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Schaublin 135 - Help upgrading electrical system


Hot Rolled
Mar 24, 2020
Hi all, I have a recently 're-juvinated' 135 which I have documented the process of on this forum 'Project Schaublin 135', a huge amount of help and pointers from the group along the way made the process much easier which is fantastic, I am in the process of adding a 2 axis DRO system to the 135 on this thread but wanted to explore another challenge too - the electrics.

I felt it needed a new thread and as always looking for help as I am NO electrician. I did completely change the electrics on a Hardinge HLV a few years back, it was tough but all worked out in the end so hence this thread. Picture posted of the finished electronic bay.

Like all machines (and classic cars/bikes) it's normally the electrical systems which are the achilles heel and in the case of my machine a few items are not working as they should, brake and high speed range spring to mind, there are others too.

The 135 has a two speed main motor (3ph), a coolant pump and the oil circulation pumps, not sure of the type or voltage on these, I do have drawings of the electrical circuits but have not fully studied them.

So clearing out the old system and its archaic components and replacing them with newer devices should/would be good IMO, it cannot be that complex for someone in the know and I would love to take a close look at what's involved and what components would be required to bring the electrics into the 21st century.

- So has anyone already done this, first obvious question and if so would they share the process/outcome/design?

- If not who might be in the know to potentially guide someone like me through the process? I have the ability to build, assemble, solder and wire such a device just would not know what components and where to start to design a replacement system.

Anyone able to guide/assist or point me in the right direction?

PS one question, my 135 is powered by a phase converter, input 240v single phase (UK) and outputs 370-400v 3ph to the machine, the load on the convertor so I have been told when high range is selected creates a voltage drop which kicks the contractor open (sprung loaded item in the original electronics) on the lathe and results in me rarely being able to use the high speed range as it trips out. It's not the end of the world as most jobs can be achieved at the mid speed range but it's one thing I would like to fix in this process.

Would prefer to deliver the power via the phase convertor as don't want to change out the motor (not sure you can get a 2 speed 240v motor of the required HP anyway). I guess what is required is when selecting high speed the motor speed ramps up over a few seconds rather than bangs right in, so designing this into the electronics to ramp up the motor speed on start up would I think be a worthy consideration. Naturally if I'm talking out of my rear end on this then put me straight :)

OK lets see what you say ........

HLV new electronic bay:

Do you have the amperage available to increase the phase converter size and are you feeding the current converter with enough single phase amps to fully utilize it's capacity? Does the motor have two separate windings or is it a Dahlander type where poles drop out of a single winding to increase speed? Those tend to be hard starting so a soft start might help. Others here will know more. Dave
Soft starters are cheap enough nowadays Put one between high speed contactor (or overload relay)and high speed leads to the motor
I am in the camp of do not fix what is not broke So I would just solve the electrical problems it has and continue with life
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Life has to be fun, if there is a challenge to along with that then great - the end result is always more rewarding.

So any electrical guru's out there who can help with this?
Peter wise words ......

This is what I have


  • wiring diagram lower front 135.pdf
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  • 135-99023.pdf
    4.7 MB · Views: 54
I am sure if I took a while and worked it out that I could make some headway but the reason for posting is I'm looking for someone with electrical credentials to assist and come up with a 'new version' that I can make with modern electronics.

I studied engineering at college but not electronics sadly.
Mark, I see both your point of view and Peter's. In your shoes I would do the following.

(0) Make minor fixes to get everything working (fix brake, fix shutoff on switch to high speed). Although I do not have a Schaublin, the wiring diagram is simple enough that I can do some handholding with that.

Once everything is working and you can use the lathe a bit more, only then would I contemplate the other two possibilities:

(1) Replace all of the old electrical components (relays, contactors, etc) with modern ones, perhaps in a new enclosure, or

(2) Modify the system to have a two phase input. This is possible if you add four VFDs, one each for the oil and coolant pumps, and then two for the spindle motor, one for the low speed winding and one for the high speed winding.

From reading your other thread through, I see that you have bought a 3-phase rotary converter which (if I understand correctly) produces enough power for the 135. If that's right, I would focus on step (0) first.

Let's start by fixing the brake, which I think will be very easy. The first thing to check is the full-wave rectifier, marked n1 in the diagram. Can you locate this device? I've attached some typical photos below. It has two inputs, coming from a transformer secondary, and two outputs. I would disconnect all power from the lathe, locate the transformer, identify the secondary windings, and trace those to the rectifier. Alternatively, figure out how the numbering in the circuit diagram corresponds to the numbering in the cabinet and locate the wires that way.

When the lathe is turned on, you should be able to measure an AC voltage (probably in the range 20-30V) on the two leads coming from the transformer and a DC voltage (probably 20-40V) on the other two leads. (It is also possible, but unlikely, that this device is made from four discrete diodes.) Try to locate it, and if you are not sure, post a photo, or ask other Schaublin owners. Alternatively, identify the wires or connector to and from it.

Regarding the spindle motor, according to the documentation and/or motor dataplate, in the high-speed setting how much current does it draw? If you measure your 3-phase power voltage between any two of L1, L2 and L3, does it measure 230VAC or 400VAC?

[EDIT] Found your motor nameplate in the Project Schaublin 135 post:

Screenshot 2023-07-19 at 10.51.13.png

So your line-to-line voltage should be 380 or 400VAC, and your motor draws 6.9A in the high-speed mode.

To fix the problem of the contacter opening when you turn on the motor in high-speed mode, I would purchase three of these NTC (negative temperature coefficient) inrush current limiters:
and put one of them in series with each of the three high-speed motor power connections. These NTC inrush current limiters are rated for 7A continuous. (Digikey UK has 22,000 in stock and they cost 1.54 quid in quantity one, so if this fixes the problem its a good deal, and if it fails to fix the problem it's not a lot of money lost.) The devices get hot (this is how they work!) when operating so I typically install them mounted high up on their legs and somewhat distant from other components. Their cold resistance (20 ohms) will limit the inrush current when the motor starts to less than about 11 amps. (This is how I successfully fixed the problem of the transformer in my Studer cylindrical grinder popping the circuit breaker of my workshop.)


Screenshot 2023-07-19 at 10.37.12.pngScreenshot 2023-07-19 at 10.36.29.pngScreenshot 2023-07-19 at 10.35.45.png
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Hi Peter,
The industrial solution to limit inrusch current is a slowstart
Sure, that's correct, but the price is also industrial, at least a few hundred dollars or Euros I think. I'm providing the hobbyist solution which has worked well for me and costs pocket change.

The basic function of an NTC inrush current limiter is as follows. When you first close the circuit, the device looks like a R=20 ohm resistor, which (together with the resistance/inductance in the motor windings) limits the current flow to say I=10 amps. That current flow generate quite a bit of heat in the NTC (I^2 R = power = 2kW) which heats it up in a fraction of a second, from it's normal 25-40C to 100C. The device has a negative temperature coefficient, so by the time it reaches that temperature, it has a resistance of say R=0.05 Ohms and the power dissipated in the NTC drops to 5W. That's enough heat to keep it at 100C, so it continues to basically act as a slightly lossy wire.

In my (very limited) experience this is a good way to avoid motor and transformer inrush currents of (say) 50 amps or more, which might be what are causing the voltage sag and contactor dropout problems.

If you have a good source for an inexpensive 7A three-phase motor soft starter, please share it.

I bought one once for a Weiler commador And it was about €160
And I found this one at RS online Not the cheapest for €164
Elsewere I found it for €125 ex vat

ABB 3 kW Soft Starter, 208 → 600 V ac, 3 Phase, IP20


Elsewere I found it for €125 ex vat

ABB 3 kW Soft Starter, 208 → 600 V ac, 3 Phase, IP20​

If the OP can spend a bit of money, then I agree that it is a much better solution that what I proposed. It also provides a soft stop, and lets you control the start ramp time, stop ramp time and the initial/end voltage level.

If I were doing this, I would probably spend 50% more and purchase a 3kW 3-phase-in 3-phase-out VFD for the motor's high-speed windings. These VFDs are available for under 200 Euro and provide soft start, soft stop, and motor protection as well as variable speed, reverse, and other features. There are also more VFDs on the used market, so it might not cost any more than a soft starter. Peter, does that make sense to you? Or am I overlooking something?
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I am a dealer and in my experience machines bring most if as original as possible
So in that perspective I would go with the soft start as that keeps alteration to a minimum and also look proffesianly
If it would be for my own use I do not have a problem with a VFD Perhaps buy one with the possibility to program 2 motors
so you need only one for both speeds
I would have gone all VFD and skip the phase convertor

Bruce wow thank you for a very informed post, I had written this off as a project as I don't have the time to learn another skill right now and was hoping to get some guidance (exactly what you have provided) - brilliant.

OK I will take a look at the suggestion for the brake first, according to the diagram n1, below is a picture of the main electronics panel and the item marked n1 is based on the wiring schematic the blue square item in the top right hand corner under the three switch units, does this look right?

The transformer according to the diagram is behind the side panel but behind the contactors so not sure I can easily gain access so its a matter of trying to check n1 and see if the wiring is obvious, can then check the voltages etc.

If the output and inputs are as you state I'm assuming that would indicate n1 is functioning correctly so if this is the case what would be the next check? When I re-built the brake I did apply a DC voltage to the coil in the brake to check it worked and it magnetised fine.

Picture of the electronics with n1 in top right ...

Hi Mark,

Blue item looks right except that I expect four connections not three. Undo the mounting nut in the middle, pull it off, and take a better photo please.

A full wave rectifier typically has the four contacts marked as follows. Two of them will have a wiggly line, indicating alternating current (AC). Those are the inputs from the transformer. The other two contacts will be marked + and -, which are the direct current (DC) outputs.

According to the circuit diagram, when power is applied to the transformer, then there should be an AC input voltage and a DC output voltage. For this kind of troubleshooting I like to start upstream and move downstream. So the very first step is to verify that when the transformer is powered on, you have that AC and DC present at the correct terminals of the rectifier. Then we'll move downstream, one component at a time.

Bruce interesting, will remove and check and take some pics. It does seem to me this item is not 'original', the soldered connections look messy and it just seems it was replaced at some point, might be an indication this part of the machine has been 'tinkered with'.

If anyone has this model 135 and can check their machine for this item and let us know if it looks original that would be a great help.

All I do know is the electromagnet works as this was checked so hopefully once the right feed of power is sorted it will function as intended.