Starts per hour based on load

[VinceS]

I am looking to reevaluate start stop conditions based on PLC / PE, (added, not OEM), control of a 15 hp motor which averages 20 starts per hour, (MG10 Max is 5.4/Hour).

Question 1: Its is easy to use MG 10 to lookup starts for a motor if load work is known. Is there a formula to use the running amp load to calculate work?

The motor uses a reduction gearbox directly coupled to the motor. A further reduction is used at the belt between the gearbox and the pulley driving a chain conveyor.

Question 2: Is there a reevaluation of MG10 starts per hour required when using reductions?

The end game.... I'm building a case for a drive install, with "$" being todays concern. Cost of loss of production and motor cost vs drive cost.

 

[VinceS]

Update

Update

Disregard question 2. The reduction has nothing to do with conditions which would affect start constraints affected by the NEMA MG 10 reference.

 

[VinceS]

Update 2

Update 2

I am example found in the American Electricians Handbook (7.226). I can convert load in amps at 480Vac to hp. From this data I should be able to calculate the duty cycle and load requirements, working backwards. I'm not sure if this will be accurate. Still looking through old ref's for a better method.

 

[Jraef]

If all you have is running load amps, then that is not a good indicator of HP, because you do not know the power factor. You will know it is unloaded if it is lower than FLA, but you will not know by how much in terms of mechanical workload. Find yourself a kW meter to determine load.

Starts-per-hour limitation is all about the thermal damage curve of primarily the rotor, because the rotor has less ability to dissipate heat than the stator does. So you can, to a certain extent, substitute absorption in lieu of dissipation. Absorption means more mass, more rotor mass automatically comes with higher HP ratings. Some motor mfrs will sometimes design custom motors with more mass in the rotor compared to the stator winding size; centrifuges are a prime example. But if you don't want to pay for a custom design, the "quick and dirty" method is just to derate. So if the motor was derated for the expected S/H duty, then it is probably OK.

So if you are attempting to justify the higher cost of a VFD as a control method on something with a high duty cycle, it does make sense if the motor is going to be replaced with a smaller one based on actual known mechanical load. If it was designed for something less by the OEM and the new control system is demanding more now, then you are also on the right track. But if you have no intention of resizing the motor and it has been working fine like this for years, then it's going to be a tough sell.