Carrier 5H - Manual (Page 18)

Motor Selection - High starting torque motors
must be used with the 5F20 and 5F30 compres­
sors if these machines are not equipped with
capacity control.
Normal starting torque motors are normally
used with the 5F40 and larger models because
of their partially unloaded starting feature. Where
the starting motor voltage tap must be kept to an
absolute minimum, coincident with actual com­
pressor starting, it is sometimes necessary to
use a high starting torque motor or a larger motor.
NEMA standards permit continuous overloading
of 40 C rise open squirrel cage motors to 15%
above the nameplate rating, when operated at full
rated voltage and frequency, and at ambient tem­
peratures not exceeding 40 C. Whether or not part
of this service factor should be used in making an
initial motor selection depends on accurate infor­
mation concerning; local voltage and frequency
variations, ambient temperatures, compressor
speed, and the maximum suction and discharge
pressures. Where these conditions are compar­
atively unknown, motors should positively not
be overloaded.
Wherever possible, across-the-line starting
is most desirable because it is less costly and
more trouble-free than the reduced voltage type
of starting equipment. However, limitations of
the power system often necessitate the use of
reduced voltage starters for motors of the horse­
power sizes required for 5F,H compressors.
In each case, the power company concerned
should be consulted and a ruling obtained with
respect to the particular application. The point
on the network where the installation is to be
made may often be of importance in obtaining
a ruling, so approval in one case may not hold
true for the entire network.
The starting torque which a motor develops
is approximately proportional to the square of
the voltage at the motor terminals. Thus, at half
voltage, a motor will develop only one-quarter
of the torque that it will develop at full voltage,
or .50
X
.50 = .25. To make starting possible, the
voltage at the motor terminals must be high enough
to provide the starting torque required at the
compressor shaft.
On step resistance starters with 75% starting
taps, the starting torque is approximately 56%
of the rated full voltage starting torque of the
motor, assuming that the voltage obtained is
actually 75% of nameplate voltage. If subnormal
line voltage exists, the starting tap voltage will
fall below the 75% and the motor torque will be
reduced in proportion to the square of the voltage
which is actually obtained. For example, a reduc­
tion to 70% line voltage would result in .70 x .70,
or only 49% starting torque.
Most power companies impose limitations on
the current drawn during motor starting periods.
While these limitations vary considerably, depend­
ing upon the power system capacity, close voltage
control and similar factors, they generally fall
into two catagories:
1. The maximum allowable current which may
be drawn from the line during the starting
period.
2. The maximum increment of current increase
during the starting period.
1. Maximum Allowable Current - When the limita­
tion is in terms of paragraph (1) above, and
the full voltage locked rotor current of the
motor being considered is in excess of this
maximum permissible limit, it is necessary
to use reduced voltage starting equipment.
Under this condition it is essential that the
motor start and accelerate under reduced
voltage. Otherwise, the full locked rotor cur­
rent of the motor will be drawn as soon as
sufficient voltage is applied. If the motor can
be started on the first step and accelerated
at the reduced voltage, then full voltage locked
rotor current will not be drawn from the line
during any part of the starting period.
2. Maximum Increment of Current Increase -
When the limitation is in terms of maximum
increment of current increase, the starting
equipment need only be selected with a suffi­
cient number of steps so that no single step
will exceed the allowable current increase.
The starting torque developed at the various
steps is not of importance, except that the
motor selected must have sufficient torque
to start the compressor when full voltage is
finally applied. The fact that compressor motor
may not start at any of the reduced voltage steps
is of no consequence during the few seconds
required for the starter mechanism to act.
Reduced voltage starting equipment is now
available in several types, both manual and mag­
netic. The best known are the Step Resistance,
Auto Transformer, Wye-Delta and Increment
Starter and Motor Combinations. No attempt will
be made here to compare the merits of each type,
although the starting equipment to be used for any
specific application should be discussed with the
power company engineers with a view to obtaining
their rulings on all matters pertaining to the elec­
trical aspects of the project.
Motors of American manufacture in horse­
power ranges used on the 5H compressors have full
voltage starting torque amounting to 125% of full
load torque on ' 'normal torque'' motors and 200%
of full load torque on "hightorque" motors. When
motors or other manufacture are to be used, the
starting torque characteristics should be obtained.
SECTION
PAGE
DATE
5F,H IXA
18
10-66
Drive Packages - Tablé 13 indicates the drive
package components for the 5F,H standard belt
drive packages. Figure T4 and Tables 14 and 15
indicate the data for the flywheel used in each of
these packages.
S U P E R S E D E S
SECTION 5F,H-1X
PAGES 1-42
Printed in U.S.A. DATE 11-63