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The Truth about DC Motors?

1927 Views 16 Replies 11 Participants Last post by  John Metric
With OEM AC motors so plentiful and cheap nowadays and aftermarket AC motors (Hyper 9, AC-50, etc.) very simple to set up and run, it seems that almost no one is looking to run a DC motor in builds nowadays. But the cool thing about DC motors is that the operating voltage and current don't seem to be nearly as set-in-stone as with AC motors. There's all kinds of grainy old videos of people pushing crazy amps through the brushed motors that used to be all anyone used in conversions.

So the question is, how much power can DC motors really push?

With heat seemingly the biggest limitation, would it not be feasible, with proper cooling, to have a massive (think 19" diameter) motor that could do 800kw+ for short bursts?

Curious as to what folks have learned from working with the good ol' DC motor.
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I have two ME1004 motors. They're powerful and compact, owing to permanent magnet rotor. I also have a Peerless/Ohio 48V/7HP motor from an industrial machine, it's massively heavy and weak for its size.
 

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DC motors are typically voltage limited (compared to other types) but with the right brushes can go as high as 200 V. They have been described as being like a mechanical transformer and will try to convert every amp and volt thrown at them into rotational motion - even if they blow up in the process. With a Z2k controller at 2000 A a WarP 9 with Helwig red top brushes can make 2000 X 200 = 400,000 watts (400 KW) for short bursts, like in drag racing. This would probably be around 400 HP after accounting for inefficiencies at those levels. Nobody makes a commercially available controller with more output that I know about.

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I'm running an 11 inch Hitachi motor with a P&S controller
I have been feeding it 1200 amps and 340 volts - I was hitting 6800rpm at the end of the 1/4 mile
Just doing a rebuild and increasing the maximum voltage to 390 volts plus changing the diff to cut the rpms by 14%

The motor is rated at 200 amps, 48 volts and runs at about 1400rpm in the forklift

One of my friends (Brian) is just building a drag bike
We have got a Chevy Volt battery pack (390 volts) and a Crown 11 inch motor
Brian has ordered a Zilla high voltage 2000 amp controller

Its looking good - but only a maniac (Brian) would ride a drag bike
 

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Series motor voltage constraints is oddly enough the gap between commutation segments and corresponding flash over which erodes the comm bar and brushes super fast. Total voltage determines what current you can push and power made. In @Duncan 's case it might zorch if he ever gets to above 75% "on" in the duty cycle at low rpm.

Having said that, realize a motor is an inductive load. The faster it spins, the more back EMF is going to resist forward voltage with zero rpm having the highest current demands. Therefore, it is possible that motor may survive the + 300 volts at redline rpm.

Kostovs with added interpole magnets are tested to survive 300 volts but also have an 8 degree retarded comm timing and do not generate the same levels of HP to prevent zorching

500 lbs isn't that odd, Kostovs are 220 lbs and 500 is about the weight of a 7 liter gasoline engine
 

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Larger diameter motor increases the inertia, which requires more torque to spin, meaning higher current needed.

So how heavy is the battery pack needed to go with the gigantic 500-lb motor?

i suppose it could all be built, but what would be the objective, or application, or benefit of a system such as this?
 

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A larger diameter motor does have more rotational inertia but produces MORE torque for a given amount of current....we'd be using slotcar motors, otherwise, for traction applications.

The unloaded spinup torque is irrelevant in a high load application unless massively geared down.
 

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Series motor voltage constraints is oddly enough the gap between commutation segments and corresponding flash over which erodes the comm bar and brushes super fast. Total voltage determines what current you can push and power made. In @Duncan 's case it might zorch if he ever gets to above 75% "on" in the duty cycle at low rpm.

Having said that, realize a motor is an inductive load. The faster it spins, the more back EMF is going to resist forward voltage with zero rpm having the highest current demands. Therefore, it is possible that motor may survive the + 300 volts at redline rpm.

Kostovs with added interpole magnets are tested to survive 300 volts but also have an 8 degree retarded comm timing and do not generate the same levels of HP to prevent zorching

500 lbs isn't that odd, Kostovs are 220 lbs and 500 is about the weight of a 7 liter gasoline engine
Hi Piotrosko
I hit 100% "load" at low rpms - but at low rpms that only needs a small voltage - so the controller only feeds the motor a small voltage

With a DC motor the three values are all linked - current, rpm and voltage

To get a high voltage you must have high rpm and high current - so at low rpm my controller can only feed a low voltage

So I can't "get to above 75% "on" in the duty cycle at low rpm" - unless the controller fails - in which case it will probably zorch the motor !
 

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Well in theory for a very short time, you can spin a 500 lb motor with 18650 cells. You can test a 13" using a 12v car battery but that drew 50 amps. I suppose 12v would spin at low rpm a bigger motor, didn't have one to test. Weight of battery kinda correlates to range depending on chemistry. 1/2 ton of FLA don't go as far as 1/2 ton of pouch Lithium.

@Duncan : does your controller have circuitry or software to alter the PWM frequency based on RPM? Soliton does software checks and balances and something Tesseract called a "sanity check" subroutine that limits maximum voltage and current output but I am not sure an igbt failure to "on" would stop full power motor destruction before the mains contactors open.
 

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@Duncan : does your controller have circuitry or software to alter the PWM frequency based on RPM? Soliton does software checks and balances and something Tesseract called a "sanity check" subroutine that limits maximum voltage and current output but I am not sure an igbt failure to "on" would stop full power motor destruction before the mains contactors open.
Nope my controller does not have a speed input

As far as the speed input on the Soliton is concerned - my friend had a Soliton running his Ute - it exploded the motor - and then died - possibly the other way around - but at the end a dead motor and a dead controller

I'm playing at silly buggers with my car - its called "motorsport" - so if I kill the motor or the controller......
 

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We made 3300ftbs torque and 1900 battery horsepower in panic. After motor efficiency it was probably 1200RWHP. we have a new motor but are working on chip shortages to build new controllers.
 
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