Cold Fusion,
No, I know very well the difference between inrush and LRA as "inrush" is not the same as LRA. I've been dealing with these applications for years.
I trust that this post will illustrate the energy efficient motors are not magically different than standard efficiency motors. It's just that they use less energy to do the same amount of work with the exception of inrush and not NEMA torque curves or code letters being the issue. I've included a lot of stuff in this post, some of which may seem like I'm preaching the choir but the whole picture must be considered not bits and pieces.
It is not unusual to not understand what happens when a motor starts.
It is often misunderstood that the LRA is the only current event that specifically occurs at the point the motor is energized or if the motor's rotor is locked and unable to turn. The value of the LRA actually continues through the acceleration of the motor.
As mentioned in my previous post the inrush is a spike of current as the result of magnetizing the iron in the motor, which quickly decays in about 1/10 second to the LRA value. This current spile of often up to 13X the FLA reaching as high a 17X with some motors. Studues have found that they can even exceed 17X on a rare occasion. The LRA value then continues through the time that it takes to accelerate the motor to about 80% of its speed. (Of course there are exceptions the the 80% just like the LRA.)
I found the following to be a good explanation of what actually occurs from this excerpt from:
http://www.lmphotonics.com/m_control.htm:
"The starting current of a with a fixed voltage, will drop very slowly as the motor accelerates and will only begin to fall significantly when the motor has reached at least 80% full speed. The actual curves for induction motors can vary considerably between designs, but the general trend is for a high current until the motor has almost reached full speed. The LRC of a motor can range from 500% Full Load Current (FLC) to as high as 1400% FLC. Typically, good motors fall in the range of 550% to 750% FLC."
This is illustrated by the attached torque curve.
There are those who would love to find the exceptions to the rule to typical LRA but the fact remains that a good rule of thumb is 7X for the average motor.
I have attached a copy of a chart that shows NEMA design A, B, C, and D torque curves, the typical torque curve of which is B. Knowing that starting current is related to torque the chart illustrates torque curves with higher starting torque than the design B that naturally would equate to higher starting current than 7x.
Energy efficiency has nothing to do with the NEMA torque curve. It simply has to do with the motor performance remaining the same and accomplished more efficiently. How would one propose that an enegy efficient motor would have a different torque curve?
NEMA Design Letter. Changes in motor windings and rotor design will alter the performance characteristics of induction motors. To obtain uniformity in application, NEMA has designated specific designs of general purpose motors having specified locked rotor torque, breakdown torque, slip, starting current, or other values. NEMA design letters are A, B, C, and D.
NEMA Design A motors have normal starting torques, but high starting currents. This is useful for applications with brief heavy overloads. Injection molding machines are a good application for this type of motor. Many of the Tatung design B motors can handle Design A requirements!
NEMA Design B motors are the most common. They feature normal starting torque combined with a low starting current. These motors have sufficient locked rotor torques to start a wide variety of industrial applications.
NEMA Design C motors have high starting torques with low starting currents. They are designed for starting heavy loads due to their high locked rotor torques and high full load slip.
NEMA Design D motors have high starting torque and low starting current, however they feature high slip. This reduces power peaks in the event that peak power is encountered, motor slip will increase.
NEMA code letters have nothing to do with NEMA code letters either and an energy efficient motor is designed to be more efficient, that is less heat for the work that it does. Heat is waisted energy. How would one proposde that these code letters below would change for enegy efficient motors?
NEMA Code Letters for Locked-Rotor KVA
The letter designations for locked-rotor kVA per horsepower as measured at full voltage and rated frequency are as follows.
NEMA
Code Letter KVA/HP
with locked rotor
A 0-3.14
B 3.15-3.55
C 3.55-3.99
D 4.0-4.49
E 4.5-4.99
F 5.0-5.59
G 5.6-6.29
H 6.3-7.09
J 7.1-7.99
K 8.0-8.99
L 9.0-9.99
M 10.0-11.19
N 11.2-12.49
P 12.5-13.99
R 14.0-15.99
S 16.0-17.99
T 18.0-19.99
U 20.0-22.39
V 22.4-and up
Because the energy efficient motor must be designed to essentialy give off less heat for the HP generated the only way that it can be done is to use a higher quality core steel and a more efficient winding design. This results in a lower resistance. Increased inrush, that increased assymetrical spike of current that decays in less than 1/10 sec, is what energy efficient motors are known for. There was a short period of time where you were caught between a rock and a hard place by the NEC where you were not allowed to set an MCP ablve 13X the FLA. Now the NEC allows settings up to 17X.
Cold Fusion, you may have misunderstood that I implied that the MCP protects the motor or the conductor. I believe that I have stated that the MCP takes the motor off line as it is failing. If the motor is failing if set correctly the MCP will limit damage. The only thing that the MCP can bring to the party with conductors is short circuit protection should there be a fault in the conductors which is not vet likely to occur with a good installation. The MCP also protects the contactor which most people don't have a clue about. Contactors are not designed to interrupt fault currents. Matter of fact if a contactor stays closed because the control circuit doesn't have a reason to open it and it is possible for the contacts to float and weld together due to fault current which may flow through the contactor.
MCP are designed to interrupt fault currents, contactors are not.
Westinghouse Electric Corporation invented the magnetic only circuit breaker because of the fires that occurred in motors when they failed. If you recall a thermal magnetic circuit breaker is design to protect cable when properly applied. Because the thermal element of a TM breaker does not coordinate with the overload protection that a motor requires and OLR with properly sized heaters for that motor are used. Then to cover short circuits the magnetic only MCP completes the protection.
