Coping with start up current on large single phase motors?

[ian maybury]

Hi guys. Over here in in Ireland/UK we rarely see 5hp single phase motors used due to the high start up current, and machines like large bandsaws are not offered with single phase motors.

At our 220V single phase the plated current is roughly 16A, but they apparently can draw x5 or up to 80A on start up. Which is a problem, since most incoming service provider fuses are only 60A, with (not always) the possibility of an upgrade to 80A.

At least some of the few items with large e.g. 4kW 'single phase' motors like Felder saws actually seem to use 3 phase motors driven by VFDs (inverters) to control the start up current.

Yet in the US many of the big bandsaws etc seem to be offered in 5HP/3.75kW sizes.

I guess i'm wondering (i have a big 3 phase bandsaw to run from single phase, and am exploring rotary converters, dual voltage motors with a VFD and conversion to single phase) how you guys get around this issue? Maybe you have a 100A supply or something???

Does anybody know if the single phase motors supplied on these machines stock single phase types, or do they use Felder's VFD/dual voltage trick or capacitors enable them to start against load?

Do stock single phase motors present any problems when starting a big bandsaw with cast iron bandwheels, or with the slightly less robust performance of single phase motors?

Thank you,

ian

[Will Overton]

If all wiring had to be rated for start up current, most homeowners couldn't run a 1 1/2 hp saw. That's where 'slow blow' fuses and breakers come into play. They allow for the momentary surge at start up.

[Dan Hintz]

I would also think 5x current draw is a bit high... 2-3x is more likely in all but the most odd of cases.

Have you considered running a VFD, as you mentioned? Hitachi makes a really robust line, and interestingly enough I was just looking at prices... several models support up to 5HP motors and only run about US$350-$500 (depending upon model). You can set it up for soft starts, etc. which would drastically reduce the surge current rating, and you get variability of speed via keypad.

[Matt Armstrong]

I would also think 5x current draw is a bit high... 2-3x is more likely in all but the most odd of cases.

Have you considered running a VFD, as you mentioned? Hitachi makes a really robust line, and interestingly enough I was just looking at prices... several models support up to 5HP motors and only run about US$350-$500 (depending upon model). You can set it up for soft starts, etc. which would drastically reduce the surge current rating, and you get variability of speed via keypad.

5x isn't unrealistic. Inrush currents are pretty intense.

[Mike Cutler]

Ian

Without going nuts with theory. The inrush peak on start-up is a short duration spike, measured in milliseconds. The breakers have a "time delay", if you will, that will not trip the breaker unless the inrush current stays high for too long, or the voltage stays too low, which would cause the current to remain high.
If you were to plot the current on initial startup, versus voltage, you would see that as voltage increases, current decreases. Remember though that this is all happening in milliseconds, or cycles. It's very fast, and highly doubtful that you would ever be able to see it on any "meter" a homeowner could be expected to own.

Wiring, and insulation resistance, are rated for continuous ratings, and peaks, at temperature, and have an "envelope" to maintain conductor integrity.

Rick Christopherson, a member here on the board, has ( or used to have ) a really nice website for understanding electrical theory as applicable to the shop. It's in pretty straight forward terms. If he still has it up, it's a good read.

[Van Huskey]

5x isn't unrealistic. Inrush currents are pretty intense.


I agree 5X is not that unusual. As others have mentioned inrush currents last a very short time on typical motors, I have rarely has an induction motor trip even a "standard" breaker even when the motor current rating is right at the breaker amperage.

[Dan Hintz]

Well, yet again I'll point to the fact that I was a solid 'C' student when it came to my power classes... didn't like it then, don't like it now. I'm just trying to imagine the large inductance of such a sizeable motor allowing 5x current in, even for a short time...

[Paul McGaha]

Hi Ian,

To use the Grizzly G0568 (5 HP, Single Phase) as an example if we were to buy that saw we would have to feed it with a 40 Amp Circuit and Wire Size #8 per the Grizzly specifications. The standard voltage for a single phase service here in America is 115/230.

If I'm not mistaken European Voltages are higher which would lend itself to lower amperages.

In your case of having a three phase motor and single phase service I think your options are two get the circuit to the band saw converted to three phase or change the motor to a single phase. I'm not sure which way would be the most cost effective.

PHM

[ian maybury]

Hi guys, thanks for that. Don't worry Dan, my 5X was just from a casual comment by a local electrics guy so your number is as good as mine.

The slow 'pop' of the mains fuse or a local breaker sounds promising Mike. I can fit a slow 'C' type breaker, but i wasn't too sure if it would cope with the saw given the heavy flywheels. Presumably the incoming fuse is 'slow' too.

An inverter would be my first choice as well Dan - it's both cheaper, and as you says controls the start current. Unfortunately the saw manufacturer (Agazzani) so far as i can establish fitted a 3 phase 400V delta only motor to the saw - not the dual voltage star & delta type than can run at 220V/3phase - which i gather are usually plated 220/400V. The inverters sold over here don't step up voltage, reportedly something to do with feeding harmonics/interference back into the system.

I've a query in on this with Agazzani, but haven't had an answer yet. I don't know how to tell from an inspection of the terminal box if this is definitively the case (photo attached), but so far as i know dual voltage types usually have copper dog bones on the terminals to facilitate the change over.

The only other suitable option i'm aware of that can deliver the required 400V/3phase is a rotary converter, but so far i've not been able to dig one up at an affordable cost. I wasn't sure about start up current either, although i've been told they limit it to about X3. New is expensive at about $900 over here, but if going that road i should really buy a much larger one that would enable me to go 3phase in future with more equipment.

Just out of interest. What is the maximum current you guys are normally allowed to draw on a single phase 220v (?) domestic supply?

ian

[Dan Friedrichs]

Just out of interest. What is the maximum current you guys are normally allowed to draw on a single phase 220v (?) domestic supply?



200A is standard.

[Dan Hintz]

A number of the newer (and bigger) homes are often supplied with 300A (and I've heard of 400A service in the big McMansions). I drooled a bit at seeing a 300A panel while doing a walk-through... that would make a nice sub-panel for a workshop and have practically zero effect on even larger households :)

[Rod Sheridan]

Hi, I have two Hammer machines with 4 HP motors rated at 16 Amperes.

The maximum allowed circuit ampacity for those two machines is 20 Amperes.

Both of them start and run on a 15 Ampere circuit at my shop.

As was previously posted, you can purchase fuses with time delay characteristics suitable for motor starting, from your local electrical supplier.

Regards, Rod.

[David Castor]

Hi, Ian. I'm mostly a lurker here, but maybe I can shed a little light on the motor issue, since I make my living as a EE.

The 5X full load value given for starting current is a reasonable guess. It could be even higher, as others have mentioned. When an induction motor is started "across the line" at full voltage, there is a very brief period of "inrush current" - this is due to energizing the large inductance that is the stator winding. This will last a few cycles at most and could be 8 X motor full load amps. After this, while the motor is coming up to speed, it will be drawing "locked rotor current". This is the 5X value mentioned earlier. The terms 'inrush current' and 'starting current' are often used interchangeably, but technically, they are not the same thing.

The motor will draw close to locked rotor current until it is nearly at full speed, then close to full speed, the current drops off drastically.

Also, on a weak system, such as a residence, there is often a lot of voltage drop during starting due to the resistance in the wiring used. This tends to reduce the voltage at the motor terminals and this does reduce (not increase) the motor locked rotor current. This is the basic principle behind reduced-voltage starters.

As far as your fuses and/or breakers: circuit breakers have two tripping elements - a thermal overload element and a magnetic short circuit trip. The thermal element is time delayed based on the amount of current. At 85 A, it might take a couple of hours to trip. At 400 A, it might be a second or two. The short circuit "magnetic" trip is instantaneous. The magnetic trip will probably actuate somewhere between 5X and 10X of the breaker rating, or 400 A to 800 A for an 80 A breaker. Fuses have somewhat similar characteristics.

So long story short, if your 5 hp motor is drawing 80 A locked rotor current it is probably NOT going to trip an 80 A breaker during starting, since the acceleration time is too short to cause the thermal element to trip.

But if you have a three-phase service, a three-phase motor is a step up in reliability and performance over a single-phase motor, all other things being equal.

Hope that helps - sorry for the length.

Cheers,

Dave

[ian maybury]

Based on what you have to say (Rod and David) it looks like a single phase motor conversion is possible. Even if it exceeded the nominal 80A of my supply for short periods, it looks like with the right breaker and fuse fitted it should be OK.

That 24in Grizzly GO568 @ 5hp with its cast iron wheels must be very similar to my 24in Aggazani Paul, so 40A @230V is probably a good bench mark. That's a bit more than 2X the name plate amps i suspect, which is not that high and again suggests that it's normal to presume that circuitry can and is routinely expected to handle short duration currents much higher than their nominal rating - presuming that is there's nothing special about the Grizzly that keeps the current down vs a stock single phase motor set up.

16, 32 and i think 63A are standard ratings for single phase sockets here, although the 63A would be pretty unusual.

Given that an inverter is likely to require me to buy a dual voltage motor as well the single phase conversion or rotary converter look like the most cost effective options - probably the latter is best given that it retains the 3 phase motor, and i gather reduces starting current to 2 - 3 X.

All i need now is for a used one of the right kW to come up for sale. Most bought here are used to power whole shops and tend to be OTO 10 - 15kW - and consequently even second hand cost about $1,500....

Thank you guys, some great info there.

ian

PS I'd hate to be paying for the power consumption in a house with a 200A supply - our electricity costs about 18.5c/unit (kWhr). Probably multiples of what you guys pay - yet another gift from the state/legacy of yet another unreformed state monopoly gone private - having been sold to maximise short term government gain and to hell with the consumer.

[Peter Quinn]

My 2HP Dewalt RAS (17A @220V on the motor plate, what they used to call a TRUE 2, probably more like a 3HP today) runs fine on a 20A circuit, draws 93A momentary on start up! My FIL (former electrician) measured it with a neat device that looks like a little hand cuff that goes around the wire. Thats nearly half my service for a few milliseconds.

Ian, I bought a Gentec American Rotary phase convertor, 10HP, to push an SCMI shaper (5 HP), cost around $700 new with slave motor and control box. Add several hundred dollars in wire and incidentals to set it up. The voltage issue will add to the cost I imagine considerably. I was converting 220 single to 220 three phase. I think Gentec American rotary also handles transformers for the voltage step up but I have no idea what the cost is. It might be a better option if three phase is the norm. Three phase is the norm here in most industrial or commercial situations, but is very difficult to get in many residential areas and cost more per Kw.

I saw a picture in a museum of a large wooden band saw being powered by two apprentices on a sort of tandem bicycle whose out put could be switched via leather belt to a lathe also. So good luck getting that big bandsaw running, and remember things could always be worse! It could have come with pedals!

[Stephen Cherry]

I've got a 24 aggazani with single phase, and I usually give the top wheel a few spins to help it get going. It starts OK on a 30 amp breaker.

I think that bandsaws are made for VFDs,

Is your supply 220 single phase, and you are trying to run 380 V three phase? What is the nameplate info, volts, amps, hz?

I was able to put a phase converter together for my table saw for just a few hundred dollars using a idler motor.

None of the solutions are really all that complex, depending on your willingness to scrounge.

My initial thought is to use a transformer to go form 220 to 440, then use a vfd to run the saw. The trick is that most 400 volt class drives are only rated for three phase supply. Some Hitachis allow the use of external dc supply, where you supply rectified filtered juice to a terminal on the drive.

[Stephen Cherry]

here is a good site for three phase info.


http://www.practicalmachinist.com/vb/

[Rick Christopherson]

....... or do they use Felder's VFD/dual voltage trick ....Ian, you mentioned VFD and "Dual Voltage" twice in your postings, so I thought I would comment on that. I believe you are confusing the versatility of a dual voltage motor with something functional with the VFD. A dual voltage motor simply means that its windings can be reconfigured for the various supply voltages it may see. In the U.S. for small single phase motors, this means the same motor can be configured for either 120 volts or 240 volts. 3-phase motors can also be dual voltage and can be configured between 208 and 480, for example.

This is a static configuration and no VFD that I have heard of is capable of switching the configuration of the motor on-the-fly as your posting seems to suggest.

As for your tool, at 5 hp, you are in the middle of the gray area where it might be more economical to use a VFD for phase conversion (3 hp and below) versus using a rotary converter (above 5 hp). Which method to choose will depend on the cost. VFD would be the better choice, but a 7 hp VFD (you need to oversize them at or above 5 hp) gets fairly expensive.

Rick Christopherson, a member here on the board, has ( or used to have ) a really nice website for understanding electrical theory as applicable to the shop. It's in pretty straight forward terms. If he still has it up, it's a good read.Yes, I am still alive, and the information you mentioned is located at http://www.waterfront-woods.com/

[ian maybury]

I'm starting to think maybe pedals wouldn't be a bad option Peter!:p That's quite some start up current on the RAS, and it doesn't even have the big cast iron wheels of a bandsaw.

Good to get another reference point on current draw on a 24in bandsaw Stephen. Thank you. Giving it a twirl isn't i think such a straightforward option for me as the saw is a Euro/UK model with an electrical and not a mechanical brake, but it tends to support the previous 40A number.

Here are the motor plate details, there's a photo of the terminal box at the top of the thread:

Seimec Modena CE
IEC 34-1 ICL F
IP 54 S1
3 09709/07 03/07
HFV 100LA 2 B5

Brake:
V06 13Nm 400V 0.2A RV1/50 178

400V (delta)+/- 5%
6.6A (could be 5.6)
3kW
2900 rpm
Cos phi 0.77

So far it seems it's a 400V delta only motor. i.e. it's not dual voltage, and so not suitable for an inverter drive.

The rotary converters sold over here normally have voltage step up included to give the full 380/400v 3 phase output - they unfortunately are quite a bit more expensive here than in the US. (shipping one over is too costly to make it a good deal here though)

I've superficially toyed with the DIY route, having seen that it's fairly straightforward - even for a mech eng. It could be a runner, and thanks for the links. One factor putting me off a little is that there seems to be a sense about that the good commercial units do some smart things to ensure that the third phase is not just a crude approximation. Do you think is this a real issue, or is it just smoke by the makers? Local electrical equipment approvals regulations are also possibly a factor.

My post on Felder's approach should have said VFD and 220V three phase motor i guess Rick - a typo as i know (superficially anyway) about dual voltage motors. Re. my motor above i'd have bought an inverter drive by now if it was a 220/400V - the 4kW Omron/Yaskawa V-1000 model is available here at decent money. The problem is that adding a new motor brings it into the same cost territory or slightly more than a rotary. It would deliver the advantage of giving better control of the start up current though. Thanks for the link to your site.

It's starting to look like it's a case of either pay up for new stuff (rotary or inverter and motor) now, or sit and wait for something used to come up, or go a DIY converter. With a single phase conversion in the background as probably feasible too....

ian

[Rick Christopherson]

It's starting to look like it's a case of either pay up for new stuff (rotary or inverter and motor) now, or sit and wait for something used to come up, or go a DIY converter. With a single phase conversion in the background as probably feasible too....

ianYou don't need the dual voltage motor, you just add a simple single-phase step-up transformer to the input of the VFD (rated for the 400V input). Autotransformers (single coil with taps) are reasonably priced.

[David Castor]

Ian,

I would recommend seriously looking at an adjustable (variable) frequency drive - at 5 hp, you can probably get a drive specifically designed to operate on a single-phase input and produce three-phase output. If that is not available, you can get an oversized three-phase drive and operate it with only one phase on the input side - it will still produce three-phase output.

With an AFD, all issues related to starting current can be eliminated. The drive can be adjusted to ramp up the motor and never exceed the nameplate current.

I'm not quite clear on the voltage question. A transformer can be placed ahead of the AFD if necessary. So replacing the motor should not be necessary.

[Stephen Cherry]

The rotary converters sold over here normally have voltage step up included to give the full 380/400v 3 phase output - they unfortunately are quite a bit more expensive here than in the US.



It's starting to look like it's a case of either pay up for new stuff (rotary or inverter and motor) now, or sit and wait for something used to come up, or go a DIY converter. With a single phase conversion in the background as probably feasible too....

ian

I was looking at the stepup rotary convertier idea, but I think you would need a big idler motor-- it needs to absorb enough current to run the saw on one of the low voltage windings. Maybe something like 20 HP. I'm guessing that you would look at the high voltage current rating of the motor to see what it can handle one only one winding.

The practical machinist site I referenced earlier has a conversion specific forum, they would eat this sort of thing up (lots of knowledge there)

http://www.practicalmachinist.com/vb/transformers-phase-converters-vfd/

[Justin Bukoski]

Ian, I'm probably missing something but it seems like it would be easier and less expensive to just buy a single phase motor. I'm pretty sure you can get a 5hp one for about $500 USD.

[ian maybury]

Rick, David. I'd love to go a VFD/inverter drive, but so far as i can see from the (UK) catalogues there's no single to three phase 400V input inverters available here.

This is a typical listing from Omron: http://www.axiscontrols.co.uk/en-GB/omroninverterprices.aspx (the makers don't sell the same VFD models here as in the US, and in fact there's only a couple one of which is Yaskawa/Omron doing 4kW single phase input models - most stop at 2.2KW)

Also i'm being told by inverter suppliers like the above guys that i do need a dual voltage motor.

There is one company selling inverters with voltage step up (presumably a transformer) to give 400V/3 phase, but they (a) cost the same as a rotary, and (b) are under heavy attack to the effect that they are not CE/regulations compliant, and that there are issues with the feeding of interference back into the mains. (see second post below) This could of course be smoke since it's mostly coming from their competition, but i've seen their rebuttal and they certainly avoid answering the question of whether or not they are compliant with the regs.

That's just to set out the position i seem to have come to. I'm all ears regarding the possibility you both mention of placing a single phase transformer on the input of a 400V (3 phase?) VFD for the reasons you say. (soft start, speed control etc) Can you possibly explain a little more regarding how this works/is wired, and exactly what equipment is needed? Presuming it's possible does it entail any risks/problems/regulatory issues?

I guess i have to wonder why the inverter makers are not selling voltage step up models over here as there's a definite market judging by what the rotary guys are offering, and what they are charging.

:) Please pardon my coming back for second helpings...

Thanks for the link Stephen. I'm getting a little behind on keeping up with the info i've been given now - i'll have to get reading over the weekend.

Single phase seems like it's feasible Justin (the info tabled by some of the guys suggests that 40A cabling/breakers or a bit bigger since our voltage is a little lower should do the business with a 24in saw. I've been stalling so far on it though - (a) our incoming supply is unlike in the US fused at 80A by dictat of our service provider and runs our house as well, (b) it'd cost about the same as an inverter, (c) it involves modding the machine, (d) a three phase motor is probably a little more robust a drive - if i can find a way to swing it.

ian

[ian maybury]

PS Warning by one (rotary converter) maker against buying step up inverters:

POWER CAPACITORS LIMITED, 30 REDFERN ROAD, TYSELEY, BIRMINGHAM. B11 2BH
© POWER CAPACITORS LIMITED 2009
6
********* WARNING *********
Electrical Products are covered by specific safety regulations.
Ensure the products you propose to purchase are safe.
MODIFIED INVERTERS OFFERING 415V 3-PHASE OUTPUT FROM 240 1-PHASE AND MOTOR CONTROL
Manufacturers of Electrical Products have a legal responsibility to ensure their products bear CE marking if a
company intends to sell such products in the European Union (EU) or member states of the European Economic area
(EEA) - Iceland, Liechtenstein and Norway
A CE mark is a manufacturer's claim that its product meets specified essential safety requirements set out in
relevant European directives.
The requirement for CE marking and the exact process a manufacturer will need to go through varies from product
to product. Different types of product are governed by different European directives. Where an item of equipment
is covered by more than one directive, it must be CE marked under all applicable directives.
A range of “modified” single phase input inverters is available for sale on the internet, sometimes referred to as a
“digital phase converter”. Unlike a standard inverter this type of equipment offers a 415v three phase output from
a 240v single phase supply with the aforementioned motor control benefits.
All inverter manufacturers offer products with a variable frequency output of 240v three phase from a 240v single
phase supply. By limiting the output of these devices to a maximum of 3kW/4hp, manufacturers such as IMO,
Mitsubishi, Omron, Teco, Eurotherm ABB and Siemens are able to sell CE marked equipment compatible
with European Power Quality regulations.
Digital Phase Converters are modified versions of three phase 415v input inverter products. Modified inverter
products do not carry any warranty support from manufacturers of inverters and may require assessment/approval
from your network operating company (i.e. the provider of your single phase electricity supply) prior to connection.
The modified product has no CE Marking and therefore does not comply with EU EMC regulations. Tests on this
equipment confirm that acceptable levels of emissions are exceeded and thus a CE mark cannot be applied
• All TRANSWAVE Converters and IMO inverters are manufactured in a BS EN ISO9000:2000 Quality Assured
Environment.
• All TRANSWAVE Converters and IMO inverters are CE marked and comply with EMC Regulations, the Low
Voltage Directive and BS EN ISO 61000-3-2:2006.
• Certificates of Conformity are available from both Power Capacitors Limited and IMO plc.
• NOT ALL PHASE CONVERTERS SOLD ON THE INTERNET ARE CE MARKED.
• SOME PHASE CONVERTERS CARRY AN INVALID CE MARK AS THE EQUIPMENT HAS BEEN MODIFIED AND NO
LONGER MEETS THE MANUFACTURER'S ORIGINAL SPECIFICATION.
• EQUIPMENT THAT HAS BEEN MODIFIED SHOULD BE RE-CERTIFIED PRIOR TO SALE, NOT SIMPLY PUT BACK IN
THE BOX AND RE-SOLD.
• CE Certification and re-certification costs manufacturers such as IMO and Power Capacitors TENS OF
THOUSANDS OF POUNDS - this is the cost of compliance.
• BY LAW ALL ELECTRICAL PRODUCTS SHOULD CARRY A CE MARK - CONSUMERS ARE ENTITLED TO SEE A
DECLARATION OF CONFORMITY FROM THE MANUFACTURER TO CONFIRM COMPLIANCE.
• WE WOULD STRONGLY ENCOURAGE YOU TO SEEK WRITTEN CONFIRMATION OF SUCH COMPLIANCE PRIOR TO
PURCHASE.
Under the Sale of Goods Act 1979, all products must be "fit for purpose". This means that products must fulfil the
purpose the customer has been led to expect and the reasons that led them to buy it. The Act also covers any
purpose that a customer asks about when the product is purchased and is guaranteed by the retailer to meet that
purpose when it is sold. If a product is not fit for purpose, the customer is within their rights to have the goods
replaced or repaired.
Power Capacitors Limited offers a service over and above the above requirements. We also offer a no-obligation
money-back guarantee (less postage costs) assuming the product is returned to us in a state fit for resale.

[Rick Christopherson]

Ian,
You're getting bad information from manufacturers, and some of it is simply badmouthing their competition. :mad:

There is nothing about a standard VFD that requires a dual voltage motor. I suspect this is from a miscommunication. As I said previously, simply put a step-up transformer ahead of the VFD and use the existing motor. The words "dual voltage" should not be coming up in your discussions whatsoever.

Oh, you are correct that you probably won't find a 400V single-phase VFD. That is still fine. You simply need to over-size the VFD to 7 hp. The reason why this is feasible is because the VFD takes the incoming AC power and converts it into DC power (like a battery) and then artificially generates the 3-phase output from this DC. So the only part of the VFD that cares about the incoming 1 or 3 phase is the rectifier. A 3-phase rectifier is more efficient than a single phase rectifier, so that is the reason for "de-rating" the VFD's output power.

As for the CE listing, that vendor appears to be making more out of it than he needs to. Yes, the listing is good in that it means the device has been examined by an outside source. However, it is a VERY expensive process to go through and does not automatically mean a poor product if there is no CE listing--as long as it is a reputable company.

For example, many of the Festool products you can buy in your country are not available to us in the U.S. simply because of the bureaucracy of the UL listing process here(similar to CE). The tools are no less safe, it is just bureaucracy.

Does your bandsaw have a CE listing? Technically is should, but would you have not purchased it simply because it didn't? Probably not.

[george wilson]

When I planned my new shop,I had a 400 amp panel,separate from the house panel put in. Also had #6 wire run to where I might someday upgrade to a 10 h.p. metal lathe.

At my last shop,funds were running low,so I skimped on electricity,which I regretted later. Always running around re-pluging things.

[ian maybury]

I do agree on the bad mouthing Rick, and have a similarly cautious attitude to the bureaucracy that much of this regulation beings - it seems to be mostly about jobs for the boys.

That said a 7HP inverter over here is only available in three phase, and anyway costs about the same as a new 5hp rotary converter.

I'm not sure what's going on over the dual voltage issue, but as before (a) it's absolutely clear that voltage step up inverters are not generally sold over here (it could be for marketing or regulatory rather than basic technical reasons), that (b) 4kW is the max single phase inverter normally available (presumably due to the 80A limit on domestic supplies i mentioned before) and (c) there's definitely a widely held view that a dual voltage 220/400V motor is required to enable the use of single phase inverters giving 200V 3 phase output. (that some three phase motors are not suitable for use this way)

On the latter - I spoke to a rewinds outfit yesterday to see if they could set up my motor to run on an inverter, and they were adamant that the motor was not suitable based on the 400V delta listed on the plate.

Despite this there's a conflicting few saying that any three phase motor can be connected to run perfectly well on 220V 3 phase too - but little hard information.

I've arranged to have an experienced industrial electrician llok at the motor early next week, maybe he'll be able to clarify the position.

ian

PS George. I'm already thinking that maybe i should have bought a largish rotary converter to run my whole shop and bought 3 phase equipment, but that said 400A is simply not an option. Mains 3 phase power was in the meantime looking like costing approaching €10,000 to install, with ongoing annual charges resulting from it too.

[george wilson]

Ian. Not sure how it is over there,but over here I believe you have to have a minimum number of 3 phase motors in your shop to get 3 phase installed. I knew an old guy who had an open back basement workshop he could drag heavy lathes into. He used them and also sold them.

He had a bunch of motors sitting around not being used,in order to meet the number of motors required. They didn't need to be in use,just in the shop.

You may not need a real big converter to run your whole shop if you only use 1 machine at a time. I was looking at a 10 h.p. DSG lathe. The owner said he'd been running it on a 3 h.p. rotary converter. He wasn't doing heavy work,so was getting away with it. I wouldn't base your decisions on this info,though.

[Todd Willhoit]

You don't need the dual voltage motor, you just add a simple single-phase step-up transformer to the input of the VFD (rated for the 400V input). Autotransformers (single coil with taps) are reasonably priced.


Ian,
You're getting bad information from manufacturers, and some of it is simply badmouthing their competition.

Rick hit it on the head.

Mains input circuits are pretty basic for single-phase in/three-phase out inverters. Generally, either it is a purpose-built unit with a single-phase input rectifier, or it is a derated unit with a three-phase input rectifier potentially, though not always, using only 2/3 of the diodes. As compared to its full-rated sibling, the derated three-phase variety will have one less wire (or PCB trace on a small unit) on the input circuit, a different rating sticker, and potentially a software modification to allow for increased DC bus ripple. Inverters usually use single-phase control power derived from two of the three incoming mains. Warranty issues aside, in the case of a three-phase unit, if the third incoming leg has no specific current or voltage monitoring for fault purposes, it is almost certain that the inverter can be used in a derated, single-phase input scenario. You will need the help of the manufacturer's tech support line or a competent distributor or repair shop to confirm.

Todd

[Rick Christopherson]

Ian, take another re-read through my posting. All of the issues you brought up are addressed.

What I am suggesting is that you use a 230 to 415 step-up, single phase transformer to power a 415V, 7 hp, 3-phase VFD which powers the motor. Yes, this VFD is rated for 3-phase input, which is why I said 7 hp VFD instead of the 5 hp VFD you could normally use for this motor if you had 3-phase available. The VFD is larger (oversized) to accommodate the single phase input. No, the sales guy probably won't tell you that you can do this, but it already appears that he is not telling you the proper information to begin with.

Because you won't be using the full 7 hp from this VFD, it will operate from a standard 32 amp circuit. It should be drawing somewhere around 25 amps at maximum 5 hp load from the 230 volt supply. (The output from the transformer-side will be 14 amps).

Also, don't forget that if you use a phase converter, you would still need to use a transformer before or after a rotary converter to give you the 415V that your motor needs to see. This is why I am suggesting that a VFD is still the best alternative in your case.

A dual voltage motor would be nice, but all that means is that its internal windings can be reconfigured to run at 230 volts. This would eliminate the need for any transformer, but since your motor cannot be configured in this manner, you don't have a choice and must power it from a 415 V transformer.

If you haven't done so already, it might be helpful for you to read through a couple of my articles. They were not written for the UK audience, but the general concepts are the same. They won't tell you how to solve your problem, but they may give you some of the background information to help make your situation more understandable.
http://www.waterfront-woods.com/Articles/Electricity/electricity.htm
http://www.waterfront-woods.com/Articles/phaseconverter.htm

[ian maybury]

Thank you very much Rick, i think i've got the picture now. I was (a) seeing non availability of 7hp single phase inverter here as a show stopper (when you meant use a de-rated 3ph unit as you describe above), and (b) mixing up my star and delta by thinking that delta related to the lower of the voltage options - which kept me searching for a way to wire the motor for 220v when it's not possible.

Pardon my slowness on this, electrics have never been my strong point. You've all been more than generous, and have given me clear options to work from. It's time now for me to go away now and do some reading.

Thanks again

ian