Does anybody know what size power supply I might need for a typical cordless drill motor? For example, I wish to power a 19.2VDC motor from black and decker/ Ryobi drill. Could I get by with a 18VDC supply? What amperage would I be looking at? I think most of those battery packs are 2.4Amps. If I put a 4 amp supply would that be sufficient or would that overpower the motor? I called DeWalt and the technical support person wouldn't tell me (I think for liability reasons). He said that people have tried to use a car battery to power a dewalt drill and something exploded or went haywire on them.
To the best of my knowledge, it's the impedance of the motor that (for a given voltage) determines how much current flows. The battery is not generally the limiting factor.
You can use a lower-voltage batttery pack, but the device's performance will suffer.
As to a car battery damaging a drill rated at 13.8 V or higher -- absurd.
A couple of things to understand: the Battery is rated in AMPERE-HOURS or AH, so a 2.4AH battery is capable of putting out 2.4A of current for one hour at rated voltage in theory. However, your drill likely will want something on the order of 15 or 20A of current, so a battery will last proportionately far less than an hour.
A 4A supply will likely not be adequate to run the drill under any sort of load. Too low a voltage will greatly overheat the drill if it turns it at all. Motors are sensitive to low voltage and low current - Imagine a DC motor as a series of short-circuits creating a magnetic field that pushes the armature on to the next short-circuit and round she goes. So, if you have Lots-O-Current but not enough voltage to turn the motor, all that current will sit across whatever winding
*sizzle*.12V (more properly, 13.6V lead-acid batteries) will do exactly that.
Do not worry about feeding the motor *TOO MUCH* current IF AT THE CORRECT VOLTAGE! The motor will only draw what it needs, no more.
Bad Idea overall. A 20A (minimum) 18V P/S will take quite a large transformer, a heavy rectifier (and that too has implications - a 12V secondary bridged will yield ~16.5+/-V) and you will need a massive amount of capacitance to smooth out the ripple. Cheaper & safer to buy a new (several) batteries and charger and this time take care of them properly so they do not fail. You could rig up three identical sealed nominal 6V lead-acid batteries and get reasonable performance, but you had also better use at least 10gauge wire on the leads and make dead- sure you do not short anything. Problem is that sealed lead-acid batteries are both relatively "slow" and do not take repeated deep discharges very well.
Read my post below. If the motor does not turn at the correct speed, a car battery that is capable of nearly-infinite amperage will fry the windings in short order. Due to my secondary hobby (R/C boats & submarines) I run LOTS of motors at LOTS of voltages, from 3.6 to 24V, and from everything from true Gates Cells through 2 @ 12V VW batteries for the big tugboat. It is low voltage at high amperage that fries motors. Not too-high voltage. An unloaded 12V motor with a properly epoxied armature and good bearings will take 24V nearly all day. Load it and water-cool it, for considerable time. A 24V motor unloaded will not do well at 12V/high amperage. Try it. A loaded 24V motor will burn up in short order on 12V. That is why resistance speed-controls have gone out of favor in the hobby except at the very lowest end. Too many motors were dying. Even in trains, the trend is to pulsed full-voltage vs. resistance speed controls. Furthermore, those motors are very specially designed for the use with massive magnets, rather heavy magnet wire and heavy bearings if they are expected to last.
Of course, train transformers are current-limited - but batteries in drills, R/C boats and submarines and such as well as vehicular batteries are not unless outside devices are installed (as most of us do anyway), relative to the use.
For it to perform properly, an extremely high current one.
Don't think they've got the same maker. Ryobi is part of the Techtronics group.
It's actually the peak current capability that determines the performance rather than the voltage - unless that's vastly different.
If you want maximum torque I'd guess at around 20 amps. If all you want to do is drill a few holes much less.
That's the capacity in amp/hours.
A supply delivers the power the device demands up to the capacity of the supply - not the other way round. But 4 amps won't be enough for tasks which require full power.
Because they probably won't know.
Make that definitely don't know - about anything. ;-)
I'd ask why you're bothering, though, given the cost of a new mains drill.
--
*Gaffer tape - The Force, light and dark sides - holds the universe together*
Dave Plowman dave@davenoise.co.uk London SW
To e-mail, change noise into sound.
The batteries are rated in Amp/Hours, that is *not* the same as Amps. A
2.4A/hr battery pack can supply tens of amps for short periods of time, and under heavy load many drill motors will draw in that range. A power supply to run that will cost more than a plug-in drill, however a car battery or gel cell will work just fine if you hook it up right.
That's because air conditioners use induction motors which are inherently constant-wattage devices, lower the voltage and the motor pulls more amps to maintain power. A permanent magnet drill motor does not behave that way. Drop the voltage down and the amperage follows it and torque is reduced. If you have an effectively unlimited amperage available from the power source, it's possible to load the motor enough that the airflow through it is insufficient for the power it's dissipating, but lowering the voltage does not automatically cause this.
A rotating motor also acts as a generator and generates a voltage that is opposite the driving voltage. This limits the current. That limited current is used to calculate the motor windings and allows for smaller wire sizes. At reduced driving voltage there may not be a fast enough rotation to generate a sufficient back emf and the current will rise. As an extreme case consider a DC motor in a stalled condition. It may rapidly overheat and fail.Many a little device like the popular Dremel tools have met a sad ending because of this. Ohm's law works as advertised, but the effects of back emf must be considered.
It depends largely on what you expect the motor to do. What kind of a load are you going to put on it?
The harder the motor has to work, the bigger of a power supply you'll need. Voltages should be close but don't necessarily have to be exact.
If properly set up, that would work just fine. I could see the biggest problem coming in the form of short circuits or poor wiring. A car battery can produce huge amounts of current for a long enough period of time to cause impressive damage. Some people who have tried this probably didn't fuse or otherwise protect the circuit...which works fine until something goes wrong! Then something might go "bang!" or worse.
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