Minilathe/Mill motors

I have had a 7x12 'chinese' lathe and mill (X1 type) for many years now, and have had the usual trials of failing control boards and motors several times. I know there are many different motors/controllers fitted to these machines, but I refer to the older versions (I think 250W or 350W brushed motor)

While fixing these problems, I have collected various circuit diagrams and info relating to these machines, and as a result have a question:

The circuits for the European and American controller boards (for the lathe, for example) are very similar ( number is FC250J, but the same question probably applies to the uprated FC350J).

The DC voltage for the motor in these circuits is derived from a bridge connected directly to the incoming AC Mains - so in Europe this should be 230 x root(2) = 325 volts DC, and in America 120*root(2) = 170 volts DC. (I think).

There are some differences in resistor values in the control circuits from the two areas, presumably to compensate for this difference in voltage, but there appears to be no difference in the bridge DC supply for the motors, implying that a much higher voltage is fed to the European version. The drive circuit (via two IRFP450 IGFTs) is apparently identical in all the circuits I have.

Does anyone know if a different motor is fitted in the two areas? There seems to be no indication on the motors themselves. I ask because I think one of the motors I have came from the States, and I would like to use it.

I think that you will find that the motors are 180v DC, I stand to be corrected though. The motors are the same but the boards are different to accommodate the different voltage supplies. Regards Pat

Edited By Pat Bravery on 28/03/2020 13:00:19

The speed of a brushed DC motor is proportional to the applied voltage. Rather than alter the DC voltage, speed control on the these machines is done by pulse width modulation (PWM) of the motor voltage. So to run a 180V motor on a nominal 325V bus you simply limit the PWM to less than 100% so that the motor never sees more than an average180V. That'll be why there are different value resistors in the control circuit. At least that's the theory, anything goes, right or wrong, with these cheap boards.

Andrew

I don't have information on the motors fitted to thee machines but there is no reaon why the same motor could not be used on both.
The motor controller uses pulse width modulation (PWM) to control the power supplied to the motor. Basically the mins is rectified to DC of around 340 or 170V (you are correct in your maths, I used 240V) this is then switched by the transistor (the IRFP450 is a MOSFET not an IGBT) to acheive the correct power. This switching is at high speed, tens of thousands of times per second, and the ratio between the on an off times (pulse width) control the power delivered. For example for 120V mains and a 170 volt motor, transistor off all the time (0%) is 0V and no power, on all the time (100%) is 170V and full power. So if it is on half the time (50%) the average power is 50% or 85V. If we leave it at 50% and change the input to 240V AC we get 340/2 =170V or full power using the same motor. Note that the peak voltage is double that of the 120V input case but the avrge current is less. The pulses are fast compared to the inductance of the motor windings so the motor responds ot the avergage value, any where between 0 and 100%
The control circuit has feedback and can be used to control voltage (speed), current (torque) or more likely in our case speed feedback with voltage and current limits.

So as long s the circuit is designed for the worst case voltage and current for the voltage range it oes not require any changes to work with both 120 and 240V. Note however that while average power is the same, in the 240V case peak voltage and current iss higher so components and motor insulation are more stressed . This may result in less reliable operation if components are marginal. Also with thi simple aproach the feedback, if digital is less precise on 240V than 110V. This is because it is only using half of the control range, 0 to 50% for 0 0 100% power.

Better designs use a switchable voltage doubling rectifier and storage capacitor arrangement that produces around 340V with either 115V (doubled) or 240 (full wave) rectification this can be link or switch selected or even automatic. This design can usually be identified by the presence of two identical large electrolytic capacitors in the rectifier circuit. These are in series for 120V and parallel for 240V operation.

The ideal set-up is a 300 Volt rated motor with a voltage doubler when used on 120V. The lower rated motor gives some overhead in the control for losses and lower than nominal mains voltage.

Robert G8RPI.

I need to type faster..... and SWMBO gave me a chore in the middle of typing.

Robert G8RPI.

Could you possibly scan the variants of schematics you have and post them here? Could perhaps then give better advice re the differences between them, and the implication regarding the motors you have in mind.

Robert, if you read as posted : (via two IRFP450 IGFT's) - you may have read IGBT, but AnOther did NOT say they are IGBT's - he is quite correct - those devices ARE insulated gate field effect transistor, also known as MOSFET's....

Also, you said - This design can usually be identified by the presence of two identical large electrolytic capacitors in the rectifier circuit. These are in series for 120V and parallel for 240V operation.

Not so...The capacitors are normally in SERIES for 240v Operation.....parallel connection for 240VAC would require a change of underwear.

AnOther, If we can see your schematics, it will be a doddle to see if the DC bus is run as a doubler, etc.

Joe

Just an observation, but is the bridge rectifier an ordinary bridge rectifier?

I've come across an arrangement like this where two of the bridge diodes are replaced with SCRs, which presumably allows the 'bridge' to chop the DC output as necessary to control the motor.

Same motor on 110V or 230Vac input - what it sees as DC is an average chopped by the circuit controlling the SCRs. (Control circuitry not shown in my diagram.)

Dunno what my mini-lathe PSU was like - never had to fix it! The motor was 110Vdc.

Dave

Thanks for the inputs - given me something to think about - just a few comments:

I have seen the operating voltage of these motors mentioned in many places, and frankly they seem totally contradictory - some sources say as low as 80VDC, others 120VDC, and yet others up to the 180VDC mentioned here by Pat, who also notes the boards are different. This may be the case, but as I said in my first post, the power supply for the motors is the same.

I know they use PWM for speed control, and that simply limiting the on/off ratio of the pulse would limit the average voltage to the motors, but as I said, I have the circuits, and the ones I have (UK and US) show no difference in the PWM control circuitry itself. There is a current feedback loop, which has a different sensing resistor for Europe (0.33 ohm) and for US (0.22 ohm) - and a startup compensation loop, which appears identical in both cases. So I agree with Andrew, and also with his comment that anything goes.

Roberts explanation is interesting - it also seems to explain why the European version suffers from damaged motors (I have three, all with internal shorts or open circuit (in the windings unfortunately). I appreciate the explanation of improved versions, but I want to remain with the existing system if possible - I have various spares, and a fixed pension

Joseph - I don't have the means to scan the circuits at the moment (and its a damn nuisance!), but I also have worked in electronics for many years, and can tell you the motor supply is definitely NOT run as a doubler. It has L&N connected to the inputs of a bridge (an S4VB as marked - also risky - its only rated at 4 amps). The positive output goes directly to the motor via the current sense resistor mentioned above (and a relay contact). The other side of the motor goes via the 2 parallel IGFTs to the negative rail. There are no large capacitors anywhere - none at all in the motor power supply - its a raw full-wave rectified supply.

The PWM switching IGFTS are switched by the PWM signal fed via two optoisolators to the IGFT gates. (I think one optoisolator turns the IGFTs on, and the other is used to speed up the turn-off - they are definitely NOT connected in parallel, or one to each IGFT). The 18V power for these IGFTs is also derived from the raw motor supply - just an R/C/Zener supply.

So as you see, a very simple circuit. It was the apparent lack of any obvious or failsafe method of limiting current through the motor/IGFT chain that prompted my question. Most of the failures I have had were shorts in the IGFTs, leading to the motors running flat-out with no speed control, or burned out motors - and now I am getting a better idea why. The IRFP450 has a continuous drain current of 14 amps, but its power dissipation is only 180Wmax. I believe the motor is rated at 3A (I stand to be corrected), so if a European motor is run at 180 volts, the power is going to be possibly over 500W (3 x180), which cannot be correct if the motors are rated at 250W, or even the newer 350 Watts - this suggests either a lower average operating voltage, or a higher current rating. It also accounts for the two IGFTs in parallel. It is not difficult to see that a stall condition could lead to high current, followed by IGFT failure, followed by motor failure - hence my questions about the motors.

Dave - you are quite right - I have a circuit for a motor controller which has the configuration you describe: the XMT 2335 fitted to Sieg C3 machines has this configuration - but as I noted above - I need to stay with the original configuration. The bridge rectifier in the FCXXJ series is just a simple silicon bridge - an S4VM in my case. I have seen a board with separate diodes fitted in place of the bridge.

Many thanks for the inputs - I'll go away and see what I can modify. (Been considering using an Arduino UNO to generate the PWM control, but its still a work in progress until I am happy with the motor power side.).

Edited By An Other on 28/03/2020 18:58:06

SOD, this is an SCR controlled bridge. These are old technology now. The switching rate is limited to 2 x the line frequency (100 or 120Hz often causing audible noise) and as (most) SCRs can't be turned off they stay on until the next zero crossing of the mains supply. They have been made obsolete in most applications by the development of high voltage and current power transistors.

Robert G8RPI.

Edit as crossed with other post.

Edited By Robert Atkinson 2 on 28/03/2020 18:57:53

I know my Warco mini lathe uses a 180V motor cos I had it in pieces yesterday. A couple of years back the original membrane controls and controller got wet from a leak in the shed roof and promptly died. I replaced it with a controller from BICL in Canada (Beel Industrial Controls Ltd) but only lashed up the control panel, including gaffer tape. So yesterday I made an aluminium panel as it was about due, hence having it in pieces.

Simon

Thanks Robert, makes sense!

Dave

this is an SCR controlled bridge. These are old technology now. The switching rate is limited to 2 x the line frequency (100 or 120Hz often causing audible noise) and as (most) SCRs can't be turned off they stay on until the next zero crossing of the mains supply. They have been made obsolete in most applications by the development of high voltage and current power transistors.

By no means obsolete - still standard technology for industrial DC drives. Currently available from Sprint Electric, Parker, Control Techniques, Baumueller & others.

Nigel B.

I have SCR controlled bridge on my Warco WM180 and WM240 lathes, the only difference is a pluggable resistor. However, one board blew and from memory I replaced the SCRs with 20A versions instead of 10A as originally fitted