VFD question

All the surplus ones I've seen accept 3 phase input. You probably don't have to oversize the VFD for normal operation, but it would give you a safety margin, with more ability to dissipate heat, etc., and I imagine more ability to withstand sudden load changes. The Allen Bradley 1305 series have good documentation and are available used on ebay at pretty decent prices.

I just bought two VFDs from Dealer Electric ( they have a new name now, can't think of it), also patronized by people in this ng.

They told me (I think) that most--or at least the ones I got-- VFDs will take either single phase or 3 phase input. The difference is, with 3 ph input, you no longer have to de-rate the VFD by

A 2 hp VFD is not that much more expensive than a 1. My 2hp was about $250. I would go 2 just for the margin, and for the unforeseen, like using the VFD elsewhere, or changing the lathe motor, etc.

If the VFD doubles the speed of your motor (I'm not sure it can, haven't really done much with mine), you'll still have only 1,000 rpm on your lathe, not very fast for a 13", imo. You can also have your motor rewound, for two-speed operation as well, for more speed latitutde. Or put in a higher rpm motor, which would raise your lower range as well. Some re-winding houses charge very reasonable rates, while others ask for the deed to your house.

Mebbe you should put your SG motor on yer lathe!

I've been curious about the pros/cons of varying grinder speed myself. Less of an issue than say a lathe, but still might be useful for differing materials.

It's your call, you can certainly do it. If it were me, I guess I'd think about the 1940's spindle bearing design and what speed it could handle when deciding how much to speed it up.

Also, electric motors have a max speed. Modern motors (especially inverter duty) usually have it on the nameplate. At some speed above max, the rotor will come unstuck due to centrifugal force, or whatever you want to call it, and expand, wrecking the motor.

If you have an old motor, I'd at least try to find some max speed data. If none can be found, I'd be conservative. Hearsay on this group suggests older motors may generate more torque. They are usually a larger frame size for the same power. A larger diameter rotor might have a lower max speed.

Good luck.

Pete Keillor

Generally one VFD per machine, so mill & grinder are irrelevant. Generally VFDs take single phase input, but many take 3 phase. It's a good idea to somewhat oversize VFDs if you can afford it.

You may have some issues ($$$) if you go more than 20% faster than the motor was designed for. Not necessarily in the motor, but maybe in the lathe.

GWE

Wouldn't it be cheaper to upsize the motor to perhaps 2hp and change the pulley setup to speed up? Your model was supplied in a 750 rpm version as well as 500 rpm.

Tom

Be aware that the older A-B drives are unfiltered and electronically noisy. If you have a lot of electronics, thermocouples around, chokes are available which help. I had to install a bunch on some older drives to stop my ContolNet network from glitching intermittently. The Powerflex family are well filtered, except maybe for some of the 120 V. input Model 4's.

Pete Keillor

check out automationdirect.com you can download the manuals for the units they sell

Yes.

No, not sure where you got that idea. Nearly all VFDs take three phase input, many will also accept single phase input, some will not. Remember that VFDs were developed as motor speed controls for industrial applications, not as substitute phase converters for home use.

It's a bad idea trying to over speed a non inverter duty rated motor by more than a few percent, particularly if it's a fairly old motor.

On VFD ratings and derating, you *never* have to derate a VFD when operated with a three phase input unless there is a separate derating factor such as a high temperature environment. When operating a VFD on a single phase input you *may* have to derate the VFD, but you need to read the manual for it to find out for sure as some VFD ratings already take single phase input into account.

As for overrating the VFD for the heck of it, if there is a reasonable probability you might eventually want to use the VFD on a different machine and the cost difference is not more than 25% higher, purchasing the next size up is reasonable.

Pete C.

There's no electronic stuff in my garage yet, but I'll keep that in mind. I do have a speed control for my Bridgeport feed that I plan on wiring in at some point.

How much current do you need? A simple buck transformer will do the job, as long as it's secondary can handle the current.

Variac of approiate size, or a triac type voltage regulator control. Big ass light dimmer ?

Best Regards Tom.

Call the Corporation Comission or the Public Utility Comission or whaever state organization regulates the power company. They have amazing powers.

Bob AZ

Are the transformers center tapped or do they just have 2 terminals for input and two for output.

If the latter they are simply 10:1 ratio transformers. If you stick 120 in you will get 12 out the secondary. Stick 240 in and you'll get 24 out.

If they are centre tapped, without seeing them I'd guess you put 240 across the full input winding or 120 across half the input winding.

Having said all that, I'd do it myself but I'm not sure I'd advise anyone else to try it.....

If you do a search on "buck boost wiring" you will find references for how to wire one of your transformers to "buck" your 249V down to 225V. Basically, the 24V secondary goes in series with the feed to your RPC, and the primary just connects to the line. The secondary amp rating at

If your transformers are large enough to handle the single phase load see single phase diagram A and you'll need only one transformer.

If the transformers are only large enough for the three phase load you'll need to use three of them after the RPC. See diagram F, which is really the same as the single phase setup in triplicate.

Do plenty of voltage checks before trying to run anything to ensure you have it wired correctly to "buck" to 225V rather than "boost" to 273V.

Rough numbers--- 5hp at 240 volts is 16 amps. That means your transformer secondary needs to handle more than 16 amps and 25 or 30 amps would be a nice SAFETY CUSHION. So, that's a big transformer, if you have that, then connect the secondary in series with one of the 240V wires, But the phase must be reversed to buck the incoming

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As an example, I run three 450 Watt Metal Halide high-bay lamps in my barn. They were surplus, thus inexpensive, but designed to run from

I purchased a "0.25KVA" (250-"watt") buck-boost transformer that has a

The three 450 Watt fixtures draw 6.5A total at 208V. So the 250 VA rating of the transformer is sufficient. ('been running them every night for 12 years off that same little grey box)

LLoyd

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That's actually not necessary, and not great practice. Transformers are designed to run essentially forever at their rating, and "regulate" best at near their rated load.

If you nearly doubled the ampacity of the secondary from what you needed, the voltage drop in 'buck' could be much greater than you anticipated.

In his _particular_ case, that's not a terrible problem, since his VFD will run down to 200V. But in some cases it could cause trouble.

LLoyd

Well, the real problem here is the VFD has a VERY restricted operating range. It must have been designed for 208 V only +/- a percentage.

What you need is a transformer with 240 V primary and 12 - 15 V secondary with 10 A output rating. You wire the primary across the mains, and put the secondary in series with one of the mains wires going to the VFD. Check the output first with the VFD disconnected. One polarity will increase the voltage, the other will drop it by the secondary voltage rating.

Now, if you can't find such a transformer with a 240 V primary, you could rig up a 120 V primary transformer between one hot and the neutral to do the same thing. The only problem is you may not have run a neutral to a purely 240 V load. The 120 V to 12.6 V filament transformer should be fairly easy to come by.

There are also "voltage adjusting" transformers that have multiple taps in small steps and 240/208 autotransformers, often found in buildings with

Jon

Most likely, the output is fully isolated, and you'd get 24 V out. If you wire the secondary in SERIES with the mains, you can either add or reduce the mains voltage by 24 V, depending on the polarity. Check with a meter before you connect the output to the VFD to make sure you got the polarity the way you need it.

Jon