Erl-1340

The thing i see about vfds is that they are portrayed as a silver bullet by a large segment of the general population.

Take this graph for a 60 htz (base motor speed) set up. If you slow the motor way down, you loose all your HP, and if over speed it up you lose all your tongue. This ignores the issue of if the motor can thermally handle really low hertz (low cooling), or physical handle high hertz (high rotational intertia).
Figure1-PE0513.jpg
 
That's a bit misleading, because HP is just torque × speed. Torque is limited in the low end by the current carrying capacity of the windings (torque is proportional to current), and in the high end you wind up being power limited.

That curve is actually a bit optimistic, since the motor loses efficiency the farther away you get from the design frequency.

Sent from my Nexus 6 using Tapatalk
 
Probably get bitten for this, but since many hobbyists I've met seldom, if ever push their machines hard enough to require full hp or torque, I suspect that most don't care if their motor is running at 100% optimization or not. (I'm not counting the folks who buy a 3/4 hp bench mill and try to push a 1" end mill...)

Or is this just a theoretical conversation? :D

Given that the OP is buying a 3000 lb. lathe I suspect that he may want max hp and torque to make those tank driveshafts or whatever he's making. RPC? But don't you have to buy 2x the motor's rated hp as a minimum? Apologies, as I don't know much about RPCs.

EDIT: Just read where the OP's lathe is a 16 speed. Wouldn't it be better to go VFD in this case as you then have the option to vary the motor frequency to fine tune your spindle rpm?
 
Last edited:
Exactly so.

Sent from my Nexus 6 using Tapatalk
 
The thing i see about vfds is that they are portrayed as a silver bullet by a large segment of the general population... If you slow the motor way down, you loose all your HP...
Too true.

A lot of people want to direct-drive their spindle off a motor with a VFD or just use a simple belt drive with no speed selection choices.

That is all good until you try to drive a 3/4" drill through steel at 800 rpm. Good possibility that you will stall with the amount of torque available at the low speed on a VFD.
 
Too true.

A lot of people want to direct-drive their spindle off a motor with a VFD or just use a simple belt drive with no speed selection choices.

That is all good until you try to drive a 3/4" drill through steel at 800 rpm. Good possibility that you will stall with the amount of torque available at the low speed on a VFD.

Been there, done that. Got the t-shirt.
 
As pstemari alluded to, its actually worse in a real world situation, some times a lot worse.
And sometimes not. Inverter rated 4P motors are designed to provide flat torque down to a few Hz and usually maintain full Hp up to 2X their base speed. Also manufactures often increase the Hp of their VFD models to compensate for decrease torque or Hp depending on if you are above or below the base speed. Sensorless vector VFDs can momentarily compensate for some of the apparent losses and this can be set by the overload restriction level which is usually around 150%. If VFDs were so bad, then you wouldn't see this level of machines using them. In the setting of this discussion and use, it is very unlikely the a VFD machine would even flinch at the loads/workload that they would see in a hobbyist situation.

In any case, whether you purchase the ERL, TRL or RML, they are all very fine machines and will probably last a long, long time. A VFD is just icing on the cake.
Motor overspeed guideline.jpg
 
Which may be why I can drill a 3/4" hole in steel at about 650 rpm. :)
 
And sometimes not. Inverter rated 4P motors are designed to provide flat torque down to a few Hz and usually maintain full Hp up to 2X their base speed. ...

Which is what that ideal graph shows—constant torque at low Hz (hence declining HP), constant HP (and hence declining torque) at high Hz.

As I understand it, getting flat torque on a real-world induction motor at low frequency is tricky because you aren't inducing current in the armature to the extent you would at higher frequency.
 
Back
Top