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basic amperage question

Discussion in 'General Electronics Discussion' started by captainentropy, Oct 12, 2012.

  1. captainentropy

    captainentropy

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    Oct 12, 2012
    OK, I'm totally inept on electrical questions. So go easy on me ;)

    I have a motor that's rated for 12VDC, 7A max. The label on the motor also says to use a 10A fuse.

    So, I have a 120V -> 12V 8A power supply. If I were to use it to run the motor would I be damaging it? Presumably, since it's an 8A power supply, I think that I would, but I'm not certain (since I don't know the basics). Also, I have 12VDC, 6A power supply. Could I use that one safely? Or do I need to match the (7A) rating of the motor?

    Thanks!
     
    Last edited: Oct 12, 2012
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    You need to meet or exceed the 7A shown on the motor.

    Also be aware that this maximum current may not include brief excursions above it as the motor starts, and may be exceeded of the motor stalls.

    I would prefer a power supply which exceeded the fuse's capacity, but that may be going too far.
     
  3. captainentropy

    captainentropy

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    Oct 12, 2012
    Thanks for the fast reply Steve.

    So, is it correct to say that a higher voltage would be bad?

    Since there are times when the motor will draw more current, how do I know what is a good amperage for the power supply? I presume that can be divined from the label saying using a 10A fuse? If so, how would I know otherwise?

    Thanks again!
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    A higher voltage would be bad. As to how bad, that depends on quite a bit. I'd say you want to keep the voltage within about 10%. Having said that, 12V is not so uncommon that it should be an issue.

    The 7A *may* be exceeded on startup or if the motor is stalled. The fuse rating suggests it probably isn't (or if so only very briefly).

    It always pays to have a power supply capable of a higher current than you need as it helps keep things well within their ratings meaning a longer cooler lifetime for them.

    The motor doesn't indicate a wattage on it does it?

    The 8A supply would *probably* work.
     
  5. captainentropy

    captainentropy

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    Oct 12, 2012
  6. davenn

    davenn Moderator

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    Watts = Volts x Amps so if its running at 12V and 7A then you are looking at the motor using 84Watts (84VA)

    on motors, transformers and such you will often see a rating in VA (Volts x Amps) rather than W


    Dave
     
  7. captainentropy

    captainentropy

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    Oct 12, 2012
    Thanks for the reply Dave. I knew that about watts (basic physics) but when it comes to electronics requirements/limits I'm kinda lost.

    If a motor says 12V, that means it can't really handle more voltage, right? That's the way I understand it. So, if a motor is a 7A motor it allows 7 amps of electron flow through it? I presume that would be due to the components its made from.

    By varying the voltage and/or amperage I can control the power output (wattage), right? Assuming that's correct is there any harm in running a motor at a lower current? From what I gather, no, although that may not be sufficient for what kind of work the motor needs to do.

    And, if I want to vary the output of the motor (i.e. run it at a slower RPM to lower the pump flow rate in my case) can I do that with some kind of potentiometer?

    Thanks!
     
  8. davenn

    davenn Moderator

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    Sep 5, 2009
    was just checking ... you seemed unsure ;)

    ---Take note of steve's comments a few posts back about high startup and stall currents
    ---no, not allows, 7 A is the current it draws under normal operating conditions --- that may be with or without load. I dont know the specifics of a certain motor but its spec sheet may state 12V 7A at 12000RPM no load. it may be 12V 10A with a given amount of load

    varying voltage or current is not a good idea with motors, specially if they have any significant load on them. it will lead to the motor being damaged

    No a pot will just burn out with the current through it
    the standard way is to use PWM ( pulse width modulation) to control the duty cycle of the current applied to the motor. What this does is vary the on time and the off time that the motor is receiving current and by that way controlling the speed of the motor

    cheers
    Dave
     
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