Using 110V AC for 230V AC power tools

[vinlove]

I have a 230V AC operated grass strimmer, which runs too fast and powerful.
If it was connected to a step down transformer 230V ==> 110V AC, and operated, it would slow down the speed by half?
But what about the current, heat and power generated by the tool in that dropped down voltage input?

Would it give any kind of mechanical or electrical strain to the tool running with the half dropped AC voltage input? Or not?

[NiHaoMike]

If it's a universal motor, just try a series diode?

[IanB]

I have a 230V AC operated grass strimmer, which runs too fast and powerful.
If it was connected to a step down transformer 230V ==> 110V AC, and operated, it would slow down the speed by half?
But what about the current, heat and power generated by the tool in that dropped down voltage input?

Would it give any kind of mechanical or electrical strain to the tool running with the half dropped AC voltage input? Or not?

I think if you used a step-down transformer to halve the voltage, then the tool would run at a quarter of the power rather than half the power. This may be more than you want.

[IanB]

If it's a universal motor, just try a series diode?

I think it is irresponsible and dangerous to suggest modifying a power tool like this. Please do not make such suggestions.

[Siwastaja]

I think if you used a step-down transformer to halve the voltage, then the tool would run at a quarter of the power rather than half the power. This may be more than you want.

This is not true. A motor is not an ohmic load. Power would drop quadratically only if the mechnical load itself has such characteristic (e.g., a fan). If the mechanical load has similar torque at half the RPM, then load current would be similar, and power would be half of the maximum, too.

Remember, the equivalent circuit of a motor is a voltage source directly proportional to the rotational speed, and a small series resistance. In steady state operation, voltage drop over that resistance is small, because that resistance value is small (especially in efficient motors). When more mechanical load is applied, the RPM drops, increasing the drop over that resistance, driving more current into the motor.

[Siwastaja]

If it's a universal motor, just try a series diode?

I think it is irresponsible and dangerous to suggest modifying a power tool like this. Please do not make such suggestions.

IMHO, it's no more dangerous than adding a transformer (which you seemed to accept).

[TimFox]

I think if you used a step-down transformer to halve the voltage, then the tool would run at a quarter of the power rather than half the power. This may be more than you want.

This is not true. A motor is not an ohmic load. Power would drop quadratically only if the mechnical load itself has such characteristic (e.g., a fan). If the mechanical load has similar torque at half the RPM, then load current would be similar, and power would be half of the maximum, too.

Remember, the equivalent circuit of a motor is a voltage source directly proportional to the rotational speed, and a small series resistance. In steady state operation, voltage drop over that resistance is small, because that resistance value is small (especially in efficient motors). When more mechanical load is applied, the RPM drops, increasing the drop over that resistance, driving more current into the motor.

In that equivalent circuit (back-emf proportional to speed with series resistance), the current through the circuit is proportional to the torque.  With no load, the speed will be (almost) proportional to the applied voltage (small current due to the torque required to overcome bearing friction).  As the load increases (more torque), the current through the series resistance drops the voltage available to overcome the back emf, and the speed falls.

[magic]

If you think of a wire suspended in magnetic field, it follows straight from the laws of electromagnetism that force produced by the wire is proportional to current and magnetic field and then it follows from conservation of energy/power that voltage across the winding is proportional to velocity (ignoring resistive losses and inductance).

A universal motor would seem to slow down by half in such case, but I'm not entirely sure because such motor generates its stator field by electromagnets and hence the field will also get weaker. I think it may end up really being four times slower?


And then there are other types of motors which care more about mains frequency than voltage, and there are possible electronically controlled motors running from rectified mains which could completely fail to work at wrong line voltage, and whatnot...

[IanB]

IMHO, it's no more dangerous than adding a transformer (which you seemed to accept).

In the transformer case, we have a consumer step down transformer that should come with a test/safety mark and which you just plug in. No modifications to electrical wiring required.

In the diode case, how would someone add a diode to the circuit? They would have to break the circuit somewhere or modify the tool. I don't think this is a simple thing to contemplate.

[IanB]

I think if you used a step-down transformer to halve the voltage, then the tool would run at a quarter of the power rather than half the power. This may be more than you want.

This is not true. A motor is not an ohmic load. Power would drop quadratically only if the mechnical load itself has such characteristic (e.g., a fan). If the mechanical load has similar torque at half the RPM, then load current would be similar, and power would be half of the maximum, too.

Remember, the equivalent circuit of a motor is a voltage source directly proportional to the rotational speed, and a small series resistance. In steady state operation, voltage drop over that resistance is small, because that resistance value is small (especially in efficient motors). When more mechanical load is applied, the RPM drops, increasing the drop over that resistance, driving more current into the motor.

While I don't doubt your description of how motors work, my experience of grass trimmers (that usually have brushed universal motors) is that they bog down really easily when you put too much load on them. My suspicion is that if you halved the supply voltage, the motor would have a very hard time maintaining a reasonable speed under load.

(I did an experiment where I installed a slightly heavier gauge trimming cord in my electric trimmer than the manufacturer specified diameter. To my surprise the trimmer completely failed to maintain a proper speed due to the increased load and became non-functional. I had to put back the correct size trimming cord.)

[Siwastaja]

A grass trimmer needs the speed because the cutting action is based on the speed. You can test the same idea with a scythe, if you move it too slowly, the grass won't get cut but instead bends or smashes, this requires more energy than cutting it in the first place. So I think a cutter is a special case where reducing speed would increase the required torque (opposite to centrifugal pumps or fans), and also result in very poor cutting. Therefore it makes no sense to slow a grass trimmer down.

So yeah, in essence, it is always important to understand what the mechanical load is. Motor theory alone does not suffice.

How can a grass trimmer be "too powerful"? I don't understand. Having it rotate at almost full no-load RPM would not pull much current either, and seem quite optimal for the purpose.

[vinlove]

Great points !! Thanks.

But my grass strimmer is not just for cutting grass.  If you look into Amazon, you will see they sell the attachments made with metal brush for grass strimmers.
Like this one,

https://www.amazon.co.uk/Viabrico-strimmer-wire-trimmer-ataptor/dp/B098TWZW5H/ref=cm_cr_arp_d_product_top?ie=UTF8&amp&tag=coa_gb-21

  When you attach it to grass strimmer, it becomes powerful dirt remover for your patios, driveways and stone paths.   For this job, it is good to slow down the rotating speed of the attachment, but get more torque for removal of the dirt, weeds and muds.    Too fast rotation is OK, but sometimes it can damage the patios, bricks on the drive ways etc.

By the way my step down transformer 230V AC to 110V AC is a stand alone transformer well made device which works well for any 110V AC operated builder tools (often made for USA), but could be used in UK.    It is a heavy transformer with lots and lots of amperage.

[Gyro]

...
  When you attach it to grass strimmer, it becomes powerful dirt remover for your patios, driveways and stone paths.   For this job, it is good to slow down the rotating speed of the attachment, but get more torque for removal of the dirt, weeds and muds.    Too fast rotation is OK, but sometimes it can damage the patios, bricks on the drive ways etc.
...

Dropping the voltage won't give you more torque, quite the opposite in fact. To achieve something like what you are describing would need some form of electronic speed control with feedback. Unfortunately 'misusing' the motor in that way would burn it out rather rapidly as you would be putting a lot more current through it with reduced cooling.

Amazon will happlily sell you anything, but trying to spin an 8" wire brush with a little strimmer motor with direct drive, designed for a single nylon line and get useful work out of it seems over-optimistic. You would be far better with something that is geared down like an angle grinder.


Edit: If you look closer at the Amazon pictures and video, you will see that they are using a petrol strimmers with geared heads. Those would stand some chance.

[vinlove]

...
  When you attach it to grass strimmer, it becomes powerful dirt remover for your patios, driveways and stone paths.   For this job, it is good to slow down the rotating speed of the attachment, but get more torque for removal of the dirt, weeds and muds.    Too fast rotation is OK, but sometimes it can damage the patios, bricks on the drive ways etc.
...

Dropping the voltage won't give you more torque, quite the opposite in fact. To achieve something like what you are describing would need some form of electronic speed control with feedback. Unfortunately 'misusing' the motor in that way would burn it out rather rapidly as you would be putting a lot more current through it with reduced cooling.

Amazon will happlily sell you anything, but trying to spin an 8" wire brush with a little strimmer motor with direct drive, designed for a single nylon line and get useful work out of it seems over-optimistic. You would be far better with something that is geared down like an angle grinder.


Edit: If you look closer at the Amazon pictures and video, you will see that they are using a petrol strimmers with geared heads. Those would stand some chance.

Mine is a Bosch 950W grass strimmer, and it comes with large extra attachment blade for cutting thicker woods.  And the motor feels more powerful than any angle grinder I have.  So, it does handle the metal brush attachment very well.  I have been using it for all summer, and it was great.  It runs all day, and next day and on and on ... I don't see any lack of power with the motor.

https://www.amazon.co.uk/Bosch-three-prong-auxiliary-protective-cardboard/dp/B01AK2DYEQ/ref=asc_df_B01AK2DYEQ/?tag=googshopuk-21&linkCode=df0&hvadid=210268466367&hvpos=&hvnetw=g&hvrand=5582847954805244351&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=1007330&hvtargid=pla-421456610051&th=1

I just wondered if there is a way to slow down the rotating speed a little, so it could even be used as stone path dirt remover.  It just would work better with slower speed.  With the harshness of the metal brush, more torque is not necessary, but it would be better too.

Problem with using angle grinder sounds good idea just to say it, but for real uses, it doesn't work at all for the garden clean up. For one thing, your back will be paining to bend over to reach the ground with the angle grinder.  And it would take 10 times longer with the small hand tool  to work large path or patios, and for most crucially, it wouldn't have right attachment to work on the stone paths, driveways or patios.  Angle grinder attachments are usally for cutting tubes or derusting sand paper types, which will wear out within minutes when applied to the stones and concrete paths. 

[IanB]

Maybe see if you can find a device like the one linked that is designed for the UK market? It might do what you want.

https://www.harborfreight.com/power-tools/routers-cutout-tools-lathes/router-cutout-bits/speed-controllers/router-variable-speed-control-dial-59386.html

[Gyro]

Ah, 950W is rather more like it. There are obviously strimmers and strimmers! You're in with a chance there.

A phase angle speed controller (as IanB suggests), running on 230V, will give you a lot more torque than a step down transformer. They used to be very common for use with single speed electric drills and the like but most have speed control built in these days. I'm sure they are still around though, and electronics magazines used to be full of projects.


P.S. I've certainly seen projects that sense back EMF to further boost available torque, I don't know if any off-the-shelf offerings do that as they probably need to be tuned to the motor. Don't forget the reduced cooling issue though.

[johansen]

You could add more turns to the field coil of the series wound motor.

A single diode definitely works, did that with one of my dads shop vacs 20 years ago. Worth a try imo

[vinlove]

Ah, 950W is rather more like it. There are obviously strimmers and strimmers! You're in with a chance there.

A phase angle speed controller (as IanB suggests), running on 230V, will give you a lot more torque than a step down transformer. They used to be very common for use with single speed electric drills and the like but most have speed control built in these days. I'm sure they are still around though, and electronics magazines used to be full of projects.


P.S. I've certainly seen projects that sense back EMF to further boost available torque, I don't know if any off-the-shelf offerings do that as they probably need to be tuned to the motor. Don't forget the reduced cooling issue though.

Sorry the thumbs down was for angle grinder for this application
Yeah, I was wondering how running the 230V AC 950W strimmer with 110V AC step down transformer will work in terms of torque, heat generation and speed.  I am sure speed will definitely slow down.  But the rest I was not so sure.

I once ran 220V AC tube radio with 110V step down transformer, and it worked OK, with reduced sensitivity and lower volume, quieter reception. It was working OK. 

[Zero999]

I say give it a go. I shouldn't damage it. Obviously if the motor runs to slow, or stalls, then switch it off an abandon the idea.