AC motor on DC?

[SamTexas]

Not sure if this is the right forum, please feel free to move it.

Simple (hopefully) and naive question: Can I run my 110V AC leaf blower directly from my 18s-22s (75-92V DC) Lithium battery pack? If so, does polarity matter?

FWIW, I know I can run a 110V lightbulb on DC and the polarity does not matter at all.

 

[Ypedal]

http://www.youtube.com/watch?v=9CvpnnTWiJc

[youtube]9CvpnnTWiJc[/youtube]

 

[salty9]

Another use for the Cycle Analyst

 

[SamTexas]

That's great! So what's the easiest way for me to know which type of motor I have in my leaf blower?

http://www.amazon.com/Weed-Eater-WEB150-2-Amp-Blower/dp/B002YKQ9B8

 

[Gordo]

+1 Ypedal, you are too quick for me :lol:

 

[ZOMGVTEK]

Most leaf blowers probably use brushed motors. You can often tell by how it starts up if you are a bit experienced.

Can you ever see sparks coming from the motor? That would definitely let you know its brushed. Polarity will matter, but it shouldn't be a big deal if you get it wrong initially. Just don't run it for hours and hours backwards, since some motors might have the brushes oriented in a way that reverse operation is not quite desirable and can cause premature failure. Of course polarity won't matter on a light bulb, its just heating up a wire that glows. It won't matter which direction the current flows. In the case of a DC motor, polarity indicates the direction the motor will spin. AC more or less has no real 'polarity' to worry about with motors, but DC does.

I vote plug it into 80-120VDC and see how it runs. You'd know just about instantly if its brushed. If it spins, its brushed. Run it on your 22S pack.

 

[SamTexas]

Thank you YPedal and ZOMGVTEK.

I couldn't tell which type my motor is from the outside so I opened the blower up. It's a brushed motor. So I plugged it into my 18s battery and it worked fine. Polarity did not matter at all. It worked even better on 22s.

One more question: Will I cause any harm to the motor by running it at a lower voltage? Even a fully charged 22s is only 92V.

 

[ZOMGVTEK]

It's not a good idea to run it on a lower voltage. It will likely run hotter and operate worse than closer to the voltage it was designed for.

I forgot about the fact that it would have a rectifier to run the brushed motor, making polarity not matter. If you never plan on using it on AC again, you can bypass this and just run the voltage straight to the motor.

 

[SamTexas]

That's good to know. Thanks again. More question.

Rectifier: This is pure speculation on my part, please correct as necessary. So this appears to be a doctored DC motor in order to be used with AC. The rectifier is there to simply ignore/discard half of the AC cycles, effectively running the motor only half of the time. So 110V AC would be the equivalent of 55V DC, correct?

Bypassing the rectifier on DC: What do I gain? Or what I do lose by NOT bypassing it when running on DC?

 

[Ypedal]

I run mine on 24 cells for 100v DC.. straight up.

lower votlages = lower RPM and less oompf.

 

[ZOMGVTEK]

It would be silly to use a half wave rectifier. Its likely a full wave rectifier that 'flips' over the bottom waveform to make 120Hz DC with 120V RMS.

The advantage to bypassing it would be simply to avoid the voltage drop it adds unnecessarily

The only thing you need to worry about with this setup is the power switch. It's more likely than not that it will NOT interrupt 100VDC and cook the first time you let go of the trigger. Not that big of a deal, just be prepared to disconnect the battery to prevent potential additional damage that might be caused from the thing eating itself for several seconds or longer. You could just run wires RIGHT to the motor with the correct polarity, and leave everything else alone. Then use 45A Power Poles or similar to power the unit. No real need for a switch unless you want one. Switching 100VDC with a load can be tricky.

 

[SamTexas]

It would be silly to use a half wave rectifier. Its likely a full wave rectifier that 'flips' over the bottom waveform to make 120Hz DC with 120V RMS.

Got it.

The advantage to bypassing it would be simply to avoid the voltage drop it adds unnecessarily

What would be a typical voltage drop, say for a 100V DC battery? The blower consumes 7.5A per specs.

The only thing you need to worry about with this setup is the power switch. It's more likely than not that it will NOT interrupt 100VDC and cook the first time you let go of the trigger. Not that big of a deal, just be prepared to disconnect the battery to prevent potential additional damage that might be caused from the thing eating itself for several seconds or longer. You could just run wires RIGHT to the motor with the correct polarity, and leave everything else alone. Then use 45A Power Poles or similar to power the unit. No real need for a switch unless you want one. Switching 100VDC with a load can be tricky.

This is a cheap $27 leaf blower. It does not have a trigger. It runs at a constant speed and the only control is an on/off switch. So do I still have to worry about changing to a more robust switch? From a functional point of view only.

 

[ZOMGVTEK]

You can expect around a 1V drop... Nothing that significant, but it's easy enough to bypass if you have the thing open anyways.

The switch is either going to interrupt the 100VDC fine, or not. It's more likely it will not. One easy way to find out is to try it. Since it sounds like you already did, I assume it works? Unless its a on/off type of switch and you had it on all the time during the testing.

Try it out. It might cook the switch, or it might work. Worst case is that the switch destroys itself and you bypass it, not being able to turn it off without disconnecting power. Probably not a big deal.

 

[SamTexas]

The switch is either going to interrupt the 100VDC fine, or not. It's more likely it will not. One easy way to find out is to try it. Since it sounds like you already did, I assume it works?

Yes, it worked so far. I guess I can/will replace it with something more robust when it fails.

 

[ZOMGVTEK]

As long as you can't hear it arching, its probably OK.

I've trashed the switches on two cheap heat guns being used as a load on 24S LiPo from switching them. Some things can take it, some just don't. It's still a bit harder to switch than AC, since 60 cycle AC crosses over 0 120 times a second, and DC never does, but if it handles a few switches, it will probably last a while.

 

[amberwolf]

It would be silly to use a half wave rectifier. Its likely a full wave rectifier that 'flips' over the bottom waveform to make 120Hz DC with 120V RMS.

Silly or not, it's what my larger Ryobi trimmer does for it's "low" speed, and a trashed B&D trimmer I got off freecycle simply ran all the power thru a big fat diode, so it only used half the AC power. Motor works fine (and fast!) on DC; had planned a bike around it but never got that far.

 

[SamTexas]

Geez! Amberwolf, what's the easiest/simplest way for me to determine my rectifier type? Half or full wave?

 

[ZOMGVTEK]

A diode in line with the AC is the best way to get 1/2 power, so its used quite commonly for two 'speeds' since its really simple and cheap.
For a brushed motor, this would not work to create a low and high. It could be used to create DC out of AC, for a single speed device, but its not common on higher power applications.

You can tell what type it has by what it looks like. If its a device that takes both of the AC wires in, and then has two wires going out to the motor, its a full wave. If its simply a thing in line with only one of the power wires going to the motor, its half wave. You already ran it off 80V LiPo, right? Did it run slightly slower than normal? If so, its a full wave. If it ran faster than normal, its half wave and you would want to reduce the input voltage a bit.

At 80V or so it should operate fine regardless.

 

[liveforphysics]

If you have 6s packs, I would run it on 30s LiPo, and it will perform very well.

If you have 5s packs, I would run it on 30s LiPo or perhaps 25s.

The difference in output between 90v and 100v is likely huge. However, the motor was designed with maintaining decent performance after going through who-knows-how-many extension cords of voltage drop, so it's likely going to rip pretty hard at just 100-105vdc local to the device so voltage drop is almost nothing.

 

[SamTexas]

You can tell what type it has by what it looks like. If its a device that takes both of the AC wires in, and then has two wires going out to the motor, its a full wave. If its simply a thing in line with only one of the power wires going to the motor, its half wave.

Great. I'll look next time I open the blower.

You already ran it off 80V LiPo, right? Did it run slightly slower than normal? If so, its a full wave.

Yes I did. It ran slowly than normal. So full wave for sure.

If you have 6s packs, I would run it on 30s LiPo, and it will perform very well.

If you have 5s packs, I would run it on 30s LiPo or perhaps 25s.

The difference in output between 90v and 100v is likely huge. However, the motor was designed with maintaining decent performance after going through who-knows-how-many extension cords of voltage drop, so it's likely going to rip pretty hard at just 100-105vdc local to the device so voltage drop is almost nothing.

I have 1s packs. So I can run at any voltage (multiple of 3.7V nominal). Good point about voltage drop through extension cords. Thanks.





Another question while we're still on topic. What would happen if I try it with a brushless motor? Does it simply not work or will I damage something?

 

[ZOMGVTEK]

DC on any AC motor that has no brushes generally just means the rotor will be 'locked' and won't rotate. They use the AC to make a rotating magnetic field that spins the rotor. DC will just generate a steady magnetic field.

It's not going to damage it, immediately... But depending on how the motor is designed and intended to be started, its likely it will take LOTS of amps and burn up in seconds to minutes.

For your particular application, higher voltages are generally preferred over lower voltages. It should be operating at least as good as it does off the wall, and its more ofter safer to run it higher rather than lower.

 

[strantor]

Are you sure there's a rectifier at all? Most angle grinders, drills, dremels, and other high speed AC power tools use series wound "universal" motors. I would expect a leaf blower to be the same. The fact that changing polarity didn't change rotation makes me think so. They can be run on AC or DC. Its basically just a series wound DC motor with low reluctance laminated stator so it can run on AC (efficiently).

 

[SamTexas]

I'm definitely NOT sure. When I opened the blower up a few days ago, my main goal was to determine the motor type (brushed or brushless). I did not look for a rectifier.

Thanks for chiming in. What's a "series wound 'universal' motor"?

 

[strantor]

I'm definitely NOT sure. When I opened the blower up a few days ago, my main goal was to determine the motor type (brushed or brushless). I did not look for a rectifier.

Thanks for chiming in. What's a "series wound 'universal' motor"?

Is basically a series wound brushed DC motor that is optomized to run on AC (but still very optimal for DC use). If you swap the polarity of a series wound DC motor, the rotation stays the same. So, it doesn't matter that the AC keeps reversing polarity, the motor doesn't care. If you want to change the direction of rotation, you have to swap the field wires.

http://en.wikipedia.org/wiki/Electric_motor#Universal_motors

 

[SamTexas]

Thank you strantor. Something new for me to learn.