VFD/ table saw question

[flashlight]

I just completed installing a VFD to a table saw with a 3 ph. 3HP motor.

After a bit of tinkering with parameters, saw appears to run fine. But when I check amps it shows 3.6, whereas the motor is rated 7.4
It's an older saw and the nameplate doesn't have FLA, or LRA, just 7.4 Amp

I can't check it under load because owner is retooling cast iron top. I told owner I believe saw will run at or near
nameplate amps when it is being used to actually saw lumber, and not just spin the blade.

Would appreciate your educated opinions on this, thanks.

 

[zbang]

If the motor is only spinning the blade, then it's doing almost no work and will not use much power. OTOH, 3.6 amps (what voltage?) seems like a lot just to turn the think- like most table saws, it probably needs a good cleaning and oiling.

 

[petersonra]

My educated opinion suggests that if you run the saw at 60 hz at full load you will probably get the same amount of current used as when you ran the saw without a VFD at full load.

The VFD is only there so you can run the saw at different speeds. It doesn't change any amount of power that is required to do whatever work the motor has to do.

 

[petersonra]

If the motor is only spinning the blade, then it's doing almost no work and will not use much power. OTOH, 3.6 amps (what voltage?) seems like a lot just to turn the think- like most table saws, it probably needs a good cleaning and oiling.

might have a power factor of like 0.4 at such a low load

 

[zbang]

Could be for the PF.

Why a VFD here at all? Maybe it's being used as a phase converter.

 

[Besoeker3]

If the motor is only spinning the blade, then it's doing almost no work and will not use much power. OTOH, 3.6 amps (what voltage?) seems like a lot just to turn the think- like most table saws, it probably needs a good cleaning and oiling.

It probably doesn't need much power just to spin the blade. The 3.6A is likely low power factor not real power.

 

[LarryFine]

But when I check amps it shows 3.6, whereas the motor is rated 7.4

Are you measuring the VFD input or output?

 

[retirede]

I setup my table saw exactly like this. Replace the 1 HP, single phase motor with a 3HP, 3 phase with a VFD.
It made the thing a beast. Before, I could barely rip 2” oak without stalling, now it cuts it like butter.
I make sure to run the speed below the max rated speed of the blade. I think right now I have 7500 rpm blade and run the drive at 100 HZ (6000 rpm).
I’ve never measured the current draw, but if I remember when I get back home next week, I’ll let you know what it is.

 

[Besoeker3]

Are you measuring the VFD input or output?

It would still be low motor PF.

 

[flashlight]

It is 230V, and yes the input power is single phase, the VFD is only there as a phase converter.

I am measuring the output of the VFD.

 

[jim dungar]

....I am measuring the output of the VFD.

It would be better to use the VFD display to see the output data.
At a minimum you would need a meter that is 'true RMS' because a VFD output is not a sine wave.

 

[flashlight]

At a minimum you would need a meter that is 'true RMS' because a VFD output is not a sine wave.

Right, I wonder how close my measurement was. VFD is second-hand and several years old, pretty sure I can't get it to display that but I will check book.

 

[__dan]

High force vfd braking of the saw blade would be good to have for safety. They can be set up with a foot switch also, so the blade is running for the minimum time, and brakes hard on release of the foot switch.

 

[GeorgeB]

I just completed installing a VFD to a table saw with a 3 ph. 3HP motor.

After a bit of tinkering with parameters, saw appears to run fine. But when I check amps it shows 3.6, whereas the motor is rated 7.4
It's an older saw and the nameplate doesn't have FLA, or LRA, just 7.4 Amp

I can't check it under load because owner is retooling cast iron top. I told owner I believe saw will run at or near
nameplate amps when it is being used to actually saw lumber, and not just spin the blade.

Would appreciate your educated opinions on this, thanks.

It is not at all unusual for a 3 phase motor to draw 40% -60% of rated amps under no load ... note that it is not converting much electricity to heat or performing work.

If you are metering (how?) the input to the drive, I'd be a little surprised, but that's more a metering question; who knows what your meter shows with the capacitor charging peaks at 180 or 360 Hz. I'd expect, assuming no bad bearings, a no load motor POWER consumption of maybe 25 to 50 watts.

 

[flashlight]

I am using a fluke I forget which model, volt-amp-ohm meter. I have been assured by others on this site that this is not a true meter but more like a simple light-bulb tester ; ) Curiously, when I check the single phase 230 V lines from panel, they show a lower amperage (2.7) than the "3-phase" lines leaving VFD.

 

[GoldDigger]

I am using a fluke I forget which model, volt-amp-ohm meter. I have been assured by others on this site that this is not a true meter but more like a simple light-bulb tester ; ) Curiously, when I check the single phase 230 V lines from panel, they show a lower amperage (2.7) than the "3-phase" lines leaving VFD.

This is expected since the VFD supplies the reactive current to the motor internally from its DC bus (averaging out to zero) instead of passing it through to the AC input.

 

[flashlight]

This is expected since the VFD supplies the reactive current to the motor internally from its DC bus (averaging out to zero) instead of passing it through to the AC input.

You learn something new every day, that's why I love this site. Thanks everyone for your insights.

 

[garbo]

I just completed installing a VFD to a table saw with a 3 ph. 3HP motor.

After a bit of tinkering with parameters, saw appears to run fine. But when I check amps it shows 3.6, whereas the motor is rated 7.4
It's an older saw and the nameplate doesn't have FLA, or LRA, just 7.4 Amp

I can't check it under load because owner is retooling cast iron top. I told owner I believe saw will run at or near
nameplate amps when it is being used to actually saw lumber, and not just spin the blade.

Would appreciate your educated opinions on this, thanks.

I worked on hundreds of VFD'S and no matter how fancy of a clamp on ampmeter always perferred to measure the input current to drive. The display touch screens are usually accurate. I had to measure DC Buss voltage on drives that I PM'D. On older drives they had a point where you could measure the DC Buss voltage. On the numerous ones that I measured this voltage it was always within 1 volt DC of display. Unloaded motors will often only run @ 50% of Full load amps. Often had 50 to 200 HP drives running @ 100% ( full speed ) but ampere on display often 10% less then nameplate amps.

 

[kwired]

This is expected since the VFD supplies the reactive current to the motor internally from its DC bus (averaging out to zero) instead of passing it through to the AC input.

You learn something new every day, that's why I love this site. Thanks everyone for your insights.

IOW the power factor on the supply side of the drive is 1.0 or very close. Any power factor on that side is probably from harmonic distortion more so than typical displacement power factor that is seen with AC motor loads. The reactive power in this situation is only flowing between the motor and the drive