DC motor reversing

[Cornish JackParticipant @cornishjack]

The photos below show a 'recovered' part of an invalid mobility lift which I was hoping to adapt as a power lift for the mill or similar. It is intended to operate on 24v DC and I have tried it successfully on a 12v power supply – but only in one direction. Reversing the live and neutral inputs just produces a low 'buzz' with no movement. Any pointers to how to connect it for selectable rotation, please?

TIA

rgds

Bill

[Cornish JackParticipant @cornishjack]

[Brian Sweeting 2Participant @briansweeting2]

A couple of thoughts, I'm no expert on these things…

Are there any limit switches in the circuit?

Is the ram mechanically sound and able to operate in both directions?

What is the small PCB for?

[John RuddParticipant @johnrudd16576]

Posted by Brian Sweeting on 17/07/2020 18:49:53:

A couple of thoughts, I'm no expert on these things…

Are there any limit switches in the circuit?

Is the ram mechanically sound and able to operate in both directions?

What is the small PCB for?

Looks like there is at least a relay for up/down operation…

No doubt our resident experts will come up with a more appropriate explanation………🤔

Edited By John Rudd on 17/07/2020 19:11:39

[Eric LucasParticipant @ericlucas71410]

I would try it using 24 volts

regards

eric

[Cornish JackParticipant @cornishjack]

Brian and John, thank you.

The column does extend/retract mechanically on the screwed drive shaft when it's rotated by hand. I'm assuming that if I can get the motor to reverse and install a chonge over switch,, it will offer two-way movement.

rgds

Bill

[EmgeeParticipant @emgee]

Hi Bill

If you have the make and/model there may be some info if you search the net.

As said there may be some limits on the screw movement limit to activate the reversing function, clearly a relay on the board.

Emgee

[Ian PParticipant @ianp]

If its a DC motor, which ever way round you connect it to a DC supply it would not buzz so there must be some AC present.

With brushed DC motor, reversing the connections to the brushes will reverse the motor. I do not think that the blue and brown wires you show go direct to the brushes of the motor

The two connections to the motor might be brown and blue but they are not live and neutral, if the unit operates on a DC supply then they will just be positive and negative

If the blue object on the PCB is a relay then that might be the device that actually reverses the connections to the motor.

More investigation needed, is there any label on the motor?

Ian P

[Bruce NewmanParticipant @brucenewman56212]

A permanent magnet dc motor will reverse direction when the input connections are reversed but a dc motor with field windings will not. That said its most likely a permanent magnet one.

I suspect the buzzing is from the relay which needs 24 volts to work ( or at least about 18V) but will not work correctly with 12V.

To reverse a motor with field windings you have to reverse either the field or armature windings but not both at the same time.

Bruce

[Ian PParticipant @ianp]

Small PCB mounted relay coils are usually driven with DC, there might be some exceptions but I've never seen a PCB one with a coil designed for AC.

In any event whilst a relay might not work on a lower voltage than its coil is intended for, it will not buzz on a DC supply.

Ian P

[Robert Atkinson 2Participant @robertatkinson2]

Do you know if it was working OK before you aquired it ?
IT sounds a lot like a worn out thrust bearing causing the worm gear to jam. One bearing typically takes more load than the other, pushing th armature one way and pulling the other.
Ideally get the worm drive disengaged and see how the motor runs then. Also try and separate the motor connections form any other circuitry.

Robert G8RPI.

[Bruce NewmanParticipant @brucenewman56212]

Relays will buzz if the voltage is too low if when the contacts make and the motor draws current causing the voltage to dip making the armature drop out before it has fully operated, this will happen repeatedly causing a buzz. I notice the psu is only 1.5A which is a bit on the low side. It needs to be tested on the correct voltage.

[Jon LawesParticipant @jonlawes51698]

It may have an electronically actuated brake.

[I.M. OUTAHEREParticipant @i-m-outahere]

I’m wondering if there might be some sort of friction clutch with a one way bearing in it to release the drag from the clutch when it is lifting , this could give a more controlled rate for lowering as the one way bearing would lock and activate the clutch . Maybe the 12v is enough to make it work in the lifting direction but not enough torque to overcome the clutch if it has one fitted ? What is hidden under that copper band ?
Just a thought !

[Pete RimmerParticipant @peterimmer30576]

Posted by Jon Lawes on 18/07/2020 01:35:28:

It may have an electronically actuated brake.

As a lifting device for the disabled, I'd say it has a brake that is held off electrically and fail-safe's to 'braked'. Probably under that copper strip.

I'd start with pulling out the motor and testing it outside the housing. Certainly don't test it with 24V if it's wedged with 12V.

[Cornish JackParticipant @cornishjack]

Thank you to all replies. Much there to check and try. The original (an Oxford Standaid) was operated via a pendant control and powered from a rechargeable battery. The point made about 'fail-safes' certainly needs checking.

rgds

Bill

[GeorgineerParticipant @georgineer]

Just a thought: I know the power supply says "Regulated" but a lot of school DC power supplies are quite primitive, basically just rectified AC with little or no smoothing (ex-teacher speaking). The resulting ripple would easily give a buzz or hum to a circuit which wasn't quite managing on 12 volts.

George B.

[SillyOldDufferModerator @sillyoldduffer]

Posted by Georgineer on 18/07/2020 10:36:20:

Just a thought: I know the power supply says "Regulated" but a lot of school DC power supplies are quite primitive, basically just rectified AC with little or no smoothing (ex-teacher speaking). The resulting ripple would easily give a buzz or hum to a circuit which wasn't quite managing on 12 volts.

George B.

Good point, or even the other way round! Bill's PSU looks modern, in which case it may automatically current limit to stop youths blowing it up by shorting the terminals. Very simple PSUs rely on a fuse, more modern (like my 1950's Triang Train Controller), have resettable thermal cut-outs, later came electronics, which play all sorts of games to protect the PSU. One tactic is chopping the voltage so the current never exceeds 3.5A: rather than clean DC the overloaded PSU outputs pulses, causing buzzing.

With luck it's simply lack of power from an unsuitable PSU. The motor is buzzing because there aren't enough amps to run it in reverse# and it's stalled. If the motor is stalled, possible to detect it by manually turning the shaft. Strong resistance in one direction, turning easily in the other means the motor is trying.

Is the motor worn, or possibly not meant to reverse? Although DC motors are reversible in theory, in practice bearings, brushes and gears can be arranged to strongly favour turning in one direction only.

Dave

# Or maybe it needs more power to run forward because years of lifting people upstairs have worn the gearbox and brushes? Reversing downstairs is light work, and the motor/gearbox might turn easier in that direction because it's less thrashed.

 

Edited By SillyOldDuffer on 18/07/2020 11:19:13

[Cornish JackParticipant @cornishjack]

Just to tidy this one up – George and S O D plus Bruce earlier got it right. I dug out another 12v source and tried it … and Bingo!. Works fine in both directions. It was originally powering the lift arm taking full body weight, so, even at 12volts it should provide useful lift. Thanks again to everyone for advice and pointers.

rgds

Bill

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