Arboga EM825 motor rewind

**WCD** · 9th April 2012, 07:55 PM

Recently I purchased an EM825, 'heavy' version. It has the usual 415V, 2 speed, 3 phase motor, and came with a Eurotherm (WEG) VFD for single phase 240V supply. The 415V motor did run OK on 240V via the VFD, and I set the parameters on the VFD as per instructions on the Inverter Drives website ( HowTo: 240V Supply to a 400V AC Motor - Application Detail ) but found there was little torque available at higher speeds - I could easily stall it with a (gloved) hand on the spindle nose. It's possible that better VFD parameter settings might improve the situation, but after few loops of parameter adjustment without improvement it was time for other options.

Options considered:
Installing 3 phase mains
Use a VFD that delivers 415V with 240V supply
Use a rotary phase converter plus transformer
Rewind the Arboga motor for 240V 3 phase operation from the existing VFD.

Aspects considered:
Cost overall.
Positives of using a VFD (speed control, soft start, braking, ease of reversing, etc).
Negatives of using a VFD (more complexity in control circuit; risk of cooking the
VFD if motor speeds are switched with VFD on).
Space required – the EM825 is very compact, and I don't want to waste that
advantage with extra items such as a rotary phase converter.

Final decision was to have the motor rewound to 240V three phase, seemingly a simple matter except for the unusual motor design, which is variously described as a 'Dahlander', 'pole changing' or 'consequent pole' motor. Phone calls to motor rewinding outfits were not encouraging, with uncertainty expressed about the type of motor, and talk of high costs if they needed to work it out from scratch. Time for some research.

It turns out that Mr Dahlander was a very smart lad who designed this circuit more than a century ago. It gives two motor speeds without using more copper (or a bigger frame size) than required for a single speed motor, but the high speed is always twice the low speed. There are winding variations for motors giving constant torque, variable torque and constant power. Testing winding resistances with a multimeter revealed that the EM825 windings are arranged in delta configuration for the low speed, and “double star” (each leg of the star has two windings in parallel) for high speed (see picture). One consequence for Arboga owners is that the windings cannot simply be reconnected internally to convert from 415V to 240V operation.

I did think about having the motor rewound as a typical 4 pole three phase motor and relying on the VFD to get up to 2800rpm. That would avoid concerns with the Dahlander two speed winding, but it was not clear to me if the stator slot configuration was suited to a standard 3 phase 4 pole winding, and the stator windings would need to be designed from scratch, so that idea was binned.

So the motor stator was removed from the EM825 and together with a diagram of the delta double star winding, and some fears about what would happen to the precious Arboga if they lost the stator, was taken to a motor repairer, who said “How much do you want to spend?” Not an encouraging start, but in the end the job was done for less than $300. On 240V 3 phase supply via the VFD, the motor now runs very well at both low and high speeds. Switching the motor speed remains a worry due to the risk of killing the VFD if the speeds are switched without first depowering the VFD, so the plan is to simply run the motor in 4 pole mode, and use the VFD to get the higher motor speeds when needed.

The motor rewinder kindly provided the following details of the job, which may be of interest to anyone else considering this:
Original nameplate specs:
3 phase 50Hz 415V speeds: 2810/1390 Amp:1.6/1.3 kW:0.66/0.48
General winding detail:
Slots 24
Coils 12
Pitch 1-6-8 (I'm not sure what that means)
Coil arrangement D, Y-Y (delta, double star)
Original coils for 3 phase 415V: 213 turns of 0.5mm diameter wire.
New coils for 3 phase 240V: 123 turns of 0.67mm diameter wire.

Removal and replacement of stator
The stator is locked in place by two setscrews, but removing these did not release the stator, and "persuasion” was required. I quote from the EM825 user manual: “..bang the spindle housing against a suitable surface until the stator comes out" (straight talk in these old manuals is a delight!) My method was less elegant and caused only minimal damage to the laminations, but using a pressing ring (see below) would have been much better.

When replacing the stator, I took a piece of 50 x 3mm MS flat bar 395 mm long and, with a soft face hammer and the horn of an anvil, formed it roughly to a circle – see photos. This ring was placed over the end of the stator to protect the pristine new windings, and the stator was pressed home (inverted truck jack in a press frame). Plan B was heavy threaded rod, nuts and washers etc, but fortunately I had access to a press (and an anvil).

All that needs to be done now is make up an extra nameplate showing the new voltage and current. But I'm short of round tuits.

Bill

**Bryan** · 9th April 2012, 11:06 PM

Tidy work Bill. Welcome aboard. We like a man with solutions.

**jhovel** · 9th April 2012, 11:18 PM

Bill,
thanks very much for that infomative article!
If you want to use the high speed (2-pole mode) of your motor, You can always switch it with the VFD off. Then you have full control over two speed (and power) ranges. As far as I can tell, the parameter setting for 2 and 4-pole in the VFD is only related to the indicated RPM. If you ignore that (or double it for 2-pole mode), then you don't have to do anything else.
I have had a 2-pole motor running on my VFDs without achning any settings at all and it ran happily at 120Hz (with a measured speed of 6430rpm). At higher frequency it started to slow down so I stopped the experiment and labelled the motor as '120Hz max.' (almost all my 3-phase motors will happily run at 120Hz bar one one which falters at 90Hz)
Joe

**neksmerj** · 9th April 2012, 11:43 PM

WCD, congratulations on acquiring an Arboga EM825 mill/drill, and your informative post.

I too have one of these machines, sitting in storage until I can find a place for it.

I faced the same dilemma as you, and in the finish, I purchased a 240V 1-phase to 415V 3-phase VFD from a company in the UK called Drives Direct.

It cost a little over $500 with remote control pod, so your price of $300 was more than reasonable.

Do have any photos of your machine?

Below are just a few links you might find of interest. Search Arboga EM825 for more.

https://www.woodworkforums.com/f65/ar...-drill-131401/

https://www.woodworkforums.com/f65/dr...rrived-130567/

https://www.woodworkforums.com/f65/ar...resent-128204/

Ken

**RayG** · 9th April 2012, 11:57 PM

Hi Bill,

Welcome to the forum

I note that one of the options you considered was 240 to 415 vfd, these are a normal teco vfd but with modified for 240 single phase in 415 3 phase out. PHASE CONVERTER, INVERTER items in DRIVES DIRECT store on eBay!

One other Arboga owner on the forum went down that path. I'm sure he would be interested to compare notes.

Rewinding the motor is a good choice, you could always put some kind of interlock on the speed switch, so you don't accidentally switch when the VFD is running. Not that you would of course..

From everything I've heard about Arboga, you've got yourself a nice machine

Congratulations on the 240V conversion.

Regards
Ray

Edit I see Ken beat me to it..

**Stustoys** · 10th April 2012, 10:08 PM

Hi Bill,
An interesting read and a job well done.
Have you tried the glove test again?

Hi Ray,
Reading Bills link, is it telling me that for 415V motors running on 240V it would be a good idea to set the base Freq to 29Hz? (PD004). But this would only make any difference if you were running the motor at less than 50Hz(assuming you have a the base freq set at 50) correct?

I'm not sure I understand the part about matching the full load current of a 400V motor to the output current on a 230V VSD?(the entries for 0.12kW and 0.18kW both having the same Amps isn't helping). I guess its trying to tell me I can run a 5.4hp 400V on a 3hp 230V VSD? Though it seems a strange way to go about it.

Stuart

**morrisman** · 10th April 2012, 10:21 PM

Originally Posted by Stustoys

Hi Bill,
An interesting read and a job well done.
Have you tried the glove test again?

Hi Ray,
Reading Bills link, is it telling me that for 415V motors running on 240V it would be a good idea to set the base Freq to 29Hz? (PD004). But this would only make any difference if you were running the motor at less than 50Hz(assuming you have a the base freq set at 50) correct?

I'm not sure I understand the part about matching the full load current of a 400V motor to the output current on a 230V VSD?(the entries for 0.12kW and 0.18kW both having the same Amps isn't helping). I guess its trying to tell me I can run a 5.4hp 400V on a 3hp 230V VSD? Though it seems a strange way to go about it.

Stuart

this is a advert , but may help you out ...

eBay.co.uk Guides - 3 THREE PHASE CONVERTERS INVERTERS 415V DIGITAL ROTARY

**Stustoys** · 10th April 2012, 10:52 PM

Hi Mike,
I was talking about the link in the first post referring to running 400V motors on 230V VSD(at reduced power)
The Drives Direct VSDs seem pretty good but they sure aren't cheap.

Maybe I should have moved it to another thread?

Stuart

**morrisman** · 10th April 2012, 10:58 PM

Originally Posted by Stustoys

Hi Mike,
I was talking about the link in the first post referring to running 400V motors on 230V VSD(at reduced power)
The Drives Direct VSDs seem pretty good but they sure aren't cheap.

Maybe I should have moved it to another thread?

Stuart

I was wondering about that myself . Like how does a 415v 3 phase motor run on 240v 3 phase . Is the motor power halved ?

MIKE

**Stustoys** · 10th April 2012, 11:10 PM

Originally Posted by morrisman

I was wondering about that myself . Like how does a 415v 3 phase motor run on 240v 3 phase . Is the motor power halved ?

MIKE

More like 60% as I understand it.

Stuart

**Ropetangler** · 11th April 2012, 12:06 AM

Negatives of using a VFD (more complexity in control circuit; risk of cooking the VFD if motor speeds are switched with VFD on).

Bill
[/QUOTE]

Bill, this issue could be sorted I would think by either disconnecting the existing speed control altogether, and using the VFD for all (electrical) speed control.
OR using any built in motor speed switching on the Arboga, to control the VFD, which will then control your EM825. In other words, you would not use the original switching controls to switch current between the VFD and the motor, but merely replicate the variable potentiometer by giving 2 set speeds, rather than a range of speeds available with the use of the pot.
You may have to be careful not to have any time when switching speeds, when no resistor network is in circuit, - it may be that the VFD might go out of control if the voltage it expects from (the normally used) pot goes AWOL, if you used a centre off switch like a reversing drum switch for example. You may need to have a make before break set of contacts, to stay kosher, I simply don't know.
Your work so far though looks great, and as others have said, your write-up most informative. Cheers,
Rob.

**neksmerj** · 11th April 2012, 03:18 PM

I reckon when the time comes, I'll switch my Arboga to the high speed setting (star connection), then tape the switch up or remove the knob.

All future speeds will then be via the VFD. Are we agreed?

Still haven't figured out where to install an emergency stop button.

Stu, you have a similar Drives Direct VFD, where did you put your E Stop?

Ken

**Greg Q** · 11th April 2012, 03:36 PM

Ken, my Arboga drill clone is wired using the low speed windings on a simple VFD. My reasoning was that the low rpm range is used for large bits where torque matters. The higher range is used for smaller bits where torque is less of a factor. This allows me to use the frequency band from 30 to 150 Hz without cooling or power loss concerns as well as having the gear box as a torque multiplier.

Greg

**Stustoys** · 11th April 2012, 03:40 PM

Hi Ken,
I think you might be better of setting it to low speed and using the VSD to increase the rpm. If you set it high and use the VSD to run it at half rpm you will have half the power. If you want low rpm you are likely running a big cutter and will want all the power you can get.

If you set it low you will have full power at low speed, when you over speed it you'll lose some power, but I wouldnt think it would be half(?) and you'll be using a smaller cutter so are less likely to miss the power

Still you can try it both ways and see.

I use the machine wiring for start,stop,E stop. Technically it isnt an Estop, but at least its all NC wiring, it just turns off spindle and powerfeed by pressing the big red botton instead of the two little red buttons.(though one of these days I may have a foot pedal switch/brake).
I dont like the Estop on the Teco drives as it uses a NO circiut.

Stuart

**neksmerj** · 11th April 2012, 04:17 PM

Thanks Greg and Stu, why aren't you blokes at work?

Low speed setting it is. Probably better to leave the high speed/low speed switch in tact, and judiciously switch between the two settings AFTER TURNING OFF THE VFD.

Maybe I don't need an E-stop anyway. The remote control pod I have has an OFF position.

Ken

Edit. Greg please reassure us you are not sitting at the controls of an Airbus with a laptop on your knee!

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