Power supply design

[Herald1360]

Originally Posted by MichaelR

Is this not true only if the motor has no attached load i.e the installation has some sort of clutch mechanism to disconnect the load.If the load is connected there could be a very large current due to the initial Torque required to rotate the motor with load.

Mike

Sort of. Whether the motor is loaded or not will not affect the initial surge peak determined by winding resistances, but a loaded motor will then take much longer for the peak to decay back to running current since torque will be required for longer to accelerate the load against its moment of inertia than for an unloaded motor with just its rotor to accelerate.

[MichaelR]

The surge peak you refer to for winding resistance will not be anything like the Surge peak you will get with a load attached.You have to generate more torque for the motor to move when a load is attached on start up.Granted the surge may only last a relatively short time until the motor gets moving and decay to steadier level. Cold cranking a petrol engine can see surge currents into the many hundreds of amps for the first few hundreds of milli seconds while the motor starts to move dropping to a much steadier but still large current until the engine fires.

Mike

[julie_m]

I know from my own experience that my (piddly little 650VA) portable petrol generator won't start with a 20 watt string of fairy lights on the output .....

The surge current drawn by a motor starting from rest won't be any greater than implied by the winding resistance. It can't, the only thing that would allow that is some sort of induced forward (i.e., in series-aiding with the supply) EMF; but that's impossible, because nothing is moving yet. Once there is enough magnetic attraction and repulsion going on to overcome friction, the armature will move (and, thanks to the commutator, when it gets there it will want to move again). The fact of moving a coil through a magnetic field induces a back EMF; and when conditions are stable, this -- in series-opposing with the power supply -- is what acts to limit the current, so the power drawn by the motor is equal to the rate at which it is doing mechanical work plus any power wasted as heat in the windings. (But the power supply neither knows nor cares what form the Joules it pushes out eventually take; all it sees is a resistor which is initially equal to the winding resistance, then increases in value).

If there isn't enough magnetic force to overcome friction, the motor will sit there stalled, drawing as much power as the winding resistance will admit until something gives way.

[Radio Wrangler]

AJS,

That's a different matter, getting a generator to come up by self-excitation is pretty futile with any sort of load.

I have a 5.5kVA diesel generator (12HP Royal Enfield opposed twin from 1953-ish) and with no batteries or permanent magnets, it relies on stray field to get going. Any load is an impossibility. I normally isolate the output, wind up the excitation rheostat and still have to blip the engine up to about 2500rpm - taking care to let the revs and rheostat down quickly before it could overvoltage its insulation.

One thing not to do is shut the engine down with the machine on-load, for then the dwindling current acts as a demagnetiser. Isolate the output, then shut down the engine and it might have a bit more field left for the next start.

This big generator weighs about a ton and still wouldn't pass your christmas tree lights test, but once up, it has run a couple of 9 inch angle grinders or a big welder.

Starting a DC motor from rest is going to take a current surge just because of the moment of inertia of the motor's own rotor, versus the rate of growth of current due to the winding inductances. Adding more moment of inertia makes this worse, increasing the time involved, and thereby also the current.

Cold starting an engine is a notorious example of huge surge current demand. If the motor does drive a blower for the organ, then the torque curve of the blower is kind to the motor, the load only really coming on as the revs build, and as you say; as that happens back-EMF is doing its job.

The start-up transient is a set of balancing acts with -Ldi/dt and moment of inertia and field strength and mechanical load characteristic all acting together.

Going any further with this has to wait for some info on the scale of the beastie, and how it's been started in the past.

David

[McMurdo]

Why not use a thyristor DC drive? That will give soft start and regulation.

In the pottery industry in the 1970's they used a variac feeding a rectifier to give you DC for potters wheels. The mains was interlocked with a camswitch so you could only turn it on with the variac at zero.

A 35A silicon bridge rectifier will probably blow with the inrush trying to start a motor like that from standstill..or else blow your supply fuse, or simply lead a short life. Maybe an old manual resistance starter can be found if you want to be authentic?

DC thyristor drives have been used for organ keyframes for years albeit with an AC motor for the blower or bellows.

[MichaelR]

Originally Posted by Radio Wrangler

AJS,

The start-up transient is a set of balancing acts with -Ldi/dt and moment of inertia and field strength and mechanical load characteristic all acting together.



David

Precisely and succintly

Mike

I want to see thie pictures, with a bit of luck that will happen on Sunday...

[DavidHunt]

Not a fairground Organ! A large domestic one by Imhof. Motor is stamped 110V and only getting 80 off load and 60 on load. Only appears to take one amp! Hope the photos help - the motor seems older than I was expecting.

So what does it drive?

[TrevorG3VLF]

What sort of motor is it? It looks as if the field is an electromagnet and it looks as if there may be a very large inductance on the top to smooth the rectified AC.

With a large drop of volts in the rectifier, it would imply that it needs a little help from some diodes across it.

[Refugee]

That is the field coil. The commutator is facing the camera and below that painted black is the second field coil.
I would replace the rectifier with a 5A bridge on a heat sink and test the capacitor and only replace it only if it is dud.
It does look like a universal motor but then again I would stick with what is there.

[Refugee]

I have looked again at the picture.
The field magnet looks like solid iron.
It will be DC only.

[DavidHunt]

Thank you refugee. I have fitted a bridge and replaced the very dead capacitor with a similar one 250u and normal voltage 110 is now resumed - even at full load (ie near stall). This motor apparently drives three pumps. One for the organ pipes, one for the vacuum controlled roll mechanism and a third for stop changing, roll reverse etc.

[Refugee]

It might be worth checking for play in the shaft. It should be pretty easy to replace the bearings if they are loose.

[Radio Wrangler]

Ah, an Imhof and Mukle 'orchestrion' rather than an organ.

A sort of mini fairground organ for tasteful indoor use, provided one had the servants to power it.

Such an early motor could well have been added to one when power became available, and then the transformer/rectifier for later mains use.

Here's one of their grandest instruments:

http://www.youtube.com/watch?feature...&v=aUM8UOTlcaM

What a great thing to get to fix!

David

[DavidHunt]

Oh no it's not!! It's an organ - as in very beautiful wooden pipes only. And no Mukle! Huge rolls of cardboard in "cassettes". David