Thread: single to 3 phase vsd confusion.

I have a rotary converter which gives me 415 Volt 3 phase from a 240 Volt supply through a 25AMP circuit breaker. I run 5.5 HP motor on my panel saw and 3 HP motors on a thicknesser and jointer. I have been running them since November and so far all seems to be well. No noticeable hot wires either on the 240 or 415 sides. I can't say that a VSD would be the same or not. Every time I use the machines I can't get over being able to run these industrial machines on a 240Volt supply. Its truly great!

Hi Lather,

5.5kw ( 7.5hp ) will need to be permanently wired by an electrician, the supply requirement exceeds power point limits. (similar to a large Aircon or stove )

Voltage spikes are what kills insulation, not current spikes, the circuit breaker for the VFD should be sized to handle the current spike on power up as the capacitors in the drive are charged. Provided the supply cable is sized for the length of the run and the peak currents, and protected by a suitable circuit breaker at the switchboard, you won't have a problem. Your electrician will choose the cable size and circuit breaker.

A vfd will produce voltage spikes on the outputs which depend on a whole host of factors, and especially if you have long cable runs between the vfd and the motor with no dv/dt filtering you might damage the motor insulation, but if your cable run is less than say 50' and it's a relatively new motor you won't have a problem.

One issue not mentioned, is that motors around that size (5hp and up) is where it swaps over from 415V star to 415V delta, if the motor in question is already wired in delta, you can't run it on 240V and get full output power

So before you start, check the motor. if it's 415 star and has 6 wires coming to the terminal strip, then you are in business.. rewire it for delta and run it on 240V 3 phase..

If it's already connected as delta, you have a problem...

Regards
Ray

my mistake, it was 5.5 hp.

have not purchased the lathe so far, until working out the extra costs involved.
plan on getting a hafco cl-38 lathe.

I think what he was refering to was large inrush current on bigger motors even though they kinder due to accel times they are not as bad as dol and this is more evident when a single to three phase vfd is used as instead of spreading the load across 3 phase you must supply the current through only 1 phase . To do that you would need seperate circuit with large conductors similar to a stove.

thanks for the replies.
the cl-38 most likely does have a 2 speed motor as it has a 2 stage switch similar to the al-1000d

this could be a stupid /confusing question, though could decide if it's worth purchasing the lathe, due to power costs on the increase.
would the larger lathe with a 5.5 hp motor use much more power, compared to a smaller lathe with 2hp if taking the same depth of cut, using the same feed rate.
it's obvious that the 5.5 hp motor will use more power due to more friction e.t.c., on top of the power needed to perform the cut, though overall would the power usage difference be negligible ?

is there any improvement if reducing the max spindle speed 50% by changing the pulleys,( don't use the high speeds),
in regards to reducing the max current running through the cable ?
power to the shed is through a 15 amp cable.

should the vsd have a higher output than the motor rating?

Hi lather,
The CL-38A has a 4kW Motor so you would need at least a 50A supply so the cable to the VSD and from the VSD to the Lathe Motor the cable would need to be at least 6mm2 and the MCCB would be 50A.
In setting up my Workshop I took these factors into consideration and ran the 6mm2 Cable, in your post you mentioned that you had a 15A supply which would mean that at best you have 20A Cable.
I suggest that if possible you should look at a new power run to your Workshop to accomodate the larger Load that your equipment is going to need.
If the Motor on the Lathe is Two Speed you need to look at how it is wired as it could be a Star/Delta 415 configeration not suitable for a 240 to 415 VSD.

Regards,
Keith.

Originally Posted by Keith_W

Hi lather,
The CL-38A has a 4kW Motor so you would need at least a 50A supply...

Are you sure about that Keith? Seems a huge amount of capacity for a 4kW motor.

Hi jack620,
The attached pdf's has the kW Rateings, MCCB Size and Cable Size for a 4kW Motor.
The VSD size I used as an example was a SINUS M 0011 2ST which is a Single Phase in Three Phase out model.

Regards,
Keith.

having to rewire power to the shed, plus the cost of at least a 4kw vsd could go over my budget.
looks like the best best option is to get a smaller lathe.

If the lathe has a dual speed motor then the slower speed will be half the horsepower of the high speed, so running your 5.5Hp lathe at the slower speed will give 2.75Hp. This is just one of those things for any multi speed AC motor, the slower speed gives less Hp.

Three phase motors are much more efficient than single phase motors, single phase units draw close to their full rated power even when there is no mechanical load on the motor. A three phase motor current draw varies with the mechanical load, so if you aren't doing 5.5Hp worth of cutting then the motor won't draw 5.5Hp. There's nothing to stop you setting up the VFD for a slow start to avoid huge start up current and also setting up an upper current limit for the motor to effectively derate the motor.

All that would be left is the short demand for current on powering the VFD up to charge it's capacitors up.

The former electrician is talking out his watsit when saying inrush current will kill the insulation, unless the wire is so tragically underrated it was getting very hot in the process. That being the case, you'd have bigger things to worry about ... your workshop possibly burning down being one of them! Further more, the VFD effectively eliminates inrush current anyway.

He was quite correct however in that you simply wouldn't run a motor this size off single phase. If you wanted the many benefits of using a VFD, then you would use a 3 phase VFD.

With regards the power consumption difference, unless you're planning on spending 18 hours a day 7 days a week standing at this manual lathe, I wouldn't worry about the difference in power consumption between a 2 or 4 KW motor doing otherwise identical work, the difference compared to all the other costs will be peanuts.

That's a pretty large lathe, what are you planning on doing with it?

Pete

Pete,
the lathe would actually be too large, there's not much choice here in Perth.
the lathe in the site below would be preferable, as it's a step up from the models with a 40mm bore, with a wider bed and in between
the H&F range from the al-1000c to the cl-38.

Engine Lathe (CQ6240X1000) - China Engine Lathe,Lathe Machine,Turning Lathe in Machine Tools

H&F do sell the al-356v with a 51mm bore, though it has disadvantages over the al-1000c as it has variable speed drive with a 2 speed gearbox, smaller width of bed, and travel of the top slide is only 68mm, e.t.c.
don't see the point of taking 1 step forward and many steps back.

anyone know if the CQ6240X1000 is sold over east ?, shipping it here would be cheaper than setting up the cl-38

Originally Posted by lather

having to rewire power to the shed, plus the cost of at least a 4kw vsd could go over my budget.
looks like the best best option is to get a smaller lathe.

Hi Lather,

Just some clarifications...

As Graziano already pointed out.. If you cranked up your 5.5hp lathe with no load, you'd find the current is nowhere near the full load rated current.

The circuit breaker supplying the power to the shed will have been sized to protect the cabling, so all that will happen is the breaker supplying your shed will trip when you load it down. Nothing is going to catch fire and burn your shed down...

However, I'd suggest that you plan to upgrade your mains power at some future date. 15amps is nowhere near enough..

As to the question of what is the maximum size for a 240V single phase to 240V 3 phase vfd, A number of VFD manufacturers offer single phase 240V input to 240V three phase out up to 10hp (7.5kw) beyond that you generally need to go to 415 3 phase

No forgetting that above 15A it needs to be permanently wired (in Victoria at least) I think NSW you can go to 20A. After that you need a permanently wired connection.

As an example of what you would need, here's my wiring for the surface grinder, which has 3hp spindle motor, and 2hp hydraulic pump motor, 1hp power cross feed and 1hp power elevation

The circuit is supplied from the sub board via a 40amp RCD, through the isolation switch you can see on the wall above the enclosure, the main spindle vfd is 4kw, the others are all 1.5kw



In addition there are circuit breakers for each vfd inside the enclosure.

When the spindle is running it rarely gets above a few amps.. no load current as shown on the vfd display is usually about 0.7 0.8 amps..

Regards
Ray

ray,
that's a neat setup.

Will be checking out the eastern states stores before selecting a lathe.
The cl-38 is too large and the only reason to purchase it is for the larger bore.

if i get desperate, may just purchase the al-1000c, though i doubt there would be much of a difference from the al-960b, apart from the price.