Out of interest, and I have no expertise here, what would be the result if the OP just used a half-wave rectifier, perhaps with a high-value smoothing capacitor. I assume that would get around any problems of too high voltage, although I suppose it might be possible to feel a vibration in the motor.
Have a look at the current waveform in a chopped DC motor before you put your foot in your mouth.
And then wonder why an impact wrench undoes rusty bolts better than just a spanner
Mine are that if it;'s AC then the motor is trying to go one way around for one half of teh cycle and teh other in teh -ve half .So it;s not really the volatage that's the problem but it's direction.
half wave has just as many peak volts as full, they just come less often
Anyway motor 'voltages' are a moveable feast.
My model planes use to use 11V on '6v' motors with hugely beneficial effects.
Add a gear box and get three times the power at less heat.
Only issue was brush wear, but the motors lasted longer than the planes.:-)
I was making the assumption, as indeed Brian was, that we were dealing with crudely full wave rectified 12V, from a 12VAC source. And ignoring any diode drops which will be significant in this case.
I have, and generally the chopping frequency is high enough to lead to a near constant current.
I find a breaking bar on a wrench gives a more controlled torque. An impact wrench is equivalent to a hammer rather than a decent press, the sort of thing you do if you don't have the right tool, or insufficient space to do anything else.
Ah well the way I read it was that NO rectifiers were present just the transformer.
------------------------ from the OP I have an 240v to 12v DC (8.5amp) transformer that was used for halogen garden lighting. I want to use this transformer to power a 12v DC motor but it doesn't drive the motor.It just stutters but doesn't turn.
------------------------- My point is that the 'stutters' have nothing to do with the voltage whether 12 10.. 17... but the fact that's teh voltage is AC i/e alternates so the motor might be trying to turn clockwise for 1/100th and then ant-clockwise for a 1/100th sec.
the most hopeless online calculator I've seen in a while. It takes no account whatever of diode drop or capacitor size. IRL there is also transformer regulation - all 3 have a considerable effect on V_out.
NT
The beauty with an impact driver is that it may help break any rust corrosion, paint or seal etc with less chance of shearing the screw. It's certainly the case here with old woodscrews. And far easier to avoid damage to the slot. Prime example being removing the door hinge screws in an old house.
Quite simply I don't understand your point, where I was answering Brian's point which started "if you use full wave rectification".
If you want to make another point about feeding AC into a DC motor it may be more wise to start another thread, then there will be no confusion where the rectified rms value and average DC value were the issues being discussed.
We are talking about percussion here, rather than torque per se.
I don't see the similarity here and chopped waveform driving a motor. It's more akin to placing a large rubber band to the end of a spanner and waggling the other end furiously. The applied torque will be greatly smoothed out. In much the same way the motor inductance will smooth out the current whilst the voltage is changing rapidly.
Not from a 50Hz FWR source
You're getting frequencies getting mixed up. A chopped waveform is typically high enough frequency to be inaudible. I can assure you even the smallest motors will be very inductive at these frequencies.
You'd normally fit capacitors sized for the maximum current draw. And transformer regulation varies by design. You may not be able to find a figure.
It's a rough guide, that's all. Might help those who think a 12v transformer will produce 12v DC under all conditions.
Not to a DC motor - usually in the 30-100Hz range, for the reasons I explained earlier, which ironically you confirmed in your first followup sentence, but you then went on to show you didn't understand it in your next paragraph.
of course it's sized for load current, ripple & reliablilty. V_out depends on size of said ripple, and that is often large.
it makes a big difference to V_out. Lots of 50Hz PSUs give about 50% above voltage spec on little load.
it's just too rough to give useful data.
NT
Not just by using resistors and capacitors alone. The symptoms suggest you're using a 50Hz mains to 12v transformer. As far as tungsten filament lamps are concerned 50Hz 12v rms works just as well as a 12vdc supply. In this case, there'd be no reason to add the losses of a rectifier to the circuit, the lamps would simply be connected directly to the 12v secondary winding.
You need to add a fullwave bridge rectifier pack between the motor and the 12v transformer winding before the motor will function. I'm assuming a permanent magnet (PM) DC motor of some, as yet, unmentioned power rating is being used.
The typical silicon diode based bridge rectifier pack will drop anywhere from about 1.5 to 2 volts depending on the rectifier current rating and the actual current drawn by the load. without the addition of a suitable smoothing capacitor, the average voltage will be slightly lower than the rms voltage even before allowing for the rectifier volt drop.
Adding a suitable smoothing cap could raise the average voltage to somewhere in the region of 14 to 15 volts dc depending on the current draw of the motor. There are a lot of variables, including the transformer regulation between no load and full load, the maximum motor current draw and the choice of smoothing capacitor used.
If we assume you're using a small 12v motor that's limited to a maximum draw of 1 amp continuous before the danger of it burning out becomes a real concern and you use a hefty 1000000uF 16v smoothing cap, the transformer is likely to provide a 3% voltage boost compared to its full
100W loading which will drive the motor at its maximum torque loading with an average dc voltage of 15 +/- 0.5v depending on actual mains voltage at the time. If the motor is able to spin without mechanical load, the voltage will increase by half a volt or so which will increase its rpm by 25% of its free spinning rpms compared to running it off a 12vdc source such as a fully charged car battery.Provided actual speed isn't critical, the motor will happily accommodate such modest speed increases quite nicely. It's sustained excess current draw due to torque loading that can prove fatal to any such DC motor. Within reason, modest speed increases due to voltage aren't a major concern (the speed variation of an unloaded DC PM motor approximates very closely in direct proportion to the applied voltage).
You might receive better advice if you can offer more information about this 12v DC motor and its planned use.
A bit late to this particular party surely?
Tim
I'm inclined to agree. The input voltage is especially confusing as it's a centre tapped 12VAC supply.
Overall load regulation on anything around 100VA is going to be around
10%, but what further messes up any simple calculation is the discontinuous current flow flowing through the resistive component of the transformer and the additional voltage drop from that.HomeOwnersHub website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.