11-77 Log #1951 NEC-P11
Final Action: Accept
(430.126(A))
_______________________________________________________________
Submitter:
Vince Baclawski, National Electrical Manufacturers Association
(NEMA)
Recommendation:
Revise 430.126(A) as shown:
(A) General. Adjustable Speed drive systems shall protect against motor
overtemperature conditions . where the motor is not rated to operate at the
nameplate rated current over the speed range required by the application . This
Overtemperature protection shall be provided is in addition to the conductor
protection required in 430.32. Protection shall be provided by one of the
following means:
Change the word ?controller? to ?system? in 430.126 (A) (2)
(2) Adjustable speed drive system controller with load and speed sensitive
overload protection and thermal memory retention upon shutdown or power
loss.
Add the following exception to 430.126(A)(2):
Exception 1: Thermal memory retention upon shutdown or power loss is not
required for continuous duty loads.
Modify 430.126 (A)(4) as indicated:
(4) Thermal sensor embedded in the motor whose communications are that
is received and acted upon by an adjustable speed drive system .
Relocate FPN from 430.126(C) to 430.126(A), and revise as follows:
FPN: The relationship between motor current and motor temperature changes
when the motor is operated by an adjustable speed drive. In certain
applications, overheating of motors can occur W w hen operated at reduced
speed, overheating of motors may occur even at current levels less than or
equal to a motor?s rated full load current. This is The overheating can be the
result of reduced motor cooling when its shaft-mounted fan is operating less
than rated nameplate RPM. As part of the analysis to determine whether
overheating will occur, it is necessary to consider the continuous torque
capability curves for the motor given the application requirements. This will
assist in determining whether the motor overload protection will be able, on its
own, to provide protection against overheating. These overheating protection
requirements are only intended to apply to applications where an adjustable
speed drive, as defined in 430.2, is used.
For motors that utilize external forced air or liquid cooling systems,
overtemperature can occur if the cooling system is not operating. Although this
issue is not unique to adjustable speed applications, externally cooled motors
are most often encountered with such applications. In these instances,
overtemperature protection using direct temperature sensing is recommended
(i.e. 430.126(A)(1), (A)(3) or (A)(4)) or additional means should be provided
to assure that the cooling system is operating (flow or pressure sensing,
interlocking of adjustable speed drive system and cooling system, etc.).
Substantiation:
Requirements for overtemperature are application dependent
and are not required in all circumstances. The revision to 430.126(A) reflects
conditions where such protection is needed, as given in the original
substantiation for the addition of this section.
The means to provide overtemperature protection are not limited to the
controller or drive and can be part of the system.
The exception recognizes that the thermal memory requirements are intended
mainly for protection of short-time, intermittent, periodic or varying duty loads.
The FPN is relocated and revised as it more appropriately applies to all
adjustable speed drives, not to only multiple motor applications, but does not
apply in all applications. The additional text in the FPN is intended to provide
guidance to help AHJs in their consideration of the analysis of potential motor
overheating. The additional text in the FPN is intended to provide guidance to
help AHJs in their consideration of the analysis of potential motor overheating
when forced air or liquid cooling systems are used to obtain the desired motor
torque capability over the speed range required by the application. When the
forced cooling is present and functioning, overtemperature protection is
provided by the overload protection mechanism. This FPN text addition
supports the NEMA proposal to remove section 430.126(B).
Panel Meeting Action: Accept
Number Eligible to Vote: 15
Ballot Results:
Affirmative: 14 Negative: 1
Explanation of Negative:
HAMER, P.: The proposed Exception No. 1 should not be included. Thermal
memory is necessary to avoid damage to the motor if it is subject to repeated
accidental overloads, even if the load is classified as ?continuous duty.?
