Speed control and reversing in electric drills

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?

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).

"Occidental"

** 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.

** 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

they just reverse the connections to the brushes (same as with a DC motor)

for ac

for DC

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

Also look up "H-bridge" for the reversible aspect.

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I have no idea, but I'd beleive Phil.

George H.

: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.

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.

"Ross Herbert"

** Shame no electric drill uses such a motor.

.... Phil

: :"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 : :

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.

"Ross Herbert" "Phil Allison"

** 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" : "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. :>

:>

:> 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?" : : :** 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 : I am prepared to concede that you are correct Phil, since the OP was asking about portable electric drills.

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