Thread: Blade Speed

Originally Posted by Mini

Blade Speed is dictated by the frequency of the mains electricity supply, some might not realise it but that is what happens and keep in mind this happens with all our electric tools.

Not quite true. Induction motors are frequency dependant, but universal (brushed motors) are not. So the multitude of portable tools like routers, hand held drills, circ saws, planers, vacuum cleaners etc, are not frequency dependant. Even some models of semi stationary tools like table saws and the very noisy domestic thicknessers (as opposed to the quieter industrial units) use universal motors.

For the gear that does use induction (frequency dependant) motors, much of it is belt or gear driven, and the drive ratios can be calculated to give appropriate speeds for the intended use.

The only areas where it becomes an issue are induction motor powered direct drive units, particularly dusties, pedistal grinders and maybe some radial arm saws.

Originally Posted by malb

Not quite true. Induction motors are frequency dependant, but universal (brushed motors) are not. So the multitude of portable tools like routers, hand held drills, circ saws, planers, vacuum cleaners etc, are not frequency dependant. Even some models of semi stationary tools like table saws and the very noisy domestic thicknessers (as opposed to the quieter industrial units) use universal motors.

For the gear that does use induction (frequency dependant) motors, much of it is belt or gear driven, and the drive ratios can be calculated to give appropriate speeds for the intended use.

The only areas where it becomes an issue are induction motor powered direct drive units, particularly dusties, pedistal grinders and maybe some radial arm saws.

Fair enough as I have been wondering about this for some time. Does that mean my TS is not speed dependent on the mains frequency? It is a three phase TS.

Do you think they change the gearing on belt driven items? I somehow am a bit sceptical about that bit.

If you have a three phase table saw, the chances are about 99% that it will have an induction motor, and so be frequency dependant, but that is a big step away from the original statement of "all of our electrical tools".

Yes I would expect the manufacturers to reconfigure the pulley sizes to optimise the machines performance for the target market. They have to change the motor anyway due to the voltage differences between the 50Hz and 60 Hz mains countries, so why wouldn't they bother.

It's not like we are some miniscule unique and remote country operating at 85Hz. Our mains supply is substantially the same as much of Europe and other parts of the world, so if they want to cater for the global market they supply a 50Hz variant to 50Hz countries and a 60Hz variant to 60Hz countries. Pretty much the same as the global car makers, they do a LHD variant for LHD countries and a RHD variant for RHD countries, and then tweek to suit specific market conditions and regs.

Mini

If changing pulleys to achieve optimum tip speed (because optimum tip speed is what you have correctly assesed) was an issue there would be little variation in speed between different sizes of saw blade too.

Malb is correct. They adjust the saw speed by changing the pulley diameter. There would be no mileage in an under performing saw and the professional woodworkers spending much more than us would have soon jumped on it.

I think you can relax on this issue.

Just on the subject you touched on, I don't know how well motors designed to run at 50hz respond to being run significantly higher via a VFD.

I can only comment on the power station generators themselves. As the US 60hz units spin at 3600rpm, compared to our 50hz units at 3000rpm, their machines are smaller diameter but longer for an equivalent sized unit.

Regards
Paul

Originally Posted by Bushmiller

Just on the subject you touched on, I don't know how well motors designed to run at 50hz respond to being run significantly higher via a VFD.

I'm running the 3 Phase 1/2HP, ~1485 rpm @ 50 Hz motor on my lathe up to 80 Hz (~2400 rpm) with no problems. It is very unlikely that 3 phase motor manufacturers would use different bearings or structures for 1450 V 3000 rpm motors. In the same way I doubt motors for 60 Hz markets would have different bearings and structures than motors for 50Hz markets.

Originally Posted by BobL

I'm running the 3 Phase 1/2HP, ~1485 rpm @ 50 Hz motor on my lathe up to 80 Hz (~2400 rpm) with no problems. It is very unlikely that 3 phase motor manufacturers would use different bearings or structures for 1450 V 3000 rpm motors. In the same way I doubt motors for 60 Hz markets would have different bearings and structures than motors for 50Hz markets.

Bob

I think that is probably right regarding bearings. The issue I was pointing to was the extra centrifugal force and it may well be that in small motors (say under 25Kw or 33HP) my concern is unwarranted.

In your case,your motor is a 4-pole and it probably does use the same bearings as a similarly rated 2-pole motor spining at 2850rpm. Even at 80Hz your motor is well under that speed. A 2-pole may well withstand 60Hz, but is there a limit? I don't know what a VFD will wind up to.

It would be interesting to know if there would be any long term detrimental effect to a 2-pole motor run at 80Hz (4560rpm). Would it hold together?

Regards
Paul

Originally Posted by Bushmiller

Bob

I think that is probably right regarding bearings. The issue I was pointing to was the extra centrifugal force and it may well be that in small motors (say under 25Kw or 33HP) my concern is unwarranted.

Yep I agree In some cases where gearing/belts/pulleys are involved there could be problems. Table saw blades are rated to about 5000 rpm but I see no point in doing this. Most of my use of VSDs are/will to reduce the need to change belt positions on machines, like drill presses and lathes, that can use variable speeds.

In your case,your motor is a 4-pole and it probably does use the same bearings as a similarly rated 2-pole motor spining at 2850rpm. Even at 80Hz your motor is well under that speed. A 2-pole may well withstand 60Hz, but is there a limit? I don't know what a VFD will wind up to.

I have 2 VFDs and one maxes out at 650 Hz! while the other at 500 Hz. Both have the ability to set a software max value which I have set at 80Hz on one and the other is not yet in use.

It would be interesting to know if there would be any long term detrimental effect to a 2-pole motor run at 80Hz (4560rpm). Would it hold together?

I think it would depend on the motor and expected work loads. I bought a ABB 1440 rpm 3HP 3 phase motor for $30 - this thing has a solid cast aluminium housing and is built like a reinforced concrete outhouse. I reckon this will run at 3640 rpm (125 Hz) without batting an eyelash. I also picked up a 3 phase 1440 rpm 1HP motor that has a housing made out of pressed steel - that one I'm going to put on my small Woodfast WW lathe as is (ie 1440 rpm max at 50Hz)) and just use the fact that I will be able to change the speed. I have a 3rd 3 Phase motor that is from an Onga pump. This is a 1HP unit with a cast iron body - I'm not sure what I will do with this one.

I kick myself for not buying my 3HP DC with a 3 Phase motor on it - that way I could have run it up to 3640 rpm - but I should be able to pick up