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post #1 of 9 (permalink) Old 10-27-2019, 11:50 PM Thread Starter
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Question Generator capacity

I have a generator rated at 4500 watts running, 5500 watts surge. It has both 110v and 220v outlets. I want to run a 2hp motor wired for 220v drawing 10 amps at this voltage. The motor is further identified as a TEFC capacitor start induction. When I turn the motor on, the circuit breaker on the generator trips. I would have thought that this generator was sufficient. Is it not or am I missing something.
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post #2 of 9 (permalink) Old 10-28-2019, 01:16 AM
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So, 4500/5500Watts @240V = 18.75/22.92Amps.
If your 2HP motor is rated at 10Amps, the starting current is likely around 3 to 4.5 times that. There should be a code letter on your engine plate. With that in hand and your ACTUAL running voltage, I can calculate your starting current (Locked Rotor Amps). If you can't find the Code Letter, give me the manufacturer and model number of the motor and I should be able to come up with it.

For example:
HP = 2
Nominal voltage = 240
Code letter = G
Max LRA @Full voltage = 52.4
Max LRA @30% sag = 36.7
Max starting kVA @30% sag = 6.2

Last edited by tabora; 10-28-2019 at 03:42 PM.
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post #3 of 9 (permalink) Old 10-28-2019, 04:53 PM Thread Starter
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The motor label shows “class A”. I recognize everything else on the label so I’m hoping that this is the code. Many thanks for your help!
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post #4 of 9 (permalink) Old 10-28-2019, 05:01 PM Thread Starter
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Here’s a pic of the label.
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post #5 of 9 (permalink) Old 10-28-2019, 08:12 PM
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Originally Posted by Cougfather View Post
The motor label shows “class A”...Here’s a pic of the label.
HP = 2
Nominal voltage = 240
Code letter = A
Max LRA @Full voltage = 26.2
Max LRA @30% sag = 18.3
Max starting kVA @30% sag = 3.1

So, 4500/5500Watts @240V = 18.75/22.92Amps. Your Max LRA is above your surge capacity. Close but no cigar. And with a motor that old, the LRA may be even higher than the calculation shows... At 30% sag, you'd think it would just squeak by, but no dice apparently.

I have a similar situation with a circular saw on my small Onan 2800Watt genset. I have to click/release the trigger 2 or 3 times on my smallest 1.5HP saw to get past the LRA hump and then it runs fine. The larger 2HP saws won't even start. If you have a way to give the motor a manual kick start to get it spinning, your generator should be able to keep it running.
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post #6 of 9 (permalink) Old 10-28-2019, 09:56 PM
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Here in the uk we have a maximum of 3000watts (13amp fuse in a mains household plug).... How do they run a large motor without the fuse blowing
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post #7 of 9 (permalink) Old 10-28-2019, 11:32 PM
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Originally Posted by speedy2019 View Post
Here in the uk we have a maximum of 3000watts (13amp fuse in a mains household plug).... How do they run a large motor without the fuse blowing
Bigger items must be hardwired, I guess... You've got a whole different system there with a single hot providing 230V @ 50 Hz... Here's an excellent video showing how a typical UK household panel works: https://hackaday.com/2019/04/14/a-pe...l-power-panel/
You'll note a 40A 230V B40 breaker to the right of center there... That's 9200Watts! There are also B6, B16 & B32 breakers in that panel. Looks like the full range includes: 1, 2, 4, 6, 10, 13, 16, 20, 25, 32, 40, 50, 63, 80, 100, and 125Amp.

In North American residential panels, we typically have 2 120V 60Hz legs 180 degrees out of phase that give 240V across the pair and each leg to Neutral gives 120V. See more here: Understanding your Electrical Service, 120V vs. 240V.
Typical residential 120V circuit breakers are 15, 20, 25, 30, 40, 50Amp
Typical residential 240V circuit breakers are 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 125Amp
Typical residential 240V main breakers are 50, 100, 150, 175, 200Amp

And then, of course, there's the rarely encountered residential 3-Phase; but I'll stop there!

Last edited by tabora; 10-29-2019 at 01:45 AM.
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post #8 of 9 (permalink) Old 10-29-2019, 09:20 PM Thread Starter
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That makes sense unfortunately. Thanks so much for your help.
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post #9 of 9 (permalink) Old 10-30-2019, 09:34 PM
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An easy rule of thumb is 1 HP = 746 Watts, x 2 = 1492. No load locked rotor current times 6 or 1492 watts times 6 = 8,952 watts. The starting capacitor appears as a dead short as the inrush current tries to charge the capacitor. The breaker trips too fast. One solution is to use a “soft starter” on your motor. Here is a link to one inexpensive solution and possibly still use your current generator.



Quote:
Originally Posted by speedy2019 View Post
Here in the uk we have a maximum of 3000watts (13amp fuse in a mains household plug).... How do they run a large motor without the fuse blowing
The key word used in the UK I see was FUSE. A breaker is a magnetic trip device (very fast) where the fuse is a thermal (slower) device. This allows enough time for the motor to start up.

My two cents worth.
Gizmo

Last edited by Gizmo; Yesterday at 05:55 AM.
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