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Converting surface grinder electronics to VFD

R

rbeard

Plastic
Joined
Apr 9, 2021
Hi,

I recently picked up a Boyar Schultz surface grinder which was wired into the shop's 3 phase supply. The machine has a 1 HP motor and hydraulics as well as a coolant pump. Looking at the existing electrical panel, it looks like there's one 3 phase source and a 110 source picked off of a step down transformer. But, I see what looks like two 3 phase contactors so I'm not sure if I would need more than one VFD. Would the hydraulic motion normally be turned on or off separately from the grinding wheel?

I'm also assuming that almost all of the existing components in this panel will go. I'll use the existing switches wired to the VFD for start/stop and I could probably connect the coolant pump to an external 110V plug rather than the transformer.

Here's a picture of the existing panel.

IMG_1834.jpg

Thanks,
Rob
 
You'll need a separate VFD if you want independent control of the spindle RPM; with a single unit the hydraulic pump RPM would change along with the spindle. To run multiple motors you need a VFD sized for more power than both motors will use on startup if you plan to start them with the VFD already active. You can get away with a little closer to both motors' summed rated power if you let them both soft start with the VFD. It's good to have thermal overload on both motors since the oversized VFD will not detect an overload of the smaller motors well.

I use a large VFD (rated for 25 HP but de-rated 40% as it's powered by single phase) to run multiple motors at once on my grinder - the hydraulic pump (1 HP) and coolant pump (⅓ HP) run off of one VFD while the spindle has its own VFD. I get the single phase 120V from the wall socket. This works very well.
 
Not sure if you have other machines or 3 phase stuff, or potential future machines ?

But the benefit of a vfd to me is the ability to adjust hz and do motor speed changes. If you wanted to just run the rated speed for the motor and grinding wheels, then I might set up a 3hp or 5hp rotary phase convertor. It would give you the option to connect other machines to it as well. You could leave your machine's wiring as is, assuming no voltage change, like 460v to 230v.
 
If the 3 phase equipment that you want to run off a VFD also has a transformer
to power some single phase 120V stuff you must not vary the frequency from 60HZ.
Motors can generally stand a wide variation in frequency but transformers
cannot. If the frequency goes too low the core will saturate and the transformer
will overheat.

If you want the vary the speed of the spindle motor then you should disconnect
the transformer and power the single phase stuff separately.
 
texasgunsmith said:
Not sure if you have other machines or 3 phase stuff, or potential future machines ?

But the benefit of a vfd to me is the ability to adjust hz and do motor speed changes. If you wanted to just run the rated speed for the motor and grinding wheels, then I might set up a 3hp or 5hp rotary phase convertor. It would give you the option to connect other machines to it as well. You could leave your machine's wiring as is, assuming no voltage change, like 460v to 230v.
Click to expand...

There is a big benefit to a VFD on a SG. I have a RPC but upgraded to a VFD on the SG.....reason? ramp speeds. 15 seconds to ramp up and down means there is no sudden jarring that can shift the wheel (there is always clearance between shaft and wheel) and upset its balance requiring a redressing. Lots of times that is a real benefit
 
Thanks everyone for the detailed replies. I do in fact have other equipment that runs off 3 phase power, but in those cases, it made sense to me to use a VFD. One is a mill and the other is a lathe so speed control and reverse made sense. I also recently got a tool and cutter grinder but for that tool, a straight VFD seemed like a good choice since it's just the one motor and some control over speed made sense to me.

However, I could see the benefit of having a rotary phase convertor to run both machines unaltered but of course would lose any ability to vary speeds. I guess since I already have VFDs for the other 3 phase equipment, it might still be about the same cost to buy VFDs for the new equipment and I would still have the control I get from a VFD. Then run the coolant to a separate 110 circuit.

Thanks again for the info!
 
Just wanted to thank everyone on the thread for the great information. I was able to remove all of the existing electronics and install two VFDs and rewire the existing switches. Everything is up and running great with the exception of an unrelated hydraulic leak.

Thanks again.
 
eKretz said:
I use a large VFD (rated for 25 HP but de-rated 40% as it's powered by single phase) to run multiple motors at once on my grinder - the hydraulic pump (1 HP) and coolant pump (⅓ HP) run off of one VFD while the spindle has its own VFD. I get the single phase 120V from the wall socket. This works very well.
Click to expand...

In vendor literature I have always seen a warning to use a VFD sized for the motor, or sized one step larger (for example, for a 2HP motor, use either a 2HP or a 3HP VFD). Here, you are using a 25HP (or 15HP after derating) VFD for a 1 1/3 HP load. Is that OK?
 
ballen said:
In vendor literature I have always seen a warning to use a VFD sized for the motor, or sized one step larger (for example, for a 2HP motor, use either a 2HP or a 3HP VFD). Here, you are using a 25HP (or 15HP after derating) VFD for a 1 1/3 HP load. Is that OK?
Click to expand...

Yes, seems to be just fine - with the caveat that a separate thermal overload of some type is used. It works well for me so far. All the motors run just fine from it. I also use it to run my 5 HP lathe. That took a little fiddling with the V/Hz curve to get dialed in (it would hang on startup now and then until that was fixed), but after that no problemo. The usual warning I've seen is to use at least one size larger VFD compared to the motor. As far as I'm aware, that's merely to be sure there's enough startup current available so the VFD isn't overloaded or always working at the limit. The current overload in a motor malfunction situation needs to be addressed if you are a lot larger on VFD than the motor, but that's easy enough.
 
Maybe it's my cheap VFD on my Boyar, but I remember a caution to use the VFD to switch the load, never a switch between the VFD and the motor. The smooth start and stop, plus speed control is a joy on an SG.
 
A thermal overload isn't really a switch. Unless there's a major motor malfunction it should never even open. Not really a factor IMO.

The other reason they say that is that a VFD rated for say 2 HP generally cannot handle the startup load of a 2 HP motor unless it's soft started by the VFD. If you have a VFD rated for 5 HP, in my experience it will have no trouble starting a 2 HP motor even if it's switched while the VFD is on. And I have seen no detrimental effects doing that. Just got to make sure the VFD can supply (and is rated for) enough current to handle the startup.

I generally use the VFD to start and stop the hydraulic pump motor on my grinder (and a separate one for the spindle motor) but I switch the coolant pump on and off while the hydraulic pump motor is running all the time with no issue.

And yes, definitely agree it is a big plus to have a separate VFD on the spindle motor. Soft start and stop and independent control of SFM is awesome.
 
Kind of off topic, but what are your feelings about installing a contactor prior to the VFD? I did install a contactor on my lathe, but didn't reinstall any contactors prior to installing the VFDs on the surface grinder. I have the original contactors, but got kind of lazy and figured I could just unplug it. Was that a mistake?
 
You could use a contactor to power the VFD on the input side I suppose, but I generally just use a guillotine type disconnect. I have seen others use a circuit breaker to switch the VFD on and off as well. What you don't want is a contactor between the VFD and the motor. That connection should be without interruption if possible. The exception would be a thermal overload or "heaters" if your VFD is of a much larger size than your motor.
 
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