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Old 12-06-2009, 06:27 PM   #1
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Horsepower Question


1 Hp equals 746 watts. So does that mean that I could power up a 5 Hp electric general purpose motor off of 4000 continous 8000 surge watts? What all factors have to be taken in account?

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Old 12-06-2009, 07:00 PM   #2
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1 hp means 746 watts, hence 5 hp consumes 3730 watts

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Old 12-06-2009, 07:04 PM   #3
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Probably not. It would probably cut out when the motor started.
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Old 12-06-2009, 07:05 PM   #4
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A couple of factors to be taken in account include Power Factor, and Efficiency of the motor.

Also consider that it takes 6 times the running Amps to start most motors. Having a generator that only has twice the running power available may or may not be sufficient to start the motor.
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Old 12-06-2009, 07:31 PM   #5
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I know this isn't gonna make since but, If I powered it initially off another power source to get it past the start winding and into run, would the same generator then keep it going once I got past the startup surge?
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Old 12-06-2009, 07:32 PM   #6
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If you have one, try putting a clamp-on ammeter on the line when you start the motor on utility power. You will be surprised at the amp draw on startup. I've seen fractional horsepower motors top 60 amps when starting.
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Old 12-06-2009, 07:35 PM   #7
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Originally Posted by Livewire78 View Post
I know this isn't gonna make since but, If I powered it initially off another power source to get it past the start winding and into run, would the same generator then keep it going once I got past the startup surge?
If you had some mechanical means to spin the shaft fast enough to disengage the centrifugal switch it would theoretically work. I'm thinking that it would be better to replace the electric motor with a gas engine (if possible) if you are running this large motor away from utility power.
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Old 12-06-2009, 07:37 PM   #8
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Originally Posted by kbsparky View Post
A couple of factors to be taken in account include Power Factor, and Efficiency of the motor.

Also consider that it takes 6 times the running Amps to start most motors. Having a generator that only has twice the running power available may or may not be sufficient to start the motor.
Full load amps times 6 or RLA times 6?
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Old 12-06-2009, 07:48 PM   #9
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Full load amps times 6 or RLA times 6?

That figure is just a rule of thumb, it depends on the code letter that is stamped on the motor that indicates the locked rotor current per KVA of horsepower. What is the code letter on the motor?

Here is a web page for you to look at:
http://www.mastercontrols.com/EngInf...r/TA_RBarr.htm

Last edited by junkcollector; 12-06-2009 at 07:50 PM.
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Old 12-06-2009, 07:59 PM   #10
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That figure is just a rule of thumb, it depends on the code letter that is stamped on the motor that indicates the locked rotor current per KVA of horsepower. What is the code letter on the motor?

Here is a web page for you to look at:
http://www.mastercontrols.com/EngInf...r/TA_RBarr.htm
Thankyou so much for the link, I'm building an invention. I dont have the 5 horse motor yet, I've been playing with a 1/4 hp and a 1 horse. I just needed to know what it would take in watts if I needed to step up my power
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Old 12-06-2009, 08:47 PM   #11
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5 hp will takes gobs more generator amperage than what your using.
Example... 5hp x code letter H (7.09 kva/hp) x (1000/240) = approx. 147 amps.
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Old 12-06-2009, 09:41 PM   #12
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A typical generator can come up with a LOT of surge current. The real question is can it produce the current long enough to get the motor up to speed. Starting a big motor with a small generator depends more on the inertia of the rotating parts than anything else.

If you had a 5000 watt generator, and it was connected to a 50HP engine, then it certainly would start the 5 HP motor. If the no-load voltage were 240, it'd likely dip to about 150 during starting. The starting torque would be about 100% of the running torque. If the same motor were connected to a source that could provide full voltage at starting current (about 150 amps), it's starting torque would be about 200-250% of running.

In real life though, I doubt that a 4000 watt generator could start a 5 HP motor, even if nothing was connected to the shaft. Speed of the motor makes a difference too. If the motor is two-pole (around 3450 RPM), it takes considerably more to start it than if it were 4 pole (around 1725 RPM), or 6 pole (around 1140 RPM). The faster it turns, the harder it is to start.

A typical 5 HP single phase electric motor is about 70-75% efficient, and will consume about 5000 watts at full load. If it was already running, then transferred over to the 4000 watt gen, it'd be able to produce about 4 HP, not 5. (Basic rule-of-thumb; figure about 1000 watts per HP for single phase motors, 850 for 3 phase ones.)

A typical 4000 watt generator will start a 1-1/2 HP motor under normal conditions, but not fully loaded.

Pushing the edge with motors and generators can be a bit of a trick, there are a lot of factors to consider. Generally speaking though, most generators can start a motor of a little less than 1/2 HP per KW. For example, a 500 KW generator will very likely start a 200 HP motor, if the motor isn't loaded too much at startup, and the rest of the electrical system can tolerate a large voltage dip.

Rob

P.S. If you're going to start the motor on utility power then transfer to gen, make sure that the utility and gen are never connected together, even for a very short time. Unless it's in synch (it won't be), it'll wreck the gen.

Last edited by micromind; 12-06-2009 at 09:49 PM. Reason: Added P.S.
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Old 12-06-2009, 10:04 PM   #13
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Horsepower Question


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Originally Posted by Livewire78 View Post
1 Hp equals 746 watts. So does that mean that I could power up a 5 Hp electric general purpose motor off of 4000 continous 8000 surge watts? What all factors have to be taken in account?
You'd probably have to take into account FLA (Full load amps) LRA (Locked Rotor Amps) when wiring the supply line. + the proper, Code compliant overload protection! (No matter what) Don't Drink and Drive, Ever!!!
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Old 12-06-2009, 10:14 PM   #14
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Horsepower Question


Micromind (Poster #12) Among other advice you mention that "The generator and utility power should never be connected together unless they were in Sync. Or else you will burn the Gen." If it's a legal setup (and I don't want to talk about the other side of the coin) then they can't physically be on together. Because you'll need a Transfer Switch. Manual or Automatic doesn't matter. Switching the breaker on and off is not sufficient and illegal! (No matter what) Don't Drink and Drive, Ever!!!

Last edited by spark plug; 12-06-2009 at 10:15 PM. Reason: Typographical error (Typo)
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Old 12-06-2009, 10:44 PM   #15
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Originally Posted by micromind View Post
A typical generator can come up with a LOT of surge current. The real question is can it produce the current long enough to get the motor up to speed. Starting a big motor with a small generator depends more on the inertia of the rotating parts than anything else.

If you had a 5000 watt generator, and it was connected to a 50HP engine, then it certainly would start the 5 HP motor. If the no-load voltage were 240, it'd likely dip to about 150 during starting. The starting torque would be about 100% of the running torque. If the same motor were connected to a source that could provide full voltage at starting current (about 150 amps), it's starting torque would be about 200-250% of running.

In real life though, I doubt that a 4000 watt generator could start a 5 HP motor, even if nothing was connected to the shaft. Speed of the motor makes a difference too. If the motor is two-pole (around 3450 RPM), it takes considerably more to start it than if it were 4 pole (around 1725 RPM), or 6 pole (around 1140 RPM). The faster it turns, the harder it is to start.

A typical 5 HP single phase electric motor is about 70-75% efficient, and will consume about 5000 watts at full load. If it was already running, then transferred over to the 4000 watt gen, it'd be able to produce about 4 HP, not 5. (Basic rule-of-thumb; figure about 1000 watts per HP for single phase motors, 850 for 3 phase ones.)

A typical 4000 watt generator will start a 1-1/2 HP motor under normal conditions, but not fully loaded.

Pushing the edge with motors and generators can be a bit of a trick, there are a lot of factors to consider. Generally speaking though, most generators can start a motor of a little less than 1/2 HP per KW. For example, a 500 KW generator will very likely start a 200 HP motor, if the motor isn't loaded too much at startup, and the rest of the electrical system can tolerate a large voltage dip.

Rob

P.S. If you're going to start the motor on utility power then transfer to gen, make sure that the utility and gen are never connected together, even for a very short time. Unless it's in synch (it won't be), it'll wreck the gen.
Wow, thanks for all the info, lots of food for thought

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