Motor Start Protection Question
Motor Start Protection Question
(OP)
I am currently looking at providing protection for a 200HP motor.
It is driving a vacuum pump.
When I sized the fuse according to the local code books one of the reviewers did not like my sizing because I did not take into account the type of load, being either a high inertia or low inertia.
I have since contacted the pump manufacturer and they have provided me with some curves for both the motor and vacuum pump but I still don't know how to relate these back to determining if it's high inertia or low inertia.
The pump curves I was provided are:
Performance curve showing volume vs. pressure
Performance curve showing HP vs. pressure
Starting torque Curce showing Torque vs. Speed (RPM)
Motor curves:
Performance under load
Performance related to speed
and Thermal Capacity
None of these curves actually show how long it is going to take to get to rated speed, I currently don't think I have the information to determine how long it is going to take.
Can anyone offer suggestions on whats next?
Thanks
It is driving a vacuum pump.
When I sized the fuse according to the local code books one of the reviewers did not like my sizing because I did not take into account the type of load, being either a high inertia or low inertia.
I have since contacted the pump manufacturer and they have provided me with some curves for both the motor and vacuum pump but I still don't know how to relate these back to determining if it's high inertia or low inertia.
The pump curves I was provided are:
Performance curve showing volume vs. pressure
Performance curve showing HP vs. pressure
Starting torque Curce showing Torque vs. Speed (RPM)
Motor curves:
Performance under load
Performance related to speed
and Thermal Capacity
None of these curves actually show how long it is going to take to get to rated speed, I currently don't think I have the information to determine how long it is going to take.
Can anyone offer suggestions on whats next?
Thanks





RE: Motor Start Protection Question
Here is one (you may want to check its accuracy from other reliable sources, this is the best you get from me for free!)
Electric Motors - Acceleration time:
Acceleration time for electric motors is directly proportional to total inertia and inversely proportional to the electric motor torque. For electric motors with constant acceleration torque, acceleration time is:
t=WK^2*(N2-N1)/(308*Tx)
where WK2 = rotational inertia in lb-ft2, (N2 - N1) = the speed difference, and Tx = acceleration torque in lb-ft.
RE: Motor Start Protection Question
RE: Motor Start Protection Question
Keith Cress
kcress - http://www.flaminsystems.com
RE: Motor Start Protection Question
Although theoretically possible, it's not considered to be a good practice, especially on a motor of this size. The fuses should be there to protect the circuit conductors and the motor windings from a short circuit or ground fault. Running overload protection should be done with a properly selected Over Load Relay (OLR). I prefer the newer Solid State OLRs that now have many of the kinds of protection features that used to come only in very expensive Motor Protection Relays.
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies
RE: Motor Start Protection Question
There is a solid state over load relay that is being used for motor protection.
My fuse sizing is related to the main incoming section on an MCC and the largest load on the MCC is the vacuum pump.
Looking at the MCC operating at full load with the vacuum pump starting to ensure the main incoming fuse does not blow.
RE: Motor Start Protection Question
So, you should have a set main over current fuses based on total panel load. Then you should have a set of branch fuses in front of the motor over current device sized at 175%. Assuming your motor starts DOL with little load, time to full speed is a moot point.
RE: Motor Start Protection Question
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
RE: Motor Start Protection Question