TM 5-688
CHAPTER 3
EQUIPMENT DERATING
3-1. Derating under 50 Hz conditions.
operated safely while overloaded continuously by 20
percent. The same motor can be operated safely with a
Derating factors for 50 Hz operation are developed
rated mechanical load and a 50 Hz power supply with
differently for different types of equipment. Derating
no horsepower derating, assuming saturation is not an
factors for HVAC, electrical distribution and protec-
issue, the thermal limit of the motor is not exceeded, and
tion, safety and security equipment, communication
the same voltage amplitude is applied. However, a 60
equipment, lighting, and other electrical equipment
Hz motor, with a 1.0 service factor, driving a rated
are discussed below.
mechanical load would have to be derated for horse
power by 20 percent, since it is not capable of handling
3-2. Heating, ventilating, and air conditioning
greater than the rated current. In summary, the user
(HVAC) for derating.
should find out the service factor and thermal limit of the
motor to determine the amount of horsepower derating
The frequency of the supply voltage affects two types of
required, and to ensure that the 20 percent increase in
components in HVAC systems: motors and controls. From
current level in the motor does not exceed the motor's
the discussion in paragraph 2-2, for the same mechani-
rating (again, assuming saturation is not a concern].
cal load and voltage level, a 60 Hz motor will draw 20
percent more current when supplied from a 50 Hz volt-
b. Another issue to be considered when purchasing
age source. This assumes the iron core of the motor does
HVAC equipment for a 50 Hz environment is that the
not saturate. Therefore, a 60 Hz motor would have to be
motor's shaft will spin 5/6 as fast as it would with a 60
capable of handling the increase in current level.
Hz supply. For a 4-pole motor, the shaft will rotate at
However, as was also mentioned in the previous chapter,
roughly 1500 RPM when run off a 50 Hz supply, where
saturation can be a serious problem when running a 60
as with a 60 Hz voltage source it will rotate at about
Hz motor off a supply frequency of 50 Hz. Developing a
18OO RPM. Consequently, equipment that is directly
derating factor to account for saturation is not possible,
coupled to the shaft of the motor will rotate at 5/6 the
since the motor designs vary from vendor to vendor, and
speed it would in a 60 Hz environment. Hence, direct-
hence, the degree of saturation that would occur, if any,
drive equipment must be derated to account for the
would be impossible to predict. Consequently, it is rec-
change in speed. In cases where the equipment is indi-
ommended that no horsepower derating be performed,
rectly coupled to the motor shaft, through the use of
and a 50 Hz motor be purchased.
adiustable pulleys for example, the reduction in shaft
speed is not as much of a problem since the required
a. However, if the vendor can guarantee the user that
speed of rotation can be obtained through the proper
a given 60 Hz motor would not saturate at 50 Hz, then
adjustment or selection of the pulleys.
the motor would need only to be derated to handle the
20 percent increase in current level. The amount of
c. Additionally, electronic HVAC controls that contain
horsepower derating required would depend on the
their own power supply may be 50 Hz sensitive. Most
motor's mechanical load, service factor, and thermal
of the vendors contacted stated that this typically is not
limit. The service factor is a measure of how much the
a problem because most controls are frequency-sensi-
motor can be overloaded continuously without exceed-
tive. If the control are 50 Hz sensitive, they must be pur-
ing safe temperature limits. The thermal limit is the mini-
chased in a 50 Hz configuration. The HVAC vendor
mum speed at which an AC motor can be operated with
must be consulted on a case-bycase basis to determine
rated amperes, without exceeding safe temperature rise.
if the controls can be used in 50 Hz environments.
The thermal limit is important because the motor's abili-
ty to cool itself will be reduced at lower speeds unless,
3-3. Electrical distribution and protection.
of course, some sort of auxiliary cooling is used. In most
In general, a 60 Hz transformer should not be used
cases, however, the minimum shaft speed necessary to
with a 50 Hz voltage source because of the potential
exceed the thermal limit is much lower than 1500 revo-
saturation problem. As with motors, a derating factor
lutions per minute (RPM, for example, for a 4-pole
cannot be developed to account for saturation because
motor), so 50 Hz operation should not be a problem,
of the many different transformer designs on the mar-
although the vendor should be contacted for verifica-
ket. It is recommended that a 50 Hz transformer be pur-
tion. A 60 Hz motor with a 1.20 service factor can be
3-1
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