Fixing motor for Myford Ml7

Hello,

I have just bought a Myford ML7 Lathe but the motor has a problem which causes it to constantly blow the 5A fuse.

I reattached most of the wiring as some was in bad condition but this didn't fix it, as a last resort I tried using a 13A fuse and using this the motor seems to work fine.

The original plug it came with was very old with round pins and no fuse inside.

I presume using the 13A fuse is potentially very dangerous though, what do people think?

Thanks

Hi James,

According to this, a 5A fuse might be too small for a 0.5HP motor. Does the motor get hot with the present fuse?

Thor

The starting amps are much higher than the running amps. I would say that every hobby mill, lathe, pistol drill etc I ever owned has had a 13amp fuse in the plug.

Just checked my stand drill with a clamp meter.1/2hp 230vac motor. Fitted with 13amp fuse. On startup it pulled 11 6 amps and dropped back to 1.6 amps no load running. Using a 5/8 drill into a 1/4 pilot hole in mild steel the motor was pulling 5.8 amps.

regards

James that looks like a very old 15 amp plug, went out with the ark. I should replace it with a new 13amp plug and fuse and try that, hopefully all will be well.

Take care

Motors are typically fitted with thermal-overload current trips. Your motor would be better fitted with that type of starter, so the 13 amp fuse is not left protecting your supply lead and (not) the motor. Make sure that the lead is rated for 3kW, if using a 13A, fuse because that is what the fuse is there for.🙂

A seven or (more likely) ten amp fuse might be sufficient.

Hmm, your plate says 3.7 A

so assuming start up current can be up to 3 times that....

Dangerous not to use a fuse, but 13A will be fine, but if that blows regularly too, then you may have a problem.

If it runs smoothly and nothing appears to get too hot then you are good to go. 

Edited By Martin of Wick on 24/03/2020 10:45:19

A 5 A fuse will not survive the start up surge from your motor. If the thermal switch operates (60C) after an hour of continuous running the motor has a problem.

Well, firstly there ain't no way a 5 amp BS1362 fuse is going to let enough energy through to start a single phase 1/5 HP (Edit correction 1/2 hp) motor. Not nohow.

13A fuse is going to be perfectly safe, provided:

Motor and wiring are adequate for 20A continuous current ('cos it takes at least 20A or thereabouts to blow a 13 amp fuse)

Earthing is good (I typed god, maybe that's a better sentiment!)

Motor and switchgear insulation is adequate.

Single phase motors take something in the region of 8 times full load current (more if they can get it) briefly while starting. The fuse has got to let enough energy through to get over this hump, after the motor is up to speed the line current falls back to something akin to what's on the rating plate, but you also need to take into account power factor (lousy). The other hazard is that running a brief transient load of about 30 amps through the house wiring and a typical 13 amp socket limits the available current because of the upstream wiring resistance, contact resistance in the socket outlet, and the resistance of those horrible bent metal pegs connecting to the fuse in the plug top.

I don't have the I squared T diagrams in the IEE Wiring reg's to hand - they're at work and I'm at home. These dictate the rights and wrongs of the fuse characteristics for this application. A 1/2HP motor should have a thermal overload device protecting it, if there is a working internal thermal trip so well and good, otherwise you need a contactor/overload arrangement as well as the 13 amp socket on the wall. A 13 amp fuse will do just fine, but don't be too surprised if it blows on repeated quick motor starts.

In short a 13 amp fuse will be perfectly OK provided the wiring is made with 1.5 mm2 or better 2.5 mm2 cable, But do please get someone with a proper calibrated insulation tester (NOT a multimeter) to check insulation at 500 V DC and earth path resistance for safety reasons.

HTH Simon

PS Waste of time my offering to do the safety checks at the moment - I'd gladly do so if you are local (West Gloucestershire) but not just at the moment.

Edited By Simon Williams 3 on 24/03/2020 11:08:56

My IEE code of practice for in service inspection and testing of electrical equipment.manual (PAT testers's bible) has it that there are only two plug top fuse ratings 3A an 13A to be fitted to protect over 0.5 and 1.25 square mm cable. On the course I did we were told the fuses are fitted to protect the cable, further protection of, lower rated, individual parts of the appliance should be by internally fitted fuses.

Edited By Don Cox on 24/03/2020 11:50:14

Trevor, the 15 amp round pin plug is covered by BS546 and still used in stage lighting and other specialist applications, though that individual plug is certainly showing its age.

Otherwise, I agree with everybody who says that a 13 A fuse will be appropriate, as long as the mains lead is 1.5 mm2 or 2.5 mm2 (sorry, can't do a superscript 2 for squared) . Even a 13 A fuse may fatigue and fail in the long term (months or years according to use) because of the repeated starting current, but that isn't an issue - just replace it. If it fails in the short term, there may be other issues.

George B.

BSc(Elec Eng), MIET, CEGB apprentice, PAT certificate and half a century of experience

Edited By Georgineer on 24/03/2020 12:10:36

Edited By Georgineer on 24/03/2020 12:11:13

Edited By Georgineer on 24/03/2020 12:11:55

I agree with Simon Don and George.

13A is appropriate if cable is large enough. Plug fuses are only intended to protect the cable.

What switch gear, Start/Stop/Reevese is fitted? This may need checking or replacement.

Robert G8RPI.

The fuse would be the least of my worries. The evidence includes 'I reattached most of the wiring as some was in bad condition'...

Now then PAT testers and electrical engineers, shame on you! Given this appliance is fitted with an obsolete plug (phased out by BS1363 since 1947) and has wiring in bad condition, why is it OK to simply replace a 5A fuse with a 13A? I suggest James has more to do!

One of the disadvantages of buying elderly second-hand equipment is the electrics might well be in poor condition, or not in accord with today's best practice. Damp, damaged, perished, unprotected etc etc. Not a problem because it can be upgraded without fuss, probably adding a NVR in the process. Assuming the machine is a bit dodgy, can the experts advise what James should look for and how he can test for electrical faults? At the moment, for all we know, this old Myford isn't even earthed...

Dave

Perhaps the "wire in bad condition" was due to age of the rubber insulated flexible cable, the outer protection cracks badly over time, where the cable is terminated the rubber insulation on each conductor perishes and breaks down because of the reaction caused by the sulphur in the rubber and the tin coated conductors.

As Dave suggests it would be wise to get a 500v insulation test and earth continuity test carried out before use, also consider fitting a DOL starter to provide overload and no volt protection.

Emgee

Edited By Emgee on 24/03/2020 13:50:14

Thank you for your replies everyone.

It's good to hear it's probably not just a faulty motor, although maybe buying a new one would make life easier with regard to getting everything running safely.

I had read the slow blow fuse type is more suitable too, so perhaps i'll get a 13A one of those.

"One of the disadvantages of buying elderly second-hand equipment is the electrics might well be in poor condition, or not in accord with today's best practice. Damp, damaged, perished, unprotected etc etc. Not a problem because it can be upgraded without fuss, probably adding a NVR in the process. Assuming the machine is a bit dodgy, can the experts advise what James should look for and how he can test for electrical faults? At the moment, for all we know, this old Myford isn't even earthed..."

Thank you for raising this. I will see how far I can go checking it myself, wiring size and earthing, i'll get someone qualified to do whatever I can't. I got the lathe for a good price and have nearly always found the old machinery is better than new unless you are ready to spend a lot, i didn't anticipate the electrics being quite so old.

"What switch gear, Start/Stop/Reevese is fitted?" Nothing, i'll get an NVR fitted.

Simon, thanks a lot for the offer. I'm in Buckinghamshire so it would be a bit out of your way.


It puzzles me that the original plug says 5A on it, can anyone offer an explanation?

Bucks is quite a big area. Which part? I went to school in Buckingham.

It puzzles me that the original plug says 5A on it, can anyone offer an explanation?

In the olden days they were two sizes of plug the one shown in your pic rated at 5A then there was a larger version of that type of round plug rated at 15A, then with the coming of the ring main 13A became the standard .

Have not read all post's but have you tried disconnecting the motor to see if the fuse blows ?

H

The round-pin style came in 4 different sizes: 2A, 5A, 15A and 30A.

Before square pin plugs and sockets, UK domestic wiring was wired on a spur system where each socket connected back to a separate fuse. Spurs could be of different current capacities and to stop people burning the house down different sized plugs were used to make sure a 2A device couldn't be plugged into a 30A spur, or a 30A device into a 15A spur. No need for round-pin plugs to contain a fuse because there was one in the fuse box.

The spur system was fine when people only needed a few sockets, but it's inconvenient in a modern home.

In 1947 the system changed to wire houses with ring mains rather than spurs. A ring main can have many sockets connected to it, which suits modern needs. Although the main is protected by a single big fuse, each appliance needs to have a fused plug. All square-pin plugs are the same physical size, but they can be fitted with different fuses as necessary to suit the appliance. I have 1A, 2A, 3A, 5A, 7A and 13A examples, but 3 and 13A are the most common.

A 13A plug and fuse would be OK for a Myford.

Maybe the lathe should never have been fitted with a 5A plug, but not all lathe owners were electricians! Sparey's "The Amateur's Lathe' is well worth reading. Excellent book apart from things like carbide inserts that came later, but his comments on 1950's home workshop electrics make it clear that quite a few chaps back then lived dangerously! It's not a problem though : the wiring and switches etc could all be replaced if necessary.

Dave

Shame me all you like, Dave; my shoulders are broad. I was responding only to the question about the fuse. Since the original poster had already mentioned that the wiring is in bad condition, I gave him credit for having the nous to get it sorted without my input.

By the way, I have a 10A BS 1362 fuse if you would like it to complete your set. Yellow, if memory serves without checking. Swap it for a 1A?

George B.

megga testing is not all it's cracked up to be, when daughter moved into her house it obviously needed a rewire, but it passed testing so was declared OK to switch on (previous occupants only moved out the day before). First time she managed to knock the pendant light in the bedroom all the lights went off. Visibly checking the insulation it was brittle and fell off when flexed. Visual inspection is at least as important as instruments, if it's old rubber wiring bin it now before it bins you.

Those two statements are mutually exclusive.....

A Condition Report on an installation would, I hope, encompass a pretty exhaustive visual examination as well as appropriate testing.

Rgds Simon