May be wrong about modern drills, but older ones used the same motor for all different output speeds. The difference was in the gear train after the motor.

One I had had two speeds, the lower speed was achieved by putting a diode in series with the motor.

 

I wonder what would happen if you put the field and rotor in parallel and then ran it from a variac? if you don't need it to run continuously, see how long it takes to heat up under load should give an Idea of the maximum you could run it for.

 

I wonder what would happen if you put the field and rotor in parallel and then ran it from a variac?

The field and rotor windings of a series wound motor may not be designed to share the voltage equally. The field windings for shunt wound motors are generally more turns of finer wire than for series wound motors.

 

I wonder what would happen if you put the field and rotor in parallel and then ran it from a variac? if you don't need it to run continuously, see how long it takes to heat up under load should give an Idea of the maximum you could run it for.

You would burn the motor. Somehow the stator and rotor need to be in series so they'd sync with the ac source. Connecting them in parallel freezes the motor.

 

Shunt the field, whichever one that is. You want to reduce the strength of the field.

With what resistor value? If you suggest a good approximation, I'll take it from there and start tweaking it.

 

You would burn the motor. Somehow the stator and rotor need to be in series so they'd sync with the ac source. Connecting them in parallel freezes the motor.

If the motor freezes with them in parallel it would be because you would need to reverse the polarity/connections to the rotor. I have a very large industrial 15HP repulsion induction motor that runs on AC , the repulsion is for starting under very heavy load, and a centrifugal switch kicks in when used on AC to run it. But we used it to drive a very old big lathe and wired out the switch. If the field coils were wired the wrong way round, it froze because it was magnetically locking the rotor to the field. unlike poles attract like poles repel.

 

I purchased a couple of 100VAC, 600RPM heavy duty drills a while ago, and I'd like to see if it's possible to make them run faster. The motors are of the universal brushed-type, with the stator and the rotor connected in series.

Short of changing their gear ratio, is there an electrical trick out there that I could use to make them run faster? Preferably at twice their rated speed. Something tells me there's no practical way of doing that, but I'm asking just to make sure.

@MaxHeadRoom

Most brush types work on DC as well - you could try a rectifier and a big fat reservoir electrolytic to sustain the peak value.

You could also make a 2 capacitor voltage doubling bridge - but it will probably chuck commutator segments all over the place.

 

As someone who has tried to deliberately blow up old universal type motors by over voltaging them I can say with good certainty that if you have a load on them you'll fry the windings from over power before they fly apart.

So far the only motor I have even managed to blow up was a old vacuum cleaner motor I ran on double its rated input voltage with no fan assy attached. It was years ago and I would guess it hit the high side of 100K RPM before commutator blew up.

As for old power drills of the ones I tried to destroy they just smoked the rotor out or burned the brushes and commutator out. They're pretty tough to kill.

Personally however, since you have a higher end milwaukee press drill unit worth $450 - $500 new I would say sell the thing and buy yourself a mid range drill press for $250 - $300.

 

One I had had two speeds, the lower speed was achieved by putting a diode in series with the motor.

But he want's to make it faster.

 

After thinking about this, doesn't field weakening allow a faster speed in a brushed motor? How about removing some windings from the fields? Get a second hand drill from pawn shop or other source and do an experiment, to see how much needs to be removed to get an increase in speed.

 

You want x2 the rated rpm, I don't see it being practical electrically, I would investigate other options, what is the actual application you need it for?
Max.

 

After thinking about this, doesn't field weakening allow a faster speed in a brushed motor? How about removing some windings from the fields?

I see this tending to increase field strength rather than reduce it. Decrease resistance current goes up.
Max.

 

After thinking about this, doesn't field weakening allow a faster speed in a brushed motor? How about removing some windings from the fields? Get a second hand drill from pawn shop or other source and do an experiment, to see how much needs to be removed to get an increase in speed.

It should to a point being the EMF generated impedance will be less for a given voltage thus pushing more voltage into the rotor.

No idea how far it can go but I know that when a universal motor gets a partial short in a field coil it doesn't take long to over-current the motor to the point of burnout despite an actual loss in operating RPM's.

I know that in high current series wound motors, like most old style starters are designed around, that the old 6 volt and 12 volt units most often only changed the field coils from being a configuration of two sets of two as a series/parallel setup for 6 volt and all 4 in series for 12 volts.

No load RPMs stayed pretty close to the same yet oddly when putting 12 volts to a 6 volt configured starter the peak RPMs would not be drastically higher than in 6 volt operation as would be assumed given double the input voltage. Crazy higher torque output at load but not a drastic gain in no/light load speed.

Given that, I agree that taking a few windings off should speed it up some but I would be concerned about passing the tipping point of where the torque power band Vs current consumption goes too far fro what RPM gains may be had for it.

I doubt he would get double the RPM's though. Maybe 10- 20% before overcurrent magnetic field saturation of the motor iron cores becomes an issue.

Personally it's not something I would be experimenting on with a $450 - $500 drill. Cheap hand drill or old vacuum cleaner motor but not on a functioning higher end power tool.

 

The whole premise with a DC motor whether shunt or series, is as the field is weakened the rpm will rise, In some shunt wound motors this can occur to the point of destruction , if or when the field is lost whilst running.
This is the main reason for field loss protection on shunt motors.
On some applications such as CNC machining, the shunt field is weakened slightly in order to gain some high rpm, at the cost of torque.
Max.

 

It just occurred to me, what if I PWM the stator and the rotor independently? That way I could change their voltage ratio through a very wide range and weaken the field, as @Ylli suggested. This while being careful of using the correct polarity on each part, as @recklessrog pointed out.

 

It just occurred to me, what if I PWM the stator and the rotor independently? That way I could change their voltage ratio through a very wide range and weaken the field, as @Ylli suggested. This while being careful of using the correct polarity on each part, as @recklessrog pointed out.

Definitely worth a try!

 

Unless this is for academic reasons, I think there are way easier ways, unless you want the original portable reasons of a magnet drill etc.
By the time you have built DC power supplies and the PWM controller you could have put together off the shelf items that would do it without as many uncertainties.
And then will the gear box withstand the x2 rpm? As well as the motor if already rotating at close to 20K rpm.
Max.

 

Unless this is for academic reasons, I think there are way easier ways, unless you want the original portable reasons of a magnet drill etc.
By the time you have built DC power supplies and the PWM controller you could have put together off the shelf items that would do it without as many uncertainties.
And then will the gear box withstand the x2 rpm? As well as the motor if already rotating at close to 20K rpm.
Max.

The motor is definitely not running at 10k. I think it's more like 4k, but I'm going to double check before proceeding.

As for the hassle of a DC power supply, the experience I already have with the design of the 90VDC motor driver is going to come in handy for this. In fact, the circuit will be much simpler in this case, I think.

It's about a dozen of those suckers I want to be able to control, Max. Not just one. I can have the luxury of possibly destroying one, since the benefits outweigh the risks.

 

The motor is definitely not running at 10k. I think it's more like 4k, but I'm going to double check before proceeding.
.

Have you tried simply increasing the voltage, a 120v Variac will often go higher, and if you were to add a bridge rectifier and large cap bank, which you will need anyway, you get higher voltage using this means.
If going this route, you can monitor the current and any other effects.
This is what I would be inclined to do first. A 240v variac would be even easier.
It could even lead to a simpler solution, if it works out.
Max.

 

Have you tried simply increasing the voltage, a 120v Variac will often go higher, and if you were to add a bridge rectifier and large cap bank, which you will need anyway, you get higher voltage using this means.
If going this route, you can monitor the current and any other effects.
This is what I would be inclined to do first. A 240v variac would be even easier.
It could even lead to a simpler solution, if it works out.
Max.

Maybe it would, but an MCU has the advantage of easily being interfaced to a PC... I'll look into the variac option too, though.

Many thanks for your input, it's always appreciated.