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shivaaa
01-14-2011, 10:27 AM
How much current a single phase single HP motor draws at 230VAC?

Current rating is important to select a relay to operate a motor?

is it Ok to select a relay with 230V, 7A ?

jpanhalt
01-14-2011, 10:34 AM
That information is on the motor dataplate. You can calculate the approximate running current from knowing 1 HP = 750 watts and then multiply by the inverse of the motor's efficiency (i.e., 80% = 1.25X).

Starting and stall current are a lot larger.

John

shivaaa
01-14-2011, 10:44 AM
is it like,

I = P/V

= 750/230 = 3.26 A?

GetDeviceInfo
01-14-2011, 03:17 PM
1phase / 1 HP / 230v = 8 amp

your 'relay' must be rated at least the horsepower of the motor. You also require overload and overcurrent protection, along with a means of disconnect.

Audioguru
01-14-2011, 03:22 PM
I bought a 1/2hp 120V motor for my furnace fan. Its max current rating was 10A and I adjusted the pulley to exactly 10A.
But 1/2hp is only 375W and it was using 1200W. Is it very inefficient?

Kermit2
01-14-2011, 03:25 PM
Just for understanding, you say you adjusted the pulley?

Adjusted it, as in "tightened the belt" till the motor was drawing 10 amps?


If so, you will be replacing that motor again real soon.

Audioguru
01-14-2011, 03:46 PM
The pulley wheel on the fan had an adjustable ratio to adjust its speed and therefore how much power it needed. I used a rubber belt that was not too tight and it was soft, not stiff so it had low drag. I positioned the motor for low belt tightness.

I know that bearings do not like to have the belt too tight.

GetDeviceInfo
01-14-2011, 03:53 PM
1/2 HP @ 115vac = 9.8 amps . Should be good but a temp check during normal run would be useful.

jpanhalt
01-14-2011, 04:02 PM
Here are some data from Leeson for a 1 HP, general pourpose , 3450 RPM motor:

http://forum.allaboutcircuits.com/attachment.php?attachmentid=26540&stc=1&d=1295024518

John

Audioguru
01-14-2011, 05:44 PM
My 1/2hp motor was a capacitor start type. It had two speeds (I think the slow speed was 1/8hp) and it got extremely hot when running at slow speed.

marshallf3
01-14-2011, 11:37 PM
Most common motor specs can be found at http://www.grainger.com

I'm seeing about 6.5A full load for a 230V 60 Hz motor, if you're on 50 Hz it won't be much different.
http://www.grainger.com/Grainger/DAYTON-Capacitor-Start-Open-Dripproof-Motor-6K321?Pid=search

A lot of my motors have adjustable pulleys on them and we adjust them while monitoring the current draw. Of course most are 480V 3 phase driving constant loads but the theory is the same. If the thing is pulling too much current you lessen the effective size of the driving pulley to a level you feel comfortable with, not the best idea in the world to push them to maximum output unless they're being operated in a fairly intermittent mode.

Audioguru
01-15-2011, 01:16 AM
The AC motor driving the fan on my old furnace was straining to produce a small air flow.
But the DC motor on my new high efficiency furnace blows 10 times harder atn probably less currert.
So the old motor was a heater instead of a motor. It was heating the air conditioning in summer!!

russ_hensel
01-15-2011, 04:59 PM
You should also be aware that motors often start with a larger inrush of current and may draw current out of phase with the voltage ( if AC ). Turning off can give inductive effects. Most relays are derated for switching motors. A ohmic resistor is not a good model for a motor.

Duane P Wetick
01-15-2011, 07:44 PM
The relationship HP x 746 / Vac x % efficiency will give approx. full load motor current for a 3 phase motor. For single phase motors, NEC table 430-148 gives 230 Vac full load current of 8 amps for a 1 HP motor.
Note, that a 1 HP motor not fully loaded (good practice) will show current draw of less than 8 amps.

Cheers, DPW Everything has limitations...and I hate limitations.

marshallf3
01-16-2011, 04:24 AM
1 HP = 745.699871582 W at 100% which is simple physics.

Duane, don't forget that in 3 phase we've got the square root of 3 involved as well.

jpanhalt
01-16-2011, 03:41 PM
For single phase, if you incorporate both efficiency and power factor into the calculation, the results come out pretty close, except for the case of no load. Using the Leeson data for a 1 HP motor, one gets:

Output HP, W(HP) Actual A Efficiency PF Calc. A
187 (0.25) 3.95 0.491 0.426 3.87
373 (0.5) 4.44 0.635 0.583 4.37
560 (0.75) 5.13 0.685 0.694 5.12
746 (1.0) 5.99 0.705 0.766 6.01
1119 (1.5) 8.28 0.684 0.839 8.49


John

shivaaa
01-17-2011, 06:40 AM
I have a motor of 230V AC, 1 HP, which am using to pump the Water to overtank. I want to use a Relay.
What would be the Ratings i should take care of the Relay?

marshallf3
01-17-2011, 08:49 AM
Simply get a relay rated for switching at least a 1 HP load at 250V, coil voltage of course depends on what you're driving it with. Relays are rated in a number of ways and the most important thing is to get one that has contacts rated for your load. Inductive loads can present problems to most, you want something rated for it.

shivaaa
01-17-2011, 08:53 AM
I bought some Relays Rated below.

1. 6V DC Coil, 250 VAC-5A-1/8 HP
2. 12V DC Coil, 250 VAC - 7A - No HP mentioned
3. 12V DC Coil, 250VAC - 10 A- No HP mentioned.

Can I choose anything from the above?

I am really confused how to buy the Relay? by telling current or by telling HP?

R!f@@
01-17-2011, 09:52 AM
That won't do..U'll burn the contacts real soon.

What you need is a Contactor (http://www.thermalinc.com/power/contactors.htm) rated same as your motor HP

shivaaa
01-17-2011, 10:21 AM
what happens if i connect two -12V DC Coil, 250VAC - 10 A- in parallel? Just for a thought?

The current rating wil be added right?

jpanhalt
01-17-2011, 10:37 AM
It's the starting current that is the problem. Your single-phase motor is running a water pump, so it presumably is starting under load. Relays in parallel will not open and close at exactly the same time.

You need a contactor rated for 1HP single-phase at 230V. If you don't have that and need to make do, I would put the two largest relays you have in series. That is not to carry more current, but rather as a safety back up. If the contacts on one weld shut, you may still be able to interrupt the circuit with the other.

John

shivaaa
01-17-2011, 10:40 AM
Understood and thanks..

Can you give me some more idea how can i do this? as u mentioned i dont have a contacter. any other way to do this?

jpanhalt
01-17-2011, 10:46 AM
I have used circuit breakers (from a distribution panel) in a pinch. They are not for routine use in this type of application, though.
John

GetDeviceInfo
01-17-2011, 11:47 AM
For single phase, if you incorporate both efficiency and power factor into the calculation, the results come out pretty close, except for the case of no load. Using the Leeson data for a 1 HP motor, one gets:

Output HP, W(HP) Actual A Efficiency PF Calc. A
187 (0.25) 3.95 0.491 0.426 3.87
373 (0.5) 4.44 0.635 0.583 4.37
560 (0.75) 5.13 0.685 0.694 5.12
746 (1.0) 5.99 0.705 0.766 6.01
1119 (1.5) 8.28 0.684 0.839 8.49


John
Code dictates that one shall use nameplate data, or calculations for the specific motor, failing those, one shall use code supplied data.

shivaaa
01-17-2011, 11:56 AM
from the Above table, it seems i can use 10A, 230VAC Relay. is it right?

GetDeviceInfo
01-17-2011, 03:41 PM
from the Above table, it seems i can use 10A, 230VAC Relay. is it right?
Motor starting currents can be 6 x FLA, so to answer your question, absolutely not. You need a dedicated motor starter which is comprised of a suitable contact and overload protection. Along with that you will need additional overcurrent protection and branch circuit isolation.

shivaaa
01-18-2011, 04:34 AM
Here are my exact Motor ratings I got from the Name Plate,

Max Current - 3.5 A

Volt - 240V

Fr-50Hz

0.37KW

0.5 HP.

also the Motor has a Capacitor rated 10Mfd

Please tell me wat Relay is suitable.

As they mentioned the Maximum current is 3.5A, still i need to have everything mentioned in last message by GetDiviceInfo?

Thanks.....

marshallf3
01-18-2011, 12:56 PM
Good relay rated for at least 1 HP at 250 VAC

shivaaa
01-19-2011, 04:50 AM
Thanks.. I will try to buy only Relay Rated 1HP, but last time when I went to buy relays i could not see any relay rated in HP except one 6v relay rated 1/8 HP.

jpanhalt
01-19-2011, 09:29 AM
Here are my exact Motor ratings I got from the Name Plate,

Max Current - 3.5 A
Volt - 240V
Fr-50Hz
0.37KW
0.5 HP.


You may also want to take not that your motor is 0.5 HP, not actually 1 HP. At least that is what the nameplate says according to this post.

John