Rotary Convertor or Variable Frequency Drive. Which is Best

[Tilly819]

Hi All.

I keep hearing this question come up on various facebook groups and web forums. So I have put together this little video in an effort to debunk some of the false information and give people the pros and conns of both devices. My hope is that this will allow home workshopers to make an informed decision so they don't end up wasting money on the wrong solution for their application. Hope you enjoy.

 

[m_c]

And your video also contains false information.

I've just replaced my old RPC with a VFD, and it quite happily powers multiple motors, which are switched after the VFD.
You've also failed to mention the issue with harmonics and possible motor damage while using a VFD, which is something to be considered on older motors which very likely don't have inverter rated insulation, and is why I have a Sine Wave filter in my system.

I am still running the idler motor with the VFD at the moment though, as I have one machine that needs the neutral, although I am looking at getting some kind of 3 phase transformer to eliminate the idler motor, but that's pretty low down the todo list.

 

[Tilly819]

And your video also contains false information.

I've just replaced my old RPC with a VFD, and it quite happily powers multiple motors, which are switched after the VFD.
You've also failed to mention the issue with harmonics and possible motor damage while using a VFD, which is something to be considered on older motors which very likely don't have inverter rated insulation, and is why I have a Sine Wave filter in my system.

I am still running the idler motor with the VFD at the moment though, as I have one machine that needs the neutral, although I am looking at getting some kind of 3 phase transformer to eliminate the idler motor, but that's pretty low down the todo list.

That's very interesting. I have a whole host of questions. Sorry.

Do you power multiple motors at the same time and switch them on and off while the VFD is running or does the VFD power down between changeovers?

How did you size the VFD. For the largest motor in the system I assume?

How have you setup you overload protection parameters? I assume the motors are different sizes? All the VFDs I have ever set up have a rated and maximum current parameter for the running and overload conditions. My concern being that is the two motors are significantly different sizes the VFD has to be sized for the larger motor and therefore will allow the smaller motor to draw too much current in an overload situation thus damaging the motor.

What make and model and size of VFD is it that you are using?

What size are the load motors and what size is the idler?

I'm not trying to be a d***, I'm just trying to better understand your system setup.

You absolutely right about the old motor insulation I totally forgot about that. I might have to try and add a text overlay or something to get that point in. Thanks for pointing that out.

 

[daleyd]

While it’s not recommended to switch downstream of a vfd when under power there is absolutely nothing wrong with having switchgear after it - you just have to either build in some interlocks that you can’t switch under load or, in the case of a small home workshop, more likely you just have to remember not to switch things while it’s under power.

It’s something that always seems to crop up but I’m not really sure what the issue is - I’ve never gone to unplug my lathe or drill while the motor has been running, I’m not sure why it would be any different?

In terms of the voltage spikes, it can be an issue when replacing older gear, but I’ve done a few big installs where there were old motors fitted and they suffered no ill effects - I’d be even less concerned if it was a 440v rated motor that was being powered from a 230v single phase fed inverter.

 

[armalites]

My VFD runs two motors. I have multiway rotary switch which switches between the motors and I have used a spare way to trigger a different program. In my situation I have 4 post a 2 post lift next to each other, I run the 4 post motor a bit slower as it's prone to tripping overload. So in the 2 post position the motor runs full speed but when I switch to the 4 post the motor runs slower.

Works perfectly

 

[m_c]

I just turn whatever motors I need on/off. Starting multiple motors goes far smoother than my old RPC with it's step-up transformer, and relays chattering away.
I opted for the biggest VFD I could theoretically run from a 32A breaker, which worked out as a 7.5KW VFD.
I just rely on the machines internal motor protection.
I bought a CNCWeiken WK310 boost VFD. The boost signifies it's a 230V in 380V out VFD, as it has built in voltage doubling.
The biggest 3 phase motor it powers is a 2HP, but it also powers my CNC lathe which is rated at 7KW, although I'd very much doubt I ever push it that hard. I still need to get the VFD installed into an enclosure, and add the old ammeter from the old RPC so I can see how much current it's actually pulling.
Existing idler IIRC is 3HP. The original RPC was only specced to power my old Harrison mill, but it ended up running my old Cyclone CNC lathe for years, but I had to upgrade for the new lathe.

All I've done is essentially what DrivesDirect have been doing for years with their digital phase converters with added sine wave filter.
I'd always thought they added some magic filtering on the output, but after querying it on another forum, somebody who designs far more advanced power circuitry summed it up with something like a VFD is essentially a PWM sine wave output. You don't have to do anything special with a 240V inverter to avoid blowing it up, so why do you think a VFD is any different?
I'm led to belief adding the voltage doubling circuitry is quite a simple thing to a 380/415v VFD, although in it's basic form probably not ideal from an electrical noise view.

The big thing is you need to oversize them quite a bit to handle start up currents.
The original 4KW boost invertor I used for testing (it's destined for a big mill conversion I'm in the process of doing) would fault due to overcurrent if I tried starting the Harrison mill with the Sine Wave filter in circuit. Removing the filter, and it started without issue.