Boxford lathe motor replacement

[Robert Atkinson 2Participant @robertatkinson2]

When using an external speed control with the typical 0-10V control signal it is fairly easy to implement a dead slow switch. The circuit below will give a fiked slow speed of about 6-7% of maximum. If slower is wanted replace the 1N4148 diode with a 1k preset for a 0-10% adjustment.

There is one possible issue with this design. Depending on the drive settings the instant speed change could trip a speed difference error. Adding a low pass filter, as below, will slow the rate of change and reduce the chance of a trip. It will slow changes to speed though with the command still creaping towards the set point after a few seconds.

Robert.

[BaldricParticipant @baldric]

Robert,

That looks a good idea, I think the motor is rated to 5Hz, so 10% is the correct number, however, may be no need for a diode/preset as 0v in should go to the lowest speed as configured in the menu. I may want to make it so that the “normal” control doesn’t go that slow, with a resistor between the potentiometer & 0v. to ensure the motor fan is going fast enough, slow speed for a few seconds when setting up should be OK.

Mark.

[David JuppParticipant @davidjupp51506]

Some VFDs have a facility for one or more pre-set speeds to be called by digital inputs (switches) – just another way to achieve the same end.

[Baldric]

On Baldric Said:

Robert,

That looks a good idea, I think the motor is rated to 5Hz, so 10% is the correct number, however, may be no need for a diode/preset as 0v in should go to the lowest speed as configured in the menu. I may want to make it so that the “normal” control doesn’t go that slow, with a resistor between the potentiometer & 0v. to ensure the motor fan is going fast enough, slow speed for a few seconds when setting up should be OK.

Mark.

You could just switch the control input from the potentiometer wiper to ground if the minimun speed isn’t zero. You can’t just ground he potentiometer wiper because if the pot is near the 10V setting it will be damaged. I tried to present a unversal solution that didn’t need parameters changed.

Robert.

[BaldricParticipant @baldric]

<p style=”margin-top: 24px; margin-bottom: 1em; color: #222222; font-family: Barlow, Arial, Verdana; font-size: 18px; background-color: #f7f9ff;”>You could just switch the control input from the potentiometer wiper to ground if the minimun speed isn’t zero. You can’t just ground he potentiometer wiper because if the pot is near the 10V setting it will be damaged. I tried to present a unversal solution that didn’t need parameters changed.</p>
<p style=”margin-top: 24px; color: #222222; font-family: Barlow, Arial, Verdana; font-size: 18px; background-color: #f7f9ff;”>Robert.</p>

Sorry,  I didn’t make it clear, but yes as per your design, but with the bottom contact in your diagram to 0v.

Mark.

[old martParticipant @oldmart]

As far as I remember from programming the VFD for the Tom Senior mill, when I chose from 25 to 75 Hz, that used the whole of the potentiometers 0- 10K span.

[PeteParticipant @pete41194]

Many thanks for the extra details Robert. Yes my VFD is a 5 hp 15J unit, Well oversized for the mills 2 hp motor, but that’s what my industrial electrical supplier recommended. There isn’t any exposed insulation on the individual wires at all, and both the input single ph. supply and then outgoing to the motor do have the proper strain relief on both. And the same into the motor. All the wiring is proper industrial grade and well oversized for the voltage. While I’ve never had any durability issues with the key pad on this VFD. I’ll now give some thought about an enclosure and the usual fwd/ rev, up/down rpm, jog and stop as a separate control station.

I’ve spent a lot of my career around some rather high voltages. 4160 V & 7200 V, with ground level cables and connections, plus a whole lot higher with equipment that was capable of touching the overhead transmission lines if I ever made a simple mistake. So I was quite careful about the wiring and the proper strain relief clamps on my mill. In fact I had a professional electrician do the VFD to motor connections since I didn’t feel I had the proper skills and knowledge to do it myself.

[Robert Atkinson 2Participant @robertatkinson2]

Hi Pete,

It is strange that suppliers recommend using VFDs that are rated at higher power than the motors they are going to be used on. It is not good practice. There are a number of disadvantages:

1/ Higher cost (the cynic might say that = higher profit for the seller)

2/ Reduced control range. The VFD is digital so has a limited, fixed, minium control step size. For example if the unit has 8 bit current control it has 256 steps. A Drive with 2A output will have a minium step of about 8mA A 10A one would be about 40mA. Added to this is the fact that the 10A drive will only be operating over the lower 20% of it’s range. This results in a control range of 50 40mA steps rather than 255 8mA steps. Note drive may have higher resolution than 8 bits but princple still applies.

3/ Higher inrush current / leakage current. More likely to trip domestic electrics.

4/ Higher interference. Larger drives are less likely to have effective EMI filters built in

5/ Requires more care setting parameters. The typical or “out of the box” settings of a larger drive will not protect a smaller motor and allow excessive current to flow.

6/ Larger. Space is valuable in most hobby workshops a bigger drive needs a bigger enclosure.

There is no need to use a drive 3 times the size of the motor.

Robert.

[PeteParticipant @pete41194]

I sure wish everything you just listed was known to me when I was buying it Robert. This VFD wasn’t exactly cheap 😒 And I certainly didn’t know about the larger minimum step adjustments on the higher HP VFD’S. Although what mine does seems to work well enough for how I’m using it. But thank you very much for the additional details.

The only real item I had to change in the factory program settings was increasing the motor ramp up time by a couple of seconds. And it was only an issue at the highest spindle rpm belt position. Every once in a while the VFD would fault out during a cold motor start. Would that be due to the VFD’s motor protection circuit or something else? So far it’s never created an electrical fault at the separate sub panel it’s wired into. Or created any signs of electrical interference for TV, radio etc. While my shop isn’t large, the mill is the only item on that wall. Space for an enclosure isn’t a problem, more expense for a larger enclosure as you said though.

The single irritation I have with it is the cooling fan noise, even during extended use the VFD barely gets warm. I’ve checked the manual and there seems to be no programmable adjustment for fan speed, and it always runs at a single speed. It may not if the VFD got hotter, but that’s unlikely to ever happen so I have no way of knowing if it’s a variable speed fan at higher temperatures or not. A shot in the dark, but you wouldn’t happen to know if there’s a way to adjust the cooling fan speed? Just remember I’m an electrical imbecile if it’s anything too complex.

[Robert Atkinson 2Participant @robertatkinson2]

Another reason not to de-rate a VFD. A larger one is more likely to have a fan. 🙁
That is probably the most obvius one for the user. None of my VFDs have a fan.
A larger VFD will work but you are paying more for something that isn’t optimal.

You could disconnect the fan or wire it via a thermostat. At your risk of course. Make sure the VFD has been disconnected from the mains for at least 5 minutes before touching any terminals. They store a charge.
https://www.tlc-direct.co.uk/Products/SGRSTAT1.html

Robert.

[old martParticipant @oldmart]

Any industrial quality VFD should be perfectly capable of running a motor of the same rating. As mentioned, it is a waste of money over rating the VFD.

[BaldricParticipant @baldric]

I have done the motor swap & fitted a VFD, here are some pictures.

Motor fitted to it’s bracket, I replaced the supplied feet with new ones that match the bracket, whilst they are chunky and have not adjustment in the feed, just the bracket, they do the job with about 20mm of adjustment. I replaced the puller with a new one, that is taper-lock, surprisingly cheap compared to an imperial one, this is 75mm rather than 3″.

Motor fitted, I used the original flexible conduit for the screened cable, that is clipped to ensure it is kept out of the way.

Electrics mounted in the cupboard, I have added fuses to allow for me to add in a DRO & tacho in the future. The only issues I had were that the number of earths I could fit in the VFD is limited, hence extending the control cable screening to the earth on the DIN rail.

The supplier suggested a 16A fuse, which i fitted, even though it is fed off a suitably fused 13amp plug.

I have used the existing hole for the suds pump, that I do not have, so the jog control, only jogs forwards, but I don’t imagine it will be an issue for me. I decided to replace all the switches as I was adding one, also replaced the rubber edging on the door. Since taking this picture, the coolant image has been painted out.

Cost for the motor, VFD, mains cable, potentiometer & most switches came to £345, items for the DIN rail, about £20, about £15 for pulley/tapper & new belt, then I added items like the bootlace ferrules & crimper, control cables and a few other sundries.

One thing I found from the manual for the motor, don’t run it below 30Hz, without external cooling, even though the motor is rated down to 5Hz, if I want to go slower, I will add that, I decided for the occasional jog, with no cut, when you hold the button, it will be OK. Perhaps I should have looked at the minimum motor speed more, I was expecting there to be a point it would need extra cooling, but not that high.

The suppliers quick start guide was useful, but adding the jog control meant I had to make some small adjustments, like make the minimum speed about 5Hz, but set the lower limit of the potentiometer to 30Hz.

I have so far only done a few test cuts, I don’t think I am getting the vibration I had, a chuck touching the inside of the cabinet cupboard used to fibrate, but time will tell.

Am I glad I went this way, yes, was it much cheaper than a pre-built package, perhaps a bit, but I got what I wanted. I have learnt a few things, including what to look out for if I replace my 2-speed Bridgeport motor with a new one, consider smaller pulley set, fun at 75Hz for top speed down to 30Hz, keeping each pulley step to have roughly the original range.

Improvements to make, work out the min/max at each pulley setting & print that out plus fit a tacho. I should also have looked at the control cabling I ordered & got extra cores so I did not use the green/yellow core as a signal.

Mark.

[HollowpointParticipant @hollowpoint]

Good job, well done! 👍

[BazyleParticipant @bazyle]

Mark,
perhaps you could give details of your motor and VFD as conversions for Boxfords seem to be a common requirement.

[Bazyle]

On Bazyle Said:

Mark,
perhaps you could give details of your motor and VFD as conversions for Boxfords seem to be a common requirement.

Sorry for the delay, I have been away.

https://inverterdrive.com/group/Motors-AC/Universal-UMHA-80K-4-B3

The VFD is

https://inverterdrive.com/group/AC-Inverter-Drives-230V/VAR-TDI20-0R7G-S2-EU

I hope this is useful information, I can’t guarantee it is the best option, but is so far working for me.

Mark,

[old martParticipant @oldmart]

Good result, Baldric, if you need to run at very low speeds for over a couple of minutes at a time, then it would not be difficult to add a fan to the compartment where the motor sits blowing in as long as the input was dust and swarf free, or using a filter if that was not guaranteed. It would not have to be switched on all the time. The range of original belt speeds and backgear are still essential for supplementing the vfd speed control. Having the speed control can often help if you get chatter by changing the harmonic frequency.

[old mart]

On old mart Said:

Good result, Baldric, if you need to run at very low speeds for over a couple of minutes at a time, then it would not be difficult to add a fan to the compartment where the motor sits blowing in as long as the input was dust and swarf free, or using a filter if that was not guaranteed. It would not have to be switched on all the time. The range of original belt speeds and backgear are still essential for supplementing the vfd speed control. Having the speed control can often help if you get chatter by changing the harmonic frequency.

I am certainly expecting to use the belt changes, maybe less than I used to.

Mark

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