What power supply do I need for this knee power feed project?

[lfoggyParticipant @lfoggy]

I'm building a power feed for the knee of my turret milling machine and need some advice on the power supply I require. I've got a Parvalux brushed 24v DC motor rated at 120watts. Motor is fitted with a worm reduction gearbox so output speed is around 90rpm. Should be more than enough as my 18v cordless drill driver will wizz the knee up and down no problem. I've done the mechanical bits and have been testing the system with a 5amp 24v DC power supply I have. It lowers the knee no problem but can't always get started elevating the knee. Once its going though it seems OK. I assume the motor draws more current when starting elevating the knee as this does require considerably more torque.

The motor data plate says 120 watts and 6.8amps which is odd as 120 watts at 24 volts is 5 amps? Is that the peak current in starting? Also I am controlling the motor with PWM type power regulator rated 9-28v and 20 amps so that should be OK. With no load on the motor it all works perfectly.

Pics of motor plate and setup below. Question is what DC power supply do I need here to make this work ? Please assume no knowledge of power supplies or their characteristics…..

Any other advice? Thanks in advance.

Edited By lfoggy on 22/01/2023 17:33:07

[lfoggyParticipant @lfoggy]

[Martin ConnellyParticipant @martinconnelly55370]

It is a good idea to have a power supply rated for a slightly larger current than peak as it will be stressed if it regularly runs at its maximum output. It is possible that the power of 120W is useable shaft power and the extra current is to cover losses in running the motor. I think there have been threads in the past where the differences in motor power stated in adverts for the same basic machine were due to some vendors quoting useable shaft power and others quoting electrical input power. The latter makes the machine sound better since if your motor uses 5.6A and 24V the consumed power of 134.4W sounds better than the useable shaft power of 120W.

Martin C

[Brian WoodParticipant @brianwood45127]

Try a reduced gear ratio, it will reduce the load on the motor on lifting and provide more torque. The downside is of course a slower lift and lowering

Brian

[John HaineParticipant @johnhaine32865]

At start the motor is essentially stalled. Start current will be much higher than 6.8 amps, and since you say it's having trouble starting it clearly is stalled. Since your controller is rated at 20A max You probably need to have at least 20A peak current available from the power supply. Your drill/driver probably has lithium batteries and could apply much more current on startup. Hopefully that controller is designed to cope with starting currents, and will have appropriate flywheel

From the plate the motor has DC excitation, i.e. wound field. How are you connecting the field windings, in parallel with the armature or separately to the 24v supply? It should be the latter. The extra current 6.8 – 5 = 1.8A is presumably the field current. How do you reverse the motor?

[lfoggyParticipant @lfoggy]

Thanks John, you clearly know much more than I do about motors and how they work (which is not difficult given my shallow level of knowledge) The DC motor only has two wires exiting the body and if you remove the cover on the bottom of the motor there are no terminals or any options to change the wiring configuration. I think the motor 'is what it is' as they say. I am changing direction with a simple switch that changes the polarity of the input voltage. Am I not right in assuming that this DC motor is 'symetrical' in operation when you switch the input polarity? Should I just get a power supply that has a peak output of 20 amps and see if that works?

[StueeeeParticipant @stueeee]

When I modified a bead rolling machine from hand operation (where you need 3 hands) to powered. I used a 24V stairlift motor of similar power to yours. The motor runs through a similar PWM controller to yours and uses a 20A 24V power unit bought on ebay.

You can get an idea of its size by the fact that the power supply is bolted to a piece of 50 x 50 RHS steel

[John HaineParticipant @johnhaine32865]

Posted by lfoggy on 22/01/2023 18:59:21:

Thanks John, you clearly know much more than I do about motors and how they work (which is not difficult given my shallow level of knowledge) The DC motor only has two wires exiting the body and if you remove the cover on the bottom of the motor there are no terminals or any options to change the wiring configuration. I think the motor 'is what it is' as they say. I am changing direction with a simple switch that changes the polarity of the input voltage. Am I not right in assuming that this DC motor is 'symmetrical' in operation when you switch the input polarity? Should I just get a power supply that has a peak output of 20 amps and see if that works?

I'm not sure what you mean by symmetrical? If I recall correctly if you reverse the supply to both armature and field windings the motor should run the same way. Maybe this is a permanent magnet field motor despite the plate. Yes I suggest a 20A PSU like Stueee. One hopes that the controller has an internal flywheel diode across its output but you could always add one externally.

[lfoggyParticipant @lfoggy]

Posted by Stueeee on 22/01/2023 19:08:29:

When I modified a bead rolling machine from hand operation (where you need 3 hands) to powered. I used a 24V stairlift motor of similar power to yours. The motor runs through a similar PWM controller to yours and uses a 20A 24V power unit bought on ebay.

That's a good looking mod Stueeee, I am impressed. Have ordered a 24V, 25 amp power supply from Amazon so should find out in a few days if this does the trick.

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