Soft start-up on DC Motor, would a Thermistor work?

[www.GymRatZ.co.uk]

Afternoon all.

I've got a drum filter (like a big washing machine) for the pond. It's driven by a 24v worm-drive gearbox DC motor thing. Earlier this year the motor/drive packed up so I replaced it with a slightly faster turning unit as the original one was way too slow and wasted loads of water.

The following page has all the the spec. etc.

OEM unit was the last one on the list GMW40W0634 which I replaced with GMW40W0505 - Speed and torque on the new unit are perfect however the initial start-up current (apparently) of the new unit overloads the 24v

2A supply PSU causing it to shut down and re-start and so the on-going cycle continues.

Manufacture has advised this doesn't happen on the correct unit (slow) drive.

If I rotate the drum to take up the slack in the gears etc, on start-up the drum rotates without any issues so presumably the fraction of a second on no-load when the slack is taken up allows things to work perfectly.

The following vid. is far more useful to demonstrate the issue.

I want to stick with the new motor/gearbox because it's way more suited to the drum cleaning operation and uses a fraction of the water to clean a greater expanse of filter screen. Could anyone suggest a suitable thermistor(?) to stick into the 24v Motor feed that would limit the inrush current to not exceed the 2 Amp cut-off of the PSU? (12 ohm ?)

Or... am I braking up the wrong tree or is there another solution? I suppose a beefier 24v supply would do the trick?

Cheers All Pete

[DerbyBorn]

Trying to think of a mechanical solution!

[Archibald Tarquin Blenkinsopp]

Don't even think about it!

You will be operating on a wing & a prayer, even if the system works a bit of cold weather or maybe a slight fall in PSU efficiency will tip the balance.

The tables are for 12V

At 24V the stall current will be considerably higher.

You will have to get the stall current rating at 24V from the manufacturer and select a supply to suit. Might be problems here though as they would probably not wish to depart from or expand on published data.

You could of course get hold of a supply, or 24V battery and measure the stall current. [Use a fuse and dont dally with the measurement].

If you go in "blind" and want to be sure, then a 24V 10A SMPS from Ebay shouldn't break the bank.

I bought a few of varying Voltages and currents, the prices are unbelievable in comparison to what they once cost.

AB

[A_lurker]

Van Doorne style automatic gearbox?

[Archibald Tarquin Blenkinsopp]

I think you have the wrong idea.

Before you initiate the start sequence of the mechanical contraption the motor is stalled.

The motor is stalled if you glue it with Araldite,

The motor is stalled if you gear the thing up so the output shaft rotates at near relatavsitic speeds,

The motor is stalled if you replace it with a gear train that takes more than a human lifetime to complete an output shaft rotation.

The motor on its own is stalled, it is stationary, it generates no back EMF.

Discounting inductive reactance the situation boils down to I = V/R

Unless your gearbox is so intelligent that it can spin the motor up before power is applied, the the problem is electrical.

The condition could be catered for fairly easily from an electrical perspective, but it's far simpler and more cost effective to buy a bigger PSU. A larger PSU would also have the ability to maintain operation if any increase in mechanical load occured.

Gear replacement cannot be a serious option anyway, the costs are outlandish.

AB

[Fredxxx]

The power supply seems able to supply a start current for a split second before switching off. The filter even rotates a small distance.

In this case I agree a mechanical solution is not an economic one.

I do feel the easiest mechanical solution would have been to have a centrifugal clutch, but hey that's still more expensive than a power supply or series resistor.

[Archibald Tarquin Blenkinsopp]

The PSU is overcurrent protected.

Electronic overcurrent protection has to actually detect overcurrent to operate.

When it detects overcurrent it folds back or shuts down the output.

Not exactly "rocket science".

If you then think things through very carefully, you will realise that the excess current flow must still adhere to the laws of physics, hence the magnetic flux and resulting brief movement.

Clutch? Resistor?

That would be a systematic problem with the UK educational system.

Brexit is another example.

AB

[A_lurker]

Don't disagree with any of that, but presumably an external load will alter the area under the initial bit of the current/time curve, so that a PSU with some capacitance in the output might succeed in starting the motor if it were more lightly loaded.

My suggestion was, of course, largely facetious, but I've always wondered whether such a gearbox could be cobbled up out of an old fan belt and some split pulleys...

[Archibald Tarquin Blenkinsopp]

You would probably find that a series resistor would drop the Voltage too low to enable the motor to turn, all the power would go to warming the resistor. It could be tried but I wouldn't even go to the trouble of opening the components drawer personally.

Capacitance is the answer, charge a very large capacitor via a resistor of course then after a time period to allow the charge to be developed switch the load in.

The trouble is the sequence would be power up the SMPS, wait, Switch in the load to the charged capacitor, wait briefly while the motor starts, switch the motor over to the SMPS output.

Timers, Relays, Damn big capacitor, time to assemble box up etc.

Obviously it is difficult to be absolutely precise, but it is virtually certain that the overcurrent event occurs well before the motor has a chance to build the speed up to provide the back EMF even under no load conditions. The PSU will no doubt give a much higher current than the rating states, but as soon as the overcurrent is detected the output will fold back or shut down. This has to be fairly fast in order to protect the semiconductors.

Hence the simple almost zero uncertainty solution, a bigger PSU.

AB

[Harry Bloomfield]

The very weak start up torque of electric clock motors, used a fine hair spring between the motor output shaft and the gearing drive.

[Archibald Tarquin Blenkinsopp]

To prevent a grid shutdown?

AB

[Fredxxx]

The protection seems to kick-in after a short time.

Ones that are sensitive to overcurrrent, tend to ramp up the voltage in a controlled way.

Of course, but the time to shutdown, and the level of overcurrent to cause shutdown are unknowns.

You seem to dispense with the 'resistor' yet is the cheapest and simplest method of reducing current. We might find that adding 1ohm to the circuit is all that is need for a start without a shutdown. 1ohm at running current will be at most a 2V drop with a 10% drop in speed.

A condescending loser Remoaner. I could guess from your post.

[Fredxxx]

The protection seems to kick-in after a short time.

Ones that are sensitive to overcurrrent, tend to ramp up the voltage in a controlled way.

Of course, but the time to shutdown, and the level of overcurrent to cause shutdown are unknowns.

If you think even more carefully you will realise it is possible to have a very fast shutdown.

You seem to dispense with the 'resistor' yet is the cheapest and simplest method of reducing current. We might find that adding 1ohm to the circuit is all that is need for a start without a shutdown. 1ohm at running current will be at most a 2V drop with a 10% drop in speed.

Clearly a loser Remoaner. I could guess from your post.

[Archibald Tarquin Blenkinsopp]

Really? You have to sense the overcurrent first. There has to be a lag to prevent nuisance tripping otherwise every minor glitch would trigger a shutdown.

If the motor was running you wouldn't need a resistor. If you add resistance the motor wont move, if it could move on a lower current it wouldn't be tripping now would it?

AB

Not a loser, not a remoaner. I think the EU is better off without Britain. Britain was kept out for a long time before Heath managed to worm his way in.

Whatever is said by Junker & friends I would guess that they are more than happy to see the back of the UK.

I certainly will not lose, I'm stciking firmly with the EU. The losers are going in another direction, what that direction is, they haven't a clue.

AB

[Fredxxx]

I wouldn't call a 4 amp load a glitch, of any duration.

The datasheet gives specs of normal running current. The system in the video implies there is very little load in normal running.

I'm surprised you claim that. The issue is starting current that I recall will be in the order of 4.5Amps and hence implies the motor has a resistance of 5ohms. The running current should be a fraction of this.

I think they want to apply some pain and grief to the process in perhaps a vindictive manner.

Apologies, it seems we have a sort of accord on this.

[Johnny B Good]

The synchronous motors needed the one way clutch effect of the 'fine hair spring' wrapped around the motor shaft to persuade the motor to run in the required 'clockwise' direction upon *every* application of mains supply to eliminate the 50:50 chance of running backwards on start up.

Most Microwave ovens with turntables seem to employ a similar synchronous motor except for the absence of a 'clockwise persuader clutch spring mechanism' since it doesn't matter which direction the motor happens to start turning in. I might be wrong but even these motors might employ some form of spring assist to get them running since they seem to alternate direction at each startup with a remarkable consistency ime.

[The Natural Philosopher]

That is so pricelsss it ought to be framed and put on a wall.

A man who has absorbed a little physics, but has completely failed to learn how to think.

Gotta be a dyed in the wool socialist.

[jrwalliker]

All the clock motors I remember taking apart used a shaded-pole phase shift er to ensure the correct running direction. A thick loop of copper was pla ced around part of each pole piece to create a rotating magnetic flux with a predictable direction. No special springs required (except in anti-backl ash gears).

John

[Fredxxx]

In my very young days I had two types of mains clocks, ones that would start, and some that would not.

The ones that didn't start required a manual push on a wheel. The others use a clutch spring as per Harry's and Johnny's description. I don't recall one that started in the wrong direction.

When power cuts were numerous it was an easy matter to determine its time and duration!

[tabbypurr]

There's no shortage of such stuff when electronic questions are asked here.

NT