AC to DC to run DC motor

Hi I really could do with some help on this one. I have a 12v DC motor pulling around 5.7 amps from a battery. I want to run the same motor from an AC supply from a small generator. So the generator is pushing out 1100v at 0.05 A. So I run it through a 1KV to 12v step-down transformer, my meter is showing a max of 8.3 amps @ 12.7 volts AC.

So I made a bridge rectifier and tried it with, and without a smoothing capacitor, but fail to get enough power to keep the motor running (although it did throw a 12v relay contacts over).

I simply don't understand why... is it the bridge rectifier or maybe the AC Hz is too high so it cant be rectified ??

The bit that is throwing me is the meter readings after the transformer. I'm assuming I have 12v @ 8.3 amps to play with, even with the diode losses surely I can start that 12v 5.7 amp draw motor.

I should mention that the change over is to happen while the motor is running, it occurs to me that I'm only switching the hot/live via a relay so the earth from the transformer is still connected to the battery, could that be it?

Most likely me being dumb Many thanks for looking at this for me.

by Inventoraptor
September 28, 2020

I have just ordered up a LM7812C to try and smooth out the bridge rectifier output in case there is a load of clutter/noise coming out of the transformer. I am trying anything to get over this issue.

by Inventoraptor
September 28, 2020

With a 100% efficient transformer, if you start with 55 watt (1100 * 0.05), you end with 55 watt too, and at 12 volt, that implies not more than 4.6 amp ( 55 = 12 * 4.6 +/- ) You need more than 68.4 watt. But I would rather see guess around 80% efficiency. So something like 90 watt to start with.

The amperage that you measure, it is done how? at the transformer coil? You should avoid running a motor without load (without the motor doing something) as well as testing a battery, or source, without (resistive) load does not necessary indicates that the battery can sustain that voltage at any delivered amperage.

I am afraid that I can't make any suggestion as far as 1.1 kV AC is implied, for security reasons

With a domestic electricity, I could suggest to use an old PC power supply, from a PC that you don't use anymore. They are much safer, at first glance, and some can deliver up to 600 watt, or more, with a 12 volt nicely regulated, already.

by vanderghast
September 29, 2020

Could you please show a schematic diagram of your setup and nameplate ratings of the main components, motor, generator and transformer. I've put together a system similar to yours with the exception that I used 120v 60hz utility supply rather than a 1.1 kv generator and had no trouble running it so it should be a solvable problem. By the way, don't worry about a smoothing capacitor following the fw bridge rectifier; the motor doesn't care much about 100 or 120 hz or higher ripple except for a bit of increased loss. The mechanical time constant of the motor is generally far longer than a cycle of 100 hz so the motor can't follow the ripple.

by Foxx
September 29, 2020

Thank you Vanderghast, my goodness you have me thinking now. I checked this a hundred times before asking the question. I will spend this evening going back through everything. My initial feeling is you are spot on as the motor not running confirms it. Thank you for your help

by Inventoraptor
September 29, 2020

Thanks for explaining the bridge rectifier Foxx. The generator is very small, about 3 inches long. The 12v motor is powering it (a battery drill motor) The output voltage is way more than my normal meter can read, so I got an old Avo that reads 1000v AC and it went off the scale. I can't give anymore information than that really as it is copyright. The problem Inventors have is that they can't sit down and talk to guys like yourself who have the expert information. When I overlap from Physics to electrical motors, I really get out of my depth.

by Inventoraptor
September 29, 2020

Ah found it, my apologies Vanderghast it was my fault, the output amps is 0.15 amp not 0.05. I have checked my meter and its is 1100 volts @ 0.15 amps AC before the transformer.

That gives 165 watts I think. Wow you had me panicking for a while then.

I had to wind the transformer myself as there is nothing available that voltage, but it seems to do the job ok.

by Inventoraptor
September 29, 2020

Right I have slept on this and Foxx is on the money for me, The issue is the bridge rectifier. So I must do away with that, which then means the DC motor is the problem. So if I have an AC motor doing the driving, all these issues simply disappear.

Its like focusing on getting out of a dead end road when in fact you would do much better not turning into that road to start with.

My thanks to Vanderghast and Foxx for being inspirational in their help.

Cheers guys.

by Inventoraptor
September 29, 2020
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1 Answer

Answer by enovikoff

Did I understand you correctly that you want to power the motor electrically from the generator, and then power the generator mechanically from the motor? Barring some other outside source of energy you didn't mention, this is the classic "perpetual motion machine." Countless inventors have tried to make a perpetual motion machine and failed. The reason is that there are losses in every part of the system due to friction and electrical resistance and electromagnetic radiation. So while I have seen machines like this run for a short while, they always slow down as the electrical energy becomes heat in the rectifier or motor or generator windings due to resistance, and the mechanical energy is lost to friction in the bearings of the motors and due to air motion. There is also radiated electronmagnetic energy from the moving magnetic fields which induces currents in the motor and generator housings that in turn produce heat. Essentially you are creating a mechanical battery that stores energy in the momentum of the shafts of the generator and motor. If you just want to make the battery last longer, reducing friction by putting the assembly in a vacuum, and then increasing momentum by having a large flywheel will allow storing more energy for longer, and then you can remove the energy as electricity when you need it, by connecting a load to the generator so it can do useful work. But if you connect the motor to the generator, then your losses will occur much faster.

+1 vote
by enovikoff
September 30, 2020

Hi Enovikoff. I understand what you say and its been discussed many times.

You cannot have a perpetual motion machine as the machine will always break.

Perpetuallity is therefore a balance of forces and of no practical use as it can only power itself.

If you only use ohm's law to resolve your electrical solutions, you will only get the results predicted by ohms law !

Right Gentlemen I'm going back to my Physics world. Once again thank you for your help with my bridge rectifier problem.

If I get stuck again I hope I will be welcome to return.

by Inventoraptor
October 01, 2020
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