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Generators & Electric Motors General Discussion Antique Generators and Old Electric Motors: Questions and answers about restoring and showing old power generation systems.

Generators & Electric Motors General Discussion

Rating of Direct Current Motor as a Generator


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  #1  
Old 08-08-2019, 06:28:29 PM
Vanman Vanman is offline
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Default Rating of Direct Current Motor as a Generator

For instance, what is the realistic power rating of this 7-1/2 hp motor when run as a generator? I wish the nameplate wasn't ruined on this one. But I think it's 57 armature amps and 2.38 field amps.

https://www.ebay.com/itm/General-Ele...ecce%7Ciid%3A1

It would seem that it would simply be rated armature current minus exciting current (being self excited), but this is of course considerably more than the machine's original rating, taking perhaps 12 or more hp input to achieve.

Or would you just figure on only getting ~5.6 kW (7-1/2 hp) electrical out, taking ~10 hp to drive it.

Ultimately the limit is heat dissipation.

If we ignore core losses for the moment, armature copper losses would be the same when armature current is the same, regardless of which way the power is flowing through the machine. And field losses are the same either way as well.

So armature core losses are all that's left to consider. Running as a generator, the core losses will necessarily be more for any given value of armature current, due exclusively to the required higher speed, and thus higher operating frequency.

So what percentage of a typical machine's losses are attributable to core losses vs armature copper losses?

The answer to that, is the answer to my original question. Anyone have knowledge of this?

Keith
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Old 08-08-2019, 06:36:51 PM
Railroads Railroads is offline
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Default Re: Rating of Direct Current Motor as a Generator

Keith, I don't know about DC machines? If it was a 3 phase induction motor you would get 5.5KW from a 7.5HP motor. You would need a 10hp engine to achieve 5.5KW.

Robert
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Old 08-10-2019, 01:30:53 PM
Zephyr7 Zephyr7 is offline
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Default Re: Rating of Direct Current Motor as a Generator

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Originally Posted by Vanman View Post
So what percentage of a typical machine's losses are attributable to core losses vs armature copper losses?

The answer to that, is the answer to my original question. Anyone have knowledge of this?

Keith
I’m not going to claim to be expert for this, but I might have some insights. The core losses vary by construction and size. Larger units will tend to me more efficient in this regard as a percentage (large transformers, for example, can be way over 99% efficient). Core losses tend to be highest at no load too, and least at full load. Magnetic are weird that way.

I would guess around 80-90% efficiency for a smaller unit like this, and that’s probably conservative. If you want more info look up “reluctance” as applied to electromagnetics.

Bill
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Old 08-13-2019, 10:05:44 PM
dkamp dkamp is offline
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Default Re: Rating of Direct Current Motor as a Generator

Hi Keith!

Okay, first off, you're asking TWO questions:

1) How much power can I get OUT of it...

and

2) How much power do I need to put INTO it, in order to get that much OUT.

Answer of the first is:

A) You can get OUT, the full rated INPUT as identified on the nameplate. If it says it draws 57A IN at X volts, it will handle that same amount OUT...

---But you'll need to provide field current to match...


B) it will require every bit of that, plus losses, plus excitation power in, to get that much out.

That being said, there's another caveat:

It's a DC motor, which is to say, also a DC generator, which means the SPEED at which you turn it, and the level at which you drive the field, determines the output.

--Increasing armature speed will increase voltage, and because of I2R losses, your ability to generate POWER can increase with higher voltage...

BUT... the armature is an INDUCTOR, which means, it has inductive reactance, which increases with frequency (armature speed)

#1 lesson of all DC motors... is that they are AC MOTORS!!! ;-) They just have a built-in INVERTER (called a Commutator).

If you study Ward-Leonard drive systems, you'll see control circuit elements that increase armature and field current of a DC motor to increase speed, and once at the motor's rated speed, they DECREASE the field current (aka 'field weakening'). What they're doing here, is reducing the field current's SIZE, so that as the poles commutate from one polarity to another, that they don't get 'caught' by the size of the standing field, and cause it to 'drag' the motor down (meaning, armature current goes straight to the sky). This allows the motor to spin FASTER than it's rated speed. Rated speed is based on 'full' field current. Knock off the current, the motor will pick up more speed... and while the torque reduces, the motor's SPEED increases.

At any speed from stop to full rated, the DC motor will be a 'constant torque' device. once at rated speed, any field weakening above that point will increase armature speed, but reduce torque, so it'll be a 'constant HP' device.

Eventually, you'll reach a speed in that DC motor where the frequency of commutation exceeds the FCO of the armature windings' inductance, and it'll spin no faster... it'll be against a 'brick wall' of electromotive physics.

If you calculate the motor's torque based on rated HP output and RPM, and come up with an engine capable of producing about 20% more than that combination of torque and RPM, plus whatever's necessary to develop excitation, you'll be doing just fine. Make sure you cool that thing well, otherwise, the magic smoke will escape.
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Old 08-15-2019, 01:49:34 PM
Railroads Railroads is offline
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Default Re: Rating of Direct Current Motor as a Generator

Keith,

What are you building?

Robert
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Old 08-15-2019, 04:13:42 PM
Vanman Vanman is offline
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Default Re: Rating of Direct Current Motor as a Generator

Hi Robert,

A bus conversion. LONG story short as I can. I have two 1950's Crown buses (my favorite make). The '57 is running, but it's only two axle 35 foot and Cummins powered (I love the antique Cummins, but I am a Detroit guy at heart). The other is a '55 and is a three axle 40 foot (my favorite Crown), but has body damage and a seized engine.

For a while, it was deemed more prudent to first outfit the running one as a motor coach. It isn't my favorite, but would be quicker, easier, and cheaper to get into service and start enjoying. It was going to be based on a 24 volt battery, but with an inverter and ac generator with the majority of loads then running on ac. A 24 to 12 volt converter would supply the small amount of 12 volt power needed.

Well, this would still be a lot of work, and expense, and would still leave my favorite bus marooned at my friend's house. So the plan was changed. We're going to go straight for the bigger project, but ultimately saving money and energy by doing only one instead of two.

This one is VERY special, and as such is going to be done with great care and due consideration for the type of equipment used. I want to make it ALL as vintage as possible. As such, it will be based on a 112 volt battery (115 ~ 120 volt). All generation will be 115 volt dc, as will the majority of loads. A small motor generator (two for redundancy) will supply the 12 volt dc bus electrical system while under way, and a motor generator will supply ac for the few pieces of equipment that require it, such as a microwave oven.

It's going to get a freshly overhauled Detroit Diesel and ten speed RoadRanger transmission as the Cummins that's in it is seized anyway. It's original engine when built was a 779 ci Hall-Scott, good for 2 ~ 3 mpg. These were almost all replaced when the price of fuel started to get stupid. The Detroit will easily do 10, probably more if I can find axles with the best ratio.

Keith
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Old 08-15-2019, 04:54:34 PM
Railroads Railroads is offline
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Default Re: Rating of Direct Current Motor as a Generator

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Originally Posted by Vanman View Post
Hi Robert,

A bus conversion. LONG story short as I can. I have two 1950's Crown buses (my favorite make). The '57 is running, but it's only two axle 35 foot and Cummins powered (I love the antique Cummins, but I am a Detroit guy at heart). The other is a '55 and is a three axle 40 foot (my favorite Crown), but has body damage and a seized engine.

For a while, it was deemed more prudent to first outfit the running one as a motor coach. It isn't my favorite, but would be quicker, easier, and cheaper to get into service and start enjoying. It was going to be based on a 24 volt battery, but with an inverter and ac generator with the majority of loads then running on ac. A 24 to 12 volt converter would supply the small amount of 12 volt power needed.

Well, this would still be a lot of work, and expense, and would still leave my favorite bus marooned at my friend's house. So the plan was changed. We're going to go straight for the bigger project, but ultimately saving money and energy by doing only one instead of two.

This one is VERY special, and as such is going to be done with great care and due consideration for the type of equipment used. I want to make it ALL as vintage as possible. As such, it will be based on a 112 volt battery (115 ~ 120 volt). All generation will be 115 volt dc, as will the majority of loads. A small motor generator (two for redundancy) will supply the 12 volt dc bus electrical system while under way, and a motor generator will supply ac for the few pieces of equipment that require it, such as a microwave oven.

It's going to get a freshly overhauled Detroit Diesel and ten speed RoadRanger transmission as the Cummins that's in it is seized anyway. It's original engine when built was a 779 ci Hall-Scott, good for 2 ~ 3 mpg. These were almost all replaced when the price of fuel started to get stupid. The Detroit will easily do 10, probably more if I can find axles with the best ratio.

Keith
Keith, That is an interesting project. I have only a passing interest in buses. I thought at first you were either going to charge batteries, or perhaps were looking for a traction generator for some project.

Have you considered one of the Fabco belt drive AC generators for the AC power requirements? https://www.fabcopower.com/generat/bgen.htm They have shown up on Iron Planet and other Gov't auction sites. I tried to bid on one that was on Ebay but I got outbid at the last second.

Robert

Last edited by Railroads; 08-15-2019 at 05:17:51 PM.
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Old 08-15-2019, 05:40:34 PM
Vanman Vanman is offline
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Default Re: Rating of Direct Current Motor as a Generator

Yes, I've always wanted to build a ~115 volt direct current system, I have a lot of appliances for it, and building a house seems like it isn't going to happen any time soon. So the bus it is!

I never really noticed buses either, until I spotted these. Mid engine, mounted horizontally under the floor, both rear axles driven. They're unlike virtually any other, giving the lowest center of gravity for the best handling, and the largest uninterrupted interior floor area.

Getting back to the question at hand. Since I have already put so much work and bought parts for the Kohler generator set, I'm thinking to still use that engine, and just couple it to a suitable generator. It's rated 12-1/2 hp @ 1800 rpm.

I want to be able to vary it's speed for varying demand, since, being dc, speed need not be dictated as it is for ac. Since dc motors are more prevalent, I wanted to consider that possibility instead of specifically looking only for dc generators. They're around, but they are almost always 1750 rpm. I could get a 230 volt one, then be able to run it down as low as half speed by reconnecting the field for 115 instead of 230, but the kW would be limited to ~1/2. It would bug me having the nameplate say 230 volt. OCD.

---------- Post added at 02:40:34 PM ---------- Previous post was at 02:17:56 PM ----------

I picked the example motor above since it seems a near match for my engine and needs. 7-1/2 hp @ 1150 rpm, so my minimum speed would likely be around 1250 rpm (generating) for reduced power, then let it run up to 1800 for full power (by weakening the field).

As a motor, the armature takes 57 amperes at 120 volts, so 6840 watts. 7-1/2 hp output is 5595 watts. So the armature losses (leaving aside the field and ignoring friction and windage) are 1245 watts, divided between core and copper and commutator losses. This is an efficiency of 81.8%. Not particularly good, but not horrible either.

Dkamp's assumptions are that we can run it as a generator with the same armature current, so 6840 watts out from the armature (again, ignoring the portion of that which is now supplying the field). If the losses remained the same at 1245 watts, the power required from the engine would be 8085 watts, or ~10.8 hp, an efficiency of 84.6%. The copper losses would be the same, but the core losses will be greater, due to the necessary higher speed, so this is not exactly accurate.

If instead the efficiency is assumed to remain the same, the input power required would be 8362 watts or 11.2 hp. The armature losses now being 1522 watts, 122% of the losses when run as a motor, and that much more heat to safely get rid of. So the power rating would have to be reduced some. I would imagine that the real world figure is going to fall somewhere between those two extremes.

I would only be getting that much power out at 1800 rpm (again, with the field weakened), so the extra cooling airflow alone should be more than adequate to remove the extra heat generated in the armature core. The weaker flux density compensates for the higher frequency as regards core losses, so that difference can be ~ignored.

Last edited by Vanman; 08-15-2019 at 08:33:56 PM.
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Old 08-15-2019, 06:45:20 PM
Railroads Railroads is offline
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Default Re: Rating of Direct Current Motor as a Generator

Keith, All I can say is get the motor and give it a shot. If it doesn't work then sell the DC motor and look for something else.

I haven't seen too many DC motors with wound armatures around my parts. The only thing that turns up are the permanent magnet types of DC motors. One of those would probably work as a generator too. Controlling the output though would be another matter.

Robert
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Old 08-15-2019, 07:49:51 PM
Vanman Vanman is offline
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Default Re: Rating of Direct Current Motor as a Generator

Remember though, approximately 50's ~ 60's tech. No permanent magnets, only wound field machines for me. Besides, that's how you control the speed.

I'm quite sure that ebay machine would work very well, though I'd like one that can go slower, and don't want to belt drive. Was just curious exactly how much power I might get out of it, while staying within it's original design temperature limits.

Keith
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Old 08-17-2019, 01:02:08 AM
Zephyr7 Zephyr7 is offline
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Default Re: Rating of Direct Current Motor as a Generator

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Originally Posted by Railroads View Post
I haven't seen too many DC motors with wound armatures around my parts. The only thing that turns up are the permanent magnet types of DC motors. One of those would probably work as a generator too. Controlling the output though would be another matter.
Depending on the way the motor is built, you might be able to wind some small coils into the pole pieces of the permanent magnets. Run a variable DC current in those coils and use it to buck or boost the magnetic field of the permanent magnets to give some limited regulation ability. I’ve never tried this, but I don’t see any reason why it wouldn’t work.

Bill
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Old 08-17-2019, 12:35:37 PM
Railroads Railroads is offline
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Default Re: Rating of Direct Current Motor as a Generator

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Originally Posted by Zephyr7 View Post
Depending on the way the motor is built, you might be able to wind some small coils into the pole pieces of the permanent magnets. Run a variable DC current in those coils and use it to buck or boost the magnetic field of the permanent magnets to give some limited regulation ability. I’ve never tried this, but I don’t see any reason why it wouldn’t work.

Bill
Bill, The motors I have come across just have big ceramic magnets epoxied to the inside of the motor shell. No pole pieces of any kind really. Cheap Chinese motors!

Robert
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Old 08-18-2019, 09:55:06 AM
armandh armandh is offline
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Default Re: Rating of Direct Current Motor as a Generator

the way to regulate the dc out is the engine power vs load
as the load increases the voltage goes down
unless the engine output is ramped up.
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Old 08-19-2019, 12:34:49 AM
dkamp dkamp is offline
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Default Re: Rating of Direct Current Motor as a Generator

Keith- one option you could look into, is the generator segments of a Monarch 10EE toolmaker's lathe. It uses a Ward-Leonard drive... 3ph AC motor drives TWO generators... one for field, one for armature, of a DC spindle motor.

You could use one of the generators to generate armature field current, then use the spindle motor to generate output...

but it'd be really, really heavy.

BTW... how are you gonna make the Cummins or Detroit driveline fit in where the Hall-Scott 'flat' engine was mounted?
And... if the Hall-Scott needs a museum place, I DO have access to a major antique truck museum that has antique truck engines on static display... if you're willing to donate it, I'll give it the cleanup and fit it for display stand...
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Old 08-19-2019, 05:06:23 PM
Vanman Vanman is offline
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Default Re: Rating of Direct Current Motor as a Generator

There's actually one of those motors and motor generator sets that's been on the local Craigslist here for quite some time. The motor has a speed range of 690 - 2700 rpm via field weakening and would make an outstanding generator for my purposes except that it is only 3 hp and is 230 volt so only good for 12 amps. If I could find a 115 volt one of these, in 7-1/2 hp, I'd be golden.

The Hall-Scott is long gone, replaced with an NHH Cummins (the horzontal version of the NH) probably in the '70's when gasoline became stupid expensive.

Both Cummins and Detroit built horizontal versions of their regular straight sixes. The NHH Cummins was the first diesel engine to be put in a Crown, in 1954. And in the early '60's the Embree's (a transportation company) adapted a 671 for horizontal operation in a Crown. Not long afterwards, GM offered horizontal 671s.

The last Crown to get a Hall-Scott gas engine was I believe in 1967. Ever since then they have had various NHH Cummins (743 ci), various 855 ci Cummins, and various 671 Detroits.

The 671TA Detroit I'm overhauling and ten speed Roadranger came out of a 1981 Crown.

Keith
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