Electric Motor - NEMA Locked Rotor Code, Design Code , Insulation Class


NEMA Code letter
    kVA/HP with locked rotor
A
0 - 3.14
B
3.15 - 3.54
C
3.55 - 3.99
D
4.00 - 4.49
E
4.50 - 4.99
F
5.0 - 5.59
G
5.6 - 6.29
H
7.1 - 7.99
J
7.1 - 7.99
K
8.0 - 8.99
L
9.00 - 9.99
M
10.0 - 11.19
N
11.20 -12.49
P
12.5 - 13.99
R
14.00 - 15.99
S
16.0 - 17.99
T
18.0 - 19.99
U
20.0 - 22.39
V
22.4 and up


With the data above it is possible to measure the In-rush current or locked rotor amps from the Locked rotor code and Rated HP of the motor. Practically it is possible to do it by mechanically strapping down the rotor, then you press the start button and measure the stator current with your clamp-on ammeter.

For this example let us assume a 3-phase 75HP motor designed to operate at 460 V with NEMA Code Letter G. How can we calculate the Locked Rotor current?

Soln

We start by calculating the starting KVA of the motor. kVA is apparent power 


 Where HP = 75 and NEMA Code G from the Chart above is 6.29 in the worst case scenario so the starting kVA(different from rated kVA) would be


VA simply means Voltage Amps ie Voltage multiplied by Amps. We already know the nominal voltage(V), we already know the power(S), so we make current(A) the subject of the formula in this equation  so that   or S x 1000 as it is initially in kV and  x 460 as it’s a 3 phase motor.

 I(A) =  =  = 592.1A


I(A )=592.1A, This is the in-rush or Locked rotor current it is usually about 6.5 times the FLA but some motors draw up to 8 times the FLA at starting.

Nema Insulation Class table

Insulation Class
Temperature Classification
Class: A
105 Degrees C
Class: B
130 Degrees C
Class: F
155 Degrees C
Class: H
180 Degrees C


The Surface temperature of the motor is typically 30°C lower than it is at the windings.The temperature classification for an insulation class is the maximum allowable operating temperature for a motor that has that insulation class.

When you check the temperature of your electric motor with your Infrared Gun or instrument, to get the true winding temperature you must add the ambient temperature to your readings.

For example, if your Infrared thermometer measures 65°C and the ambient temperature is 35°C the real temperature of the winding is 65°C+35°C = 100°C.

 Finally, check the name plate for the insulation class of the motor. Let’s assume it is B then there would be no cause for alarm as the table informs us that the winding insulation is designed to handle temperatures up to 130°C.
 .



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