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f_lunn@yahoo.com
08-24-2011, 07:52 AM
I was reading about the electrical prototypes in EAA SportAviation, July issue. The articles mention various power plants that deliver 27 hp with 20 KW and 54 hp with 40 KW and various voltages---no amps discussed. Is there a basic article or forum that explains the electrical parameters of motors/generators used in this airplanes? Thanks:confused:

Dana
08-24-2011, 11:08 AM
HP and kW are both measures of power. 1HP = 0.746kW so 27HP = 20kW, etc.

The current (amps) depends on the voltage. Volts X amps = watts so your 20kW motor would draw 833A at 24V, 416A at 48V, 200A at 100V, etc.

f_lunn@yahoo.com
08-24-2011, 12:33 PM
Got that--and helpful---so, how do you calculate how long the battery will operate the prop ? Obviously depends upon work load but is there a typical formula for prop/gross wt of plane calc?
Thanks

Matt Gonitzke
08-24-2011, 05:06 PM
The battery's capacity will help you determine how long you can operate the electric motor. For example, a 500 A-hr battery would deliver 50 amps for 10 hours, or some other product of current and time that multiples to 500 A-hr. So the 20kW motor being operated at 100V draws 200A and would have a battery life of 2.5 hours with the fictional battery I made up as an example.

I guess I'm not completely sure what you mean when you refer to 'prop/gross weight of plane' calculations. If I may assume you are trying to size a powerplant and propeller for a particular aircraft, a good aircraft design or aircraft performance book will contain the numerous equations necessary to size the powerplant and propeller.

Dana
08-24-2011, 08:11 PM
The battery's capacity will help you determine how long you can operate the electric motor. For example, a 500 A-hr battery would deliver 50 amps for 10 hours, or some other product of current and time that multiples to 500 A-hr. So the 20kW motor being operated at 100V draws 200A and would have a battery life of 2.5 hours with the fictional battery I made up as an example...

...and before anybody gets excited, that's a 20 kW-hour battery. That's a big battery. With current Li-po battery technology, that's 220 lbs of batteries. For only 27HP.

Matt Gonitzke
08-24-2011, 08:27 PM
Indeed. That was made up to make the numbers easy, and highly unrealistic with today's battery technology.

KJ Hamblin
08-24-2011, 09:08 PM
Now don't quote me on this because it is coming from memory, but I believe the HP rating for a combustion engine and electric motor are specified differently. I believe an electric motor is rated on the amount of continuous power it can deliver while a combustion engine is rated by the peak power it can deliver. That difference is very important.

So if you need a 200HP combustion engine to make your aircraft fly, an electric motor that will give you the same performance will have an HP rating lower than 200HP. Really you have to get into the mathematics of the whole system. You need to figure out the amount of thrust you need that will result in the airspeed you need to produce the amount of lift you need to get the payload(aircraft+people+stuff) in the air.

Once you have that thrust you can calculate how fast the propeller needs to spin(RPM) to produce that thrust, then check the datasheet of the electric motor to get an estimate of the current draw at the calculated RPM and a given voltage. Expect the actual current draw to be 10-20% more.

What are you thinking about doing f_lunn? I'm an electrical engineer and pilot, so I like where you seem to be heading.

Matt Gonitzke
08-24-2011, 09:42 PM
As far as aircraft performance is concerned horsepower is horsepower. The equations don't care what is producing the power. It is up to the designer to use the appropriate numbers for each flight condition. An electric motor can produce maximum torque at essentially any RPM, and also would not exhibit performance degradation with altitude as an air-breathing powerplant would.

KJ Hamblin
08-24-2011, 10:00 PM
True, 1HP will always be 1HP. I just wanted to point out the difference in how the motors advertised HP rating are derived since I don't believe that is common knowledge.

Aaron Novak
08-25-2011, 07:49 AM
True, 1HP will always be 1HP. I just wanted to point out the difference in how the motors advertised HP rating are derived since I don't believe that is common knowledge.

Incorrect, in the aviation and marine world engines are rated and a specific RPM that is typically where you would "prop" it, or whats called the "rated RPM". What you are speaking of sounds more like automotive salesmanship. HP is HP (rpmxtorque)/5252.

KJ Hamblin
08-25-2011, 08:19 AM
Well then my apologies. I will admit that I am not well versed in aviation or marine engines yet. With the 1HP will always be 1HP, all I was meaning to say is along the lines of 1 meter is exactly equal to 1 meter. 1 inch is exactly equal to 1 inch. 1 watt is exactly equal to 1 watt and so on. That was in reference to Matt's "The equations don't care what is producing the power." It was a reference to the units used in equations.

f_lunn@yahoo.com
08-25-2011, 08:48 AM
Now don't quote me on this because it is coming from memory, but I believe the HP rating for a combustion engine and electric motor are specified differently. I believe an electric motor is rated on the amount of continuous power it can deliver while a combustion engine is rated by the peak power it can deliver. That difference is very important.

So if you need a 200HP combustion engine to make your aircraft fly, an electric motor that will give you the same performance will have an HP rating lower than 200HP. Really you have to get into the mathematics of the whole system. You need to figure out the amount of thrust you need that will result in the airspeed you need to produce the amount of lift you need to get the payload(aircraft+people+stuff) in the air.

Once you have that thrust you can calculate how fast the propeller needs to spin(RPM) to produce that thrust, then check the datasheet of the electric motor to get an estimate of the current draw at the calculated RPM and a given voltage. Expect the actual current draw to be 10-20% more.

What are you thinking about doing f_lunn? I'm an electrical engineer and pilot, so I like where you seem to be heading.

KJ,
I was intrigued with the " FlyNano" on pg 32 of the July issue of SportAviation. This 155 lb flying jet ski sounds far fetched--but I wouldn't mind building a more conventional amphib with electric power. I live on a lake and fly a trike Tundra. I could put floats on it, but I'm sensitive to the noise. My wife and I do not like the one jet ski on the lake and know the neighbors and the loons don't like it either.
Sounds like the batteries will significantly increase the weight of the FlyNno !

Ron Blum
08-26-2011, 11:27 AM
IMO, all of you are correct. Power is power, and it doesn't matter what is driving the prop. With that said, though, the prop is still the limiting factor. Also, many people claim that electric motors are much better because they produce max torque at "zero" RPM. This is great for ground transportation (cars, busses, tractors, etc.), but it doesn't matter in an airplane. Here's why. The prop still produces thrust based on its RPM (... and forward flight speed, (variable) pitch and cross-section). At zero RPM a propeller produces zero thrust. The huge advantage that airplanes have over cars is that they operate at a constant RPM for the vast majority of time ... and the variation can be 'slow' (per the regs, jet engines need to produce takeoff thrust in <8 seconds (props are better at <~5 seconds)). This would be an eternity in a car.

One small clarifying point on horsepower ratings, though. Airplane engines are certified to run at maximum power for 5 minutes/flight and 75% thereafter. "Typical" electric motors run at their rated power for a few seconds and 50% thereafter. Car engines rarely run at rated horsepower and at interstate cruise speeds use about 12 horsepower (squat compared to an airplane). Bottom line: Airplanes require a lot of power. -Ron

steveinindy
08-26-2011, 12:33 PM
...and before anybody gets excited, that's a 20 kW-hour battery. That's a big battery. With current Li-po battery technology, that's 220 lbs of batteries. For only 27HP.

Yeah, welcome to why most of us (over the age of 30) will never see anything bigger than a small LSA powered by electric motors in our lifetimes. All of this effort and money are being spent on the aviation equivalent of a sideshow attraction.

Ron Blum
08-27-2011, 06:17 AM
I agree with the battery size issue, but who says it has to be batteries? There are really big companies working on this issue: Boeing, Sikorsky, GE Aviation, Honda, Toyota, GM, the government, etc. On top of all that, there are a lot of venture capitalists pouring money into this new industry. How long did it take the world to go from the Wrights' 12 Hp motor to the jet engine? Looking back, it wasn't very long. This could very well be the next generation.

PS. I am over 30, but know that i will see practical electric airplanes in my time.

steveinindy
08-27-2011, 08:52 AM
but know that i will see practical electric airplanes in my time.

Define "practical". To me a two-seat LSA that cruises at less than 130 kts is not very practical for anything other than bug smashing (which I enjoy, don't get me wrong). I just think we are differing on our definition of practical here.

Ron Blum
08-27-2011, 11:22 AM
My definition of "practical" is "useful", and LSAs that fly at 130 KCAS are defined as "illegal". There are several thousands of C150/172, Cub, Ercoupe, Piper and Beech pilots/owners that would love to see 130 KCAS (that also use their airplanes for business). The stuff that the big boys are working on (in addition to electric propulsion for transport category airplanes) could very well be enough power for a Bonanza, C210 or etc.

As a perfect example, Boeing spent A LOT of money putting an electric motor in a Diamond (motorglider). The hydrogen fuel cell alone was over $1M. Also look at Boeing's "N+3" and "SUGAR" programs. Sikorsky has also put a lot of research into their electric helicopter, "FireFly". Boeing's research won't lead to them building an electric 172 (they are actually looking at APU type appliances), but it could shed technology that allows someone else to produce one.

We are often limited by our own lack of imagination.

f_lunn@yahoo.com
08-27-2011, 02:17 PM
Well, I should have finished reading the July issue of SportAviation before posting this thread--on pg. 90 is an article titled " Practical Electric Airplanes" by Peter Lert. He answers most of my questions. Also, he agrees with Ron and believes electric airplanes will reach "parity" with gasoline in 5 years and "perhaps surpass them" in 10 yrs. Very informative article.

steveinindy
08-27-2011, 02:33 PM
Eh....it's not a lack of imagination on my part. I just find it to be a bit less likely to productive than a lot of other potential courses of research including my own into crash survivability, occupant protection, reduction of risks of post crash fire, etc which is yielding patents and applicable technology in the short term rather than the "Well, maybe we can get this to market sometime around 2025". We haven't seen the billions of dollars poured into electric cars produce a truly viable alternative (the hybrids notwithstanding) and I'm pretty up to speed on a lot of the stuff that Boeing and those guys are up to. It's a lot of talk and speculation at this point planning for something 20-30 years or more down the line most likely by conservative estimates.

I might be focused on what I'm doing, I'm definitely more pragmatic (you might call it pessimistic), but I haven't seen anything approaching a usable design that lives up to the hype once it is actually put in to the testing phases. It's the same problem we see with the flying car concepts (except here we don't have the increased risks associated with that particular operating model). We all get excited and then it turns out that it doesn't work in the "real world" as well as it did on paper. Then those who were enthralled get let down and eventually the cycle starts anew with the next engineer who thinks he has the problem licked and not enough sense to keep from spouting off performance standards to the public before he has the technology (either off the shelf or a test-proven new design) to actually achieve it. That's one reason you'll notice why I never try to get into too specific of detail about the aircraft I am designing. I have a few benchmarks (fuel efficiency, range, speed, inclusion of new safety features like I am working on, etc) picked out that I will discuss publicly but it is because I know the technology is there to do it.

I find the electric propulsion idea interesting, as I do many things (birdwatching, scuba diving, photography, wine, etc) but I maintain sufficient distance to allow myself to glean what is useful but not get so close as to lose my scientific objectivity.

I like problems that can be solved within a reasonable amount of time. I try to follow the advice of James Watson (the bold ones are the ones I feel are pertinent and emphasis is, therefore, my own):


Knowing “why” (an idea) is more important than learning “what” (a fact).
New ideas usually need new facts.
Think like your teachers not your peers.
Seek out bright as opposed to popular friends.
The sooner you narrow your creative interests, the better.

Keep your intellectual curiosity broad.
Work on Sundays.
Exercise when you feel intellectually dull.
Have a big objective that makes you feel special.
Always have an audience for your creative work.
Avoid boring people.
Science is highly social.
Leave a project or field before it bores you.
Choose an objective apparently ahead of its time.
Work on problems that take 3-5 years to work out.

Never be the brightest person in the room.

Stay connected to intellectual competitors.
Work with a teammate who is your intellectual equal.

Constantly share what you learn.
Immediately write-up big discoveries.

Travel increases your creative prowess.
Be the first to tell a good story.
Read out-loud what you write.
Two obsessions are one too many.
Don’t take up golf.
Close competitors should publish simultaneously.
Schedule as few appointments as possible.
Never dye your hair or use collagen.

Likewise, I have "rules" posted for my research and those who work with me. This is geared towards the day I am a professor and have a formal "lab" but bear with me:


If you utter the word “accident”, you have to put a quarter in the jar. The proper term is "crash".
Breakthrough ideas often come to prepared minds that have all the facts, but are unfocussed and even bored with what they are doing at the time.
If you think you have a better idea, let’s hear it.
Don’t address Steve as “Dr. ________” unless you want something. (NOTE: At the moment, I don't have my doctorate yet so it doesn't matter one way or the other)
It helps to have top cover when you are fighting entrenched opposition.
Good scientists need to be multi-skilled.
The quickest way to get kicked out of this lab is to lose your desire to learn.
Sometimes you can’t tell the authorities what you are actually doing because the authorities need deniability.
In transmitting new ideas, face to face contact is usually the only effective way
Always put in a provisional patent before you publish anything.
Be nice until it is time not to be nice.




LSAs that fly at 130 KCAS are defined as "illegal".

Wow...I knew they were slow but dang. All the more reason why I've never understood the fixation some folks have with the new generation of aircraft geared towards that market. If I want to putter around, I'll be more than happy to get an Aeronca or a vintage Cub (which I plan on doing anyhow). I guess my mindset is more towards "Point A" to "Point B" rather than what transpires (beyond the safety and cost aspects) between the two.

Ron Blum
08-27-2011, 02:36 PM
Now, for the real kicker ... reality. For the majority of the EAA crowd, aviation is about passion, but for the OEMs, it's about finances ... yes, one has to make a profit or the business closes. Olive Ann Beech was once asked (a long time into her career), "What do you think is the biggest change in aviation has been since you started?". The "reporter" was looking for ... composite materials, new avionics, new radios, new... Her answer was simple and elegant. There is no difference. We started the business to make money, and that is still our goal today.

On a great note, both the "visionaries" and the OEMs are seriously looking at electric airplanes. There IS a future.

steveinindy
08-27-2011, 02:42 PM
On a great note, both the "visionaries" and the OEMs are seriously looking at electric airplanes. There IS a future.

There is, but for the immediate future (<20 years) it's going to likely be very limited applicability. If you knew half the crap the OEMs are investigating at any given time in terms of propulsion systems that never gets mentioned in the press, you'd probably not put so much faith in the fact that they are "seriously looking" at any given topic. Until it's on the assembly line, it's all just bloody speculation and theorizing.

Ron, I'll even offer to buy you a steak at Oshkosh if they put an electric airplane into production that can best 150 kts in cruise flight before 2020. Actually, you know what, if you'll be at Oshkosh next year, I'll buy you a steak anyhow. I am always up for a lively exchange of ideas and it's always best to surround yourself with folks who aren't in creative lockstep with you.

Frank Giger
08-28-2011, 01:56 AM
Wow...I knew they were slow but dang. All the more reason why I've never understood the fixation some folks have with the new generation of aircraft geared towards that market. If I want to putter around, I'll be more than happy to get an Aeronca or a vintage Cub (which I plan on doing anyhow). I guess my mindset is more towards "Point A" to "Point B" rather than what transpires (beyond the safety and cost aspects) between the two.

Too funny - I have the exact opposite mindset. If I really want to get from Point A to Point B reliably I'll either drive or call Delta.

Probably why I'm building a single seat, open cockpit biplane that cruises at 55 MPH.

:)

Seriously, I think the focus on LSA (1,320 gross weight limit, 120 kts cruise, two seats only) in electric powerplants is more about ready-made production platforms that have lower engine requirements than anything else. Add in composite material construction, advanced electronics, etc., and it's a good way to start out.

This is why electric cars focused on sub-compacts as development beds. Personally, I'd of opted for a light truck or mini-van and loaded up the batteries for greater range; clearly they did the math and the lower power demand trumped power storage capacity.

LSA's also fit another handy design parameter - range (or, more precisely, duration of flight). The FlightDesign CTLS, for example, carries 34 gallons and burns 5.5 gallons of mogas an hour, for a flight time of six hours (total, without taking off the 30 minute required reserve). The CTLS actually cruises at the max allowable - 120 kts - for a nominal range of 660 NM. This is pretty typical for LSA's, and while pretty steep for electrical aircraft gives a nice benchmark goal to shoot for.

In a different direction, I have serious doubts that a hybrid aircraft would be desireable. Beyond the weight penalties of having one engine required to make another engine work, I would not want my soft pink flesh relying on a combustion engine that had to automatically start, shut down, and restart in irregular intervals to keep the batteries up to snuff.

All of this avoids the net negative environmental impact of electric aircraft, which seems to be actively overlooked. To achieve zero emmissions at the end of the pollution stream it's upsourced the supply chain and then dumped down once the plane is on the ground.

The materials for composite aircraft and in manufacturing the batteries are far more demanding on the environment than aluminum skinned (or fabric covered) aircraft using an internal combustion engine. Electric generation is our single greatest area of emissions (40% of the USA's total GHG production comes from coal fired electric plants), and the alternatives for generating even more are unworkable or politically difficult.

On the downstream, batteries that have outlived their life cycle are environmentally nasty. And they're going to need to be replaced on aircraft far more frequently than in cars. I take the cue of ELT's - expect the FAA to mandate replacement at 50% of the expected life expectancy. Even if they're recycled there's a huge amount of toxic waste generated.

I hate to be the naysayer, but pound-for-pound nothing beats the power storage capacity of liquid hydrocarbons....

steveinindy
08-28-2011, 10:11 AM
Seriously, I think the focus on LSA (1,320 gross weight limit, 120 kts cruise, two seats only) in electric powerplants is more about ready-made production platforms that have lower engine requirements than anything else. Add in composite material construction, advanced electronics, etc., and it's a good way to start out.

I would tend to agree, although I don't think the advanced electronics are really necessary for anything that is day VFR only. An airspeed indicator, altimeter, maybe an attitude gyro, radios and a fuel gauge are all that's necessary. Plugging a glass cockpit into one of these LSAs is a bit like my urge to plug one of the higher end, 1200 hp PT6As into my design: it'd be nice, but it's not essential for the mission of the aircraft. That's why I'm going with a much lower horsepower engine (most likely either a Rolls Royce or GE engine) that will do the job at a much lower fuel consumption rate.



In a different direction, I have serious doubts that a hybrid aircraft would be desireable. Beyond the weight penalties of having one engine required to make another engine work, I would not want my soft pink flesh relying on a combustion engine that had to automatically start, shut down, and restart in irregular intervals to keep the batteries up to snuff.

As a safety researcher, I am in complete agreement with you there, but then again I also feel the same way about mogas engines in aircraft and electric powerplants too.


The materials for composite aircraft and in manufacturing the batteries are far more demanding on the environment
On a somewhat related note, welcome to one of the toughest problems in composite design: finding a way to make the stuff not either burn or not be as toxic when it does burn in a crash.

Ron Blum
08-28-2011, 03:02 PM
Guess it's my turn to "speak". First, Steve, I'll email you privately with other information ... somehow I can't believe that you're an engineering student.

A hybrid airplane is just a bad idea ... it's good for a car, though. Car and airplane engines have totally different operating environments. A car's engine is up and down all the time (equates to inefficiency), so making it a hybrid makes sense because the engine can run at peak efficiency all the time it is running. (except for airplanes doing aerobatic routines) Airplane engines run at high (efficient) power all the time, and they rarely change RPM (another good thing for efficiency).

Yes, liquid hydrocarbons have a 72:1 weight advantage over current batteries, but that ratio is starting to come down. In addition, electric propulsion systems can be made more efficient than engines (for multiple reasons). I like Frank's comment about 6 hours of range. Who ever uses that? (rhetorical question). Even the mid-range capable Citation IIIs (Cessna Model 650s - not CJ3s), The mean flight time is 1:15, and modal flight time is 0:42. In other words, 150-200 NM trips.

My background is supervising/managing OEM Flight Test Engineering departments ... and running DO-160 labs. The Hawker Beechcraft composites pass all the FAA requirements.


I hate to be the naysayer, but pound-for-pound nothing beats the power storage capacity of liquid hydrocarbons....

Although I currently agree with this, the Prius would be a total flop. Sorry to say, but green sells ... and that keeps people employed.

steveinindy
08-28-2011, 04:17 PM
My background is supervising/managing OEM Flight Test Engineering departments ... and running DO-160 labs. The Hawker Beechcraft composites pass all the FAA requirements.

Still don't want to inhale what result when you burn it. Even if the "usual suspects" of incomplete combustion (CN, CO, HS, etc) don't get you, then you wind up with a nasty pulmonary fibrosis from the inhalation of the fibers.

Frank Giger
08-29-2011, 02:28 AM
Burning composites is a tiny problem environmentally.

On the issue of fire safety, they beat the next lightest covering - fabric - all to heck.

Manufacturing them is where the pollution comes from. Not to say that they aren't a good trade off for heavier materials in the long run when energy requirements for aircraft over a thirty year (if not more) lifespan of an aircraft are taken into account.

Recycling a typical GA aircraft today is simple - gut it, chop it up, and melt down the aluminum. Not so much for composite aircraft, AFAIK.

Both hybrid and electric cars sell because they are fashionable and the illusion of savings. They cost more and are ultimately more polluting than internal combustion ones. In the future the price will come down, but the pollution for the increased demand for electricity will go up. One is simply trading emissions from a tailpipe for one coming out of the stack of a coal fired plant - and due to wasteage of electricity over lines in the grid much more is required.

On glass panels, it's hard to find an LSA on the market that doesn't have one, and from a marketing standpoint it is almost required for any aircraft that bills itself as high tech.

steveinindy
08-29-2011, 08:35 AM
Burning composites is a tiny problem environmentally.



I'm not thinking environmentally outside of the immediate 'environment' of the cabin for purposes of survivability and the health of my brother and sister firefighters who have to respond to crashes.


On the issue of fire safety, they beat the next lightest covering - fabric - all to heck.



This is very true although that is no reason to rest on one's laurels and not try to improve things, which is all I am advocating for.


Manufacturing them is where the pollution comes from.

Take a look at the pollution associated with aluminum production since it involves large amounts of electricity.


On glass panels, it's hard to find an LSA on the market that doesn't have one, and from a marketing standpoint it is almost required for any aircraft that bills itself as high tech.

Point taken, but maybe it's the nostalgia buff in me that find something appealing in old steam gauges when flying low and slow. That's one of the reasons I want to own an L-bird some day. Stick and rudder, basic instruments and cheap to operate. No need to shell out more money for something that's "high tech" but low speed.

Ron Blum
08-29-2011, 10:02 PM
Point taken, but maybe it's the nostalgia buff in me that find something appealing in old steam gauges when flying low and slow. That's one of the reasons I want to own an L-bird some day. Stick and rudder, basic instruments and cheap to operate. No need to shell out more money for something that's "high tech" but low speed.

If there ever is a practical electric airplane, the first one should be done with "steam" gauges. Cheaper, easier and requires less (no) power (and failure modes are well known). Flying IFR, in icing or at night is a much different story ... and power requirements are significantly higher.

Steve - I think you're giving me enough inspiration to go after some ideas that I have been thinking about for years now to make air data systems better. Thanks. ... ah, after the 1911 is done.

steveinindy
08-29-2011, 10:58 PM
Steve - I think you're giving me enough inspiration to go after some ideas that I have been thinking about for years now to make air data systems better. Thanks. ... ah, after the 1911 is done.

Not a problem. Can I help?

shadow738
09-01-2011, 10:27 AM
I'm also a pilot and electrical engineer. With regard to horsepower, an electric motor can deliver much more than its rated power, BUT there is no such thing as a free lunch. The reason is that you can put up with short term heating as long as you don't fry the motor. You will be taking the power from the battery.

This means you can size the motor for cruise, and overload it short time for takeoff and initial climb. When speed and altitude permit, you throttle back to "cruise climb"/MECO power just like any other airplane.

Electric heating effects are proportional to the SQUARE of the current: "Twinkle, twinkle little start, power equals I (current) squared R (resistance)."
A motor on low voltage will put out the same power, but it will draw more current and usually fry itself. I work in a hardware store and we sell big, 240-volt air compressors; if you try to run them on 120 the thermal breaker in the motor will trip--that's the good news. The motor is trying to draw twice times the amps resulting in four times the heat.

--- Doug Drummond

Dana
09-02-2011, 08:42 PM
... I work in a hardware store and we sell big, 240-volt air compressors; if you try to run them on 120 the thermal breaker in the motor will trip--that's the good news. The motor is trying to draw twice times the amps resulting in four times the heat.

Uh... not exactly. Power (wattage) equals I²R or V²/R. Halve the voltage and you get 1/4 the power. What is probably happening is that the motor has a starting circuit (which has much less resistance, resulting in higher amps during starting until a centrifugal switch cuts out the start circuit), and the lower voltage can't get the motor up to speed to get it off the starting circuit, so it quickly overheats.

shadow738
09-04-2011, 07:01 PM
Sorry Dana, I have to disagree. W = I^2*R, but a motor is not a resistor. When just starting or anytime running slower than its proper speed, a motor looks a lot like a short circuit. I work with industrial strength motors and have recently taken classes. A motor is also a generator and what limits the current is not so much the internal resistance but the "Counter Electromotive Force" [voltage] being generated.

The simplest case is small DC motors (eg model trains) which have an internal resistance of about 10 ohms. When you first put 12 volts on them, they draw over an amp for a fraction of a second. Once they are up to "full" speed [proportional to the voltage] they draw about 1/4 of an amp. I maintained controls for these puppies for four years as a full time industrial controls technician.

Doug.

Dana
09-04-2011, 07:21 PM
Sorry Dana, I have to disagree. W = I^2*R, but a motor is not a resistor. When just starting or anytime running slower than its proper speed, a motor looks a lot like a short circuit...

Mmmm... good point. Inrush current is often much higher. Though small DC motors are often speed controlled by feeding them lower voltage.

Ron Blum
09-04-2011, 11:48 PM
"I work with industrial strength motors and have recently taken classes."

Out of curiosity I am wondering where one would take a class(es) in motors/motor design. I want to learn all I can. Thanks, Ron

steveinindy
09-05-2011, 01:58 PM
I was thinking the same thing. Not that I want to even try to actually build one but I'd like to sound as little like a blithering idiot when I talk shop with the guys from RR.

shadow738
09-06-2011, 04:38 PM
"I work with industrial strength motors and have recently taken classes."

Out of curiosity I am wondering where one would take a class(es) in motors/motor design. I want to learn all I can. Thanks, Ron

I took industrial controls and other practical courses at my local community (2 year) college, basically to catch up on stuff that has been invented since I got my electrical engineering degree from Purdue in 1968. I'm semi-retired and trying to get a full- or part-time technical job. My strength has always been that I understand and can do the practical side as well the theory. My wife is a librarian at the college so we get an employee discount on classes. I'm hoping to get a job teaching this stuff part time at one of the local colleges--I can do that without a master's degree.

The classes were "Programable Logic Controllers, National Electrical Code [for my electrician's license], Motor Controls, Industrial & Commercial wiring [to upgrade electrician's license], and Industrial Electricity" [mostly advanced motor controls]. I also take writing classe. Sorry about getting off topic, guys.

To go beyond the tradesman/technician level, I buy really old books on the subject (back when electric motors where high tech] and check out similar books from the college and public library.

It may be possible to take classes at a local 4-year college but this might be expensive.

--- Doug

shadow738
09-06-2011, 04:49 PM
Mmmm... good point. Inrush current is often much higher. Though small DC motors are often speed controlled by feeding them lower voltage.

Absolutely correct. I use Permanent Magnet DC motors as an example because the speed is almost exactly proportional to the voltage, and the amps are proportional to the load [torque]. The electricity being converted to mechanical energy is the internal voltage (Counter EMF) times the amps. The amps are the (line_voltage - Counter_EMF) divided by the internal resistance.

--- Doug

Ron Blum
09-07-2011, 08:06 AM
Thanks for information on the classes that you have taken. This is really helpful. It is also cool to know that you are trying to get a teaching/technical job after "retiring". Thanks, Ron

shadow738
07-17-2015, 06:15 PM
I don't think anyone has said that 1 horsepower = 746 watts. Of course nothing is 100% efficient -- electric motors are typically 85-92% efficient, so if your motor is 85% efficient, then you need 877.65 watts to produce 1.0 horsepower. Note: in the metric system, watts are used as the unit for mechanical power as well as electrical power. I am also a pilot and retired electrical engineer. I've been going to Oshkosh since 1974.

Ron Blum
07-18-2015, 09:33 PM
I've been going to Oshkosh since 1974.

Shadow738: Can I meet you for a couple moments at Oshkosh this week? I would love to learn good sources of motor design. In other words, what makes a motor more (or less efficient), higher torque, etc.

If so, please call 316-295-7812 to arrange a time to meet up. It would be fun (Oshkosh since 1976).

Thanks!
Ron