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Sizing VFD for Nuttall Lathe 1phase to 3 phase

HobbyMaker

Plastic
Joined
Feb 5, 2021
I hoping someone could offer some insight on sizing a VFD for an old Nuttall Lathe.
Any advice, help, tips, tricks, pointers or links would be appreciated.

Problem
Sizing and selection of budget VFD for single phase 240V to 3 phase conversion for Lathe and Motor outlined below. Ideally with a total current draw of 10amp (but possible up to 15 amps with minor upgrades to circuits)

Background
I have acquired a couple of old 3 phase Nuttall Lathes (ex TAFE technical collage) and was hoping to get one up and running in my home shop. Unfortunately the current power option is single phase 240V 50HZ 10 amp (15 amp possible with small upgrade). This is a hobby project whilst it maybe possible to get 3 phase connect but it would require new switch board etc so big $$$ and probably not practical.

I have read numerous theories about VFD selection, most resulting in circular arguments about it's all about the application of the VFD but limited solid practical advise on sizing VFD. Several wild ranging opinions from it's fine to have it under the HP of the motor (as you never run at 100%) to double the rated HP (because single phase to 3 phase conversions required more 1.6 times amps(?) then round this up to double). As this is a budget project doubling the rating arbitrarily (which presumable would solve the problem) but seems a little wild without a solid technical reason to justify the $$$ increase.

Basically I'm a little confused!:confused: and hoping someone can offer some solid advice on the way forward.

Hardware
Nuttall Lathe (Model & Year unknown)

Motor:
Noyes
5 HP
RPM 1420
3PH
50 Cycles
400/440 Volts
7.2 Amps
Delta
Class: E
Frame: C213
 
Motor:
Noyes
5 HP
RPM 1420
3PH
50 Cycles
400/440 Volts
7.2 Amps
Delta
Class: E
Frame: C213

Problem 1: your motor wants 400 volts.
Problem 2: your motor wants 3 phase power
Problem 3: your power available is honestly pretty limited.

One option is a step up transformer to get the 400 volts, and then a 400 volt capable VFD.

Your motor at a bare minimum is looking for 3.75 kW minimum, at 100 percent efficiency. In the real world it probably wants 5 kW of power to develop the full 5 hp (which as you say you don't need). You will loose another ten or fifteen percent in the step-up transformer. So you need to see if any VFD can run this motor at a lower power rate.

Consider changing this motor to a 2-3 hp one, 240 volt three phase. This would be a better fit for the 10 amp power available. No step up transformer needed and the VFD would probably be a less expensive model. Is there something special about this particular motor, in this lathe?
 
Thanks for the reply. My electrical knowledge is fairly rusty so I will try to clarify my thinking a bit

Problem 1: your motor wants 400 volts.
I was under the impression that the configuration of the coils could be changed from 'delta' to 'star' dropping the voltage from 400V to 240V. From memory on most motors this can be achieved by a configuration of the power input terminal strips, but I haven't looked closely at the motor setup in the lathe and the name plate specifically says 'delta' (being an older unit this maybe a lot harder thing to change over than say:confused:).
Again from memory if I understand correctly the compromise with changing from 'delta' to 'star' is an inverse relationship with the current so it goes up by 1.66 times to achieve the same total power. This is a bit of a worry as the rated current is 7.2amp implying 12amp is required (I can get a 15 amp circuit of the existing board with a dedicated feed circuit.)

Is this in keeping with your experiences / knowledge or am I way off the path?

Problem 2: your motor wants 3 phase power
So if I understand correctly the correct VFD would overcome this with a single phase to 3 phase conversion but output voltage would be capped at 240V (implying a need to change the configuration from delta to star)


Is there something special about this particular motor, in this lathe?
Nothing special about this motor other than it exists and fits the machine. It is an old giant chunk of a motor so not sure how easy it would be locating something that would just drop in without major modifications.
Obviously not really busting to strip out the existing motor and hunt around for a suitable replacement then workout how to make it couple to the unit (I suspect the frame number may point me in the right direction if need be) but just feels like asking for a lot of problems.
 
Chances are you can probably rewire the motor to star - post the inside of the terminal box and the nameplate.

230>400V step-up VFDs are available, but not from reputable manufacturers and I wouldn't recommend them.

You're probably going to need a 32A circuit.
 
If you can rewire from delta to star you can use a single phase to 3 phase designed VFD. Hitachi makes some and I like them. No additional derating needed. They only come in up to 3HP version I think, but you don't have the power for more than 3hp anyway.

If it were me I would try a VFD such as this:3 HP Hitachi Variable Frequency Motor Drive WJ200-022SF-CP Variable Frequency Drive | Variable Frequency Drives | AC 3 Phase Motors | Electric Motors | www.surpluscenter.com
(It requires a heatsink for full load operation)

There is also this one for cheaper, though I haven't used this particular brand: 3 HP Regal-Beloit T217 Variable Frequency Drive 1ph input/3ph output | New Arrivals | www.surpluscenter.com

If you have issues you very well may have to change the motor for a 3hp version.
 
If the motor plate actually says "delta" for 400/440 V you are basically stuffed and will need to change the motor as its already hard wired in the low voltage configuration.

Delta wound 400/440 volt motors are somewhat uncommon but not exceptionally rare in the 5 to 15 hp range.

Clive
 
Whoops, yeah. Delta is for lower voltage than star. A motor built for 400V delta has each coil seeing 400V. To run in star, you'd need to feed it 690V.
 

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You are SOL on that motor without wasting a bunch of money in drives and transformers. How about an overall picture of the motor? If it's a typical NEMA C face motor you have lots of options. You can probably get a 2HP motor and VFD for under $500 from surplus center. That would run nicely on your 10 amp supply but would certainly appreciate the 15 amp upgrade.
 
If the motor plate actually says "delta" for 400/440 V you are basically stuffed and will need to change the motor as its already hard wired in the low voltage configuration.

I was worried I might have it backwards or upside down. :confused:

So from what you are saying in a star configuration the line voltage between nodes would be 400V (or say 240V in my supply case) but the phase voltage would be less across each coil 240V between each node and neutral point in the centre of the star (or say 141V in my case)
Where as in the the delta case the the line voltage of say 400V would equal the phase voltage across the respective coils.
Thus in the case of this motor is designed to operate with 400V across each coil and this must always be 440V across the coil (???)

My understanding was that the VFD would supply 240V directly to each coil in star configuration (between the respective node and neutral) thus "simulating" a 400V delta connection for the motor
:confused:


So from what you are saying I see at this stage my options are:
1) Try to run under voltage at reduced Frequency (something like this article HowTo: 240V Supply to a 400V AC Motor - Application Detail)
2) Replace the motor with a 240V 3 phase motor (possibly a lower HP) and a VFD.
3) Replace the motor with a Single Phase motor (possibly a lower HP) and upgrade the feed wires for a significant current draw. (Something like Vevor 3.7kw 5hp 1440rpm Single Phase Electric Motor Asynchronous Motor





– Vevor AU
assuming the mounting and output shaft can be sized or adapted correctly)
4) Byte the bullet and upgrade to 3 phase (I'm investigating this option and getting a quote but I suspect it will be outside the budget)
5) Run up the white flag. Clean the machine up then pass it onto a new owner and look out for a nice smaller single phase machine.
 

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How about an overall picture of the motor?

Attached are a few pictures of the motor, my apologies its hard against the wall and a little tricky to get a good photo. (presumably also to get out the motor but we will cross that bridge when need be.)


If it's a typical NEMA C face motor you have lots of options. You can probably get a 2HP motor and VFD for under $500 from surplus center. That would run nicely on your 10 amp supply but would certainly appreciate the 15 amp upgrade.

The name plate gives this as a "Frame: C213"

As perhttps://www.engineeringtoolbox.com/nema-electrical-motor-frame-dimensions-d_1504.html
I can't find a C213 in this table but I do see a 213 CT
So I am guessing its some kind of standard mount, it would make life so much easier if I could just drop in a motor of the correct dimension & using the existing pulleys and brackets etc.

You are SOL on that motor without wasting a bunch of money in drives and transformers.
I'm guessing "SOL" ==> "Sh.. out of luck"
if so perhaps you are correct but still trying to work my way through the problems:o.
 

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If it's a belt drive then swapping the motor is the solution here, and it's an easy one.

Yoink the motor and measure it to some other frame sizes and see what will fit.
 
So from what you are saying I see at this stage my options are:
1) Try to run under voltage at reduced Frequency (something like this article HowTo: 240V Supply to a 400V AC Motor - Application Detail)
2) Replace the motor with a 240V 3 phase motor (possibly a lower HP) and a VFD.
3) Replace the motor with a Single Phase motor (possibly a lower HP) and upgrade the feed wires for a significant current draw.
4) Byte the bullet and upgrade to 3 phase (I'm investigating this option and getting a quote but I suspect it will be outside the budget)
5) Run up the white flag. Clean the machine up then pass it onto a new owner and look out for a nice smaller single phase machine.

Another option

(6) motor re-wind shop, rewind your existing motor for 240 volt service, and then a VFD.
 
One option if you don't need full power: feed the 400V motor 230V.

You will get full torque up to 29Hz, and then flat 2.9HP from there. Program the VFD to tell it motor nameplate speed, frequency, voltage etc are 57% of what's on the label. Current stays the same.

Get an oversized VFD and you'll have quite a bit of room for short-term overload too.

Edit: this is your solution #1...

As per the link:
If the maximum speed is set to 50Hz or more, the Motor may possibly reach these speeds, but it will become progressively "Under-Fluxed" (Torque will reduce). However, this will only be evident if the Motor is fully loaded. If it is, the Motor will look for more current to meet the load. A correctly set Inverter will provide protection against over-Current by reducing the speed automatically to bring the load current under the maximum set.
 
Most name brand VFD's are sized to the motor amps, you do not undersize or need to oversize in most circumstances. Under-size is a bit of a misnomer as VFD's can be programmed for much higher output for short periods of time in addition they are not 100% efficient. If you have a 3 phase input VFD running on single phase then the derating for single phase ranges by VFD model, but usually a factor of around 1.7-2.0. So for the most part you need a VFD sized to the motor load, exception may be the generic Chinese models which often ones need to upsized because they are built to minimal specs. The other aspect is you have an older motor, the insulation tends to breakdown with time, and do less well with the PWM output from VFDs and also have a narrow operating RPM. Rewinding or rewiring the motor of this age may be a bit impractical, often the expense is more than the cost of a new motor so more practical for motors that do not have off the shelf replacements or replacements are expensive.

Cheapest "budget" approach would be to replace the motor with one suitable to your input power, 2nd replace the controls/wiring as they look a bit crusty, or get a 3 phase motor of the appropriate voltage and add a VFD with low voltage run controls. Cost would be about the same whichever approach you take, as opposed to trying to adapt the current motors. There are a few voltage doubler VFD's out of the UK and also direct from China, the (UK ones) will probably cost more then replacing the motor and getting a lower voltage VFD. Single phase input VFD's tend to go up in price significantly beyond 3 Hp, and typically say for a 5Hp motor you would use a derated 3 phase input VFD in the 7.5-10Hp range. If you plan to replace the motor with a 3 phase one, it may be more economical to use a 3 Hp motor/VFD, you can also change the belting ratio on the motor to give a wider usable speed range.
 
also keep in mind that the vfd derate required for single phase is according to the motor's mechanical load output.. not the motor's default horsepower.


if you buy a 5hp single phase input 230volt vfd.
you can then get 3hp out of the motor without overstressing the VFD, at 29hz or 2/3rds nominal rpm.

being limited to 10 amps on the line (or 15 if you put a bigger circuit breaker in.. don't tell anyone i said that)
you will be limited to about 60 percent of 15*230vac or about 2.7 hp. 60% is the power factor of a single phase rectifier. a 15 amp circuit breaker might pass 20 amps for 5 minutes, so keep that in mind.

you should be able to get 2 hp at the spindle continuously and 3hp for 10-60 seconds, if you just buy a 5hp single phase 230v vfd, at 29hz.
 
also keep in mind that the vfd derate required for single phase is according to the motor's mechanical load output.. not the motor's default horsepower.


if you buy a 5hp single phase input 230volt vfd.
you can then get 3hp out of the motor without overstressing the VFD, at 29hz or 2/3rds nominal rpm.

being limited to 10 amps on the line (or 15 if you put a bigger circuit breaker in.. don't tell anyone i said that)
you will be limited to about 60 percent of 15*230vac or about 2.7 hp. 60% is the power factor of a single phase rectifier. a 15 amp circuit breaker might pass 20 amps for 5 minutes, so keep that in mind.

you should be able to get 2 hp at the spindle continuously and 3hp for 10-60 seconds, if you just buy a 5hp single phase 230v vfd, at 29hz.

I would like to mention that the 3hp VFD I suggested requires no derating for single phase input, and would be a good candidate for this approach too.
 
A 5 Hp single phase input VFD will cost about 3X that of a single phase 3 Hp, there are very few native 5 Hp VFD's and often you end up with a 7.5Hp 3 phase input and derate it for single phase input. To put on a 5Hp VFD and try to get 2 Hp output and fuse for that, let alone just about anything else on a 10 or 15A 240 circuit is really questionable with out causing a fire. The no load amp draw will be around 40-60% of the FLA on a 5 Hp motor. At least in the US, even if a circuit is rated for 15A you can only pull 80% continuously, and input fusing of VFD's is technically not rated on the output draw but on the input rating. Look at most 240V input single phase input VFD's you will see recommend breaker of 30A for 2 and 3 Hp, and 50-60A for 5 Hp . Sure I have run 2 Hp VFD's on a 20A circuit, that's about the minimum unless you plan to only use it to JOG. Put in a 30A circuit, replace the motor with a 3 Hp and get a 3 Hp VFD.
 
In Australia, the 'typical' circuit configuration is a 16 or 20A breaker, feeding a fairly large number of 10A sockets, or very occasionally 15A sockets. The breaker is intended only to protect equipment/cable up to the socket (the fixed installation); overload protection for the plug and flexible cable to the machine is the responsibility of the machine designer.

This generally means that the machine should either have a fuse/breaker onboard, or otherwise be designed so that it can't draw more than the 10A (or 15A) rating.

Some digging shows that a 4kW drive is likely to draw around 30A input current. A 2.2kW drive is likely to draw around 12A. These can potentially be improved with input reactors/chokes.

The reason to oversize the drive is that many drives are rated for a motor a size larger when used for 'fan/pump', 'low overload', or 'variable torque' applications, compared to hard/frequent starting applications like a compressor. Check the manual for what overcurrent they can do for how long.
 








 
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