Motor thermal overload equation

[SceneryDriver]

I'm working on a test rig for induction motors. The intent is 15HP or smaller. Since we plan to be able to connect just about any size motor >= 15HP, I need to ensure I will have overload protection that would shut down the drive, regardless of the FLA of the motor. With smaller motors, I will be below the turndown limit of the drive, and I plan on providing an external true-RMS current transducer that will monitor motor current and report to the PLC. The PLC will then be in charge of determining when to command the drive to shut down on overload, based on nameplate info entered into the test rig's touchscreen.

I've found plenty of sometimes contradictory info online regarding the equation to use for overload current VS time. Any pointers towards a easy-to-understand set of equations that I can reference? I imagine that if I simply monitor for FLA * 1.1, I will receive nuisance trips on motor start if I don't have at least a time delay programmed. I also know that the time delay should be overload dependent; a small overload would have a longer trip time than a large overload. How to quantify that is the question.


Thanks,

SceneryDriver

 

[don_resqcapt19]

The typical thermal overload in a magnetic starter is a "Class 20" overload...that permits the motor to draw 6x the full load current for 20 seconds. There are also Class 10 overloads that are specified for some motors and Class 30 for motors that are starting large inertia loads.

 

[drcampbell]

Incorporate a thermal model into your software that measures how much energy goes via in the wires, how much comes out via the shaft, estimates the amount of heat dissipated and the amount of heat stored, and calculates the temperature.

Or just make a snap-on temperature probe with a magnetic base, and actually measure the motor temperature.

 

[Jraef]

A good generalized motor thermal model is an I^2t (I squared t) curve, with a pick-up point at 115% of FLC and then pick your time point for the other end (600%) to get the slope, i.e. 10 seconds for Class 10, 20 seconds for Class 20 etc. If you are not aware, most STANDARD OFF THE SHELF motors made to NEMA standards in the size range you are looking at are Class 20 with a 1.15 Service Factor (SF), but ALL motors made to IEC standards are Class 10 and no SF (or consider the SF to be 1.0). Submersible pump motors are also usually Class 10, but a few are even Class 5.

 

[ptonsparky]

<=

 

[SceneryDriver]

<=

I blame fat fingers.

 

[kwired]

What is the intended use of the "test rig"?

If just running short time and even unloaded after making repairs or alterations to something, you probably don't really need to worry about overload protection other than make sure you don't leave circuit closed if motor fails to accelerate.

If it is for testing something at it's normal loading conditions then probably best to be able to set overloads to the application every time you change whatever is being supplied.

 

[ptonsparky]

I set current limits on all PLC monitored motors. Adjustable via HMI. Just exceed ___ amps for ___ seconds, down it goes. Never to the level of Jraefs post. I didn't know how for one and it wasn't what I was looking for either.

 

[SceneryDriver]

What is the intended use of the "test rig"?

If just running short time and even unloaded after making repairs or alterations to something, you probably don't really need to worry about overload protection other than make sure you don't leave circuit closed if motor fails to accelerate.

If it is for testing something at it's normal loading conditions then probably best to be able to set overloads to the application every time you change whatever is being supplied.

We often have need to spin motors on our scenery automation projects before the drive racks are turned on and commissioned. This is being built into a Pelican case so as to be portable, and may spin a motor anywhere between 1/2HP and 15HP. Thus the need to have external current sensing.

I like ptonsparky's suggestion for "exceed [x] amps for [y] seconds and trip. I can compute the tripping amperage setting from the FLA of the motor being spun; that info will be entered on the included touchscreen.

Obviously, there exists the possibility of someone entering the wrong motor FLA and smoking a small motor on overload. This will only be used by our trained field techs, and not left in the hands of the client.


SceneryDriver

 

[Jraef]

... This will only be used by our trained field techs, and not left in the hands of the client.

Lol. Famous last words...

 

[SceneryDriver]

Lol. Famous last words...

HA! I suppose with the proper password protection on the touchscreen and the proper programming in the PLC, it could be a legit single-axis controller for simple moves. I'm sure I'll end up sending it out on a job as a client-operated controller eventually...


SceneryDriver