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Carrier 5H User Manual

Page 18

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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