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Fan Load — Parallax Forums

Fan Load

ArchiverArchiver Posts: 46,084
edited 2001-08-09 20:35 in General Discussion
General question: When you have a simple circuit of a DC brushless
fan powered by say, a 15V power supply, and you want to know what the
current is in your circuit, do you just put an ohmmeter across the
two fan pins to find its resistance then plug into I=V/R?


Thanks

Comments

  • ArchiverArchiver Posts: 46,084
    edited 2001-08-09 09:11
    I'm not a motor expert but doing what you suggest won't give you a
    real accurate value I don't think. It may be close to what the
    starting current of the motor is, but once the motor builds up speed
    it turns into a generator and generates counter EMF which will cause
    the current to go down. Also the current will vary with the amount of
    load placed on the motor or how "hard" it is to turn the shaft. If
    for instance 6 months down the road the bearings in the fan start to
    go bad and start to bind up, the motor will draw a higher current to
    maintain its rated speed.

    Best and easiest thing you could do to see what the current draw is
    would be to hook up a digital multimeter in series with the motor to
    read the current. You could then somehow mechanically lock the fan
    blades so they could not turn and start the motor and watch what the
    current is on the DMM. DO NOT let your motor sit there stalled like
    that for any length of time however. A few seconds should be enough
    to get the reading and not harm the motor. The value you record
    should be about the max current that your motor will ever draw.

    Also your DMM is probably only able to handle current up to around 10
    or so amps, so if you have a big fan motor this will probably result
    in a blown fuse in your meter.

    Or, if you have the spec sheet for the fan you could just read that!


    --- In basicstamps@y..., pstrittmatter@y... wrote:
    > General question: When you have a simple circuit of a DC brushless
    > fan powered by say, a 15V power supply, and you want to know what
    the
    > current is in your circuit, do you just put an ohmmeter across the
    > two fan pins to find its resistance then plug into I=V/R?
    >
    >
    > Thanks
  • ArchiverArchiver Posts: 46,084
    edited 2001-08-09 13:44
    yes,

    My rule of thumb for locked rotor current draw is 2 times running current .
    . . . . .so about 300 ma for start-up and 140 ma for running

    Richard

    Original Message
    From: <pstrittmatter@y...>
    To: <basicstamps@yahoogroups.com>
    Sent: Thursday, August 09, 2001 9:02 AM
    Subject: [noparse][[/noparse]basicstamps] Re: Fan Load


    > So if, on the back of my motor it says 12VDC and 140mA, is 140mA what
    > it will draw at 12 volts?
    >
    >
    > --- In basicstamps@y..., tbanez@h... wrote:
    > > I'm not a motor expert but doing what you suggest won't give you a
    > > real accurate value I don't think. It may be close to what the
    > > starting current of the motor is, but once the motor builds up
    > speed
    > > it turns into a generator and generates counter EMF which will
    > cause
    > > the current to go down. Also the current will vary with the amount
    > of
    > > load placed on the motor or how "hard" it is to turn the shaft. If
    > > for instance 6 months down the road the bearings in the fan start
    > to
    > > go bad and start to bind up, the motor will draw a higher current
    > to
    > > maintain its rated speed.
    > >
    > > Best and easiest thing you could do to see what the current draw is
    > > would be to hook up a digital multimeter in series with the motor
    > to
    > > read the current. You could then somehow mechanically lock the fan
    > > blades so they could not turn and start the motor and watch what
    > the
    > > current is on the DMM. DO NOT let your motor sit there stalled
    > like
    > > that for any length of time however. A few seconds should be
    > enough
    > > to get the reading and not harm the motor. The value you record
    > > should be about the max current that your motor will ever draw.
    > >
    > > Also your DMM is probably only able to handle current up to around
    > 10
    > > or so amps, so if you have a big fan motor this will probably
    > result
    > > in a blown fuse in your meter.
    > >
    > > Or, if you have the spec sheet for the fan you could just read that!
    > >
    > >
    > > --- In basicstamps@y..., pstrittmatter@y... wrote:
    > > > General question: When you have a simple circuit of a DC
    > brushless
    > > > fan powered by say, a 15V power supply, and you want to know what
    > > the
    > > > current is in your circuit, do you just put an ohmmeter across
    > the
    > > > two fan pins to find its resistance then plug into I=V/R?
    > > >
    > > >
    > > > Thanks
    >
    >
    > To UNSUBSCRIBE, just send mail to:
    > basicstamps-unsubscribe@yahoogroups.com
    > from the same email address that you subscribed. Text in the Subject and
    Body of the message will be ignored.
    >
    >
    > Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/
    >
    >
  • ArchiverArchiver Posts: 46,084
    edited 2001-08-09 15:02
    So if, on the back of my motor it says 12VDC and 140mA, is 140mA what
    it will draw at 12 volts?


    --- In basicstamps@y..., tbanez@h... wrote:
    > I'm not a motor expert but doing what you suggest won't give you a
    > real accurate value I don't think. It may be close to what the
    > starting current of the motor is, but once the motor builds up
    speed
    > it turns into a generator and generates counter EMF which will
    cause
    > the current to go down. Also the current will vary with the amount
    of
    > load placed on the motor or how "hard" it is to turn the shaft. If
    > for instance 6 months down the road the bearings in the fan start
    to
    > go bad and start to bind up, the motor will draw a higher current
    to
    > maintain its rated speed.
    >
    > Best and easiest thing you could do to see what the current draw is
    > would be to hook up a digital multimeter in series with the motor
    to
    > read the current. You could then somehow mechanically lock the fan
    > blades so they could not turn and start the motor and watch what
    the
    > current is on the DMM. DO NOT let your motor sit there stalled
    like
    > that for any length of time however. A few seconds should be
    enough
    > to get the reading and not harm the motor. The value you record
    > should be about the max current that your motor will ever draw.
    >
    > Also your DMM is probably only able to handle current up to around
    10
    > or so amps, so if you have a big fan motor this will probably
    result
    > in a blown fuse in your meter.
    >
    > Or, if you have the spec sheet for the fan you could just read that!
    >
    >
    > --- In basicstamps@y..., pstrittmatter@y... wrote:
    > > General question: When you have a simple circuit of a DC
    brushless
    > > fan powered by say, a 15V power supply, and you want to know what
    > the
    > > current is in your circuit, do you just put an ohmmeter across
    the
    > > two fan pins to find its resistance then plug into I=V/R?
    > >
    > >
    > > Thanks
  • ArchiverArchiver Posts: 46,084
    edited 2001-08-09 20:35
    I would say yes. Like I said before, hook up a DMM in series through
    the fan motor and watch what the current is on the meter.

    --- In basicstamps@y..., pstrittmatter@y... wrote:
    > So if, on the back of my motor it says 12VDC and 140mA, is 140mA
    what
    > it will draw at 12 volts?
    >
    >
    > --- In basicstamps@y..., tbanez@h... wrote:
    > > I'm not a motor expert but doing what you suggest won't give you a
    > > real accurate value I don't think. It may be close to what the
    > > starting current of the motor is, but once the motor builds up
    > speed
    > > it turns into a generator and generates counter EMF which will
    > cause
    > > the current to go down. Also the current will vary with the
    amount
    > of
    > > load placed on the motor or how "hard" it is to turn the shaft.
    If
    > > for instance 6 months down the road the bearings in the fan start
    > to
    > > go bad and start to bind up, the motor will draw a higher current
    > to
    > > maintain its rated speed.
    > >
    > > Best and easiest thing you could do to see what the current draw
    is
    > > would be to hook up a digital multimeter in series with the motor
    > to
    > > read the current. You could then somehow mechanically lock the
    fan
    > > blades so they could not turn and start the motor and watch what
    > the
    > > current is on the DMM. DO NOT let your motor sit there stalled
    > like
    > > that for any length of time however. A few seconds should be
    > enough
    > > to get the reading and not harm the motor. The value you record
    > > should be about the max current that your motor will ever draw.
    > >
    > > Also your DMM is probably only able to handle current up to around
    > 10
    > > or so amps, so if you have a big fan motor this will probably
    > result
    > > in a blown fuse in your meter.
    > >
    > > Or, if you have the spec sheet for the fan you could just read
    that!
    > >
    > >
    > > --- In basicstamps@y..., pstrittmatter@y... wrote:
    > > > General question: When you have a simple circuit of a DC
    > brushless
    > > > fan powered by say, a 15V power supply, and you want to know
    what
    > > the
    > > > current is in your circuit, do you just put an ohmmeter across
    > the
    > > > two fan pins to find its resistance then plug into I=V/R?
    > > >
    > > >
    > > > Thanks
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