-
Welcome back Guest! Did you know you can mentor other members here at H-M? If not, please check out our Relaunch of Hobby Machinist Mentoring Program!
[Newbie] Why Use A Switch/button Panel?
- Thread starter AxeMaker
- Start date
- Joined
- Feb 8, 2014
- Messages
- 11,144
Great question!
Let's take each case one at a time.
On/Off
The VFD must be wired directly to the motor, you can not put a switch between the VFD and the motor. Disconnecting the VFD from the motor with the system powered up would most likely fry your VFD. You could put the switch ahead of the VFD, and use it to power up the system. But you still need a way to start and stop the spindle.
For/Rev
Since you cannot put the reversing switch between the motor and VFD, the only way to reverse the motor is using that function in the VFD. Putting a reversing switch ahead of the VFD will not reverse the motor. On some VFDs swapping two motor wires may not reverse the motor either.
Adjustable Speed
I found it very convenient to be able to adjust the speed from the VFD even though I had a vari-drive like you do. Especially for operations like power tapping and reaming, where I wanted a slower speed than would be afforded by the veri-drive in high speed. Saved shifting into low speed for a quick, low torque operation. Your new Sensorless Vector VFD will give you near 100% torque at near 0 rpm. You could just set the VFD at 60Hz and not use the variable frequency function.
I mounted my original VFD right on the head in place of the switch because I have spent about 45 years reaching up into that area to control a mill and just used the VFD keypad to run things.
When I converted the veri-drive to direct drive and installed a Sensorless Vector VFD, I built a control station and mounted it on the head
And mounted the VFD on the back of the electrical cabinet. (Don't panic, most of those cables have nothing to do with the VFD
)
Let's take each case one at a time.
On/Off
The VFD must be wired directly to the motor, you can not put a switch between the VFD and the motor. Disconnecting the VFD from the motor with the system powered up would most likely fry your VFD. You could put the switch ahead of the VFD, and use it to power up the system. But you still need a way to start and stop the spindle.
For/Rev
Since you cannot put the reversing switch between the motor and VFD, the only way to reverse the motor is using that function in the VFD. Putting a reversing switch ahead of the VFD will not reverse the motor. On some VFDs swapping two motor wires may not reverse the motor either.
Adjustable Speed
I found it very convenient to be able to adjust the speed from the VFD even though I had a vari-drive like you do. Especially for operations like power tapping and reaming, where I wanted a slower speed than would be afforded by the veri-drive in high speed. Saved shifting into low speed for a quick, low torque operation. Your new Sensorless Vector VFD will give you near 100% torque at near 0 rpm. You could just set the VFD at 60Hz and not use the variable frequency function.
I mounted my original VFD right on the head in place of the switch because I have spent about 45 years reaching up into that area to control a mill and just used the VFD keypad to run things.
When I converted the veri-drive to direct drive and installed a Sensorless Vector VFD, I built a control station and mounted it on the head
And mounted the VFD on the back of the electrical cabinet. (Don't panic, most of those cables have nothing to do with the VFD
)
Last edited:
- Joined
- Feb 17, 2013
- Messages
- 4,406
+1 to what Jim said.
I'll add (just for additional clarification) that with a VFD, you'll have wiring in two different voltage regimes. The main on/off switch, between the line and the VFD, runs 240 (or whatever) volts of AC at several amps. The start/stop, speed control, and direction controls, and maybe a remote speed readout, are low voltage DC. It's bad electrical practice (and in some instances a code violation) to mix high and low voltage circuits in the same enclosure (OK, can be done if suitable barriers are installed). Anyway, the easiest solution is frequently to add a separate box/panel/whatever just for the low voltage controls (switches, speed pot, etc.)
I'll add (just for additional clarification) that with a VFD, you'll have wiring in two different voltage regimes. The main on/off switch, between the line and the VFD, runs 240 (or whatever) volts of AC at several amps. The start/stop, speed control, and direction controls, and maybe a remote speed readout, are low voltage DC. It's bad electrical practice (and in some instances a code violation) to mix high and low voltage circuits in the same enclosure (OK, can be done if suitable barriers are installed). Anyway, the easiest solution is frequently to add a separate box/panel/whatever just for the low voltage controls (switches, speed pot, etc.)
- Joined
- Oct 13, 2014
- Messages
- 6,633
It boils down to this, if you want to use the machines original controls you must connect them directly to the VFD. Also if the machine has other 3 phase motors such as a coolant pump or axis drive you will need to get a separate VFD for those motors as well.
- Joined
- Jul 2, 2014
- Messages
- 7,594
The VFD already has high and low voltage connections in the same box, and high and low voltage wiring directly to it. Installing a switch or a potentiometer in a couple of those low voltage circuits does not seem too bad to me, and I am quite sure it is commonly done. Naturally, it is a very good idea to wire things so the two sides do not present a threat to each other or a shock or fire risk. Or am I missing something? I am for sure not an electrical genius...
- Joined
- Dec 8, 2013
- Messages
- 2,651
Use a contactor for the main on/off. You'll have at least one made redundant by the VFD.
- Joined
- Jun 12, 2014
- Messages
- 4,808
Jim's explanation is well outlined and explains that VFDs operate based on low voltage DC signals, usually 12 or 24VDC. Main power, usually 120 or 240VAC is kept separate from the control wiring. The motor is directly connected to the VFD, and should not be switched.
I would keep it simple. First off NEC considers low voltage anything under 600V. What they do not allow is the mixing of power and control wires in conduit and junction boxes with some exceptions. There are dividers that can be put into some junction boxes to provide some separation. It is also not a good idea to put control wires (at a lower voltage) close to power line wires because of noise/induction can be significant enough to generate falso control signals. It is definitely a bad idea to tie together power and control wires, or run them in the same conduit. Grounding and shielding can also be important aspects. But almost every control system has both power coming in at a higher voltage than the control wiring (at a lower voltage), and VFD installs uses a single enclosure. Some eclosures will bring the signaling wiring in at a different location (away) from the power lines. What I think the point being made is that the control box for the speed and VFD input controls should be kept at low voltage and not be mixed with higher voltage AC power. If you have a main power disconnect for the AC disconnect, it should not come into the control wiring box. So either put the main power disconnect before or in the VFD/electrical enclosure, or uses a contactor that is then connected to a "lower voltage power" that uses a remote switch at the front panel. This is often why machines will have transformer hum even when the main power switch is off, the transformer always remains on to power the power on contactor. Alternatively you could have a separate remote power switch in a separate enclosure.
This is recommended, but not cast in stone. Example is the PM1340GT lathe, it has low voltage 24VAC going to the front panel for the spindle controls and power light, but the coolant switch is a direct connect to 240VAC, all on the same panel. Not what I would recommend, but you often see this.
I think what Jim has on his mill is the most practical and easiest approach to a VFD install on the mill, it also keeps VFD away from the chips and possible contamination. He has a separate control system box at the front of his mill that controls the speed, direction, run and stop controls. The run and stop are usually momentary switches NO, using what is known as 3 wire controls. The direction switch only closes and input contact that tells the VFD to run in reverse. If the power fails, the machine restarts in the stop mode. I do not advise using 2 wire control with a sustained forward/reverse switch, if the power goes out and then comes back on the machine will restart, although there are ways to program the VFD not to do this.
I would keep it simple. First off NEC considers low voltage anything under 600V. What they do not allow is the mixing of power and control wires in conduit and junction boxes with some exceptions. There are dividers that can be put into some junction boxes to provide some separation. It is also not a good idea to put control wires (at a lower voltage) close to power line wires because of noise/induction can be significant enough to generate falso control signals. It is definitely a bad idea to tie together power and control wires, or run them in the same conduit. Grounding and shielding can also be important aspects. But almost every control system has both power coming in at a higher voltage than the control wiring (at a lower voltage), and VFD installs uses a single enclosure. Some eclosures will bring the signaling wiring in at a different location (away) from the power lines. What I think the point being made is that the control box for the speed and VFD input controls should be kept at low voltage and not be mixed with higher voltage AC power. If you have a main power disconnect for the AC disconnect, it should not come into the control wiring box. So either put the main power disconnect before or in the VFD/electrical enclosure, or uses a contactor that is then connected to a "lower voltage power" that uses a remote switch at the front panel. This is often why machines will have transformer hum even when the main power switch is off, the transformer always remains on to power the power on contactor. Alternatively you could have a separate remote power switch in a separate enclosure.
This is recommended, but not cast in stone. Example is the PM1340GT lathe, it has low voltage 24VAC going to the front panel for the spindle controls and power light, but the coolant switch is a direct connect to 240VAC, all on the same panel. Not what I would recommend, but you often see this.
I think what Jim has on his mill is the most practical and easiest approach to a VFD install on the mill, it also keeps VFD away from the chips and possible contamination. He has a separate control system box at the front of his mill that controls the speed, direction, run and stop controls. The run and stop are usually momentary switches NO, using what is known as 3 wire controls. The direction switch only closes and input contact that tells the VFD to run in reverse. If the power fails, the machine restarts in the stop mode. I do not advise using 2 wire control with a sustained forward/reverse switch, if the power goes out and then comes back on the machine will restart, although there are ways to program the VFD not to do this.
- Joined
- Aug 16, 2016
- Messages
- 193
Okay thanks for the feedback... I am a little confused though. If there is service power to the VFD and the VFD supplies power to the mill motor, why doesn't the mill act as though it is wired directly from the service power? Of course I am speaking from a newbie/logical point of view and not argueing. It seems to reason that if the mill motor is getting power, whether it is from the service, or the VFD, the switches on the mill would still work, including reversing the motor.
i.e... if I cut the plug off of the motor electrical cord, hook it up to the motor connections of the VFD, then run some 10/3 from the junction box to the connections on the VFD what changes? Why does the motor and its switches not work any longer?
i.e... if I cut the plug off of the motor electrical cord, hook it up to the motor connections of the VFD, then run some 10/3 from the junction box to the connections on the VFD what changes? Why does the motor and its switches not work any longer?
Last edited:
- Joined
- Feb 8, 2014
- Messages
- 11,144
The change is that you have added a very sophisticated electronic control system to the mix......... the VFD.
The VFD does a lot more than convert single phase to 3 phase, it is not a static or rotary phase converter which could be wired as you suggest.
You can not have a switch between the motor and the VFD, but the motor is going to work fine. Your existing switch reverses the motor by swapping two wires, the VFD does this electronically by inverting the phases. (OK, not exactly inverting, but close enough for this discussion)
You are either going to have to mount the VFD in a convenient location to be able to operate the motor from the keypad, or build a remote control box to run it and hang the VFD on the wall or the back of the mill.
EDIT:
:+1:
This is one possibility that I didn't remember to mention. The original switch could be connected to the VFD control terminals. All of the high voltage wiring would be removed from the switch, and it would be wired into the low voltage control portion of the VFD. This would eliminate the need for a new control station.
Last edited:
- Joined
- Feb 17, 2013
- Messages
- 4,406
I may be confused here. I don't recall your specifying whether your mill currently has a single phase motor or a 3 phase. And is the "service power" you refer to single phase or three phase?
A VFD is used either to convert single phase input power to three phase output power (and by varying the frequency of the output, control the speed of the motor) -or- convert 3 phase 60 Hz input power to a different frequency of 3 phase output power for speed control. In either case, it's only good for driving a three phase motor (OK, there may be ways to drive a single phase motor with a VFD, but IIRC, they tend to significantly reduce motor power).
The circuitry of the VFD is designed with the assumption that the (electronically driven) outputs will be continuously connected to the motor windings. Interrupting the connection, either to start/stop the motor, or to reverse it, will likely overload/fry one or more of the output devices, releasing the magic blue smoke from the VFD. I suppose a VFD could be designed to handle such surges/overloads. But it would add a lot of cost for little or no benefit.
A VFD is used either to convert single phase input power to three phase output power (and by varying the frequency of the output, control the speed of the motor) -or- convert 3 phase 60 Hz input power to a different frequency of 3 phase output power for speed control. In either case, it's only good for driving a three phase motor (OK, there may be ways to drive a single phase motor with a VFD, but IIRC, they tend to significantly reduce motor power).
The circuitry of the VFD is designed with the assumption that the (electronically driven) outputs will be continuously connected to the motor windings. Interrupting the connection, either to start/stop the motor, or to reverse it, will likely overload/fry one or more of the output devices, releasing the magic blue smoke from the VFD. I suppose a VFD could be designed to handle such surges/overloads. But it would add a lot of cost for little or no benefit.