3 phase motor wired for low voltage, controlled by VFD at 480vac

[Ingenieur]

increasing speed mechanically would reduce torque proportionally

perhaps start with a 100 hp/3600 rpm and slow it down mechanically

 

[Besoeker]

increasing speed mechanically would reduce torque proportionally

So does running it over base speed by frequency change.

 

[winnie]

We did quite a few in the 56kW to 200kW typically capable of 20,000 to 30,000 rpm for machine tool applications but I agree with you - these are special purpose and relatively rare applications in the great scheme of things.

I also wonder why a gearbox/v-belt/morse pulley arrangement wasn't used. Space maybe? Perhaps the OP knows the reasoning behind that.

The faster the motor goes, the greater the horsepower/kw you can get out of a give frame size.

Double the motor speed and you double the horsepower. If the load needs the higher speed (so you aren't adding a gearbox) this 'extra' horsepower is almost free.

As I said, you don't get something for nothing; you have more bearing wear, you need a larger inverter, etc. Also losses will increase. But if you need the higher speed there is very little reason to not use the available motor capability.

-Jon

 

[Besoeker]

The faster the motor goes, the greater the horsepower/kw you can get out of a give frame size.

Double the motor speed and you double the horsepower.

Not unless you double the voltage.

 

[winnie]

Not unless you double the voltage.

Absolutely agree...which is the whole point of this thread, wiring the motor for nominal 240V at 60Hz and then connecting it to a 480V inverter.

-Jon

 

[Jraef]

I see that people have a hard time with this concept, but honestly, it's done all the time with no significant issues.

In this application with shear rollers that need to run at different speeds, the mechanical designer had two choices; run the top roller at full speed and use gear reduction to slow down the bottom one, or run the bottom one at full speed and increase the speed of the top. Slowing down the bottom means lower throughput (by comparison). So then increasing the top speed has two choices; use a gearbox to increase the speed, or use a VFD. The gearbox results in less torque, so they must start with a larger motor, then get lower operating efficiencies. Increasing with a VFD would result in lower torque /larger motor, ULESS you do the voltage trick. So all in all it's the best choice.

 

[kwired]

I see that people have a hard time with this concept, but honestly, it's done all the time with no significant issues.

In this application with shear rollers that need to run at different speeds, the mechanical designer had two choices; run the top roller at full speed and use gear reduction to slow down the bottom one, or run the bottom one at full speed and increase the speed of the top. Slowing down the bottom means lower throughput (by comparison). So then increasing the top speed has two choices; use a gearbox to increase the speed, or use a VFD. The gearbox results in less torque, so they must start with a larger motor, then get lower operating efficiencies. Increasing with a VFD would result in lower torque /larger motor, ULESS you do the voltage trick. So all in all it's the best choice.

IIRC OP said this runs around 3000 RPM. I can only recall seeing 4 pole motors in the HP range OP is dealing with, and have a suspicion that a 2 pole is either non existent or not so easy to get, and could be a factor in why they are overdriving the one they have instead of using a 2 pole and slowing it down to what they need.

I run into motors that size that drive a load at a higher then 1800 RPM speed but always use mechanical means to increase speed.

 

[drummin89]

There has already been some very good info posted.-Jon

Yes, tons of great information in this thread. Thank you everyone that has posted. Highly appreciated. I have learned a lot. I am going to take some notes and put them in the file for this machine so I can reference it in the future or for the next guy.

I see that people have a hard time with this concept, but honestly, it's done all the time with no significant issues.

In this application with shear rollers that need to run at different speeds, the mechanical designer had two choices; run the top roller at full speed and use gear reduction to slow down the bottom one, or run the bottom one at full speed and increase the speed of the top. Slowing down the bottom means lower throughput (by comparison). So then increasing the top speed has two choices; use a gearbox to increase the speed, or use a VFD. The gearbox results in less torque, so they must start with a larger motor, then get lower operating efficiencies. Increasing with a VFD would result in lower torque /larger motor, ULESS you do the voltage trick. So all in all it's the best choice.

I believe what you said sums up the manufactures intent exactly. We questioned the manufacture about this technique when we first learned about it but they assured us they do this all the time without any known issues. And in fact our machine ran for 5-6 years, probably around 300 days a year, before the motor had to be replaced and I believe for reasons unrelated to this setup.

 

[mike_kilroy]

At the plant I work at we have a piece of equipment that came from the manufacture with a 50HP 3 phase motor wired for low voltage (I need to verify this, not sure on the voltage) that is controlled by a VFD that is supplied with 480vac. The motor nameplate parameters in the VFD list the motor as being 45kW, 84 FLA, 460V, 104Hz, 3016 rpm. I am waiting for the machine to shut down so I can grab the actual nameplate data off of the motor.

I really didn't understand a whole lot about this and how it is done. I ran across the following post by Jraef in an older thread that helped me to understand things a little better. http://forums.mikeholt.com/showthread.php?t=115549&page=2&p=1074268#post1074268 Post 18 if it doesn't take you to his post. The manufacture is based out of Germany (equipment for chocolate refining) and the tech that flew over here tried explaining this to me a few months ago but he/what is was explaining was difficult to understand.

What Jraef described matches the setup I believe I am dealing with. The manufacture uses a motor that is undersized for the application, not 100% sure why they do it this way, I know a larger motor and gearbox probably wouldn't fit in the machine so that might be a reason. I believe they also do this to gain a higher speed. We constantly run this motor between 85 and 104Hz

Anyways my question is, what affect does this have on the current that the motor pulls? How would you size the VFD for this application? The VFD that is used is I believe rated for 150-200HP.

We have 1000+ motors and a couple hundred VFDs at my plant but this is the only one setup like this.

Thank you for your time!

Everyone explained how this works so I won't add to that. But no one mentioned how your mfgrer cheated you out of not only your "free" Xtra hp, but also baseline rated torque.

By doing that they guaranteed you cannot overheat or overload this motor - very slick of them. As u see, not even engineers delving into here caught on. Sly trick mr. German oem!

I won't do the math for u, but they used the motor PF value to limit amps to 84a on your 120a rated motor. They also entered 60hp instead of the 104hp it makes. Lots of drives do not use that kw entry so that may be mute point. But they certainly reduced ur torque and hp a bunch!

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