new motor is 24v, when run at 12v it consumes 0.6A ( whilst I hold it) and 0.48A in a free run... The PSU has a -12V at a max of 0.7A or I could just run it at 12V and hope the motor is okay with about 25% pwm!

I believe the motor should be okay at half it's designated voltage but 10Ncm was just a tad too low.. The new one would break my arm with 100% PWM at 12v.

Should I chance the -12 ~ +12 at 0.7A or keep it at 12v @ 17A ( Torque would be better at 24v )

Comments welcome.

 

[Pommie]

Not sure where the 17A came from (typo?). However, at 24V it'll draw twice the current at stall and still around 1A when running. If you're always going to pwm it then you could try a big capacitor on the negative supply. I think I'd try it and see.

Mike.

 

[JimB]

How often, here on ETO, do we advise someone that you cannot get a quart out of a pint pot, and that the components that they are proposing to use are just not up to the job.
I think that this is going into the same territory.

What would I do?
Get a second PC ATX PSU and connect the high current 12v outputs in series to give 24v.
Obviously you need to ensure that the second 12v supply can float at 12v above ground. If both 12v supplies are referenced to chassis ground, this scheme will not work.

Or,

Get a suitable 24v transformer and rectifier to provide the 24v.
Probably no need to use a capacitor to smooth the output which is to be PWM'd, I dont think that the motor will care less if it is supplied with a PWM'd 100Hz rectified supply.

JimB

 

[Pommie]

you cannot get a quart out of a pint pot

Come on Jim, we can change the laws of physics.

Mike.

 

[Ian Rogers]

The 24v motor is only 24v @ 0.6A ... The PSU has 12v @17A out and a -12@0.7v out so theoretically will work... BUT!! its working fine at 12v, I just hope it'll stay working... What would running at 12v do to the motor?.. Don't give a rats arse about the current, like I said the 12v rail can supply 17A.

I read a note on a web site that said the motor wouldn't come to any harm... Thats all I'm asking..

I should reiterate... The torque seems fine at 12v!

 

[Nigel Goodwin]

If it's working fine, then it should be fine, as it's so under driven it won't be getting hot, and it's heat that damages motors when the cooling fan isn't going fast enough to cool it sufficiently.

 

[gophert]

A 24v max motor will work fine at 12VDC if it provides the torque and rpm you need. Voltage control was a very common way to control a motor's speed before PWM became a common and easy alternative.
I think your question is very confusing so you received confusing answers.

 

[JimB]

The 24v motor is only 24v @ 0.6A ... The PSU has 12v @17A out and a -12@0.7v out so theoretically will work

Sorry I misunderstood your original post, I did not interpret it like that.

What would running at 12v do to the motor?.

It should be fine.
When running on 12v it will run at half speed.
The only likely problem is cooling, with the fan running slower.

But this is a welding turntable, and I guess that you are not setting up a production line of circular welded things, so its duty cycle will be low.

So yes, run it off the 12v ATX PSU and it will probably (almost certainly) be fine.

JimB

 

[Nigel Goodwin]

A 24v max motor will work fine at 12VDC if it provides the torque and rpm you need. Voltage control was a very common way to control a motor's speed before PWM became a common and easy alternative.

Very true, and often is wasn't even that good - simply switching resistors in series with the motor was a very common technique - and was probably the poorest possible way to do it.

 

[Ian Rogers]

Thanks Gophert... That's the message I was getting... I'll run it down to the speed I need and if it doesn't stop when held, I'll be good to go.

The old 12v motor got down to nearly the correct speed but when it stops, you really need to crank the PWM back up to restart, which is less than ideal.. Once my son is back in from furlough I'll do a video... You'll then see why it has to be sooooo sloooowww..

The PSU I have is slimline so it all fits together well..

 

[Ian Rogers]

Ah well!! Sack that.... 24v motor with a 12v PWM is not good... I have it now running -12 ~ +12v.. I'll try the PWM again tomorrow.. I just need to alter the micro with a 5v reg so can run it from -12v...

 

[Visitor]

The –12v rail may not be adequate because of the starting current of the motor.

 

[Ian Rogers]

The –12v rail may not be adequate because of the starting current of the motor.

That's the bit I was worried about..
I have a 24v PSU coming soon... Also a proper PWM controller on the way.

 

[alec_t]

It sounds as though you need a greater reduction ratio on the gearing. Can you fit extra gears/pulleys?

 

[Visitor]

I had several 12 volt fans connected to a switching power supply that had something like 4× the needed capacity for their run current. When it was plugged in the fans would start to run, the power supply would cut out and the fans would stop. Power supply would start again and the process would repeat.

A beefier supply solved the problem.

 

[Ian Rogers]

It sounds as though you need a greater reduction ratio on the gearing. Can you fit extra gears/pulleys?

I tried… The first had a 65:1 this one has 150:1 I'll get there.. I need to I have 100 tubes to weld..
We used to have a AC machine with a variable v belt setup... It was perfect.. But unfortunately all were left behind..

 

[gophert]

I tried… The first had a 65:1 this one has 150:1 I'll get there.. I need to I have 100 tubes to weld..
We used to have a AC machine with a variable v belt setup... It was perfect.. But unfortunately all were left behind..

work gears are great for slow speed stuff. Look for a broken two-stage snowblower.
EDIT- never mind, not many two-stage blowers in the UK, I assume.

 

[shortbus=]

Normally you don't choose a SMPS for motors, especially motors that need to do some work. Linear power supplies are what is usually best for motor driving. Just a suitable transformer and bridge rectifier, filter cap not really needed.

Before the fall out, do people use SMPS for motors? Yes. Are they ideal? No

 

[Visitor]

Why do I use a switch mode supply for motors? Because I usually have some on hand and they are cheap.

An X Box power supply like the one pictured will supply regulated 12VDC at 14 – 16 amps depending on the version and I can buy them at the thrift store for a few bucks.

These supplies also include a 5V at 1 amp output, with the 12v output being controlled by a 5v logic signal – it might be possible to get a PWM output using this enable line.

 

[rjenkinsgb]

Normally you don't choose a SMPS for motors, especially motors that need to do some work.

Virtually all industrial machine tools & robots, with massive motors, use switched-mode supplies.

The power supply needs to be rated to stand the possible stall current (or drive-limited maximum current) of the connected motors.

A basic switched mode PSU with internal protection will fault on overload; you have to use one rated at the worst-case load, and/or the drive(s) running the motors need internal current limiting.
A simple analog PSU will handle short-term overloads & just get hotter.

It's down to appropriate design, rather than the type of PSU.

As an example, the machine I'm working on at present has a 95KW (~ 200A input at 415V) switched-mode regenerative PSU running the drive systems..