Sizing 3 Phase Gen for Specific Load

[NewbieEC]

I found many people on here saying to go to generator manufacturer's websites to size generators, but I haven't found one that allows for adding specific enough info to get what I need.

First of all, if you know of a calculator that will help my situation, share it with me and I will go there first. Otherwise, here's the details.

I have a customer that bought a 3 Phase 240V dual head wide belt sander for his "under the radar" cabinet shop. He currently has a 200A 120/240V Single Phase service to his shop. The sander has two main motors, a 25HP and a 20HP, with a couple small (fractional to maybe 2HP) motors for the drive belt and table raising.

A phase converter seems to be out of the picture as a large enough unit to power the whole sander would have too high of a primary current to be added to his current service.

He also doesn't want to upgrade his service and tip off the governing bodies that he is running a business in his shop.

We thought about running a phase converter small enough to handle just the larger of the two main motors along with the control motors, so big enough for 30HP. The only other option we are entertaining at this point is a dedicated generator to power the whole sander. Would I be wrong to calculate it as such?

kW=(running load of 3 smaller motors + LRC of largest motor)*240*1.732

179.16kW=(6A+6A+54A+365A)*240*1.732

Thus, I would need a 200kW generator or a 180kW if I can find one.

Or, would it be fine if I calculated it this way to keep the size down, with clear communications on how to start the machine (larger motor first).

kW=(running load of 2 smaller motors and largest motor + LRC of second to largest motor)*240*1.732

153.8kW=(6A+6A+68A+290A)*240*1.732

Both generator options seem quite over the top for a single piece of equipment. They seem more like the size a whole commercial building would run with.

Thoughts? I'm open to anything at this point. Thank you in advance!

 

[Jraef]

There are 746 watts in a HP. So 45HP x .746 = 33.57kW. Total load. The smaller motors are likely not going to run while the others are under load.

If you want to keep the generator size down, implement a control scheme that forces him to start one motor at a time. General rule of thumb is that the generator kW must be 3x the largest motor that will be starting Across the Line. Largest motor is 25HP, so 18.65kW, ergo you need a 55kW generator. If both motors are on VFDs, you only need 35kW.

 

[NewbieEC]

There are 746 watts in a HP. So 45HP x .746 = 33.57kW. Total load. The smaller motors are likely not going to run while the others are under load.

If you want to keep the generator size down, implement a control scheme that forces him to start one motor at a time. General rule of thumb is that the generator kW must be 3x the largest motor that will be starting Across the Line. Largest motor is 25HP, so 18.65kW, ergo you need a 55kW generator. If both motors are on VFDs, you only need 35kW.

Thank you! I was assuming my calculations were WAYYY overkill, but that's what a used generator dealer told be to go by. I appreciate your feedback!

 

[Jraef]

but that's what a used generator dealer told be to go by.

Sell more generator…

Actually yo be fair, that formula would be appropriate for a backup emergency generator because in that use case, you might not have control of which motor starts first. So if the largest motor starts AFTER everything else is running at full load, AND you must avoid a voltage drop that may cause coils to drop out, then your generator size must cover that worst case scenario. But when powering a single machine where you control the starting sequence and VD is not as big of an issue, it doesn’t require all of that “fudge factor”.

 

[NewbieEC]

Sell more generator…

Actually yo be fair, that formula would be appropriate for a backup emergency generator because in that use case, you might not have control of which motor starts first. So if the largest motor starts AFTER everything else is running at full load, AND you must avoid a voltage drop that may cause coils to drop out, then you r generator size must cover that worst case scenario. But when powering a single machine where you control the stating sequence and VD is not as big of an issue, it doesn’t require all of that “fudge factor”.

Very good insight! Thank you.

 

[Jraef]

Very good insight! Thank you.

20+years of putting power systems on portable rock crushers. Owners in that industry don’t want to spend 10 cents more than they have to, so they taught me not to trust the generator salespeople when it comes to powering one machine. 90% of their generator sales are for emergency backup applications. They have one tool in their belt, it’s a hammer, so all problems look like nails to them.

 

[Knightryder12]

I found many people on here saying to go to generator manufacturer's websites to size generators, but I haven't found one that allows for adding specific enough info to get what I need.

First of all, if you know of a calculator that will help my situation, share it with me and I will go there first. Otherwise, here's the details.

I have a customer that bought a 3 Phase 240V dual head wide belt sander for his "under the radar" cabinet shop. He currently has a 200A 120/240V Single Phase service to his shop. The sander has two main motors, a 25HP and a 20HP, with a couple small (fractional to maybe 2HP) motors for the drive belt and table raising.

A phase converter seems to be out of the picture as a large enough unit to power the whole sander would have too high of a primary current to be added to his current service.

He also doesn't want to upgrade his service and tip off the governing bodies that he is running a business in his shop.

We thought about running a phase converter small enough to handle just the larger of the two main motors along with the control motors, so big enough for 30HP. The only other option we are entertaining at this point is a dedicated generator to power the whole sander. Would I be wrong to calculate it as such?

kW=(running load of 3 smaller motors + LRC of largest motor)*240*1.732

179.16kW=(6A+6A+54A+365A)*240*1.732

Thus, I would need a 200kW generator or a 180kW if I can find one.

Or, would it be fine if I calculated it this way to keep the size down, with clear communications on how to start the machine (larger motor first).

kW=(running load of 2 smaller motors and largest motor + LRC of second to largest motor)*240*1.732

153.8kW=(6A+6A+68A+290A)*240*1.732

Both generator options seem quite over the top for a single piece of equipment. They seem more like the size a whole commercial building would run with.

Thoughts? I'm open to anything at this point. Thank you in advance!

try here

https://www.cummins.com/na/generators/home-standby/generator-size-calculator

You also have to be concerned with starting current on that 30HP motor.