Ask someone you don't like to put their finger in the way. :-)
John Woodgate OOO-Own Opinions Only
J M Woodgate and Associates www.woodjohn.uk
Rayleigh, Essex UK
On 2017-12-19 18:45, Wiseman, Joshua wrote:
I’ve done similar techniques. The only problem I’ve run into is
waiting for the motor to stop spinning in the case of fans. The
spinning can cause some erroneous readings too.
Josh
*From:*John Woodgate [mailto:j...@woodjohn.uk]
*Sent:* Tuesday, December 19, 2017 1:18 PM
*To:* EMC-PSTC@LISTSERV.IEEE.ORG
*Subject:* Re: [PSES] Temp rise measurement
*EXTERNAL SENDER: Verify links, attachments and sender before taking
action*
I made a box with a double-pole, double-throw switch, an appliance
toggle switch which is inherently break-before-make. This disconnects
the supply and connects the ohmmeter in about 0.2 seconds. It takes
longer to read off the resistance!
John Woodgate OOO-Own Opinions Only
J M Woodgate and Associateswww.woodjohn.uk <http://www.woodjohn.uk>
Rayleigh, Essex UK
On 2017-12-19 17:55, Ted Eckert wrote:
I’ve used the rise of resistance method without problems. The
challenge is making the measurement before the windings cool off.
The rise of resistance method can be more challenging when you
have a small winding that doesn’t hold heat well. It is
practically impossible to make the measurement the instance power
is removed. You may want to record the resistance over time to see
the winding cooling off, allowing you to extrapolate back to time
zero.
In a previous job, I had a product with a motorized impeller fan.
The engineer from a NRTL asked me to use thermocouples. When I
explained that the winding was spinning with the impeller, the
engineer finally figured out that thermocouples wouldn’t work. It
took 60 – 90 seconds to access the terminals on the winding from
the time power was removed. I measured the resistance and recorded
every 5 seconds to get enough information to extrapolate back to
the time when power was removed. The windings were enclosed well
enough that they were not getting much cooling from the moving
air. As such, I did not expect that there was overshoot when power
was removed and the fan stopped.
Ted Eckert
Microsoft Corporation
The opinions expressed are my own and do not necessarily reflect
those of my employer.
*From:*John Woodgate [mailto:j...@woodjohn.uk]
*Sent:* Tuesday, December 19, 2017 9:00 AM
*To:* EMC-PSTC@LISTSERV.IEEE.ORG <mailto:EMC-PSTC@LISTSERV.IEEE.ORG>
*Subject:* Re: [PSES] Temp rise measurement
Yes, there is a much better method. It measures the average
temperature of the whole winding, rather than the temperature at a
particular spot. IEC 62368-1 incorrectly says that the
thermocouple method measures the average temperature. You will
find it explained in Annex E of IEC 60950-1. Incredibly, both IEC
60065 and IEC 62368-1 refer to the 'resistance method' without
either detailing it or providing a cross reference. This is a
serious omission.
John Woodgate OOO-Own Opinions Only
J M Woodgate and Associateswww.woodjohn.uk
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Rayleigh, Essex UK
On 2017-12-19 16:12, Scott Xe wrote:
I will be conducting the temp rise measurements on windings of AC
motors for electrical appliances such as washing machines, dishwashers, etc.
It said the thermal couple method is commonly adopted for safety assessment. I
can dismantle the windings and place the couples inside the windings using
cyanoacrylate adhesive. However it may change the original shape of windings
giving non representative measurement result. Is there any practically
acceptable solution?
Does the measurement result include the overshoot temperature after the
power is cut?
Thanks and regards,
Scott
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