Results 1 to 12 of 12
  1. #1
    Join Date
    Feb 2013
    Location
    Lamar, SC
    Posts
    603

    Measuring Amps on Variable Speed Blower Motors

    As a part of our PM's we measure the amps that the blower motors are pulling. When you pull the door off of a variable speed unit the motor ramps down and any measurement taken then is not true to conditions. Just wondering how everyone else is handling this. I have tried a few different ways but none seem very easy or accurate for that matter. Is there a meter out there that will hold the peak value so I can recall the amperage after shutting it up inside the unit?
    "If you've eliminated all other possibilities whatever remains must be the truth."

  2. #2
    Join Date
    Jan 2007
    Location
    Stongsville Oh
    Posts
    931
    Quite a few meters with min max. I use an inexpensive amprobe digital mainly because it is small (to small, what a mistake) but it does min max
    ckartson
    I didn't write the book I just read it!

  3. #3
    Join Date
    Sep 2012
    Location
    Central Florida
    Posts
    887
    Quote Originally Posted by jmsmars1 View Post
    As a part of our PM's we measure the amps that the blower motors are pulling. When you pull the door off of a variable speed unit the motor ramps down and any measurement taken then is not true to conditions. Just wondering how everyone else is handling this. I have tried a few different ways but none seem very easy or accurate for that matter. Is there a meter out there that will hold the peak value so I can recall the amperage after shutting it up inside the unit?

    Fieldpiece wireless handle with a wireless receiver? The units that I tend to work on have the vfd mounted in a control panel so you could take the measurement at the vfd without opening the blower door. Also some vfds can be programmed to display the motors amp draw.

  4. #4
    Join Date
    Feb 2009
    Location
    Tallahassee, FL
    Posts
    6,051
    Open the disconnect?

  5. #5
    Join Date
    Aug 2004
    Location
    North Richland Hills, Texas
    Posts
    14,915
    Quote Originally Posted by jmsmars1 View Post
    When you pull the door off of a variable speed unit the motor ramps down and any measurement taken then is not true to conditions.
    Even with PSC motors, if you measure the blower amps with the panel off, you are not getting the true reading of what it is pulling under normal conditions with the panel on because the motor loads up, causing the amps to go up, when the panel is removed.

    For measuring the amps on ECM motors you need a true RMS meter, so if you did something wireless, you would need something like the the Fieldpiece SC57, or the Fluke 381 or CNX setup. The Fieldpiece clamp accessory head is not true RMS, so wouldn't work.
    If more government is the answer, then it's a really stupid question.

  6. #6
    Join Date
    Oct 2008
    Posts
    819
    Quote Originally Posted by SBKold View Post
    Open the disconnect?
    What he said...

    Leave the doors/panels intact. Go to the disconnect or gain access to motor supply wires at a breaker in a distribution panel. Read your amps from there. Your airflow will be unaffected. You're getting your reading while maintaining the exact conditions under which the motor operates.

  7. #7
    Join Date
    Feb 2013
    Location
    Lamar, SC
    Posts
    603
    I have done that on the occasions where they have 1) installed a disconnect and 2) it had enough wire/room in it for me to get the clamp around. Unfortunately in my area that adds up to about 1/4 of the time
    "If you've eliminated all other possibilities whatever remains must be the truth."

  8. #8
    Join Date
    Apr 2003
    Location
    Maryville, Tennessee
    Posts
    427
    Quote Originally Posted by mark beiser View Post
    Even with PSC motors, if you measure the blower amps with the panel off, you are not getting the true reading of what it is pulling under normal conditions with the panel on because the motor loads up, causing the amps to go up, when the panel is removed.

    For measuring the amps on ECM motors you need a true RMS meter, so if you did something wireless, you would need something like the the Fieldpiece SC57, or the Fluke 381 or CNX setup. The Fieldpiece clamp accessory head is not true RMS, so wouldn't work.
    doesn't "true RMS" refer to the way that voltage is measured? What difference would it make in Amp mode?
    -Marty

  9. #9
    Join Date
    Nov 2008
    Posts
    1,673
    Quote Originally Posted by m singer View Post
    doesn't "true RMS" refer to the way that voltage is measured? What difference would it make in Amp mode?
    Voltmeters and ammeters can respond to average or RMS
    values.

    Early on they responded to average values but read out in RMS values for a sine wave because sine waves were common.
    Now with all the weird waveforms seen, an RMS meter is better and a scope is even better.

    http://www.google.com/search?client=...Gba44AOWjoCACQ

  10. #10
    Join Date
    Apr 2009
    Location
    Mobile, AL.
    Posts
    88
    the amps with a VFD drive are relatively meaningless. The power is the amps X volts X Powerfactor X efficiency.
    The standard VFD varies the voltage and the current in the F to V ratio. This means that at low power and RPM settings the current may be high, but the voltage low and therefore the power is low.
    We often see currents fairly high at low RPM (low voltage as well). If you just look at amps you would think that the power is relatively low instead.
    Most VFDs will read out power as well as amps and this is the best way to measure the actual power to a vfd.
    This is not necessarially the case with an ECM that might also be variable speed. In this case the input power to the motor SYSTEM might relate to the power to the motor.
    Also in this case the amps to the motor might not be indicative of the actual power unless you also know the voltage applied. IF we are talking a DC motor then the power formula would be Volts X amps and power factor would not be a factor.

  11. #11
    Join Date
    Aug 2014
    Posts
    14
    I just bought this ideal Ideal 61-775 1000A AC-DC TightSight-amp;reg; Clamp Meter with TRMS
    It works well. I have used amprobe for years but I don't think they make a good clamp meter that does Ac and dc amps, Hz, and capacitance. Anyway this Ideal product has a min max button. good luck.

  12. #12
    Join Date
    Aug 2014
    Posts
    14
    Sorry just realized this thread is dead. I am new to threads and blogs and chats and .......

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  
Comfortech Show Promo Image

Related Forums

Plumbing Talks | Contractor Magazine
Forums | Electrical Construction & Maintenance (EC&M) Magazine
Comfortech365 Virtual Event