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.
 .



Comments

Post a Comment

Popular posts from this blog

How to check continuity over long distance

Image
When I began my career as an electrical technician my first challenge was the problem of checking the continuity of a wire that spans a long distance. My problem was compounded due to the physical layout of the wire under test. Let me use a simple picture to layout the nature of my problem. A bunch of Armoured cables outgoing from the  PCC (Power Control Center) in Building 1 feeds a group of distribution panels in Building 2 over 100ft away. Ideally, the cables should be tagged but due to aging and poor maintenance, most of the tags were not visible.  The problem was that some Distribution boards in building 2 had power while some others didn't.  The DB of interest didn't have power and thorough checks showed that there was no power on the cable feeding that DB.   All outgoing cables from Building one PCC had power so I suspected that the cable was no longer continuous. Anyway in the initial design of the facility ...
Read more

Making sense of a transformer nameplate data

Image
The nameplate of a power transformer contains the following details as per standard, then additional information could be provided varying from manufacturer to manufacturer. Name Of Manufacturer Serial Number Year Of Manufacture Connection Symbol – This tells you the HV winding and LV winding configuration of a transformer and difference in phase angle between them. Example DYN11 where the first letter represents the HV winding and indicates that it is connected in delta, the second letter represents the LV winding and indicates that it is connected in Star(wye)  and the third letter N indicates that the LV winding has a Neutral while 11 denotes a 30 degree lead in phase angle. This article explains more on the Vector group of a transformer and why it is important.  (Transformers connected in parallel must have the same vector group i.e. same phase angle shift to avoid circulating currents. This is a situation where one source will become load to the ot...
Read more

Relay vs Contactor , what are the differences.

A Relay is an electrically operated switch, it basically consists of an electromagnetic coil which builds up a magnetic field when current is passed through it. This magnetic field is used to operate the relay contacts to an OFF or ON state. A Contactor is an electrically operated switch, it basically consists of an electromagnetic coil which builds up a magnetic field when current is passed through it. This magnetic field is used to move the contactor contacts into an OFF or ON state. So what is the difference between a relay and a contactor. In the most simplistic terms, a contactor is a  relay  is used to switch a large amount of electrical power through its contacts . Other notable differences are stated below. 1)     Relays tend to be for logic switching while contactors are for Power switching 2)     Relays don’t come with auxiliary contacts while many contactors have provision for auxiliary contacts 3)   ...
Read more