PDA

View Full Version : ac/dc resistance

dian
05-02-2013, 03:43 PM
recently i converted this tool to something usefull:

http://i973.photobucket.com/albums/ae218/romandian/DSC00815_zpsd428a935.jpg (http://s973.photobucket.com/user/romandian/media/DSC00815_zpsd428a935.jpg.html)

its getting 8 volts from the transformer and now does its job.

i took the trouble and installed the rectifier in the tool (i had a 16v capacitor in the handle, but it died a fast death, i dont know why, probably because it was 15 years old), as i didnt want to use a thick cable and my understanding was, ac loses less voltage than dc.

but on second thought, is this really true and why?

The Artful Bodger
05-02-2013, 04:09 PM
Resistance is resistance and both AC and DC are effected by it but AC is also effected by 'reactance'.

Paul Alciatore
05-02-2013, 04:31 PM
Your losses, which are primarily due to heat from the resistance, will be almost exactly equal if the Voltages (RMS Voltage for the AC) and currents are the same.

AC can have less loss in the wires IF you use transformers to convert it to a higher Voltage and then convert it back to the original Voltage at the other end. Since for a given amount of power that is carried by the wire, the product of Voltage and current is a constant but line losses are given by I^R, lower current equals less loss. It is not AC vs. DC that makes the difference, just the lower current when a higher Voltage is used. This trick is more useful with AC because it is a lot easier to change the Voltage level of an AC current than it is to do it with DC. This is why AC and high transmission Voltages are used by the power companies for long distance power transmission.

But the transformers also have losses and you would be carrying them around and they have weight. I would just use a wire that is heavy enough for the current you have and live with it.

05-02-2013, 04:40 PM
. This is why AC and high transmission Voltages are used by the power companies for long distance power transmission.

.

Not in my province, we have 15 Hydro generating stations capable of 5485MW, the method now is to convert to DC for transmission and back to AC at the consumption end. Less losses claimed.
Max.

Lew Hartswick
05-02-2013, 05:07 PM
Not in my province, we have 15 Hydro generating stations capable of 5485MW, the method now is to convert to DC for transmission and back to AC at the consumption end. Less losses claimed.
Max.Yes but at what voltage? It has been more "efficient" for a number of years now to use DC for long distance transmission. This has come about with the advent of switching components of adequate voltage rating.
I don't know all the details but the long lines are in the 100KV range. The
...lew...

ed_h
05-02-2013, 05:08 PM
At 50/60 Hz, losses in the cable will be the same for AC and DC with the same current. Only at higher frequencies would there be significant difference.

05-02-2013, 05:32 PM
At 50/60 Hz, losses in the cable will be the same for AC and DC with the same current. .

I assume they know what they are doing, they have been doing this for a considerable time, and they are just about to install a new transmission line at a cost of 3.28 Billion dollars, I have a couple of relatives who are engineers employed by the Hydro Co. I have been meaning for some time to get in depth details on this transmission method, so I will bring it up at the next B.Q.!!
Max.

ed_h
05-02-2013, 05:38 PM
Max--

I was actually responding to the OP.

I do know that there is a tend toward HV DC transmission. I haven't researched it, but it may have something to do with not having to synchronize AC wave forms between and among plants. Also there is a small skin effect with AC that might ake a difference at very high voltages and currents. Though I've never worked on anything af this scale, long distance power transmission engineers may have to take reactances into account with AC.

Lew Hartswick
05-02-2013, 06:22 PM
At 50/60 Hz, losses in the cable will be the same for AC and DC with the same current. Only at higher frequencies would there be significant difference.
Not when the lines are quite long. This was the reason that in, either Norway
or Sweden, the LONG transmission lines were made DC back a Long time ago.
...lew...
Perhaps the ones in Canada are not long enough to exhibit that loss.

dp
05-02-2013, 08:08 PM
The Pacific inter-tie between Washington state and California is a one-way DC wire. There are multiple wires for redundancy, but current flows only one way. The return is through the Earth. Both ends of the connection have huge grounding farms to carry the current into the Earth at low loss.

ed_h
05-02-2013, 09:03 PM
Not when the lines are quite long.

Lew--The cable I was referring to was the one in the OP. I think you're right about long lines.

darryl
05-02-2013, 09:50 PM
There's a couple of basics at work here- one is that battery operated tools use higher currents because of the lower voltage. When you have higher currents, there's higher losses in wiring and rectifiers. Secondly, any capacitors involved are subject to higher ripple current. Every pulse of current that goes into or out of a capacitor does its share of heating the series resistance of the capacitor. In a normal power supply, the filter cap gets its few big pulses as it initially charges up, then operates with lower ripple currents after that. If it's subject to constant cycling between peak voltage and something a lot lower, say half peak voltage, it's got a hard job to do. Switching caps are best for use like this- their main spec after voltage and capacity ratings are low series resistance, ESR. They are also usually rated for a higher temperature, 105 degrees vs 85 degrees.

Something to consider when developing a power supply for low voltage, high current applications is where to put the rectifier and capacitor. If you put them close to the transformer, there will be less resistance to the charge current and the peak voltage could come up faster and sustain longer. If you put them in the tool, with a length of wire between the transformer and the tool, you will lose average voltage at the tool. Also, with the cap and rectifier in the tool, the physical size of these components is limited. This will result in a lower current rating and higher voltage drop for the rectifier than might otherwise be the case, and it will definitely result in the capacitor having a capacity rating lower than would be desired.

Over the years, I've wired up several cordless tools, mostly drills, to separate power supplies. I've come to a few conclusions- first it's nice to have consistent performance from the tool, and you are not waiting for a battery which might be no good anymore to charge. Also, because you have a cord now, plus another device which must be plugged in, it's less convenient than a regular corded drill. Another point- assuming the power supply is up the task of supplying high currents with livable voltage drops, the wire you put between the supply and the tool needs to be fairly thick. By the time you have a thick enough gauge of wire, covered with insulation, it becomes much stiffer than you would otherwise like. More than once I've been on the search for a heavy gauge of wire that is flexible enough to live with.

A cordless tool running on 9 volts can easily draw 20 amps. It's not hard to lose 5 volts at 20 amps through a length of wire, so the performance of the tool can be weak, even if the unloaded rpm seems to be right up there where it seems it should be. And 20 amps is just a realistic figure for run of the mill cordless tools. I bet some these days can easily pull 50 amp surges- depending of course on what it's doing. Turning a small drill bit will be pretty easy, snipping twigs and brush or turning a hole saw will be demanding applications. It's hard to beat having the batteries right in the tool to supply high current through very short wiring.

Evan
05-03-2013, 01:33 AM
The primary reason for using DC in transmission lines is that there is no need for rephasing capacitor banks every few hundred kilometres. Secondly, as mentioned, with the advent of very high powered and high voltage solid state switching systems the need to synchronize networks is eliminated. Skin effect isn't too much of an issue except at very high currents which require very large conductors, usually at sub stations.

Another major factor is corona loss. Ac inherently produces greater corona loss due to the changing polarity in respect of the space charge around the wires. If the wire voltage is constant in respect of the space charge it tends to suppress corona generation. If it alternates corona is enhanced because of the doubling of the voltage in respect to the space charge every half cycle.

The amount of power lost to corona can be severe, especially at high altitudes in poor weather. It can reach values in the >1 percent range per 100 kilometres. We have power lines in BC longer than 1000 kilometres. That includes the triple circuit 3 phase 545KV system a few klics down the road from me. It is that system that supplies the Pacific Intertie from the WAC Bennet Dam in Northern BC.

Here is an example of severe corona loss during seriously smoky conditions during a major forest fire.

http://ixian.ca/pics10/coronaloss.jpg

Paul Alciatore
05-03-2013, 01:35 AM
Jeeeeezz! I was only trying to illustrate a point. Yes, some power distribution is done with DC. But most is via AC and that was the primary consideration that lead early power companies to use AC and transformers. DC Voltage conversions were not that easy back then and even now, I suspect that other considerations (like avoiding the need for sync) may play a strong influence when DC is used.

My point was that the losses in a small scale system as the OP has are almost precisely the same with AC or DC if the Voltage and current is the same. And I did say "almost precisely" as I am sure someone will find some minor effect that proves that one or the other is slightly more efficient in a small system like this.

dian
05-03-2013, 03:56 AM
well, so i could have saved me some trouble.

Barrington
05-03-2013, 06:55 AM
Back to basics...

As has been said, in a simple rectified/smoothed power supply, the current from the transformer is effectively pulsed. The current flows near the peak of the AC waveform and drops to zero elsewhere. The timing depends on the load current and size of smoothing capacitor.

It may not be obvious that this pulsed waveform has a higher RMS value than the averaged DC current flowing from the smoothing capacitor to the load.

(e.g. and to simplify the maths, let's assume a waveform of a rectangular pulse, 5A on:20% and off:80%, for an averaged DC output current of 1A. If this DC output current is passed through a 1 ohm resistor it will dissipate I^2*R = 1W.

However if the pulsed current before the capacitor is passed through a 1 ohm resistor, it will dissipate I^2*R = 25W during the pulse, i.e. for 1/5 of the time, for an averaged dissipation of 5W.

Or - to put it another way, the RMS value of of this pulsed current is sqrt(5^2 / 5) = 2.236A. An RMS current of 2.236A through 1 ohm disspates 2.236^2*1 = 5W.)

So, putting the rectifier/capacitor at the far end of the cable therefore increases the dissipation in the resistance of the cable compared to running the smoothed DC down it.

All that being said, at the potentially high currents involved in the battery tool and with a small capacitance, any smoothing may actually be rather ineffective, so the difference in current waveforms either side of the capacitor may be less than in the general case.

Cheers

.

taydin
05-03-2013, 08:00 AM
A cordless tool running on 9 volts can easily draw 20 amps. It's not hard to lose 5 volts at 20 amps through a length of wire, so the performance of the tool can be weak, even if the unloaded rpm seems to be right up there where it seems it should be. And 20 amps is just a realistic figure for run of the mill cordless tools. I bet some these days can easily pull 50 amp surges- depending of course on what it's doing. Turning a small drill bit will be pretty easy, snipping twigs and brush or turning a hole saw will be demanding applications. It's hard to beat having the batteries right in the tool to supply high current through very short wiring.

The high current requirement is spot on. After the batteries of my 12V, 2,4A cordless Bosch have died, I bought a 12V, 30A power supply and thick (4mm2) copper wire and mostly use the drill this way. Once I get around to making a battery spot welder, I will rebuild the battery pack myself (currently, an original battery pack costs about 180\$, grrr, if I rebuild them, it will cost around 70\$ with original cells and 25\$ with chinese POS cells)

flylo
05-03-2013, 09:49 AM
Back to the tool. I have an 18V Hitachi cordless set that charges in 20 minutes but the best part it came with a fake battery with about a 20' cord that twist locks into the charger to make it work off house current is you run down all your batteries. You can also charge your battery while using this. They didn't make them long but what a great idea!

dian
05-03-2013, 01:52 PM
barrington, thanks for that, ill have to unpack my books that are still in the attic and read up on that. btw, the voltage was 10v with the 1 mf cap, as measured with my multimeter (believe its rms) so i am kind of glad it died, as this might have been too much in the long run.

valleyboy101
05-03-2013, 09:31 PM
DC has some advantages for long distance transmission of large amounts of power. The transmission lines need only 2 wires vs the 3 wire for 3 phase AC so lines are cheaper to build. Also voltage control is much simpler, as AC lines generate high voltages when lightly loaded requiring the use of reactors to absorb MX and when they are heavily loaded the voltage sags requiring the use of capacitors to maintain voltage.
For DC the terminal equipment is very expensive making the addition of tapped stations along the line cost prohibitive.
Michael

05-03-2013, 09:39 PM
The Hydro engineers I have spoken to all say the Main reason for DC is the lower losses soon recoup the cost for any DC conversion equipment.
Max.

co_farmer
05-03-2013, 10:10 PM
The Pacific inter-tie between Washington state and California is a one-way DC wire. There are multiple wires for redundancy, but current flows only one way. The return is through the Earth. Both ends of the connection have huge grounding farms to carry the current into the Earth at low loss.

Actually, the Northern end is just up the hill from the Dalles Dam in Oregon. The Southern end is in the Los Angeles area, I don't remember just where. In the 1970's the Portland Amateur Radio Club had a tour of the Oregon end of the facility. A member was an engineer for PPL, the operator. We got to go through the entire facility, including the rectifier house. I have some pictures, somewhere of the mercury vapor rectifiers. Each was about the size of a VW beetle. They were mounted on ceramic insulators around 10 feet high and behind wire cages, so you could walk around them.

Power from the dam to the rectifiers is 18 phase, so it is almost DC when all are combined by the rectifiers. The transmission is 250K volts with return on wire, not ground. The initial design was for ground return, and voltage at 500k volts, but the ground currents charge every piece of metal that gets in the way, ie. well casings, farmers electric fences, plain old wire fences, anything that has at least one end in the ground. The political uproar was such that the ground return and higher voltages were never implemented.

At the visitors center, there is a movie of the system's reaction to an earthquake at the LA end of the line. The whole system is computer controlled. In the 1970's, they were DEC mini-computers. The computers safely shut the system down in just a few seconds. Orderly start and stop are necessary to protect the dam generators. We were told, it would be possible to lock the dam's generators if the load was applied too quickly.

The system also can be reversed so LA power can be sent to the Northwest when water flow is low. Not sure if they do that anymore or not.

I haven't been back in years, but I bet visitors don't get near the actual components, anymore.

Paul, KD7HB in Central Oregon

valleyboy101
05-03-2013, 10:22 PM
Yes, DC does have very low losses over long distance. As a control centre operatorI have seen a station putting out 3000 MW into 2-500 kV AC lines and losing about 50 MW in less than 200 miles as well as requiring a lot of voltage support at the sending and receiving end. Our power system infastructure and operation are interesting to me, but gauranteed to make eyes glaze over at parties.
Michael

Evan
05-03-2013, 11:28 PM
You will notice in the image I posted that there are 4 conductors for each phase. That spreads out the space charge over a much larger volume of air. By doing that it reduces the voltage gradient which reduces corona. Another method is to use hollow conductors. A lot of people think that is because of skin effect but that isn't the primary reason. Again, it reduces the voltage gradient because the conductor surface is larger and the air volume adjacent is much larger. High altitude increases corona and so does low barometric pressure. Thinner air is not as good an insulator. Rain and fog increase losses too.

Fasttrack
05-03-2013, 11:41 PM
Back to the tool. I have an 18V Hitachi cordless set that charges in 20 minutes but the best part it came with a fake battery with about a 20' cord that twist locks into the charger to make it work off house current is you run down all your batteries. You can also charge your battery while using this. They didn't make them long but what a great idea!

It's about time! I've been wanting to see this for ages... I try to buy the best tools I can afford and it is really hard for me to justify buying battery operated tools because I know I'll need to replace the batteries in a couple of years. When most batteries cost 40-80 a pop... well having a cord so I can still use the tool would be awesome (if the cord attachment was made well).

Darryl already said I was going to mention about the capacitor. You'll need a high quality cap capable of handling the ripple currents. There are some aluminum organic polymer caps that would do the job or you can put a large aluminum electrolytic in parallel with a couple of high quality, Multi-Layer Ceramic Capacitors (MLCCs). If you can manage small, surface mount capacitors, there are a number of MLCCs in the 1-4 uF range that are rated at ~2 amps at 60HZ. Be aware that most ripple current ratings are at 100kHz or 300kHz but the datasheet should give you a scaling coefficient for different frequencies. You might see the coefficient is 0.6 or 0.8 for a frequency of 100 Hz.

darryl
05-04-2013, 12:28 AM
You need real capacity for voltage smoothing with high currents. 4uf isn't going to do anything for the tool, but it will help knock down voltage spikes.

I'm wondering how a cap of that small a value can have a rating of 2 amps at 60 hz, - unless the voltage is high there won't be enough rate of change to force that level of current flow.

At any rate, there's one other thing I should have mentioned in conjunction with my other comments- without the battery pack in place, the tool will be unbalanced and may not feel good in your hands. This can be gotten used to, but it sure doesn't feel right at the start. One of my drills got an mdf base added to the end where the battery normally goes, allowing it to stand for one thing, and to feel a little better for another.

A while ago I picked up three cordless drills with chargers and flashlights that use the same battery pack. I think I paid \$5 for the whole works. There were no batteries. I think what I'll do with these is build a stand to include a 12v battery, a charger, and a home for each of the drills. They will be wired, and will have a bit of a base attached so they can stand also, but the battery box will have holes that they can each be dropped into. In my case, one drill will have a chamfer bit, another will have a combination pilot and clearance bit, and the other will be used for a driver bit. This will make a convenient 'assembly' trio. I don't know about anyone else, but I'm getting tired of always having to change the bit to get the three functions done each time I want to set a screw.

ptjw7uk
05-04-2013, 03:57 AM
Corona also takes place in the UV region and as such is not visible in the visible. In the 60's we designed and built a low light camera with specific UV filters so that corona could be viewed. The transmission company used the camera fro ma helicopter and found lots of insulators exhibiting high levels of what they called silent corona, they then changed the insulators before they broke down.

Peter

ptjw7uk
05-04-2013, 03:58 AM
Double post, damn internet!!!

Peter

dian
08-08-2014, 11:58 AM
so the tool has been working for over a year now, but not very well. it will run for about three minutes and then it starts to slow down. after a while it stalls. i have a kbpc1506 in there. it was getting pretty hot. i took it out and bolted it to a larger heat sink. it was still getting hot. so i bolted it to a chunk of aluminum (using conductive paste) and it doesnt get warm. but it still does the same thing. whats going on?

i measure around (multimeter) 9v/4a ac and 7.3v/4a dc (1.8v ac).

so heat is not the problem. unfortunatly i dont remember what the original voltage was. 5v maybe. but if the motor gets higher voltage, its not going to slow down after a while, its goin to burn out, right.

btw, after 10 minutes or so, tool runs well for another 3 min.

any thoughts appreciated.

Paul Alciatore
08-08-2014, 02:41 PM
You need to do some troubleshooting. Fortunately, it only takes three minutes for it to slow down. So, while it is running OK, take several Voltage measurements:

At the output of the transformer
At the end of the cord, as it enters the trimmer
At the input to the rectifier
At the output of the rectifier
After the ON/OFF swtich

Then let it warm up and slow down and take the same readings again. Somewhere along the line you are losing it.

That will give us something to go on.

PStechPaul
08-08-2014, 07:40 PM
Interesting ancient thread! The slowing down of the tool after a few minutes and speeding back up after some time sounds like heating and cooling. Copper has a temperature coefficient of about 0.4% per degree C, and it is quite possible for the wire in the motor and the transformer to attain 100C or higher without burning the insulation. So for a 75C temperature rise that would be a 30% increase in resistance, which will lower the current that can be pulled by the motor, and torque is proportional to current. It will also lose speed because the voltage at the motor will be reduced as well.

It is unclear if the rectifier is a full wave bridge or a single diode. If it is a single diode, you will get about the same peak voltage when unloaded, but with a normal load it will be about half, although this depends somewhat on the capacitor. A full wave bridge will be the most efficient.

Troubleshooting this properly will involve measurements of voltage as well as current, using a true-RMS meter and perhaps an oscilloscope. You can also measure the DC resistance of the drill itself and the windings of the transformer (both primary and secondary), when cold and hot, to get an idea of how hot the wires are. It is also possible that the brushes on the motor could be heating up and causing losses due to resistance or reduced conductivity of the surfaces to the commutator. It will be interesting to learn what is causing this problem.

BTW, I am working on a project to use LiFePO4 cells to replace the failing/dead NiCd cells in my Ryobi drills and other portable tools. I'll be adding a Battery Management System (BMS) to handle the safe charging and discharging as well as continuous monitoring of the State Of Charge (SOC) by means of blinking LEDs in the case as well as possibly a Bluetooth wireless connection that can provide more details. I am also considering Nickel-Zinc AA cells which have advanced considerably and are safer and more cost-effective than Lithium, but I have some LiFePO4 cells and I might get some more at a really good price. For more details see:

darryl
08-09-2014, 12:02 AM
A typical silicon bridge rectifier is a rather lossy thing at low voltages and higher currents. You should probably be looking for a bridge made using shottky rectifiers- these have lower voltage drops, and the higher the rated current the lower the voltage drop will be at lower currents. They are very common as dual rectifiers, and you can just parallel them right at the package leads. Four of these will make a bridge, and it won't be very large either as they are usually in a TO-220 package. You can get common cathode and common anode types, and getting two of each will simplify building the bridge.

Something else I thought to mention is that when the supply voltage to a cordless tool drops below about 10 volts, the internal switching device which is part of the variable speed circuit will start to run hot. You'd be better off boosting the voltage to 12 or more, even if the tool is designed for 9 or whatever. And if the supply voltage has a lot of ripple due to high current draw, there will be 120 times per second that the voltage has dropped far too low for the control circuit to work safely with. If you're using a switching power supply, the voltage probably won't drop as much between pulses as the frequency is much higher, but the relatively small output filter cap is doing a tough job and often gets weak. A high frequency riding down the wires to the tool can interfere with the tools own control circuit as well, so it may not be easy to track down the real cause of the problems.

dian
08-10-2014, 11:11 AM
so i did a lot of measurements and i dont understand what i see at all.

when cold, there are 3.6adc, 7.2vdc and 2.0 ohms across the two motor wires. i see a consistent rise in current and drop in voltage to 5.0adc and 5.4 vdc. the ripple voltage being around 1vac and not changing. the voltage to the bridge recrifier drops from 8.6vac to 7.6vac. that is consistent with the increase of current for this small transformer. how the current can rise and the voltage can drop is beyond my understanding, however.

the resistance is consistent when cold, but i measure weird things when hot. im using two expensive (true rms) multimeters and half a dozen "cheap" ones. my experience is, that at around 1 ohm, multimeters are no good. i tend to trust the cheap ones over the expensive ones. my best guess is that the resistance gets up to 3-5 ohms when "hot" and falls to 2 ohms eventually. one meter read 0.9 ohms, however, another one was jumping between 9 and 14 ohms, eventually settling down around 5 and dropping to 2 after a while.

the current/voltage implies 1.1 ohms when "hot", so, as i said, i dont know whats happening.

the tool has no speed contol btw and no cap for the measurements.

(my scope is somewhere in the attic and even if i were to find it, i suspect i would have to find out how it works.)

RichR
08-10-2014, 12:09 PM
Hi dian
The picture in your original post shows a tool that has 4.8v imprinted on the side, yet you are hitting it with 7.2V and puzzled that bad things are happening.
Have you tried hooking it up to a solid 5V power supply? If you have an old PC power supply, that should be adequate to try.

dian
08-10-2014, 12:19 PM
well, the idea was to make the whimpy thing work better. so i thought 1.5 times the original voltage would be ideal.

PStechPaul
08-10-2014, 02:41 PM
Accuracy of low resistance measurements with any multimeter is largely a function of lead resistance and the integrity of the connections for the banana plugs, and the range switch. Before taking a resistance measurement, always touch the leads together to see what zero reads, and it should be less than one ohm. Wiggle the connectors at the meter and turn the switch a bit to see how much it changes. Really good multimeters use a Kelvin four lead connection, where two leads just supply a known current, and the other two measure the voltage of the resistance. There are DLROs that use this method to inject 100 amps or more and can measure down to a few micro-ohms.

When the motor is running, the situation becomes more complex. The motor will generate a back-EMF (like a generator) depending on its speed. It will draw more current when it generates torque, and the effective resistance drops. It is normal for the voltage to decrease while the current increases. If you lock the rotor, there will be no back EMF and the current will be determined by the source voltage and the resistance of both the source and the motor. You could easily have 20 amps and read only 2 volts. At that point, all of the power will contribute to heating and probably eventual failure of the motor or the supply.

darryl
08-11-2014, 01:24 AM
The basic power supply will show the highest output voltage when no current is being drawn, and that voltage will drop and continue to drop as higher and higher currents are taken. No surprise there. A switching supply will maintain voltage as current draw rises, but at some point it will start having a bird- it might just shut down, it might limit by dropping the voltage way down, it might begin to oscillate as it tries to supply current and then realize the load is too much- or it might freak because it's driving another switching power supply which is looking to draw high peak currents each cycle, even though the average may be well within the rating of the first power supply. I have one which will not drive a 12v led module which normally takes about 250 ma or so. The supply is rated at 1.5 amps, and it will not power up the led.

Anyway, why would a motor work well when cold, then bog when hot, then work well when cold again- sounds like the armature is expanding with heat, and it has probably oozed out some of its insulation and it's rubbing the magnets. I've seen this in hand blenders and countertop mixers, also in similar cheap mixers. I got to take home some of these appliances, and I fixed some of the motors by sanding the OD of the armature.

I doubt that you can readily take apart the motor in that clipper, but maybe. You very likely can see the bushings, so I'd lube them for sure. Sometimes that makes a lot of difference- other times no change. Also it sounds like you may have shorting in the armature wires when it's hot. That would account for the lower resistance measurement, and poor performance with higher than normal current draw.

dian
08-11-2014, 12:47 PM
" It is normal for the voltage to decrease while the current increases." i didn know that.

so it seems the motor is the problem. i will leave the contraption alone, it works.

its interesting, though, that there has been no overheating or bad smell from the motor, whatsoever. the tool gets used onece a week.

Black_Moons
08-11-2014, 12:59 PM
Spinning motors don't limit current depending on DC resistance. they draw current based on inductance and back EMF generated by the spinning action of the motor.

Additional DC resistance will mainly just cause the motor to heat up more per amp of current passed through it, as usually the DC resistance is a small faction of the inductance.

RichR
08-11-2014, 03:27 PM
It's also possible the motor doesn't like the pulsating DC which goes to 0V 100 or 120 times a second. If you have a computer supply, Try running it
from the 5V output just to see how it behaves.

Doozer
08-12-2014, 04:47 PM
So at the end of the day, after the home shop quarterbacks have their say,
maybe you should have bought a tool that plugs in to the wall instead of
battery powered. Someone start designing a cordless tool that receives
microwave energy transmitted by a plug in microwave energy transmitter.
That's how folks seem to roll around here. Someone else come up with a
signal seeking microwave antenna to keep things pointed right. Soon we
will have this dead battery bit solved, come heck or a trip to the moon.

-Doozer