# Current of 1HP Motor

Discussion in 'General Electronics Chat' started by shivaaa, Jan 14, 2011.

1. ### shivaaa Thread Starter Member

Jan 5, 2011
55
0
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 ?

Last edited: Jan 14, 2011
2. ### jpanhalt AAC Fanatic!

Jan 18, 2008
5,699
909
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

3. ### shivaaa Thread Starter Member

Jan 5, 2011
55
0
is it like,

I = P/V

= 750/230 = 3.26 A?

4. ### GetDeviceInfo Senior Member

Jun 7, 2009
1,571
230
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.

5. ### Audioguru New Member

Dec 20, 2007
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896
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?

6. ### Kermit2 AAC Fanatic!

Feb 5, 2010
3,854
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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.

7. ### Audioguru New Member

Dec 20, 2007
9,411
896
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.

8. ### GetDeviceInfo Senior Member

Jun 7, 2009
1,571
230
1/2 HP @ 115vac = 9.8 amps . Should be good but a temp check during normal run would be useful.

9. ### jpanhalt AAC Fanatic!

Jan 18, 2008
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Here are some data from Leeson for a 1 HP, general pourpose , 3450 RPM motor:

John

10. ### Audioguru New Member

Dec 20, 2007
9,411
896
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.

11. ### marshallf3 Well-Known Member

Jul 26, 2010
2,358
201
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.

12. ### Audioguru New Member

Dec 20, 2007
9,411
896
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!!

13. ### russ_hensel Distinguished Member

Jan 11, 2009
820
47
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.

14. ### Duane P Wetick Active Member

Apr 23, 2009
408
19
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.

15. ### marshallf3 Well-Known Member

Jul 26, 2010
2,358
201
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.

16. ### jpanhalt AAC Fanatic!

Jan 18, 2008
5,699
909
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:
Code ( (Unknown Language)):
1.
2. Output HP, W(HP)      Actual A     Efficiency    PF           Calc. A
3.      187 (0.25)             3.95         0.491      0.426         3.87
4.      373  (0.5)             4.44         0.635      0.583         4.37
5.      560 (0.75)             5.13         0.685      0.694         5.12
6.      746  (1.0)             5.99         0.705      0.766         6.01
7.     1119 (1.5)              8.28         0.684      0.839         8.49
8.
John

17. ### shivaaa Thread Starter Member

Jan 5, 2011
55
0
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?

18. ### marshallf3 Well-Known Member

Jul 26, 2010
2,358
201
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.

19. ### shivaaa Thread Starter Member

Jan 5, 2011
55
0
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?

20. ### R!f@@ AAC Fanatic!

Apr 2, 2009
8,793
771
That won't do..U'll burn the contacts real soon.

What you need is a Contactor rated same as your motor HP