DC motors, PWM controllers and DC supplies

[Munkul]

so... I have a 40V DC motor on this welding positioner I've just bought. Originally it was powered by an automatic welding power source with an encoder for feedback. I just want to power it and make it variable speed.

It is a Dunkermotoren GR53x57 nominally rated at 1.9A and fault current at 20A.

I already have a suitable PWM speed controller, tried and tested on a 24v motor, it's rated up to 60v and 10a. I'll protect the motor with a 2 amp fuse, if I can't limit it via the supply.

So I need a power supply.

Finding a switched mode or linear power supply rated at 40V for under £100 seems to be a fruitless task. All I can find are 36V and 48V supplies.

So... I'm wondering, could I just feed it 36v and accept the small increase in working current? Or is this a silly idea? I certainly don't want to burn it out, it's an expensive German motor. I also don't want to under-power the positioner.

Similarly, 48v sounds like a bad idea too.... unless I can get an adjustable PSU and manage to turn it down far enough... OR is it? given that it's going through a PWM controller...

Lastly, I could just buy a lab adjustable power supply and run it off that, but it's not cheap and it seems like a waste, when I just want to build a neat little enclosure with the supply and controller, 240v in, and a socket/plug for a foot switch.

Any suggestions?

 

[Munkul]

ahhh... hang on. Found a whol load of electric scooters take 42VDC charging.

https://www.amazon.co.uk/TangsFire-Charger-Lithium-Electric-Batteries/dp/B0995RQNYB/ref=sr_1_5?crid=1SJY5XJ6O5SDW&keywords=42v%2B3a&qid=1653032743&sprefix=42v%2B3a%2Caps%2C50&sr=8-5&th=1

42v sounds a whole load better than 48v.

Now it's chinese, but there's lots of them available next-day if the first one breaks... trouble is, will it be a smooth enough output to use?

 

[mikegolding]

Some switch mode power supplies have adjustable outputs .

https://uk.rs-online.com/web/p/switching-power-supplies/1065853

Not saying this is the right one for you but goes up to 39.6 v

Mike

 

[Munkul]

Some switch mode power supplies have adjustable outputs .

https://uk.rs-online.com/web/p/switching-power-supplies/1065853

Not saying this is the right one for you but goes up to 39.6 v

Mike

That's a nice spot, thanks!

I'd definitely rather buy a proper switch mode PSU than a charger of dubious spec.

So, if i had the choice between a 100w 36v running to 39.6v, and a 100w 48v running down to 43.2v... I wonder which would be better? I'm leaning towards the 48v supply turned down...

 

[sako243]

So... I'm wondering, could I just feed it 36v and accept the small increase in working current? Or is this a silly idea? I certainly don't want to burn it out, it's an expensive German motor. I also don't want to under-power the positioner.

You won't increase the current.

A simplified view is that the voltage controls the speed that the motor turns at and the current provides the torque. Running it at 36V will simply result in a reduced overall top-speed. The current being pulled is dictated by the loading on the motor, operating at a lower voltage does not affect that - if you stall the motor then the current pulled is simply the stall current irrespective of what the power supply is capable of. You can't "push" current.

The reasons that PWM applications are generally preferred is down to simplicity, efficiency and "better" control. At slow speeds all motors have an inherent inertia to overcome, due both to their mass but also the magnetics involved - you'll often find that you require a higher voltage to get the motor started but then once it's running you can slow it down below that initial start-up speed. Due to the inertia you need more current at start-up to get the thing turning - once it's going the only current being drawn is to compensate for losses (the load that you are turning is treated as a loss in this description). Due to a motor being very slow (compared to the electronics) it doesn't care whether it sees a constant 20V or 40V chopped with a 50% duty cycle (provided it's chopped quickly enough), the magnetics are slow so the 50% 40V signal "looks" the same to it.

The benefit you get is two fold by doing this - the first is it's usually far more efficient in the electronics (as well as being simpler) because MOSFETs like operating either hard-on or hard-off, if you only partially turn them on then they are more resistive and therefore disapppate more power so also need to be physically bigger in order to deal with that heat. The second aspect is that you can achieve lower operating speeds with the PWM approach from stationary because you are delivering a full-voltage kick briefly, which overcomes the initial inertia a little better. There's still some hysteresis involved but you can usually improve the lower operating speeds a little by using a PWM approach vs. linear voltage.

The only significant downside to doing a PWM approach is that you are chopping waveforms which results in a noisier electronic environment which in some applications may not be preferable. Also if the frequency of the PWM is too low then the motor can pulse but in this day and age that shouldn't be an issue because even simple electronics can switch much faster.

 

[sako243]

An addition for your application.

Depending on the motor design and wiring - leaving the motor stalled for a long period of time may not be advisable because you could end up heating up the windings in the core and burning them out. It depends on the motor design, after all some motors (steppers for example) are meant to be operated in a stalled condition 100% of the time so they are designed to handle it. A simple DC motor on the other hand, unlikely - the datasheet may say how long it can stay stalled for.

Equally "under-voltaging" a motor will do nothing, it will either turn slowly or not at all. Over-voltaging it will result in it spinning faster than intended (mechanical limitations such as bearing ratings may come into play) but equally the motor may start arcing internally more than intended so you'll start damaging the internal components.

So, if i had the choice between a 100w 36v running to 39.6v, and a 100w 48v running down to 43.2v... I wonder which would be better? I'm leaning towards the 48v supply turned down...

What is the SMPS actually outputting - a smoothed 43.2V or a chopped 48V effectively giving 43.2V? If the motor can't handle 48V repeatedly then don't use a chopped one, but if the output is smoothed (i.e. they've got some big capacitors across the output) then you might get away with it.

 

[AKN]

You could try a second hand bench power supply off Stewart of Reading http://www.stewart-of-reading.co.uk/Power Supplies.htm tell Dwayne what you want and your budget.

 

[Munkul]

Thanks Sako for the clarifications.

Those SMPS deliver pretty "clean" DC output, max ripple/noise 200mV on the datasheet, so I imagine it should be pretty nice to feed into the PWM unit. And as far as I'm aware, all the PWM unit is doing is chopping that 43.2v up into smaller spikes to control the motor, which as you say, doesn't care too much about the precise voltage as long as it's not overspeeding or stalling.

So... motor protection?
I can order 1.6a or 2.0a fuses in either fast-blow, slow blow or time delay. I'm thinking either a fast blow 2a or a slow blow 1.6a would keep the motor from dying, and the supply will shut off at under 4 amps anyways. If it stalls, I'll know about it anyway.

Decided that a slight overvolt will be fine.

 

[sako243]

Hmm., not found it directly on Dunkermotoren's website but came across this instruction manual which implies maximum electrical voltage is 42V
https://www.manualslib.com/manual/1960137/Dunkermotoren-Gr-53-Si.html?page=9#manual

I have two Dunkermotoren GR-63s (60V) sat here in my office and with a reduction gearbox on them, shame they are owned by Heathrow...

 

[Munkul]

Hmm., not found it directly on Dunkermotoren's website but came across this instruction manual which implies maximum electrical voltage is 42V
https://www.manualslib.com/manual/1960137/Dunkermotoren-Gr-53-Si.html?page=9#manual

I have two Dunkermotoren GR-63s (60V) sat here in my office and with a reduction gearbox on them, shame they are owned by Heathrow...

I've got that manual already, but thanks

Like you say though, with PWM chopping up the voltage and as long as it's not flat-out, it shouldn't be a problem, given that I'm sure the motor is a little overspecc'ed for the rating...

My main problem now is I've no way to work out if the gearbox is a 1:64 (ideal) or a 1:225 (bit slow)

 

[Munkul]

Happy days I tested out the motor last night with the 24v supply I had on hand. The PWM controller worked great near the top end, but didn't have any umph down low. I'm guessing this is because there wasn't enough voltage to drive it at the low speed. But at full speed (24v) it did 1rpm on the turntable nearly exactly. This makes me think it has the 1:225 ratio gearbox which gives a turntable speed range of 0.07 to 1.4 rpm. Quite slow for small stuff, but pretty decent for around 50mm diameter upwards on TIG. Was plenty torquey, although I didn't want to push it as I hadn't put any overcurrent protection in.
So I've ordered that 48v power supply, as well as some connectors and a foot switch. May as well make the control side as neat and tidy as the hardware itself.

 

[Munkul]

Hooked up with 40v, still using the cheapo PWM unit. Got a kettle lead socket, a fuseholder, a fwd/rev switch and a foot pedal, but still waiting for two bulkhead plugs/sockets coming to finish it all off.
Tonight measured a speed range of 0.14rpm to 1.40rpm, I'll take that. Gonna be brilliant for TIG. Still keeping my eye out for a higher ratio motor though.