72 Rockwell Automation Publication 825-UM004D-EN-P - November 2012
Chapter 6 Configuring Protection & Logic Functions
If the Locked Rotor Ampere rating for the motor is unknown, but the motor
has a Motor Code designation, use the following table as provided by NEMA
(National Electrical Manufacturer Association) to determine the locked rotor
ampere value.
➊ Locked kVA per horsepower range includes the lower figure up to, but not including, the higher figure; e.g., 3.14 is designated by letter
A and 3.15 is designated by letter B.
Thermal Element Setting
A 4000V 600 Hp motor is protected using the 825-P Thermal Overload Element. Motor
data sheet includes the following:
• Rated Horsepower = 600 Hp
• Rated Voltage = 4000V
• Rated Full Load Current = 80 A
• Rated Locked Rotor Amps = 480 A
• Safe Stall Time at 100% Volts:
• Cold = 18 seconds
• Hot = 15 seconds
• Service Factor = 1.2
Phase current transformers with 100:5 A rating and the MCM20 module are selected
for application. 825-P settings for application are calculated as shown below:
• Current Transformer Ratio: := 100/5 := 20
• Full Load Amps (Ie): := 80 A primary
• Service Factor: := 1.2
• Locked Rotor Amps: := 480.0/80.0 := 6.0 x Ie
• Hot Locked Rotor Time: := 15.0 seconds
• Run state time constant: := Auto
NOTE: To prevent nuisance tripping from occurring when a motor operates at the rated service
factor, set Service Factor to 0.01 plus the motor nameplate value.
Table 12 - Motor Codes
Letter Designation kVA/HP ➊ Letter Designation kVA/HP ➊
A 0…3.15 L 9.0…10.0
B 3.15…3.55 M 10.0…11.2
C 3.55…4.0 N 11.2…12.5
D 4.0…4.5 P 12.5…14.0
E 4.5…5.0 R 14.0…16.0
F 5.0…5.6 S 16.0…18.0
G 5.6…6.3 T 18.0…20.0
H 6.3…7.1 U 20.0…22.4
J 7.1…8.0 V 22.4 and up
K 8.0…9.0
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