# VFD Question



## gredpe3 (Oct 19, 2014)

I have only had my mill 3 months and already am tired of the speed change procedure.It is a Grizzly 0705.It has the 3 pulley set-up on top with 12 speed changes.I will eventually put a VFD on it.When i do,will i put it in the fastest position and leave it or what?I am just tryin to get it sorted out mentally.


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## Karl_T (Oct 19, 2014)

You'll want to put it in one of the middle speeds. otherwise you'd have no power at all in the lower speeds.

Horsepower is torque times RPM. VFDs give constant torque below nominal speed. So when you run 1/2 speed you have half the horsepower. OTOH, you won't get more Hp above rated speed. In general you can run most motors twice rated speed to a max of 4000 RPM. NOTE: your machine may not take this - look at that too.

IMHO, by far the best value in VFDs is at Automation Direct. I'd suggest you pop for a brake resistor when you get one.

I've set up a HUGE number of VFDs. Ask for help when you are ready.

Karl


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## Catpower (Oct 26, 2014)

Karl is right, but I have found better pricing at Factorymation, unless Automation Direct has lower their prices recently


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## gredpe3 (Oct 26, 2014)

I have some planning to do,gonna have to move some stuff around.I have a 2x72 that needs to be converted also.So i need it and the Mill next to each other.They are currently across the shop from each other.it will also be a major expense with two motors and a VFD.I wonder if Santa still comes to B,ham?


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## mzayd3 (Oct 30, 2014)

Catpower said:


> Karl is right, but I have found better pricing at Factorymation, unless Automation Direct has lower their prices recently



Wow!  looks like a good value!  "jawdrop:  Thanks


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## Wireaddict (Oct 31, 2014)

Do the Grizzly mill & the 2X72 both have 3-phase motors?  Most, probably all, VFDs have 3-phase outputs [even though they have single-phase inputs].


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## lens42 (Oct 31, 2014)

Even if the machine doesn't have a 3-phase motor, it's worth it to swap out the motor for a 3-ph one. I've bought 3-phase motors for a few machine conversions off eBay. All were in new or near new condition and I never spent more than $100 including shipping. You can also get VFD deals on eBay, but stick to known brands and make sure you can find the manual on-line before buying.


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## Falcon67 (Oct 31, 2014)

If you swap the motor, get one rated for use with a VFD (inverter rated?) or you'll burn it up trying to vary speeds that widely with the VFD.  I have a stock Grizzly 1 HP 3ph on my mill/drill and I make it a point to only vary 10% from nominal in any direction, and only if it's really needed.  Get past 20% and the motor doesn't like it.


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## gredpe3 (Oct 31, 2014)

I will have to purchase motors and the VFD,I am not looking to get more speed out of either machine, just want more control.My mill has a 2hp and the grinder is 1 1/2. I will likely get 2hp for both and of course a VFD . My mill has enough power for the stuff I do,the grinder could use a little more .


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## lens42 (Oct 31, 2014)

One eBay example:
http://www.ebay.com/itm/Dayton-2N99...987?pt=LH_DefaultDomain_0&hash=item53fa6785a3

I've also heard warnings similar to Falcon67, but I have to counter that I've done several VFD conversions without "inverter rated" motors and have never had a problem. Now if you run the motor slow (below 50% of rated PRM) with heavy loading, its internal fan does not blow enough air at low RPM and there *is* risk of overheating, but that has nothing to do with inverter rating or not. In that case you may want to add a fan (like a computer fan) to add some cooling for the motor. That being said, I have never even done that, but my slow speed operation is usually not heavily loaded or for extended periods of time.

I have never heard of *any* 3-phase motor "not liking" a 20% speed reduction. If that's happening, I think there is something wrong in the system.


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## gredpe3 (Oct 31, 2014)

Not sure I understand where this is going.If I have a 1750 rpm motor and run it at 900 rpm with a VFD am I at risk of motor damage?Assuming the correct motor and VFD are mated,I really dont see why this would cause damge.The very reason for me  wanting a VFD is to be able to vary the speed with out changing belt position on the pulley system.


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## Karl_T (Oct 31, 2014)

2nd what Lens42 says

Karl


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## JimDawson (Nov 1, 2014)

A few random thoughts on VFDs and motors


Most 17xx RPM (1800 RPM nominal) motors will run just fine at twice the base speed.  Don't try this with a 3600 RPM nominal speed motor.


Most systems will run just fine from about 15Hz to 120Hz as long as you don’t try to run in constant horsepower mode (increasing torque at reduced speed).  There are special motors that will operate from 0 to 240 Hz (or higher).  If you need full torque at reduced speed, then you need a forced air-cooled BLDC (Brushless DC) motor.  The torque curve on these motors is almost flat from 0 to their rated RPM


Typically, as the speed decreases, the torque decreases with the square of the speed and the horsepower decreases with the cube of the speed.


Running continuously at very low speed can cause overheating if the torque requirement is high.  If the torque requirement is light, most motors will run just fine as low as about 15Hz.  Lower than that a standard squirrel cage induction motor starts to get a bit unstable.  Operating at greater than the rated speed at high torque requirements can cause overheating also, as well as inverter trips due to over current.


I have heard stories of non-inverter rated motors failing, but in 30 years of working with VFD’s I have never seen it in light use.  I have seen inverter rated motors fail when operated in their normal envelope, but this is in continuous heavy industrial use.  The last big system install I was involved with had about 700 VFDs/Motors in the system.  The ones that did fail, did so in testing, or were defective right out of the box.  These were Allen Bradley VFDs and Baldor motors.


I ran my old BP clone with a 2HP Taiwanese motor at about 15 to 120Hz for years without a problem, and it is still running just fine.  In most cases I left the belt on the 660 setting.  I did change the belt position or switch to low range if I needed more or less speed and/or higher torque for a given operation.


My new (to me) mill has a 3HP Baldor inverter rated motor on it, normally I run between 30 and 90Hz because it is also a variable speed belt.  I sometimes use lower settings for light power tapping and reaming rather than shifting into low range.  When running at maximum variable speed setting on the belt, I do not run the VFD at greater than 60Hz, just to keep the mechanics in their normal operating envelope.


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## mksj (Nov 4, 2014)

I had thought that "as the speed decreases, the torque decreases with the square of the  speed and the horsepower decreases with the cube of the speed" applies to the characteristics of the load, i.e.

VARIABLE TORQUE LOADS:
Load types require reduced torque when driven at speeds less  than the base speed of the load. Conversely, such loads may require  increased torque when driven at speeds greater than the base speed of  the load. These are classified as variable torque loads. Many variable  torque loads decrease with the square of the speed. This is  characteristic of centrifugal pumps, and certain types of fans and  blowers. Typically, as the speed decreases, the torque decreases with  the square of the speed and the horsepower decreases with the cube of  the speed.

CONSTANT TORQUE LOADS
With constant torque loads, the torque loading is not a function of  speed. Typical applications are: Traction drives, Compressors,  Conveyors, Positive displacement pumps and Hoists. As the speed changes,  the load torque remains constant and the horsepower changes linearly  with speed. Constant torque loads cause motors to draw relatively high  current at low speeds when compared to variable torque applications.  This is why the same size drive may have a lower HP rating for Constant  Torque applications.

Many VFDs can be set up with VT or CT type settings. Base speed is their RPM at the nameplate motor rated Hz (usually 60Hz): Many 3 phase inverter rated motors are rated for a CT ratio of around 10:1, they can deliver constant torque below their base speed of 60Hz down to around 5-6Hz. AC Vector motors (CT ratio around 1000-2000:1) provide full torque essentially down to ~0 RPM. They loose horsepower in a 1:1 ratio below their base speed, they maintain HP above the base speed to around 2X their base speed. They maintain full torque below base speed (as noted above), they loose torque in a somewhat linear fashion above to 1.5-2X their base speed (non-linear above that). As you mentioned, most manufactures rate their inverter (4 pole) motors to a maximum RPM around 2X their base speed, smaller (2-5HP) vector motors nameplate often go to 4-5K. Yikes. Most of the smaller Vector rated AC motors are TENV, and do not have the usual overheating problems with lower or above base speed of cooling associated with the usual forced air cooling units.

One intriguing idea I was considering for my lathe was to use a 6 pole 1200 RPM vector motor instead of a 4 pole 1800 RPM. A 6 Pole 1200 RPM vector motor provides higher full load torque (~150% of 4 pole) and maintains constant horsepower to  about 2X base speed, so a usable working RPM range of 0-2400 RPM. Unfortunately the 6 pole motors are 1-2 frame sizes bigger then their 1800 RPM version, and did not fit in my application. 

Great reference I saw posted elsewhere on [FONT=&amp]Feed your VFD with the right power[/FONT]:
http://machinedesign.com/motorsdrives/feed-your-vfd-right-power

Mark


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