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AC Vs DC

23222 Views 107 Replies 10 Participants Last post by  johnsiddle
Is there a post on this wiki or in the main forum comparing the pros and cons of AC motors vs DC motors?
Thanks everyone!
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Discussion Starter · #81 ·
Hi - looks good

You will need to find something that matches that spline - probably not difficult

How heavy is it?

What top speed/rpm do you need? - as a bigger motor it will have a lower "burst speed" - I have been told my Hitachi 11 inch will be OK at 6500 rpm - a 13 inch - maybe 5000 rpm??- we need Major to tell us about this one
It's listed as weighing 250 lbs. I'm aiming to cruise at 70 mph. This requires an rpm of 2600 or thereabouts in my jeep (it'll be a direct drive).
Thanks Duncan!
 

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For people worrying about water ingress in DC-motors (forklift motors in particular)

I wouldn't recommend it...but the (dual) DC motors in our little (7meter) boat have gone trough some abuse in the last 7 years...have been under water at least 10 times, sometimes run under water...

We did change to sealed bearings when converting the boat, that's higly recommended.

There is a thread on the forum...but due to the 'new' forum the pictures don't work anymore :(
https://www.diyelectriccar.com/forums/showthread.php?t=55318

https://www.youtube.com/watch?v=BeDhIDnUrnM
 

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Sunking's information on the higher revs in AC motors reminded me of another question about how a motor behaves under a load.
Let's take a simple brushed DC series wound motor for this example. It's rated as 1700 max rmp @ 48V. We overvolt it to 144V. It's max revs are now 5,100 rpm ( 3x 1700 - in theory).
We've calculated the max power at this voltage and the current it draws. We are happy that this is sufficient for the chosen vehicle's weight, CD etc.
It's a direct drive - no gearbox.
This vehicle used to do 70mph when the ICE was at 2500 rpm in 4th gear (1:1).
What will happen when the DC motor at 144V, tries to reach 5100 rpm, but can't, because it hasn't got enough torque? While it struggles to reach 5100 rpm and fails to, will it overheat?
OK a few points you need to understand about motors, especially Series Wound DC Motors. Remember that wirty dord Back EMF and Motor Resistance?

Lets start with Motor Resistance and how LRA works with controllers. Lets say you have that 48 volt motor and it has a Resistance of say 0.167 Ohms and peak power of 15,000 watts or 15 HP. What size controller are you looking for? If you are going to use say 48 volts, just call it 50 volts. The absolute maximum current a 50 volt battery can push is 50 volts / .167 Ohms = 300 amps. Here is the point I am trying to make I see a lot of golf cart guys making an expensive mistake. They go out and buy a 500 amp controller they have no use for nor would they want if they new how much money they wasted and the damage it could cause. All a 500 Amp Controller means is the Controller can supply up to 500 amps before it limits current and folds back voltage to limit the current. . It does not mean it can push and force 500 amps into a motor if there is not enough voltage to push 500 amps. You could use a 50 amp controller and all that means is you limited torque.

Back EMF is your friend and protects your motor from DIY's and beginners ignorance, but only up to a point. Take that same 48 volt golf cart motor where if running at 5100 RPM produces no Torque or draws current. It quit producing Torque and drawing current because at 5100 RPM back EMF = Battery Voltage, thus leaving you ZERO VOLTS across the motor. Back EMF prevented your motor from burning up and over speeding causing it to fly apart. That's a good thing and the way the manufacture intended it to work. Now you apply 144 volts. Issue is your motor can only produce up to say 60 volts Back EMF and you have a 1700 amp controller. What happens? Smoke On The Water and Fire In The Sky turning your balls Deep Purple. So now you have 144 volt battery - 60 volts Back EMF = 84 volts across the motor. That would mean your motor is drawing 84 volts / .167 ohms = 500 amps on a motor than can only handle say 75 amps continuous, or 300 amps peak LRA for a few seconds. That is a real problem that would show up first time you used the motor, and be a very expensive lesson. Loosing a lot of money is a great educator. Education is not cheap learning that way.
 

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OK a few points you need to understand about motors, especially Series Wound DC Motors. Remember that wirty dord Back EMF and Motor Resistance?

Lets start with Motor Resistance and how LRA works with controllers. Lets say you have that 48 volt motor and it has a Resistance of say 0.167 Ohms and ...
Hello Sunking,

Realize that the real resistance of a series wound golf cart motor is more on the order of 0.0167 ohms and that of a lift truck motor, half that. It requires a Wheatstone bridge to measure.

Also LRA is a term rarely used with these types of DC motors. LRA is common with induction motors.

Regards,

major
 

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I'm confused about the motor plate on DrGee's 250 pounder. Is that actually a Series wound motor?

Why does it list an RPM at all? Doesn't say max RPM, and 1300 is quite conservative if it was. Doesn't appear to necessarily be for a very specific known purpose.

Is it perhaps a SepEx or some other type?

Clearly has 4 terminals for the 2 sets of coils so it's not permmag.
 

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Hello Sunking,

Realize that the real resistance of a series wound golf cart motor is more on the order of 0.0167 ohms and that of a lift truck motor, half that. It requires a Wheatstone bridge to measure.

THX and yes I know. It was a KISS exercise in theory. As for requiring a Whetstone Bridge, not so much IMO, at least not the actual Test Set. A battery, lock the rotor down, volt meter, and amp meter will get you just as close to a real working measurement. Essentially the same thing as a Bridge or DRLO Meter.

While I am thinking about it, if you have a DC Motor and no info, you have to do some experiments to find working values. You would need to know Locked Rotor Current. Motor KV RPM, No Load Current on shaft, and Eb if I recall correctly. LRA gives you gives you Max Peak Power, No Load Current gives you max Continuous Power, and Eb gives you max safe battery voltage right?

I ask because DC motors are not my area of expertise. My thing is Batteries, Battery Plants, Power Generation/Distribution, and Electronic Communications related to telecom. Had to go through all that when I designed my first golf cart when I used DC Series Motor and that has been a while. Today I use a HPEV AC15 Induction Motor operating at 96 volts with a 650 amp controller.
 

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...
While I am thinking about it, if you have a DC Motor and no info, you have to do some experiments to find working values. You would need to know Locked Rotor Current. Motor KV RPM, No Load Current on shaft, and Eb if I recall correctly. LRA gives you gives you Max Peak Power, No Load Current gives you max Continuous Power, and Eb gives you max safe battery voltage right?
...
Hi Sunking,

No, that is not right. Here is a tutorial which turned up quickly on a google. It appears pretty good.

https://www.electrical4u.com/types-of-dc-motor-separately-excited-shunt-series-compound-dc-motor/

Also some other parts.

https://www.electrical4u.com/working-or-operating-principle-of-dc-motor/

Regards,

major
 

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Hi Sunking,

No, that is not right. Here is a tutorial which turned up quickly on a google. It appears pretty good.

https://www.electrical4u.com/types-of-dc-motor-separately-excited-shunt-series-compound-dc-motor/
Thx Major. Honestly I do not see anything on the link under Series Motors that invalidates what I have said or expressed. From what I can see mathematically, theoretically, and application are correct. Not arguing or debating, just my observation.


The formulas in the link are pure Ohm's Law which I believe I have stated accurately. I see a lot of info missing, and the application the link addresses is NOT applicable to Continuous use and Variable Speed required for EV use. They specifically say, direct battery, no current or voltage controls. They further add if operated other than a Starter Motor would burn up which I agree with when used in this manner. The link is a very specific application of short burst of low RPM very high torque for starting operation of like an engine or to get a Load moving like a craine making the initial pull to get RPM's up and then use a SEPEX or Induction motor take over.



To use a Series Motor in an EV requires regulating both current and voltage to prevent a Series Motor from self destruction and some control of speed which is going to be poor to start with using a Series Motor. Had to go back and look at my notes because like I said DC motors are not my daily thing. But when I ran through this fire drill 5 or 6 years ago, I had good specs on Golf Cart Motor that included a full set of peak/continuous power power, voltage, current, torque, and temperatures. Interesting to note they give two sets of specs, one for 36, and once for 48 volts.



I designed just off the curves, but those curves match the specs and you can use the math to recreate the curves. Example they give you Rm of the resistance of both Ra and Rs totaled, max peak power, max continuous power, Eb and so fourth. Basically from what I can tell is thermal and mechanical RPM limits.



Where am I off at?


That link is fine is you want to use a 12 volt starter motor for a diesel engine to put on a wheel chair and use a 24 volt battery. Speed control with an off/on switch operating a 1000 amp Contactor to connect the battery directly to the motor. Would work at least once and knock you unconscious when you turn the switch on, flips the wheel chair backwards upside down and landing on your head while the motor flies apart. :D

 

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I'm confused about the motor plate on DrGee's 250 pounder. Is that actually a Series wound motor?

Why does it list an RPM at all? Doesn't say max RPM, and 1300 is quite conservative if it was. Doesn't appear to necessarily be for a very specific known purpose.

Is it perhaps a SepEx or some other type?

Clearly has 4 terminals for the 2 sets of coils so it's not permmag.
It's more information than on most motors!

9,x kW at 1380 RPM
254 A @ 36V = 9 kW

1380 RPM will probably be the nominal speed at 36 or 48 volt

then there is the part number:
5BT1366B137B
http://www.jwarfieldelectric.com/shop/5bt1366b137b-g-e-drive-motor/
http://bullseyeindustrialsales.com/ge-5bt1366b137b-36-48v-98kw-forklift-motor-138937

All four connections are the same size, so probably a series motor.
 

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LRA gives you gives you Max Peak Power, No Load Current gives you max Continuous Power, and Eb gives you max safe battery voltage right? ...
I'm sorry that I can't tell you why, but these three statements are incorrect. I can't speculate on your thought process or logic used to arrive at those conclusions. But please read up on the basic theory and don't rely on nameplate data. Older text books are great sources. Internet offers the good, bad and ugly. Sometimes you can find lecture notes which are outstanding, like from the Navy.

Regards,

major
 

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How are you defining nominal?

Speed at any voltage on a series motor is nonsensical without known load.
Yes, but that placard gives a speed in the context of power output and both current and voltage. The load would be 9 kW worth; the nominal condition is the condition described by all of these values, and could be either the speed and load for maximum continuous power, or the design conditions (intended application).
 

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Discussion Starter · #93 ·
Hi Sunking,

No, that is not right. Here is a tutorial which turned up quickly on a google. It appears pretty good.

https://www.electrical4u.com/types-of-dc-motor-separately-excited-shunt-series-compound-dc-motor/
Thx Major. Honestly I do not see anything on the link under Series Motors that invalidates what I have said or expressed. From what I can see mathematically, theoretically, and application are correct. Not arguing or debating, just my observation.


The formulas in the link are pure Ohm's Law which I believe I have stated accurately. I see a lot of info missing, and the application the link addresses is NOT applicable to Continuous use and Variable Speed required for EV use. They specifically say, direct battery, no current or voltage controls. They further add if operated other than a Starter Motor would burn up which I agree with when used in this manner. The link is a very specific application of short burst of low RPM very high torque for starting operation of like an engine or to get a Load moving like a craine making the initial pull to get RPM's up and then use a SEPEX or Induction motor take over.



To use a Series Motor in an EV requires regulating both current and voltage to prevent a Series Motor from self destruction and some control of speed which is going to be poor to start with using a Series Motor. Had to go back and look at my notes because like I said DC motors are not my daily thing. But when I ran through this fire drill 5 or 6 years ago, I had good specs on Golf Cart Motor that included a full set of peak/continuous power power, voltage, current, torque, and temperatures. Interesting to note they give two sets of specs, one for 36, and once for 48 volts.



I designed just off the curves, but those curves match the specs and you can use the math to recreate the curves. Example they give you Rm of the resistance of both Ra and Rs totaled, max peak power, max continuous power, Eb and so fourth. Basically from what I can tell is thermal and mechanical RPM limits.



Where am I off at?


That link is fine is you want to use a 12 volt starter motor for a diesel engine to put on a wheel chair and use a 24 volt battery. Speed control with an off/on switch operating a 1000 amp Contactor to connect the battery directly to the motor. Would work at least once and knock you unconscious when you turn the switch on, flips the wheel chair backwards upside down and landing on your head while the motor flies apart.
You're just hilarious Sunking! 😂😂
 

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I'm sorry that I can't tell you why, but these three statements are incorrect. I can't speculate on your thought process or logic used to arrive at those conclusions.
No you can't, but I can shine some light on it. Reference this material from Mizzou Eng, and note Motor Data Table Specifications, motor 118748 a 48 volt nominal motor.


I have a different opinion from a user/engineer that name plate ratings do not tell you everything except once extremely important number from an engineer's POV supplying power. Nameplate VOLTAGE tells you the voltage for maximum Efficiency. I agree you can go Higher or Lower but there are trade-offs doing so and two of them are at the expense of Efficiency and more waste heat generated as a result. So if a manufacture tells me the most efficient voltage is say 48 volts nominal, I would consider important data when used in an EV trying to squeeze every mile of range possible with limited battery capacity.

OK let's start with LRA. I stand corrected in terminology. You are correct LRA is typically specified in Induction motors. The correct Term in a DC Series Motor spec is Stall Current or Starting Current. Term is different, but is the same characteristic as LRA in AC motors. Guilty as charged using wrong Term. Reference Starting current = 1.56 amps in the Tables. Input Power = 48 volts x 1.56 amps = 75 watts. My understanding this is the maximum peak thermal limit of the motor windings. Heat is watts or power and directly related to Ohm's law. Your controller for this motor would need to be at most is 1.5 amps or les to protect the motor windings. Voltage based on nameplate is 48 volts. So maybe that shines some light on that logic. Now find Terminal Resistance (motor winding resistance = 30.9 Ohms) and see if it squares up with my logic using Ohms Law. We have a Stall Current = 1.56 amps squared x Terminal Resistance of 30.9 Ohms = 75 watts. Being a Thermal Limit from what I can see is just short of Fussing Current of a given wire AWG. Example Fusing Current on say 12 AWG Solid Copper = 235 amps. Just I^2R heat protection on the motor windings from my prospective. Does not mean thou shall push the Thermal Limits. One just as easily could use say a 1 amp controller and eliminate most heat and stress issues.



I stated maximum continuous power is Motor No Load Current x Battery Voltage.I got that wrong and erred on faulty memory. For some reason I confused it with AC term FLA. The correct DC Series motor term is Nominal Continuous Current .356 Amps from Table. If you used No Load Current as I sated would be 6.95 mas @ 48 volts is 1/3 a watt. Again like Peak Power is a Thermal Limit based on what the motor winding can handle. So Max Continuous Power is .325 amps x 48 volts 15.6 watts, let's just say 15 watts. I was close but no cigar on that one, thx for making me look and taking a refresher. FWIW what No Load Shaft Current can give you is rpm/volt, however the Table gives you that in the specs.
 

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Discussion Starter · #95 ·
For people worrying about water ingress in DC-motors (forklift motors in particular)

I wouldn't recommend it...but the (dual) DC motors in our little (7meter) boat have gone trough some abuse in the last 7 years...have been under water at least 10 times, sometimes run under water...

We did change to sealed bearings when converting the boat, that's higly recommended.

There is a thread on the forum...but due to the 'new' forum the pictures don't work anymore

https://www.diyelectriccar.com/forums/showthread.php?t=55318

https://www.youtube.com/watch?v=BeDhIDnUrnM
Hi Boekel,
Didn't realise I hadn't replied to your post..
That YouTube clip is amazing!
I'll still protect my motor from moisture, but I feel alot better about it now.
 

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Here is Boekel's boat project in PDF form so images are visible.

Let me know if you have any issue with that, and I can remove it. :)
No problem with it. it does stop after 20 pages unfortunately.

so what browser / browser plug in can you use to have the images work?
Such a shame they don't fix it :(

attaching images sucks...
 

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I use Mozilla Firefox, which has a "Toggle Reader View - F9" button in the address bar. I think it is supposed to be for portable devices like phones and tablets. When I switch back to standard view the images are still visible. Then I just used Nitro PDF Pro to print the page. I did notice that there were three pages although page 3 did not have much content.

I like your boat project! :)
 

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I use Mozilla Firefox, which has a "Toggle Reader View - F9" button in the address bar. I think it is supposed to be for portable devices like phones and tablets. When I switch back to standard view the images are still visible. Then I just used Nitro PDF Pro to print the page. I did notice that there were three pages although page 3 did not have much content.

I like your boat project! :)
Thanks!, did another one beginning of last year:
http://boekel.nu/foto/17/2017-03-zwaansloep/

But we might be getting off-topic here ;)
 
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