Powering cordless drill motor with DC transformer

[eljainc]

Hello,

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.

Mike

 

[William Sommerwerck]

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.

 

[[email protected]]

Hello,

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?

Mike:

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.

Peter Wieck
Wyncote, PA

 

[[email protected]]

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.

William:

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.

Peter Wieck
Wyncote, PA

 

[William Sommerwerck]

On Nov 2, 4:44 pm, "William Sommerwerck" <[email protected]>
wrote:

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.

So you're arguing that a low voltage will cause too much current to be
drawn. Okay. It happens with air conditioners during brownouts.

 

[Dave Plowman (News)]

Does anybody know what size power supply I might need for a typical
cordless drill motor?

For it to perform properly, an extremely high current one.

For example, I wish to power a 19.2VDC motor from black and decker/
Ryobi drill.

Don't think they've got the same maker. Ryobi is part of the Techtronics
group.

Could I get by with a 18VDC supply?

It's actually the peak current capability that determines the performance
rather than the voltage - unless that's vastly different.

What amperage would I be looking at?

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.

I think most of those battery packs are 2.4Amps.

That's the capacity in amp/hours.

If I put a 4 amp supply would that be sufficient or would that overpower
the motor?

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.

I called DeWalt and the technical support person wouldn't tell me (I
think for liability reasons).

Because they probably won't know.

He said that people have tried to use a car battery to power a dewalt
drill and something exploded or went haywire on them.

Make that definitely don't know - about anything. ;-)

I'd ask why you're bothering, though, given the cost of a new mains drill.

 

[James Sweet]

Hello,

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.

Mike

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.

 

[James Sweet]

So you're arguing that a low voltage will cause too much current to be
drawn. Okay. It happens with air conditioners during brownouts.

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.

 

[Charlie Bress]

Hello,

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.

Mike

Do a Google search on the term "back emf"

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.


Charlie

 

[William R. Walsh]

Hi!

Does anybody know what size power supply I might need for a typical
cordless drill motor?

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.

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.

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.

William