The risk of fire for the blower may be mitigated by the selection of the power
source. If you use an LPS to provide power, the risk of fire should be fairly
low. By limiting the power source, you can ensure that there won¢t be enough
energy supplied to the blower to cause a safety issue.
NRTLs may still require some type of approval for the blower. My experience
with UL is that they are going to either ask for a UL recognized fan or they
will require you to test it in your end application. The latter option can be
a bit difficult on a brushless motor. UL¢s test procedures for fans and
motors are largely based on AC induction motors. A brushless motor has a
simple electronic commutation circuit. If it is a single speed motor, you may
be able to do a locked rotor test. Many of these brushless motors have locked
rotor detection built in. They will shut off current to the windings when the
rotor is locked. I had a project where I had an electronically commutated DC
motor that did not have the required internal UL thermal protection on the
windings. This was for an air conditioner, so UL brought in their UL 507 fan
group to look at the fan. UL required me to test it in my end application by
overriding the locked rotor protection. That couldn¢t be done without
disabling the motor completely. UL and I eventually came to the agreement to
run a test by loading the motor as much as possible before the locked rotor
circuit kicked in. Even then, the windings were quite cool relative to the
safety limit.
Since this is a 24 VDC blower, you can probably avoid testing by having
acceptable external protection. Talk to your NRTLs, but they are likely to
accept an unapproved part if you can show that you have protected the power to
the point where there is not enough energy to cause a problem. Many of the
people in UL¢s fan and motor groups come from a mechanical background and not
an electrical background. Try to keep the project with an electrical group.
Once the motor groups get dragged in, requirements that are not applicable to
this motor type may be applied.
Ted Eckert
The opinions expressed are my own and are not necessarily those of my employer.
--- On Wed, 6/25/08, Kunde, Brian <brian_ku...@lecotc.com> wrote:
From: Kunde, Brian <brian_ku...@lecotc.com>
Subject: DC Brushless Blower Motor
To: emc-p...@ieee.org
Date: Wednesday, June 25, 2008, 7:58 AM
Our engineering group wants to use a small cross flow blower that has a
brushless DC motor rated 24Vdc at only 1 amp. The blower assembly and motor
does not have any safety agency approvals though it is built by a reputable
motor company. Our company has a general rule that all motor devices must
have UL, CSA and either TUV or VDE agency approval. Unfortunately, this
blower assembly is not common and engineering cannot locate a compatible
blower that has agency approvals.
Being that the blower motor is only 24Vdc at 1 amp and it is brushless,
I
assume the risk of fire is low. We could perform the locked rotor test and
limit the input current. What other safety tests could be performed to
adequately evaluate the use of this blower? Are there any safety standards
specific to dc brushless motors? Would an NRTL require this kind of motor to
have agency approvals?
Thanks in advance,
The Other Brian
_________________________
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