View Full Version : VFD confusion, helllp!

03-31-2004, 05:00 PM
I am interested in retrofitting my 1.5 HP 3phase mill motor with a VFD. Research has simply added to my confusion. I know there are some smart guys out there! Your inputs are appreciated.

I have the choice of 1ph or 3ph 230V input for the VFD. I know what a basic VFD does, but what the heck is a "flux vector" vfd, and/or a "sensorless" VFD?

This is a manual mill. Rather than buy something I won't be happy with, can anyone suggest what type of VFD to buy? I'm leaning towards a 3HP VFD for possible future motor upgrades, or to use the VFD on another machine. I'd like the VFD to be easily reversible, be easy to set up and use. It MUST have a rheostat-type dial for speed; I hate using membrane keys to change the speed. Beyond that, I don't think I need a bunch of bells and whistles.

Thanks everyone!:p

03-31-2004, 05:36 PM
First off, go with the 3ph in if you have it, then you won't have to derate the VFD, hp wise. The most important thing on a VFD is not to exceed the rated current. If you have a 1.5hp motor the 3hp should be fine. Flux vector is a way of controlling the speed in lieu of a feedback device which is a sensorless VFD, usually if you have a digital tach or encoder you can obtain the type of VFD that operates in the torque mode and accurate speed control is possible, If you operate a sensorless vfd, try not to drop down in rpm too low with high loads as you will probabally find it will bog down. Most VFD have the ability to just add a pot for speed and provide a voltage out to feed the pot. Make sure it has an external mode as well as keypad setting.
You can get sophisticated type that have auto tuning and will run your motor through a series of tests and automatically plug in the parameters.(Mitsubishi for one).
If you have a 4 pole motor (1700+ rpm) then you can usually set the max frequency to 120hz and run up to almost 3600 rpm, This is as long as the motor is well balanced, the limiting factor is if you start getting vibration at high rpm.

03-31-2004, 06:03 PM

Having had both sensorless (no encoder) and vector drives (with encoder feedback) on a cnc knee mill, I can say I really like the vector drive (might be a Baldor trademark name). It has excellent and accurate speed control and lots of torque for tapping, without having to shift gears. But, of course for cnc, a person really needs to know that the machine can maintain an accurate spindle speed in order to calculate the correct feedrate for tapping. On a plain manual mill, this would not be a requirement, because you would be feeding the quill down by hand, I suppose.

03-31-2004, 06:35 PM
Thanks guys, this helps quite a bit. Since this is a manual machine, with no forseeable CNC conversion, I think I am going to go with a sensorless drive, as that'll save quite a bit of $$. There's an outfit here in Dallas that deals in Hitachi VFD's, and the Hitachi unit has some nice features.

03-31-2004, 07:06 PM
This is a good place to buy a VFD. They are very helpful even after the sale. Helped me wire mine when I couldn't figure it out. Give them a call. Great prices too.


04-01-2004, 10:50 AM
InventIt, thanks for the link. What VFD did you end up buying, if I may ask? Anyone who can recommend a specific brand and model, having had a happy experience with it, I'd appreciate it.

Can't wait to "dial-a-speed" :D with my step pulley mill! Should I set the belts up for a midrange setting? I guess you can always still swap belts to optimize torque or speed.

04-02-2004, 09:11 PM
Update: I picked up a Hitachi L100 3HP VFD today from Driveswarehouse.com, along with some conduit and assorted goodies from home despot. I'll take some pics of the installation. Should be interesting! I would have bought the Hitachi SJ100, which is a more capable unit, but they were out of stock. I already own a smaller L100 and was impressed by it.

Stay tuned...

06-14-2004, 11:20 AM
What is the major difference betweent he L100 and SJ100?
I have a 1 hp J head and need to purchase a VFD. I have 230 single in so I assume I need
a 2 hp converter. I noticed that they mention dynamic braking with and without an external resistor.
What is this?


06-14-2004, 02:05 PM
Dynamic braking means that the invertor will produce a controlled stop of the motor, rather than just allowing it to coast. To do this, it needs a place to waste the energy produced by the motor acting as a generator when stopping. There is usually some built in capacity for dissipating some heat energy in the invertor heat sinks. The external resistor simply boosts the energy dump capacity.

06-14-2004, 03:48 PM
Originally posted by gtslabs
I noticed that they mention dynamic braking with and without an external resistor.
What is this?

Steve, you usually require external braking, over and above the internal if you try and decel very quickly, The voltage produced cannot be dissipated fast enough at the rapid decel rates and causes an over-voltage alarm, and the motor control switches off and the motor will coast to an uncontrolled stop.

06-14-2004, 07:05 PM
One thing worth mentioning is that when braking the motor it will act as a generator. This produces
a voltage and current back into the reservoir capacitor in the VFD. And the voltage over it (DC-bus) will increase. Then happens what Al describe. If you have an external braking resistor it will be connected
to the DC-bus when braking and thus converting some of the voltage*current to heat thus pulling the DC-bus voltage down.

For many applications you don't need to brake very hard because there are frictions in the system doing most of the braking. Try first without, then use the resistor if necessary. If it's a spindle motor for a machine with a screw-on chuck, beware. Then braking may be highly undesirable and you should choose the coast to stop option.