1725 or 3450 rpm????

[rbertalotto]

Since you folks answered my last question so quickly about a VFD being either 120v or 230v input....Here's another question.

The milling machine currently has a 1.5hp / single phase /1725 rpm motor

Is there any advantage to replace it with a 3450 rpm motor....

Is there any power loss with a VFD and should I up the HP to 2hp or maybe even 3hp?

Thanks again....

 

[PNW_Guy]

VFDs are usually rated in Watts or HP. Get one that can handle the motor you're driving (1.5hp) or possibly a bit more (2 hp). There's no benefit to going much over the motor size - you won't gain anything with a 3hp VFD.

The mill is designed to be used with a 1725 RPM motor. Yes, you could replace it with a 3450 but everything will run twice as fast. In that case, you'll need to to turn down the VFD. There's two reasons to not do this:
1. A VFD will lose some torque at the bottom end. You'll take an even bigger torque hit if you're running at reduced speed.
2. Motors are fan cooled (mostly) and the fan is connected to the motor shaft. If you're running at half speed, so is the fan.

 

[rbertalotto]

Thank you....Makes perfect sense, especially the fan cooling issue. And a mill is not intermittent use. Some machining has the mill running for quite some time.

 

[mksj]

No advantage with replacing it with a 2P motor, motors are both mechanically limited as to maximum speed and performance limitations when you go above and below the motor's designed base speed. A 2P motor will have less torque than a 4P, but one also needs to look at the mechanical advantage (gearing/pulley ratios) as to the delivered torque. When you run a motor on a VFD below the base speed torque can remain relatively flat (this is the constant torque ratio of the motor), but you loose the mechanical advantage. If you plan to run the motor at say 20Hz, then you would want to oversize the motor so the performance would be better at that speed.

With a newer inverter rated 4P motor they will normally spin upwards of 2X their base speed with no mechanical issues, there is some performance loss above 90-100 Hz. Usually a 2P motor is rated for 1.25x it base speed as to maximum operating speed. There are inverter/vector motor that will operate up to 3X their base speed with constant Hp. This depends on the size/Hp of the motor, so my mill has a 3Hp motor which operates as a single speed from 20-200Hz and a back gear. If you have a gear head, or a variable mechanical speed head you would not want to change the motors top end speed, you may also have limitations to the drive gears, pulley, spindle bearings, lubrication, etc. if you overdrive it to higher RPM's.

If you want a wider operating speed range using the variable speed of the VFD then you would probably want to go to the next larger Hp size, so from 1.5 to say 2Hp. There are also advantages to changing the motor drive pulley ratio so you might operate a 4P motor from say 20-120 Hz, with the same spindle speed that you currently get at 60Hz, i.e. a 4P motor will be in constant Hp from 60-120 Hz more or less. A good example is the conversion of a dual speed constant Hp motor (4P/2P) often used on mills for more speed range, when converting to use with a VFD you wire it for 4P only and run it to 120Hz. This gives you the same mechanical top speed as the 2P and the lower speed torque of the 4P at 60 Hz.

 

[rbertalotto]

Thank You! This Mill has three stepped pullies to change spindle speed. When I convert to VFD/3 Phase, should I retain this feature or run the belt directly from the motor to the spindle without incorporating stepped pullies?

 

[PNW_Guy]

Keep the step pullies.
1. It's cheaper that way
2. You can run it at the mid speed most of the time, but if you need to really drop the RPM (big hole saw in steel, for example) you can use mechanical speed reduction as well as the VFD.

 

[RJSakowski]

Keep the step pullies.
1. It's cheaper that way
2. You can run it at the mid speed most of the time, but if you need to really drop the RPM (big hole saw in steel, for example) you can use mechanical speed reduction as well as the VFD.

Plus have double the torque to the spindle.

 

[Shotgun]

You can also move the step pulleys around, so that you step the motor down, then run a higher RPM motor.
I mounted a motor from a commercial treadmill that had a large flywheel. I chucked the flywheel into the lathe and cut a pulley groove. That let me have any ratio I wanted.

 

[fixit]

Since you folks answered my last question so quickly about a VFD being either 120v or 230v input....Here's another question.

The milling machine currently has a 1.5hp / single phase /1725 rpm motor

Is there any advantage to replace it with a 3450 rpm motor....

Is there any power loss with a VFD and should I up the HP to 2hp or maybe even 3hp?

Thanks again....

In my other life in the glass industry. We found it was better to over speed a motor instead of running it below rated speed. I talked to several motor/VFD suppliers and they all agreed. An 1800 rpm motor has no problem runnung at 3600 rpm. We used MANY 1 to 3 hp 460 volt motors on our inspection lines and equipment. The VFD's replaced the original DC drives that had a high failure rate for various reasons. We operated 24/7/365 and bean counters didn't like to stop and fix things.

 

[machPete99]

If you mill does not have a "back gear" you will want to go with the lower speed motor, as it will have more low end torque for larger drilling and milling jobs. In my case I went with a 6 pole motor which runs at something like 1250 RPM for my Rockwell mill.