Single phase 230v supply: Powering 2 speed dahlander motors with VFD

[JS540]

I have a 230v 40A single phase supply to a workshop, bringing 3 phase onto my site would cost me over 7,000 along with a higher kWh tariff so I have a problem powering an Arboga drill press and large pedestal grinder.
Both motors are 380v proprietary units built into the machines and research on the internet says people have attempted to break out the windings to no avail. I have several other old machine tools with 220/380v motors, spindle motors, coolant and table drives, all of those could be wired for 220v operation and each fed via a single phase input overrated VFD. I am aware that main motor energising or speed selection must be done with a stationary spindle and the VFD in stop condition.
I do not want an RPC or static converter in the shop and have no wish to discuss them. There is a small business in the UK that modifies Teco drives to accept 1PH 230v in to 3PH 380v out using I believe a voltage doubling circuit. I am not too keen to buy because the company is tiny, if it goes bump my warranty is dead and a one year warranty for an £800 to £1,300 spend is not good.
My thoughts are to buy a 5kVA 230v-380/415v single phase isolating transformer and feed a 3 phase VFD of suitable size. The transformer I have a quote for of £430, suitable candidates for drives can be bought second hand for £200. The idea being that the transformer should last my lifetime if protected and second hand VFD only get cheaper so can be replaced at reasonable cost.
I am a one man operation and rather than wire my shop for 3 phase my intention would be to mount the transformer and VFD on a small cart which would not be more than 3 metres from any of the machines and use plugs on the machines. I save on copper and keep the VFD output lead could be kept to 1 metre by moving the cart closer to the machine requiring power.
Is there anything badly wrong with this idea?

 

[waross]

230 V @ 40 Amps = 9200 Watts max.
That will max out at 9 or 10 HP total at any one time.
The 5 KVA transformer will max out at about 5 HP.
Mount the transformer on the wall. The cables feeding the transformer will be heavier and more expensive than the cables from the transformer to the VFD.
For two speed motors use the higher HP connection.
If the motors are similar, it should work.
Part of the set-up of a VFD is an auto-tune that matches the VFD to the motor.
I'll defer to jraef as to the possible problems and possible compromises when using different motors with one VFD setting.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

I have an old (1940's) Holbrook lathe running from my domestic 240V supply via a 240/415V step-up transformer and an old A-B 1336+ drive I acquired on the aftermarket. It works well, even under a fairly heavy cut.

Thoughts...
[ul]
[li]Run the VFD as a scalar drive. Performance won't be quite as good as a vector drive, but likely good enough for what you're doing. The benefit is that a scalar drive doesn't need to be tuned to the individual motor.[/li]
[li]The Dahlander motor will be old, so there's a fair chance that the VFD could kill the winding insulation unless you use a dv/dt or sinusoidal output filter.[/li]
[li]Make sure the 3-phase input VFD you choose has access to the DC link terminals so you can add external capacitance to hold up the DC bus. There probably won't be enough capacitance in a drive designed for 3-phase input for it to be happy when it is fed from single phase, especially if you're working the machine hard. My 15kW 1336+ drive has 4700uF of external capacitance propping up the DC link, running a 5.5kW motor.[/li]
[/ul]

Hope this helps.

 

[JS540]

Scotty the adding of capacitance for maintaining the DC link, will simple overkill and an output filter smooth the way for the windings?

 

[ScottyUK]

No, it won't help the windings unfortunately.

The DC link capacitance is needed because a rectified 3-phase supply is fairly smooth and has relatively little ripple even without capacitance, as below

whereas a rectified single phase supply will drop to zero twice per cycle unless there's a significant reservoir capacitance:

Both images grabbed from the internet.

 

[jraef]

ScottyUK,
Adding additional capacitance runs the risk of over burdening the pre-charge resistor in the drive. Or did you put in your own pre-charge circuit on the external capacitors? That would be fine, but I wouldn't recommend adding caps without it. If you burn out the pre-charge circuit, the caps fail shortly thereafter and go boom.

It should be noted however that SOME 3 phase designed drives cannot accept single phase input regardless of de-rating. Those are often evidenced by reading the manual and looking for a "Phase Loss" protection feature, then seeing how to disable it. Those that have that protection feature and offer no way to disable it are the ones that cannot be fed with single phase power.

The rules of thumb I use on feeding a 3 phase designed VFD with single phase are to use a 50% de-rate (double the side of the VFD) with regard to the motor FLA IF the VFD has a DC bus choke, or use a 65% de-rate (motor FLA / .35) if it does not as is the case on many smaller low-cost Asian designed drives. This is because of the way the capacitors are designed into the new smaller-cheaper-faster drives now. The latest generation of drives use larger but fewer capacitors so they tend to run hotter and the added burden of the DC bus ripple in a single phase feed makes them exceed the design temperature limits fairly quickly, they swell and fail. So more de-rating allows for less over heating. Alternatively, you could de-rate the drive at 50%, but ALSO de-rate the ambient to 25C as well, but that's unrealistic for most industrial applications.

Side note: Another quirk with the latest generation of drives now is that you should probably do an auto-tune to the motor even if running in V/Hz (scalar) mode. The faster rise-time of Gen IV IIGBTs and the more "lean" design of modern energy efficient motors makes it more critical than it used to be that the firing algorithm be tweaked to the motor equivalent circuit, regardless of operating mode.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[itsmoked]

JS540; What is your largest motor?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[ScottyUK]

Hi jraef,

The 1336 is an old drive (as you know!) and possibly the design has a bit more margin on everything than newer drives. The pre-charge circuit is original and it's never missed a beat.I imagine that the impedance of the step-up transformer also acts to calm down the peak current in this type of arrangement.

I hadn't realised that gate drive circuits adapted themselves on the fly to suit the load, that's completely new to me.

 

[JS540]

Keith, the largest motor is 2kW.
jraef, so 50% derate on drive choice and observe temperature rating.

I read conflicting information on the internet (normal) regarding the energising of ancillary motors while the spindle motor is running. On the one hand it is a strict NO and on the other people report that if the motor is a low power coolant or table motor that the drive will tolerate the change of load, I would like to know without frying a drive to find out.
I believe the spindle motors on my machines are all between 0.75kW and 2kW including the two dahlander units. The table and coolant motors are between 90W and 200W. If I cannot energise the table or coolant motors while the drive is set to run then my plan B is to use three drives on the intended transformer cart. The largest drive for the spindle motors and two small drives for the coolant and table motors, none of the machines has more than one table motor.
I understand I will have my work cut out for me in regards to wiring the cart/truck and the machines too. I don't mind having to move a cart and connect cables by plug and socket but walking around the back of a lathe to energise the coolant is a schlep too far. I would need to provide some control gear on each machine and try and achieve a common wiring scheme from machine to power supply and possibly use a multipole plug and separate screened cable for the control inputs.
In my old place I had the luxury of 3 phase, I miss it but the view is nicer here.

 

[waross]

If the largest drive is 2 kW I imagine that the transformer will be about 2.5 KVA.
Feeder to the transformer,- 2.5 KVA @ 230V = 11 Amps
Transformer to drive,- 2.5 KVA @ 460V = 5.5 Amps.
Do you have local code restrictions on 460 Volts on a flexible cable?
Are you concerned with the safety of a portable cable at 460 Volts?
Possibly the cost of 460 Volt plug connector versus the cost of 230 Volt plug connectors?
I am wondering why you want to drag the transformer around the shop instead of putting it on the wall and feeding the cart with 460 Volts.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[JS540]

"Do you have local code restrictions on 460 Volts on a flexible cable?
Are you concerned with the safety of a portable cable at 460 Volts?
Possibly the cost of 460 Volt plug connector versus the cost of 230 Volt plug connectors?
I am wondering why you want to drag the transformer around the shop instead of putting it on the wall and feeding the cart with 460 Volts".
Waross I am unaware of any code restrictions and will check. Yes the transformer will indeed be weighty but I should not need to move the thing more than a few feet or few yards. It may be possible to mount the lot on a post and reach each machine without the cable fouling anything or being near my reach, behind or to the side of the machine to myself I mean.
Edit: The transformer I asked to quote was for 5kVA continuous in case of additional machinery with a more hungry motor.