Page 179 - HVAC Pump Handbook

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Pump Drivers and Variable-Speed Drives

176 HVAC Pumps and Their Performance

7.2.4 Types of polyphase motors and code letters
NEMA has established four different motor designs and has given
them a letter for each design, namely, A, B, C, and D. Each of these
four designs has unique speed-torque-slip relationships. The type
used on HVAC pump motors is usually design B.
Polyphase motors from 1 to 200 hp have been designated by NEMA
with reference to their speed-torque relationships; NEMA has developed
these code letters that include percentage of slip. Full-load speed for an
induction motor differs from synchronous speed by the percentage of
slip. All induction motors are designed to various amounts of slip and
are included in the design letters from A to D as described in Table 7.2.
Most HVAC pump motors are NEMA design B with a maximum
slip of 5 percent. There is little need for high-slip motors since they
are used on pump applications with high starting torques. There may
be some very large cooling tower pumps that are high specific speed,
mixed flow pumps that do require high-slip motors.
In the past, 1200 rev/min, six pole, induction motors had full-load
speeds of around 1150-rev/min speeds while 1800-rev/min, four-pole,
induction motors had full-load speeds around 1750 rev/min. Today,
most HVAC pump installations utilize high-efficiency motors where
the full-load speeds are nearer to 1180 and 1770 rev/min.

TABLE 7.2 Comparison of NEMA Designs for Induction Motors

NEMA Locked-
design Percentage Starting rotor Breakdown
letter of slip current torque torque Applications
A Max. 5% High to Normal Normal Broad application
medium including fans
and pumps
B Max. 5% Low High Normal Normal starting
torque for fans,
blowers, and
pumps
C Max. 5% Low High Normal For equipment
with high inertia
start such as
positive displace-
ment pumps
D — Low Very high — Very high inertia
starts, choice
of slip to match
the load
5 to 8% — — — Punch presses, etc.
8 to 13% — — — Cranes, hoists, etc.
SOURCE: AC Motor Selection and Application Guide, Bulletin GET-6812B, General
Electric Company, Fort Wayne, Ind., 1993, p. 7; used with permission.

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