Power supply design

[DavidHunt]

I have to design a power supply for a 100 volt DC motor (for a fairground organ). I am wondering if I strictly need a capacitor to smooth the Dc, and if so how to specify ti. Any advice gratefully received.

I wouldn't think so, and most DC motors are quite happy on AC if they have laminated iron. Having said that a bridge rectifier is cheap, it is well worth putting some suppresion on the brushes to reduce interference and increase brush life. If it runs OK and doesn't get hot it's OK

A picture (as always) would be useful.

[elanman99]

Originally Posted by merlinmaxwell

I wouldn't think so, and most DC motors are quite happy on AC if they have laminated iron. Having said that a bridge rectifier is cheap, it is well worth putting some suppresion on the brushes to reduce interference and increase brush life. If it runs OK and doesn't get hot it's OK

A picture (as always) would be useful.

I presume you meant to say 'unhappy'.

DC motors might not bother with a bit of ripple (which could be defined as a sort of AC) but generally wont run at all on an AC supply and would most likely be damaged.

As to the original question, a transformer and bridge rectifier correctly rated will be OK for a brushed motor, things get more complicated if variable speed is required but there are quite a few ready made DC motor controllers and they might work out to be cost effective.

Ian

[Tractorfan]

Hi,
I thought that if a brush motor was series connected, then it is considered a "universal" motor and, therefore, will run just as well on AC or DC. This is what is usually fitted to vacuum cleaners, etc.
I think, but I'm not certain, that a shunt connected motor will be OK as well. Reversing the connections to the field coils OR the armature will reverse the rotation.
Just my 2 cents worth.
Cheers, Pete

[Radio Wrangler]

It's probably rather an old DC motor, most likely once run from a showman's engine.

The armature will be laminated, but we can't be sure that the field stator core is laminated.

A a pair of large ex-telecomms 48v regulated supplies might be an answer.

What are the motor ratings? Horsepower and amps?

I assume it's main load is a blower, so that gives it a soft load for starting.

David

[elanman99]

Unless the original poster gives some more details our suggestions are only guesswork.

I've tried reading between the lines but there is not a lot to go on, all he said was, DC motor and fairground organ. From that I would assume old, brushed, fairly high wattage and not very efficient so the current might be high too. Its unlikely to have a permanent magnet field but it might just have a solid stator that should only be energised with DC.

Ian

[DavidHunt]

Original poster here! I have no more info at present; the motor arrives on Sunday. It is reputedly built c.1900 and 100V DC with no specification plate. There is a power supply built in the 50/60s with a selenium(?) rectifier and the resulting voltage is about 60. I have a 35 amp bridge in my paw awaiting the job - the result will no doubt be too high a voltage. It is caps for smoothing which are concerning me as the originals are no doubt beyond their sell by date. I would expect such an inefficient motor to be drawing amps rather than mA. Many thanks for thoughts so far.........David

[elanman99]

When you say the voltage will be too high, are you referring to the lower voltage drop of Selenium as against silicon? If so I would have thought it would not make much difference to the motor.

What voltage comes out of the transformer? I hope there is one!

Ian

[Radio Wrangler]

Something has passed me by... Isn't 60V lower than 100V, so a lower rectifier drop would be a help?

Unless the '100V' motor running at 60V happens to suit the fairground organ.

As an aside, I saw one of those jobs-from-hell a few years ago at Lathalmond steam rally. One of those mobile clutch fitting firms was hard at work under a seven-and-a-half ton truck, pulling the gearbox, while above them a rather large Belgian fairground organ was going full tilt, drums cymbals, the lot.

David

[DavidHunt]

Hmm. I am planning to replace a 60 year old (Selenium?) rectifier with a new bridge. I am assuming/hoping that the voltage will rise! - from the present 60 to 100. (Or possibly higher than when the original was new?). This however is not what concerns me as may be seen from the original question. I am uncertain how tio specify an appropriate smoothing capacitor for such a high current power supply. David

[Refugee]

If it is just a motor it will not need smoothing as the currant draw from a smoothed power supply will give the transformer a harder time and it will be an old one. Model railways never had smoothing and they did not have problems.

[Tractorfan]

Hi,
I don't think smoothing caps will be necessary either. I can't see that some ripple from a rectified supply will cause any problems. A motor of this age will most likely have been supplied from a dynamo driven by a showman's engine way back then, and I'm sure that the output was far from pure DC. It's probably built like a battleship anyway, so wouldn't suffer too much from an incorrect supply or waveform.
If it were my project, I'd just wire it up, throw the switch and see what happens!
Cheers, Pete

[Herald1360]

I might go for something rather more substantial than a diddly 35A rectifier unless there's any sort of soft start (stud switch and wirewound resistor sort of thing) controlling the motor. The starting current on a dc motor is only limited by the winding resistance which on the rotor at least will be as low as possible for best efficiency. When up to speed, the back emf comes into play, reducing the current to just what is needed to supply the torque demand including motor mechanical losses. The old selenium jobs would likely have had enough internal resistance to limit the surge to something they could cope with.

You can probably get something with suitable overkill ratings within the likely budget for this sort of job, unless the outfit is run on a shoestring!

Originally Posted by DavidHunt

I am uncertain how tio specify an appropriate smoothing capacitor for such a high current power supply. David

There basically isn't one. If any smoothing were to be done I would expect a choke to be the principal element.

[MichaelR]

Originally Posted by Herald1360

The starting current on a dc motor is only limited by the winding resistance which on the rotor at least will be as low as possible for best efficiency.

.

Chris

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

There basically isn't one. If any smoothing were to be done I would expect a choke to be the principal element.

The motor itself will do that.

[Radio Wrangler]

Hmm,
I've come across organs with 2 to 15 horsepower blowers as classical, fixed instruments and cinema organs are in the same league.

Let's take a guess at 2kW for the motor, which may be a bit on the low side. On 100v that's 20 amps running current. There isn't much margin for start-up surge.

I think I'd be looking at making a big heatsink structure using at least 100A stud-mount diodes, possibly 200A types. I'd also introduce some resistance in series with each diode to limit surges, especially if a smoothing capacitor is used. If the old supply with its selenium rectifier and capacitor worked OK, then the low risk approach is to replicate it.

Instead of monster diodes, you could use paralleled arrays of more mundane types, each with a current sharing resistor. The reservoir would be a bank in parallel. A big variac might be a gentle way to bring things up.

On an alternative view, 3-phase motors are pretty cheap on the surplus market because most homes and small businesses are single-phase. A nice meaty motor could replace the DC one, and one of the single-to-three-phase motor control inverters would make it variable-speed, soft start and provide protection. I have one on a motor for my lathe and it's superb. Induction motors are a lot more reliable than brush/commutator types.

David

[DavidHunt]

Help!! So many ideas - for which many thanks. Perhaps I should wait until Sunday when I have the motor and dead power supply in my hand and can work out some facts. David

[Refugee]

Judging by the fact that it is a selenium rectifier that was there before we might be in for a shock.
It could well be the player roll driver running at less than half an amp.
I am pretty sure those sort of instruments had a direct drive fan from either diesel or steam piston engines.

[broadgage]

If the motor is indeed for feeding the roll or sheet music then it will be very small. Some were not electric at all but clockwork, others used a small belt drive from the blower.

If it is for an organ blower, then the load is probably substantial, I have seen blower motors of up to 5 HP or about 50 amps.
The air pressure needs to be constant for proper working of the organ, but this is usually achieved mechanicly and does not require a well regulated motor speed.

The original supply would have been from a dynamo on a steam engine.
Depending on the load I would try a 110 volt building site transformer and a large bridge rectifier.
Starting originaly was probably by a manual faceplate DC starter, and some means will need to devised to limit starting current. A large variac as suggested above sounds good, but does not protect against supply failure and unexpected re-instatment. A latching contactor could be added for this.

[Refugee]

Most of those yellow transformers have a 100% intermittent rating and will deal with the start up in the way they do when you connect up a large disc cutter to them.
A 2KW cutter will stall most 2KW petrol generators instantly.