Power Usage

Re: power usage

PostBy: lsayre On: Thu Apr 06, 2017 10:47 am

Generally I believe that an induction motor can briefly peak at up to about 5 - 7 times its running (under load) amp draw. This could very briefly exceed the required wire and fuse capacity of the circuit. That is why fuses have time delay ratings.

Don't quote me, but I think the general rule is to assume 250% of full load current and size your motor circuits wires and breakers somewhat above this value.

For the example of a 1/2 HP motor (single phase, 120V) I would take this to mean:

373 Watts/110 Volts = 3.39 amps (allowing for a bit of commonly seen voltage drop)

3.39 x 2.5 = 8.5 Amps

The wiring and breaker for this motor must be selected to allow for some margin of safety above 8.5 amps, so 10 amp capable wiring (at a minimum) and a 10 amp breaker (maximum) are the choices here.

I need McGiever to review my logic here and correct it as needed.

EDIT: If your motors spec plate indicates its LRA (locked rotor amps) this value (I presume) should be fairly representative of its peak draw. At the moment the circuit closes, and just before the rotor breaks free and the motor spins, it is effectively (and very briefly) in the state of "locked".
lsayre
 
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Re: power usage

PostBy: coaledsweat On: Thu Apr 06, 2017 11:13 am

They generally draw about 3X the nameplate at startup. But that is a very brief event. The average motor should draw about half its rating in actual service, at least that is my experience with them. Running over 80% of its nameplate will pretty much assure a limited lifespan.
coaledsweat
 
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Re: power usage

PostBy: Lightning On: Thu Apr 06, 2017 2:25 pm

Wow, I'll need to get one of those meters that wraps around the wire. I just assumed they used what they were stamped. Now I'm curious to see what they really draw.
Lightning
 
Hand Fed Coal Furnace: Overmodified/Bored out Clayton 1537
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Re: power usage

PostBy: lsayre On: Thu Apr 06, 2017 3:18 pm

If you know their rated HP, you know what Watts they draw. If they are conventional fixed speed motors, and they are not running at their rated HP, then you were sold a bill of goods when you bought them. Variable speed is another matter.

1 HP = 746 Watts
lsayre
 
Stoker Coal Boiler: AHS S130 Coal Gun
Coal Size/Type: Blaschak Anthracite Pea
Other Heating: Resistance Boiler (13.5 KW)

Re: power usage

PostBy: Lightning On: Thu Apr 06, 2017 3:27 pm

Yeah, they are fixed speed and stamped 2.8 amps each.
Lightning
 
Hand Fed Coal Furnace: Overmodified/Bored out Clayton 1537
Coal Size/Type: Anthracite/Awesome Size

Re: power usage

PostBy: coaledsweat On: Thu Apr 06, 2017 3:28 pm

Kill A Watt easy meter, $20 @ Home Depot.

http://m.homedepot.com/p/P3-Internation ... /202196388

Current (amps) goes up as the load increases, has nothing to do with HP rating, they only make enough HP to perform the task.
coaledsweat
 
Stoker Coal Boiler: Axeman Anderson 260M
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Re: power usage

PostBy: oliver power On: Thu Apr 06, 2017 7:36 pm

Lightning wrote:I thought they would draw what they are rated since it seems to work that way with light bulbs and heating elements lol.
Hi Lightning. I'm no electrical engineer. However, I've always been told an electric motor draws the rated amps at start up. Once up to speed, it draws about half.

I've been around some big electric motors, with amp gauges on them. And when turned on, the gauge shows exactly that. .....and every time, I think about what I've been told. Some things just get etched in ones brains for life. :)
oliver power
 
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Re: power usage

PostBy: lsayre On: Thu Apr 06, 2017 8:22 pm

Inrush AMPS (Single Phase) = ((Locked Rotor value per code letter) X HP X 1000)/VAC
(where inrush amps are the motors peak start-up current)

Locked Rotor Code Letters and their values
(low-high and midrange)
A 0.00 - 3.14 1.6
B 3.15 - 3.54 3.3
C 3.55 - 3.99 3.8
D 4.00 - 4.49 4.3
E 4.50 - 4.99 4.7
F 5.00 - 5.59 5.3
G 5.60 - 6.29 5.9
H 6.30 - 7.09 6.7
J 7.10 - 7.99 7.5
K 8.00 - 8.99 8.5
L 9.00 - 9.99 9.5
M 10.00 - 11.19 10.6
N 11.20 - 12.49 11.8
0 12.50 - 13.99 13.2
R 14.00 - 15.99 15.0

The motors locked rotor code letter should be on its spec plate.
lsayre
 
Stoker Coal Boiler: AHS S130 Coal Gun
Coal Size/Type: Blaschak Anthracite Pea
Other Heating: Resistance Boiler (13.5 KW)