Thread: Any Advice on Motor Wiring?

Originally Posted by Gestalt

Hi Guys, I'm just in the finishing stages of rebuilding an old Nolex radial arm saw. I'm thinking it would be nice to fit a VFD to give me some more blade speed. I would imagine that this saw was designed and built for carbon steel blades as the motor speed is only 2200 RPM. The blade size is 350mm. It seems that a tipped blade of this size can run about 4300RPM.

That will almost certainly be the maximum recommended speed for that blade. It may not be it's optimum cutting speed.

[QUOTE}Would there be any tables available on the coil resistance for various motor configurations?[/QUOTE]
[EDIT] while the resistances between any two connections on the same motor (wired as either ∆ or Y) are not the same, unfortunately no amount of external probing of an unknown motor is usually able to determine whether it's ∆ or Y wound..

Unless you do a lot of cutting, using a VFD to increase (especially cross) cutting speed on a saw seems like it will make very little difference. About the only reason I can If anything I would use a VFD would be to slow down a saw to reduce the possibility of wood burn.

Hello Bob, Thanks for the reply. It was just an assumption on my part that a higher speed for carbide would produce a cleaner cut.
I would have thought that a star configuration would have twice the resistance of delta. Can you elaborate on this Bob?
Thanks again.

Originally Posted by Gestalt

Hello Bob, Thanks for the reply. It was just an assumption on my part that a higher speed for carbide would produce a cleaner cut.

As long as the blade is sharp, blade speed shouldn't make much difference. Reducing the feed rate will probably do more for a cleaner cut.

Originally Posted by Gestalt

Hello Bob, Thanks for the reply. It was just an assumption on my part that a higher speed for carbide would produce a cleaner cut. .

A reasonable assumption but tooth profile is as important as speed, perhaps more so. if you want a baby smooth cut I would invest in a high tooth count "triple chip tooth" blade such as is used for melamine. They cut slowly but produce a very smooth cut. It is my "leave it on the my TS" blade and I only swap it out when ripping

I would have thought that a star configuration would have twice the resistance of delta. Can you elaborate on this Bob?
Thanks again.

It depends what you mean by the resistance of either configuration - remember you can only measure the resistance between two connection points and these motors have 3.
The resistance between any two ∆ wired motor connections (with the 3rd left floating) is 2/3rd's the R of one coil while for Y it is twice the R of one coil.
If any two of the connections are shorted, the resistance between the non shorted and shorted connections is 1/2 R of one coil for ∆, and 4/3 R for Y
The ratio of the "non-shorted Resistance" to the "shorted Resistance" is 4/3 in both cases , so simple R measurements at the connection box cannot discriminate between ∆ and Y.
One problem is that coil R alone is not the final determinant of power drawn.

How sure are you about the motor being 3 phase and 2200 RPM?

It is a particularly odd configuration, as 3 phase motors in tools are generally induction (brushless) motors, and semi mains sychronous, i.e their unloaded or free run speed is proportional to the mains frequency, but they slip or slow about 4-5% to run slower at full load. Typical speeds for 50Hz supply are 3000RPM free (around 2850 at rated load), 1500 free (1440 rated load), and 1000 free (960 rated load). Depending on the designers slip allowance, there is a little wiggle room for loaded speed, but only about 30RPM at the top speed and 15RPM at the lowest speed. The design of the motor (number of pairs of magnetic poles 1pr, 2pr, or 3pr) determines which speed range the motor operates in.

As you can see 2200 RPM is a speed that is midway between the fast and medium speed motors and could not be obtained directly from an induction motor at 50Hz. An allternative would be to use a universal (brushed) motor similar to an angle grinder or circ saw, but these are as rare as in 3 phase applications because they would need 6 brushes instead of 2. Univeral motors have a speed proportional to voltage and design parameters selected to target a given loaded speed and power output, so using a VFD would not have the effect of controlling the motor speed as you wish. Another alternative would be that the motor is geared to the saw arbour with either a speed reduction from high speed, or step up from a low or medium speed induction motor. In this case, a VFD would adjust the speed, but you may achieve a similar end by changing pulleys if the drive system is belt driven.

Do you have 3 phase power available to you in your shop? If you do, you could use a 415V VFD (if appropriate to the motor type) connected between the mains supply and motor to achieve a speed increase without concerning yourself about the winding configuration of the motor. Winding configuration becomes an important consideration if you need to operate from a 240V VFD due to the reduced supply voltage. Alternatively if you only have single phase, there is an expensive 240 to 415V VFD available from the UK tht uses voltage stepup technology to provide 415V drive for motors that are dificult to convert for 240V operation. From memory these are based on a Teco VFD with a modified power input stage but limited to about 2HP or 1.5KW.

With regard to blade speed, I would not advise operation beyond around 70% of the blades branded max speed to ensure a safety margin. The designer/manufacturer will have also designed the unit with some safety factor beyond the rated max, but most saws derate blade speed to around 70% of the rated max for the current generation of blades.

Hope this helps.

Hi guys, Thanks for all that great info. Sorry I haven't replied earlier, but this is the first time on the PC this year. I've been pretty much bed ridden since new year's eve. Everything has come to a complete stand still. Hopefully I'll begin to feel human again in a few days. I'll make some detailed comments then.
Gotta get back to bed.

Originally Posted by elanjacobs

As long as the blade is sharp, blade speed shouldn't make much difference. Reducing the feed rate will probably do more for a cleaner cut.

Thanks for that info elanjacobs. I was totally unaware of the insignificance of the actual blade speed. As mentioned earlier, I would have thought that faster was always better!!

Originally Posted by BobL

A reasonable assumption but tooth profile is as important as speed, perhaps more so. if you want a baby smooth cut I would invest in a high tooth count "triple chip tooth" blade such as is used for melamine. They cut slowly but produce a very smooth cut. It is my "leave it on the my TS" blade and I only swap it out when ripping



It depends what you mean by the resistance of either configuration - remember you can only measure the resistance between two connection points and these motors have 3.
The resistance between any two ∆ wired motor connections (with the 3rd left floating) is 2/3rd's the R of one coil while for Y it is twice the R of one coil.
If any two of the connections are shorted, the resistance between the non shorted and shorted connections is 1/2 R of one coil for ∆, and 4/3 R for Y
The ratio of the "non-shorted Resistance" to the "shorted Resistance" is 4/3 in both cases , so simple R measurements at the connection box cannot discriminate between ∆ and Y.
One problem is that coil R alone is not the final determinant of power drawn.

Hello Bob, Thanks for the tip on the triple chip tooth blade. i just came across this reference for blade geometry. http://justsawblades.com/ten/ToothGeometry.pdf I was unaware there was such a diverse range of configurations. Thanks also for the Ohms Law update.

Originally Posted by malb

How sure are you about the motor being 3 phase and 2200 RPM?

It is a particularly odd configuration, as 3 phase motors in tools are generally induction (brushless) motors, and semi mains sychronous, i.e their unloaded or free run speed is proportional to the mains frequency, but they slip or slow about 4-5% to run slower at full load. Typical speeds for 50Hz supply are 3000RPM free (around 2850 at rated load), 1500 free (1440 rated load), and 1000 free (960 rated load). Depending on the designers slip allowance, there is a little wiggle room for loaded speed, but only about 30RPM at the top speed and 15RPM at the lowest speed. The design of the motor (number of pairs of magnetic poles 1pr, 2pr, or 3pr) determines which speed range the motor operates in.

As you can see 2200 RPM is a speed that is midway between the fast and medium speed motors and could not be obtained directly from an induction motor at 50Hz. An allternative would be to use a universal (brushed) motor similar to an angle grinder or circ saw, but these are as rare as in 3 phase applications because they would need 6 brushes instead of 2. Univeral motors have a speed proportional to voltage and design parameters selected to target a given loaded speed and power output, so using a VFD would not have the effect of controlling the motor speed as you wish. Another alternative would be that the motor is geared to the saw arbour with either a speed reduction from high speed, or step up from a low or medium speed induction motor. In this case, a VFD would adjust the speed, but you may achieve a similar end by changing pulleys if the drive system is belt driven.

Do you have 3 phase power available to you in your shop? If you do, you could use a 415V VFD (if appropriate to the motor type) connected between the mains supply and motor to achieve a speed increase without concerning yourself about the winding configuration of the motor. Winding configuration becomes an important consideration if you need to operate from a 240V VFD due to the reduced supply voltage. Alternatively if you only have single phase, there is an expensive 240 to 415V VFD available from the UK tht uses voltage stepup technology to provide 415V drive for motors that are dificult to convert for 240V operation. From memory these are based on a Teco VFD with a modified power input stage but limited to about 2HP or 1.5KW.

With regard to blade speed, I would not advise operation beyond around 70% of the blades branded max speed to ensure a safety margin. The designer/manufacturer will have also designed the unit with some safety factor beyond the rated max, but most saws derate blade speed to around 70% of the rated max for the current generation of blades.

Hope this helps.

Hello malb, I just rechecked the motor ID plate and found that the RPM is actually 2900 and not 2200 as first mentioned. I don't know where I got that figure from. Seems like that miss information had you scratching your head somewhat. Sorry about that. And yes it is 3 phase.
It's interesting you mention the 415V VFD. It seems the ones that I've looked at on Ebay require the motor to be delta configured. This is what prompted my initial posting.
I've always wondered about blade speed and safety margins. I built a twin bladed rip saw several years ago to process old hardwood fence palings and geared the speed to the maximum recommended by the manufacturer. I was always concerned about tips flying off, but I've processed hundreds of fence palings since then and the blades are still intact.

Hi, you didn't leave me scratching as such but I figured something was not quite right with the info given. 2900 makes more sense, its a low slip (abt 3%) 2 pole motor.

The pommy mob who do the step up (240 - 415V) converters is Drives Direct. I just looked at their direct website and they have the product categories showing but no content in the categories. However there is plenty in their ebay store up to at least 10HP, but they are expensive compared to 240- 240 or 415-415 units. It would be more practical to convert the motor to delta and use a 240-240 if that is feasable.

Re blade speed, most 10 in tables saws target around 4200-4500 RPM, but the blades are rated at 6500-7000 RPM max. Circ saws tend to have a similar safety margin. I have worked with 14 in panel saws, but never had to purchase blades so I am not sure what speed range the blades operate at or are rated at but I assume that a similar margin would apply.

Originally Posted by malb

Hi, you didn't leave me scratching as such but I figured something was not quite right with the info given. 2900 makes more sense, its a low slip (abt 3%) 2 pole motor.

The pommy mob who do the step up (240 - 415V) converters is Drives Direct. I just looked at their direct website and they have the product categories showing but no content in the categories. However there is plenty in their ebay store up to at least 10HP, but they are expensive compared to 240- 240 or 415-415 units. It would be more practical to convert the motor to delta and use a 240-240 if that is feasable.

Re blade speed, most 10 in tables saws target around 4200-4500 RPM, but the blades are rated at 6500-7000 RPM max. Circ saws tend to have a similar safety margin. I have worked with 14 in panel saws, but never had to purchase blades so I am not sure what speed range the blades operate at or are rated at but I assume that a similar margin would apply.

Hello malb, I think given all the feedback regarding blade speeds it's probably not worth my while going down the VFD route. Thanks for the help.

I was given a Nolex ... currently in three pieces ... does this look like yours?

There are three input wires ... plus neutral.

The 2300rpm applies to a 40hz power source, so 2875rpm from 50hz ... which is normal.

(What's going on with uploading pictures recently?)

20140111_141658 (Medium).jpg 20140111_141958 (Medium).jpg 20140111_142118 (Medium).jpg

Originally Posted by Gestalt

Thanks for that info elanjacobs. I was totally unaware of the insignificance of the actual blade speed. As mentioned earlier, I would have thought that faster was always better!!

Wouldn't go so fas as to call it insignificant, having the proper blade speed for the material you're cutting can definitely affect the result, but certainly not the most important factor. Too slow and it simply won't cut, too fast and it can create excessive heat leading to burning the timber and premature wear on the blade.

Originally Posted by pmcgee

I was given a Nolex ... currently in three pieces ... does this look like yours?

There are three input wires ... plus neutral.

The 2300rpm applies to a 40hz power source, so 2875rpm from 50hz ... which is normal.

(What's going on with uploading pictures recently?)

20140111_141658 (Medium).jpg 20140111_141958 (Medium).jpg 20140111_142118 (Medium).jpg

Hello pmcgee, yes that's the one. Identical except for the switch. Mine still has the original bakelite one. Also no neutral. Just the 3 input wires.