View Full Version : VFD and Magnetic Starter

02-28-2005, 09:14 PM
Does a VFD Work with Magnetic Starters or does one have to by-pass them. If not suggestion on what size and where to by one for my 3hp 14X40 Lathe and my 2hp 9X42 Step pulley Bridgeport clone.Thanks in advance for your help.

Semper fi

02-28-2005, 09:39 PM
I assume you are wanting a VFD for its phase converter features and variable speed. You will not need an expensive magnetic starter if you use a VFD. I have used AC Tech SCM series VFD's on a couple of machines and their price is hard to beat. You can buy the same units from any Leeson Electric dealer (Leeson calls them the SM series)with a Leeson badge on them but the AC Tech units are generally a bit less expensive. See this link for more info about these units. I DON"T work for AC Tech or Leeson - I'm just a satisfied user.

02-28-2005, 09:41 PM
I forgot to add that the SCM series is available in single phase input/three phase output up through 3HP so you should be covered on both machines.

02-28-2005, 09:56 PM
FinnBear, What I need to know is if I put a Vfd on my Lathe will I have to by-pass the magnetic starter or just wire in as is.

02-28-2005, 10:24 PM
Yes, you need to bypass or remove the magnetic starter. A VFD is designed to connect DIRECTLY to the 3 phase motor, with no disconnects or anything else that could cut power to the motor. The reason is that the VFD can be damaged if the load is disconnected while the VFD is running. Also, the VFD has built-in overload protection.

I also have an AC Tech SCM series drive and like it.

If you are on a budget, you could buy a single 3 HP VFD and use it for both your machines, but only one at a time and you would have to figure out how you want to control the start/stop for the drives.

bernie l
02-28-2005, 11:26 PM
VFD's have advanced quite a bit in the last couple years. In the past it was strictly forbidden to have a contactor (or starter) between the VFD and the motor as the VFD could be severly damaged when the contacts would open. With a lot of the newer drives this isn't quite a big concern and some drive manufactures will tell that it won't hurt them. The is no reason to use a starter or contactor in the circuit, as mrennie stated the overload protection is provided by the drive. However there is one nagging exception. For installations that are required to meet specific safety recommendations a contactor may be required. The reason for this is that the VFD is a solid state device and as such could fail in the "ON" condition, with no way for the operator to shut it off. Many safety organizations require a "air gap" device with force guided contacts in motor circuits. This device can not be controlled by the VFD or logic controller (PLC) if present. In can only be a hard wired circuit that is closed when the motor is running. As a home user you are probably not required to meet those requirements, but if you were to use this machine in a place of business you may want to address these issues. One final note, as mentioned earlier this technology is constantly changing, I have heard that Siemens has a drive available that meets safety requirements without a external contactor, (it may have a contactor integral to the VFD).
I don't know what type of drive, (I use drive interchangebly with VFD) you are installing, and certainly don't know every drive that is available, but all the ones I've seen have set-up parameters. For most simple installations the ones you need to concern yourself with are the basic ones, operating voltage, motor FLA, probably acceleration time and deceleration time, max speed, min speed, etc. Different manufactures use slightly different terminology so read the manual carefully. I've seen drives with over 400 different parameters, fortunately we didn't need them all. Oddly enough one parameter that can cause you trouble is the deceleration time. The user tends to want to set this to a pretty low value so that the machine stops pretty quickly, however on many brands of drive this will result in a drive fault, it doesn't hurt the drive, but the drive will shut down and the machine will coast to a stop. If the drive has fault codes it may indicate something like "buss overvoltage". What happens is when you try to stop the motor quickly the inertia of the load wants to overrun the drive, this inertia will cause the motor to act as a generator, actually putting power back into the drive. The drive will put up with a certain amount of this but at some point it will trip out on buss overvolage. Bottom line if you get some sort of weird error that says something about buss voltage, lengthen you decel time and try it again. Also note on a large lathe the inertia could vary quite a bit depending on workpiece size. There's other ways to address some of these issues, but I've run on too long as it is.

Take care, have fun


03-07-2005, 12:56 PM
You won't need the mag starter anymore with the VFD. You can download the installation/usage manual from the AC Tech website for wiring info and informative reading on using these units.

<font face="Verdana, Arial" size="2">Originally posted by ConfederateSon:
FinnBear, What I need to know is if I put a Vfd on my Lathe will I have to by-pass the magnetic starter or just wire in as is.

03-07-2005, 01:04 PM
Yes, remove the contactor.

Use the Low voltage controls coming off the terminal strip on the VFD to reverse, forward run the inverter, or the keypad if it has one and you can mount it safely away from fluids or chips.

I have a cheap-bought at auction one with a twist lock plug, several items have twistlocks to match, on a drop cord coming out of the vfd I have a guitar Waa Waa pedal hooked to a speedpot. Running a power belt sander or the power hammer off the same inverter and speed control from the pedal.
Unplugging, or replugging with power on can blow the inverter. I always power it down. I can run a dozen different items with the same inverter by plugging them in.

I never trusted the vfd while putting my hands on the mill changing out tooling. I have a brake on my mill wired independent.


Bruce Griffing
03-07-2005, 01:41 PM
I would keep all of the parts you have, but wire them differently. The contactor can be used as an on/off switch for the VFD. VFD's have a very large turn on surge current - something the contactor can handle. You can either get other control switches for the contactor and use the old ones on the VFD to control the lathe/mill or do it the other way around. But you will need a couple more control switches for this scheme.