Speed control and reversing in electric drills
Occidental
speed and reversible. "reversible" in a DC motor is trivial, but how
is it achieved in an AC motor? And how, in both cases, is trigger-
controlled variable speed implemented?
ne...@jecarter.us
Think DC.
That AC drill probably has a DC motor - can you figure out the rest of
the speed control circuit?
John
Occidental
> >Most corded (ie AC) and cordless (ie DC) electric drills are variable
> >speed and reversible. "reversible" in a DC motor is trivial, but how
> >is it achieved in an AC motor? And how, in both cases, is trigger-
> >controlled variable speed implemented?
>
> Think DC.
>
> That AC drill probably has a DC motor - can you figure out the rest of
> the speed control circuit?
>
> John
So you are saying that the corded drill contains an AC -> DC
rectifier. The speed control must be a voltage control of some kind. I
presume that the effective voltage going into the rectifier can be
controlled via a circuit similar to that in a dimmer switch. A bit of
smoothing and voila! You have voltage controllable DC entering the
motor. But how is this done in a cordless drill?
whit3rd
Simple switches can do all this. For an AC motor, the relative phase
(time-dependence) of the rotor and the stator can be same (they
have like magnetization and repel) or opposite (they attract),
which gives direction reversal. If there are brushes in the
motor, their connection sequence is reversible.
For either AC or DC motors, the speed can be controlled by winding
the motors with multiple taps, and the trigger can be the selection
switch. I've also seen some DC devices with two cells, and
the switch connects one cell for slow, and both cells for fast
speeds.
It's also cheap to implement AC controls with SCR switches that
function a lot like a light dimmer, or DC with some other kinds of
electronic controls (probably MOSFET).
Phil Allison
"Occidental"
>
> Most corded (ie AC) and cordless (ie DC) electric drills are variable
> speed and reversible. "reversible" in a DC motor is trivial, but how
> is it achieved in an AC motor?
** AC drills use "universal" motors that will operate on an AC or DC supply.
The direction of rotation can be determined by the relative polarity of the
connections to the commutator brushes and the fixed coils on the outside
that provide the stationery magnetic field.
> And how, in both cases, is trigger-controlled variable speed implemented?
** With AC drills, it is done by use of triac phase control - which produces
a PWM version of the AC wave.
DC motors are controlled by PWM of the DC voltage using mosfets.
..... Phil
Jasen Betts
> Most corded (ie AC) and cordless (ie DC) electric drills are variable
> speed and reversible. "reversible" in a DC motor is trivial, but how
> is it achieved in an AC motor?
http://en.wikipedia.org/wiki/Electric_motor#Universal_motors
they just reverse the connections to the brushes (same as with a DC motor)
> And how, in both cases, is trigger-
> controlled variable speed implemented?
for ac
http://en.wikipedia.org/wiki/Phase_control
for DC
http://en.wikipedia.org/wiki/Pulse-width_modulation
Rich Grise
> On Jul 5, 3:10�pm, n...@jecarter.us wrote:
>> On Sun, 5 Jul 2009 11:28:37 -0700 (PDT), Occidental
>>
>> <Occiden...@comcast.net> wrote:
>> >Most corded (ie AC) and cordless (ie DC) electric drills are variable
>> >speed and reversible. "reversible" in a DC motor is trivial, but how
>> >is it achieved in an AC motor? And how, in both cases, is trigger-
>> >controlled variable speed implemented?
>>
>> Think DC.
>>
>> That AC drill probably has a DC motor - can you figure out the rest of
>> the speed control circuit?
>
> rectifier. The speed control must be a voltage control of some kind. I
> presume that the effective voltage going into the rectifier can be
> controlled via a circuit similar to that in a dimmer switch. A bit of
> smoothing and voila! You have voltage controllable DC entering the
> motor. But how is this done in a cordless drill?
Pulse-Width Modulation, or PWM. You interpose a transistor or, more
likely these days, MOSFET, between the battery and the motor, and
control it with a little pulse-width modulating circuit.
You could look up "pulse-width modulation" on google.
Good Luck!
Rich
Nobody
>> So you are saying that the corded drill contains an AC -> DC
>> rectifier. The speed control must be a voltage control of some kind. I
>> presume that the effective voltage going into the rectifier can be
>> controlled via a circuit similar to that in a dimmer switch. A bit of
>> smoothing and voila! You have voltage controllable DC entering the
>> motor. But how is this done in a cordless drill?
>
> Pulse-Width Modulation, or PWM. You interpose a transistor or, more
> likely these days, MOSFET, between the battery and the motor, and
> control it with a little pulse-width modulating circuit.
>
> You could look up "pulse-width modulation" on google.
Also look up "H-bridge" for the reversible aspect.
George Herold
I have no idea, but I'd beleive Phil.
George H.
Ross Herbert
wrote:
:Most corded (ie AC) and cordless (ie DC) electric drills are variable
:speed and reversible. "reversible" in a DC motor is trivial, but how
:is it achieved in an AC motor? And how, in both cases, is trigger-
:controlled variable speed implemented?
Reversing the direction of a single pole ac shaded pole induction motor can be
achieved quite easily as shown in this patent application.
http://www.freepatentsonline.com/3743903.pdf
This is the method used in my 25 year old Hitachi top loading washing machine,
as I discovered when I had to repair it recently. The pump motor had shorted out
and blown the driver triac on the controller pcb, and had also used a copper
track as a fuse in the process. While I had the pcb out I noted two other triacs
were used to control the direction of the main agitator/spin motor.
Phil Allison
"Ross Herbert"
> :Most corded (ie AC) and cordless (ie DC) electric drills are variable
> :speed and reversible. "reversible" in a DC motor is trivial, but how
> :is it achieved in an AC motor? And how, in both cases, is trigger-
> :controlled variable speed implemented?
>
> Reversing the direction of a single pole ac shaded pole induction motor
> can be
> achieved quite easily as shown in this patent application.
** Shame no electric drill uses such a motor.
.... Phil
Ross Herbert
:
:"Ross Herbert"
:
:
I agree with the sentiment - although I am sure you didn't mean it that way.
While I am aware that electric drills don't use this type of motor, I was
responding to the general question posed by the OP, ie. "...how is it (reversal)
achieved in an AC motor?" Perhaps the OP was not even aware that an AC drill
used a so-called "universal" motor. The OP may have been wondering how an AC
"induction" motor was able to be reversed. Now he has answers for both types.
Phil Allison
"Ross Herbert"
"Phil Allison"
> :"Ross Herbert"
> :
> :> :Most corded (ie AC) and cordless (ie DC) electric drills are variable
> :> :speed and reversible. "reversible" in a DC motor is trivial, but how
> :> :is it achieved in an AC motor? And how, in both cases, is trigger-
> :> :controlled variable speed implemented?
> :>
> :> Reversing the direction of a single pole ac shaded pole induction motor
> :> can be
> :> achieved quite easily as shown in this patent application.
> :
> :
> : ** Shame no electric drill uses such a motor.
>
>
> way.
> While I am aware that electric drills don't use this type of motor, I was
> responding to the general question posed by the OP, ie. "...how is it
> (reversal)
> achieved in an AC motor?"
** The OP did not ask that question.
He asked how an AC drill motor was reversed.
Clearly because he had seen the buggers do it !!!
..... Phil
Ross Herbert
:
:"Ross Herbert"
:
I am prepared to concede that you are correct Phil, since the OP was asking
about portable electric drills.
Rich Grise
That'll work on a PMDC motor, but for a "universal" (series-wound)
motor, you have to swap either the commutator leads or the stator
leads, but not both.
Hope This Helps!
Rich