Brochure

19
5/98
© 1998 Square D All Rights Reserved
Thermal Unit Selection
General
All tables are based on the operation of the motor
and controller in the same ambient temperature,
40°C (104°F) or less. Always be certain the
correct thermal units are installed in the starter
before operating the motor. Each thermal unit
shall be installed such that its catalog number is
visible. On melting alloy thermal units the ratchet
wheel must engage the pawl assembly.
Selection Procedure
1. Determine motor data:
a. Full load current rating
b. Service factor
NOTE: If motor full load current is not known, a
tentative thermal unit selection could be made,
based on horsepower and voltage. Refer to
Page 22.
2. Motor and controller in same ambient
temperature:
a. All starter classes, except Class 8198:
1. For 1.15 to 1.25 service factor motors use
100% of motor full load current for thermal
unit selection.
2. For 1.0 service factor motors use 90% of
motor full load current for thermal unit
selection.
b. Class 8198 only:
1. For 1.0 service factor motors use 100% of
motor full load current for thermal unit
selection.
2. For 1.15 to 1.25 service factor motors use
110% of motor full load current for thermal
unit selection.
3. Motor and controller in
different ambient
temperatures:
a. Multiply motor full load current by the
multiplier in Table A. Use the resultant full
load current for thermal unit selection.
4. Locate proper selection table from index,
Pages 19 and 20.
a. The proper thermal unit number will be found
adjacent, to the right of the range of full load
currents in which the motor full load current
or resultant full load current falls.
5. See Page 21 for calculation of trip current rating.
Slow Trip Thermal Unit Selection
To select Type SB slow trip thermal units, the
selection table for a standard Type B thermal unit
may be used with the following modifications: For
continuous rated motors having service factors of
1.15 to 1.25, select thermal units from the
standard Type B table using 93% (102% for Class
8198) of the full load current shown on the motor
nameplate and then substitute an SB for the B in
the thermal unit type number.
Example: A motor with a full load current of 14
amps controlled by an 8536 SCG3 would require
B22 thermal units for standard trip applications
and SB19.5 thermal units for slow trip
applications. The SB is selected by multiplying 14
amps times 93% for 13.02 amps and using this
value to select B19.5s. Then add the S prefix to
arrive at SB19.5s.
For continuous rated motors having a service
factor of 1.0, select thermal units in the same
manner using 84% (93% for Class 8198) of full
load current shown on the motor nameplate.
NOTE: SB thermal units are used on Size 0, 1, 2
and only some Size 3 applications. Check thermal
unit tables for current ranges.
Table A: Selection of Thermal Units for Special
Applications
Class of Controller Continuous Duty Motor Service Factor
Melting Alloy and
Non-Compensated
Bimetallic Relays
Ambient
Temp.-
Comp.
Relays
Ambient Temperature of Motor
Same as Controller Ambient
Constant
10° C
(18° F)
Higher
Than
Controller
Ambient
Constant
10° C
(18° F)
Lower
Than
Controller
Ambient
Constant
40° C
(104° F)
or Less,
for Any
Controller
Ambient
Full Load Current Multiplier
All
Classes,
Except
8198
1.15 to
1.25
1.0 0.9 1.05 1.0
1.0 0.9 0.8 .95 0.9
Class
8198
1.15 to
1.25
1.1 1.0 1.15 1.1
1.0 1.0 0.9 1.05 1.0
Thermal Units
Melting Alloy Bimetallic
Type of
Trip
Thermal
Unit Type
Type of
Trip
Thermal
Unit Type
Standard
A
Standard
AR
BAF
CAU
CC E
DD
Quick FB
Slow SB