Motor Start Protection Question

[hanksmith]

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

 

[rbulsara]

You will need rotational inertia of the load and output torque of the motor. Google for this and you will get a few hits with motor calculation formulas.

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.

 

[Kiljoy]

Are you starting the motor across-the-line or with a soft starter?

 

[itsmoked]

Hopefully with a Soft Starter.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

I'm a bit concerned that you appear to be trying to use the fuses as running overload protection. Correct me if I'm wrong.

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

 

[hanksmith]

The starter is across the line,

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.

 

[Kiljoy]

Are trying to use the fuses for group protection? If the fuses are protecting the entire MCC, then protecting the branch circuit with the 200HP motor should be a second set of fuses. It sounds like the fuses you are sizing are for feeder protection. We don’t know the sizes of other loads in the panel, but I don’t think you can use a single set of branch fuses to protect a 200HP and say a 2HP motor.
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.

 

[electricpete]

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.

You are correct. You don't have the info to determine how long it is going to take. You need the inertia of the motor and the vacuum pump which is not among the info you listed (although it is sometimes listed on one of those drawings). You can't calculate starting time without it. Also if there is a gearbox involved, that speed transformation will be relevant.


=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[cherry2000]

My suggestion..approach the pump vendor with the serial no / model no of the pump, and the required Data sheet can be made available for further number crunching. The Moment of Inertia is a basic data that all Pump & motor manufacturer's, especially for medium voltage drives, readily provide. Secondly, for the load torque curve ( torque vs speed), if the torque starts from 0 (at 0 speed) and increases exponentially, you may not have to worry, since motor torque at starting is always a positive figure, and the accelerating torque will be sufficient for successful starting. One thing to kept is mind is that, if you allow terminal voltage drop at atrtaing to be 80%, the motor (driving) torque will drop to 64%, and there shoudl always be a healthy difference with the load torque. Otherwise, you may end up with a jammed rotor.