# DC Motor power rating vs AC

Guy who tries to sell me his 90v 3/4 HP Balder motor is saying that DC motors three times as powerful for the same power rating compared to AC motors. Is it true?
Thanks, Alex
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no, that is not true. A DC motor can have greater starting torque, but it is not "more powerful" - DC horses are not bigger than AC horses. One HP is 3300 ft-lbs/sec no matter whether it's from a turbine, or a horse, or an electrical motor (and few horses can deliver one HP for long)

Bill
www.wbnoble.com
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Bill probably meant to say, "1 HP is 33,000 ft-lb minutes, same as 550 ft-lb seconds and an electrical HP is 746 watts". Looking at HP in more personal terms; if a man weighed 225 lbs and he ran up steps of 12 inch tread at the rate of 2 treads per second, he'd be working at the rate of one HP. Essentially, he'd be lifting 225 lbs. X 2 per second = 550 lb-ft per second, or 1 HP. IOW, a man might be able to work at the rate of 1 HP, but he couldn't sustain it for very long. The first 2 steps would probalby do me in.!
Bob Swinney
in message wrote:

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Remember this formula- Torque X RPM = HP An AC motor runs at a steady RPM, therefore giving a steady HP output. On most machines this output is generally multiplied through belt, chain or gear reductions. A DC motor with variable speed will not perform equally, because as you reduce the RPM you reduce the HP and also lose the armature inertia which is a factor in the overall equation. If you have a drill press with a 1 hp motor and you want to change to a DC motor and control the speeds by varying the motor RPM, then you will need about a 3hp DC motor to acheive the same sort of performance.

motors three

true?
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The "equalizer", or constant relating AC to DC is RMS (root mean square). AC and DC powers are equal if AC voltage is expressed in RMS terms.
Bob Swinney

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I hear this even from guys in the business who should know better, e.g., you need twice as much gas engine HP as electric motor HP for the same job.
In practical terms, this is sometimes true, but not because gas HP are any different than electric HP. A gas engine can deliver it's rated HP but it will slow and even stall if loaded much above rated HP. A gas-powered genset will not deliver 2X rated output even for a few seconds. It'll stall if a breaker doesn't trip first.
An AC induction motor can deliver considerably more than rated power for short durations (a couple of minutes) if it can cool down a bit between such periods.
If loaded to more than 3X to 5X it's rated torque and power, though, it will slow to the point where torque starts dropping and then it'll stall. Torque of a DC motor, particularly a series-wound DC motor, just keeps increasing as speed drops all the way to zero. The limiting factor is heat, which will build temperature to overheat level over any sustained period of overload.
The DC motor in a certain electric winch can deliver 1.5 HP for short periods -- a minute or less. That motor measures 3" dia x 5" long.
So the difference is peak available power vs averaged sustained power. This, often x some MFF (marketing fudge factor) is "Sears HP". Maybe they test them while immersed in refrigerated fluorinert with 30-amp breakers on the 115 volt line or something.......
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Alex writes:

In principle, yes.
A DC motor rating is the continuous *rating*, that is, what it can deliver for a long time without overheating. It is *not* a measure of the continuous power *possible*.
A typical DC motor will deliver multiples of its continuous rated horsepower, if the load and power supply are suitable.
Look at the torque and speed characteristics to verify this.
But of course, that doesn't mean you can operate it that way for anything but brief periods.
DC motors do have a "gutsy" characteristic compared to AC, and this makes a DC motor feel more powerful than the equivalently rated AC motor if you have intermittent excess loading.
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On Wed, 15 Feb 2006 13:10:45 -0600, Richard J Kinch

Right. So in practical terms, a job or application that will work with a 1 HP electric motor might require a 3 HP gas engine to operate reliably, if the load typically draws 1 HP or less but has short periods where it draw up to 3 HP.
The situation is similar with DC motors vs induction AC motors. Note, however, that an AC universal motor can display characteristics similar to a DC motor, as in most corded portable power tools. Try to stall a corded 1/2" Milwaukee electric drill!
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Don sez:
"> The situation is similar with DC motors vs induction AC motors. Note,

Yep! Got one of those of the "hammer" persuasion. AFAIK, AC universal motors will still run on DC - I know that originally they were designed to be, welll, errr, universal.
Bob Swinney
PS: Bad reminder of the torque capabilities of 1/2 inch drill motors. Once on the edge of a grain elevator in KS, my boss was drilling a mounting hole for an antenna. The drill bound up in the hole and the handles (outriggers) of the drill caught him under the knee and proceeded to pass him on to his reward - 120 feet below. It was kind of comical as I remember him almost sitting on the bound-up, revolving, drill motor with each revolution slapping him on first one leg and then the other. It looked like one of those ruskie cock-ass dances.
wrote:

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snipped-for-privacy@truetex.com says...

For permanent magnet motors it's also important to limit peak current, and thus torque, to a level that will not demagnetize the field magnets. The ratio of instantaneous peak to continuous rated current for a run of the mill PM motor is usually around 2 or 3:1, and as high as 8:1 for high performance motors with advanced magnet materials.
Ned Simmons
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I am running my belt grinder on 1.5 HP AC motor and can hardly stall it I want to upgrade to variable speed motor and wonder if 3/4 HP Baldor DC motor 90v will give me the same performance?
BTW Speed control is Speed-a-matic PN2400-8000 (1/4 to 1HP for 115VAC and 1/2 to 2 HP for 230 VAC ) Input 14.5 A max Outpt 10 A max Motor Baldor# CD3475, 3/4 HP, volts 90A/100/50F, amp 7.8/.6/1.2F, RPM 1750
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It will obviously only have about half as much torque (and power) running at full speed as your 1.5 HP motor does. However, if the field current is kept constant it will be able to deliver the same amount of torque at reduced speeds because torque is proportional to current and the current rating of the motor doesn't change with speed. The 10 amp limit on the controller will limit maximum torque to about 1.5 X rated torque.
Your present 1.5 HP motor can probably deliver at least 2X it's rated torque for short periods, and it's rated torque is 2X that of the DC motor since both deliver rated power at 1725 RPM.
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Using the standard formula and allowing 1.5 times peak torque at any given RPM, if you put that motor on your grinder and then reduce the speed to 1725 RPM, your rating would be about 1/3 HP. Most DC motors are rated at a higher RPM, about 5-6000, so to get the same belt speeds you will need to keep the motor at about 30% of its rated speed. If you want higher belt speeds, then you can crank it up and have almost the same power as your AC motor, but if you want lower speeds, your best bet is to simply invest a few dollars and hours into a Jack shaft setup to reduce the speed of your existing motor.
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wrote:

The motor in question is rated at 1725 RPM at rated HP.
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