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  1. #1
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    Question 2hp / 240v = 12a??

    OK, so this is more an electrical question, but it's on a dust extractor... W318 - CD-2A Industrial Dust & Cyclone Separator Collector | Hare & Forbes Machineryhouse

    Can someone explain to me how this motor is 12A?


    I know I only did high school physics, but I'm quite certain that 1500w / 240v = 6.25A

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  3. #2
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    Taiwanese amps?
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  4. #3
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    12A does sound a bit high, but let's redo your calculation:

    Input current = 1500W/240V/Efficiency/Power Factor

    A motor of that size (and price range) could be expected to have an efficiency of 0.75 to 0.85 and a power factor between 0.80 and 0.90. So a typical input current would likely be between about 8.2A and 10.4A.

    In the US the National Electrical Code specifies our premise wiring requirements. It tabulates current values by motor HP to be used for sizing conductors and circuit breakers. It might just be coincidence, but the value it calls out for a 2HP motor at 230V is 12A:
    NEC 1P Motor FLA.JPG
    So it's possible that the number on that nameplate was driven by perceived market requirements rather than actual characterization testing.

  5. #4
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    I'd say it's a plain and simple cock up. I tested quite a few 2HP DCs (including some that are about 25years old) and never seen them draw more than 8A at full load - ie fully opened impeller.
    Now adding an old school cyclone like those to the impeller will just choke the impeller more than usual so I'd say the motor is never going to see even 8A.

  6. #5
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    We have some of these in my Men's Shed, do you have any questions?

    Yes they are 12A and require being hooked up to a 15A circuit, which we have done.

    Mick.

  7. #6
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    Quote Originally Posted by BobL View Post
    I'd say it's a plain and simple cock up. I tested quite a few 2HP DCs (including some that are about 25years old) and never seen them draw more than 8A at full load - ie fully opened impeller.
    Is it the math or the NEC that you think is a cock up? 2HP from 240V/8A would be a premium high efficiency motor. Are you sure the DCs were actually demanding 2HP?

  8. #7
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    Quote Originally Posted by dmorse View Post
    Is it the math or the NEC that you think is a cock up? 2HP from 240V/8A would be a premium high efficiency motor. Are you sure the DCs were actually demanding 2HP?
    I'd say its either a labelling cock up, or MAYBE someone mention start up currents and checked out the start up current and found them to be ~30A so they said they have to go on a 15A circuit so lets label them 12A.

    A 2HP motor should easily ride on a 10A GPO. Yes a 30A start up currents is high but very brief and standard 10A circuit wiring with a "D" breaker can handle these with ease.

    I should add I also have a small dyno that I use to test motor Torque/HP/current etc. The only time I have seen a 2HP motor draw 12A is when it was close to stalling and producing close to 3HP. Almost every motor is like this.

  9. #8
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    I reckon that the Taiwanese guy who stamps the labels was stamping labels for 110 volt motors on his first day and they gave him all the figures to stamp in the right squares. After lunch they told him to stamp up some 240 volt labels and didn't tell him he needed to change the other figures.
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  10. #9
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    A 2HP motor will draw about 9 amps on reasonable efficiency. A 2kW motor will draw around 12 amps. Is that a generic label made for the US and labelled HP but is actually a 2kW motor? We need someone (Optimark?) to put a clamp meter on it and see how much it actually draws. Actually, I don't need to know but someone who is looking to purchase may be able to save some money on a 15A outlet install.
    Those were the droids I was looking for.
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  11. #10
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    Well, as it happened, today we had a sparky at the shed doing some work in a room adjoining our shed. I put the question to him, his reply (after looking at the motor and the specification stamp) went along the lines, that the start up on some cheaper electric motors, can sometimes be quite high. Also, the way the motor has been configured, can also be an issue.

    Regardless, a friend has a clamp meter, maybe by next week I may be able to measure start-up and running draw.

    Mick.

  12. #11
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    Isn't the purpose of start capacitors to reduce the inrush or start current? & Start + Run capacitor designs to do the earlier plus improve the single-phase electric motors efficiency.
    Mobyturns

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    Quote Originally Posted by Mobyturns View Post
    Isn't the purpose of start capacitors to reduce the inrush or start current? & Start + Run capacitor designs to do the earlier plus improve the single-phase electric motors efficiency.
    The purpose of the start cap is to generate a strong spinning magnetic field that will spin the rotor and bring it up to speed.

    Once it is up to speed it will sync with the non-rotating field generated by the single phase motor.

    The spinning field can be maintained by the run cap but it is no longer required to be as powerful as was required to accelerate the rotor initially.

    The start cap actually draws a lot of current and if kept on will burn out the motor.

  14. #13
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    Quote Originally Posted by NCArcher View Post
    ...We need someone (Optimark?) to put a clamp meter on it and see how much it actually draws.
    Quote Originally Posted by Optimark View Post
    ...a friend has a clamp meter, maybe by next week I may be able to measure start-up and running draw.
    If you want to measure full load current you need the rated load on the motor. Unless you can somehow measure the actual power demanded by the load you won't know that. Near full power the current draw is roughly proportional to the load power. So a 2HP motor with a 10A FLA will draw 10A with a 2HP load, a bit over 7.5A with a 1.5HP load and about 11A with a 2.2HP load.

    You can't measure start current with a common clamp on ammeter, it will be way too slow. There are high speed meters with a peak hold function that will do the job but they're rare. The industry standard way to make that measurement is to immobilize the motor shaft and then briefly power the motor. The current measured in that situation is called Locked Rotor Current (LRA) and is the true value of the startup current.

    In the US a NEMA compliant nameplate includes a LRC code letter that lets you determine the start current (within a range of values).

  15. #14
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    Quote Originally Posted by dmorse View Post
    If you want to measure full load current you need the rated load on the motor. Unless you can somehow measure the actual power demanded by the load you won't know that. Near full power the current draw is roughly proportional to the load power. So a 2HP motor with a 10A FLA will draw 10A with a 2HP load, a bit over 7.5A with a 1.5HP load and about 11A with a 2.2HP load.

    You can't measure start current with a common clamp on ammeter, it will be way too slow. There are high speed meters with a peak hold function that will do the job but they're rare. The industry standard way to make that measurement is to immobilize the motor shaft and then briefly power the motor. The current measured in that situation is called Locked Rotor Current (LRA) and is the true value of the startup current.

    In the US a NEMA compliant nameplate includes a LRC code letter that lets you determine the start current (within a range of values).
    Great to know, if I can get my mates meter, I'll measure what I measure, if you know what I mean.

    Mick.

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    Quote Originally Posted by Optimark View Post
    Great to know, if I can get my mates meter, I'll measure what I measure, if you know what I mean.

    Mick.
    That's all we need. Measure it with a couple of blast gates open and that will tell us enough.
    Those were the droids I was looking for.
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