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Fuse replacement -- CX-706

trlvn

trlvn

Ultra Member
Hi:

I just blew one of the two fuses in my Craftex CX-706 lathe (10X22). The lathe came with a couple of spare fuses so I'm up and running for the moment but now I want to buy more spares to have on hand.

If you assume I know almost nothing about fuses, you'll be very close! ;)

First, this fuse is just upstream from the motor and is marked "10A 250V". The wiring diagram in the manual shows "FU2 15A UL". Are these supplied fuses undersized? The other fuse, on the front panel, is a slightly different style and is rated 15A. Or is the variable speed motor really only supposed to draw 10A?

The blown fuse (no continuity with the multi-meter) looks like this:

IMG_5460.jpg


Canadian Tire appears to have a 15A version of this fuse (a "Microwave Fuse") but nothing in a 10A variety.

I do know that different fuses are designed to tolerate over-current situations for different amounts of time. I want to get the 'right' fuse for this application.

BTW, this is the first blown fuse in 3 years with this machine. I was reducing 1.75 inch Dura-Bar (continuous cast ductile iron) to 1.5". The lathe was in high-speed range and I think this was my problem. I stalled the machine on the first two attempts--started at 0.100" dia. reduction and then 0.080". I then had 2 successful passes at 0.040" and the fuse blew when I was mid-way through a third pass. I'm going to try again in low-speed range and I imagine I'll be able to take a more aggressive DOC. Or I'll be headed right out for replacement fuses! ;)

Craig
 
With the replacement fuse, I went back to roughing down the diameter of the part I was working on. I increased to 0.060" per pass and doubled the feed rate while roughing. To my ears, the machine was not working that hard and I think I could have pushed it a lot further. No fuses blown.

I'm never in a huge rush--I'm retired and this is a hobby! So I think I will stick with a 10A amp motor fuse. If blowing them starts to be a regular occurrence, maybe I'm go up to 15.

@skippyelwell that would be awesome!

Craig
 
I guess the other thing is that I don't really know the details of the motor in CX-706. I would have to disassemble a bunch of stuff to get a look at it.

My understanding is that it is a "1 HP" DC motor, aka 750 watts. (Brushed? Brushless?) I've learned a bit about AC motors but my most extensive experience with DC motors was with a race car set when I was 12! Not helpful! AIUI, the Craftex, PrecisionMatthews, King, etc 10X22 variable speed lathes are all pretty much alike. What powers them?

Craig
 
trlvn said:
I guess the other thing is that I don't really know the details of the motor in CX-706. I would have to disassemble a bunch of stuff to get a look at it.

My understanding is that it is a "1 HP" DC motor, aka 750 watts. (Brushed? Brushless?) I've learned a bit about AC motors but my most extensive experience with DC motors was with a race car set when I was 12! Not helpful! AIUI, the Craftex, PrecisionMatthews, King, etc 10X22 variable speed lathes are all pretty much alike. What powers them?

Craig
Click to expand...
I'm pretty sure that they have mostly gone brushless with pulse width modulation, which varies the power of the motor by pulsing the voltage on and off, so that the average voltage can be varied thereby varying the speed output. PWM has the benefit that it does not use a resistor to vary output and does not produce waste heat the way old RC cars used to...

The amplitude of the output voltage is never varies, it is always at the maximum 110v and therefore full power output. Only the duration of the pulses changes.

So given the previous calculation I = W/V , if the output voltage is always 110v, and the power rating of the motor is a constant 750 watts, the amperage is always 6.81 amps for the duration of the pulse. As the duration of the pulses increases the longer the full load amperage is presented to the fuse, the more it heats up.. and vice versa.

I dont think you should have a problem blowing fuses on a regular basis at 10 amp fuse rating.
 
My CX706 manual shows 15A fuses in both locations. IIRC one of them had been replaced with a 10A from BusyBee. These lathes appear to pass through what's called a "special inspection" by CSA. The law back when I was working was that you could not sell or even demonstrate mains type electrical equipment in Canada unless it was CSA/UL approved or if it was not you could get a CSA special inspection, basically the CSA guy shows up, inspects the equipment, looks at the fuses, usually does a megger or HI-POT test , inspects the grounding, does a flame test if there is plastic and if everything appears to be not to blatantly dangerous puts a CSA special inspection sticker on it. It would make sense based on the 1HP motor they would have told them to use a 10A fuse.

Mine popped once, similar situation. The motor is driven by a KBLC or derivative type board and that board has over current protection, so a 10A fuse is probably on the edge, but seems like it might actually save the day when were pushing too hard.

Side topic but I hear (not sure if it's true) that the lobbyists were able have these laws "tweaked" for the better of the economy and we're now flooded with non CSA approved imported equipment and much of it is absolutely unquestionably dangerous. I have inspected some of this equipment and the absolute must have safety features we had to design into products is no where to be seen. The reason I'm mentioning this is if your buying something from your favourite online store that plugs into mains, make sure the ad states CSA/UL approved and shows the sticker CSA/UL sticker and don't get fooled by the stickers with CE mark. The CE mark stickers can be very misleading, first there is CE mark from Europe that if legit has to meet similar safety criteria as CSA/UL, even stricter in many cases, the problem is that CE mark is self regulating (no 3rd party evaluation) , so uncrupulious vendors just stick a CE sticker on and no testing required. The othe CE mark obviously made to mimic the original CE mark is from China and simply stands for China Export and with those stickers anything goes.
 
Thanks for the explanation of PWM. I guess another factor, though, is efficiency. AIUI, for fractional horsepower motors, 3 phase motors are inherently more efficient than single phase. Is there any rule of thumb comparing comparing the efficiency of PWM DC motors to AC motors? Is PWM DC usually better or worse than a single phase AC? Or does variable speed mean "it depends"?

The PWM DC setup is creating waste heat both at the speed control board and the motor, right?

Craig
 
trlvn said:
Thanks for the explanation of PWM. I guess another factor, though, is efficiency. AIUI, for fractional horsepower motors, 3 phase motors are inherently more efficient than single phase. Is there any rule of thumb comparing comparing the efficiency of PWM DC motors to AC motors? Is PWM DC usually better or worse than a single phase AC? Or does variable speed mean "it depends"?

The PWM DC setup is creating waste heat both at the speed control board and the motor, right?

Craig
Click to expand...

PWM creates far less heat than a resistor based speed controller so less heat is created in the controller. There is some due to the switching but not so much.

The same applies to the motor, by omitting rotor windings found in ac motor, the brushless DC motor has less electrical resistance in the circuit ergo less waste heat when operating. There is also less slip between the rotor and stator than in brushed dc motor. The electro mechanical conversion rate of a brushless DC motor is between 85-90%.

Also by using permanent magnets instead of secondary windings to create the magnetic field the brushless motor is about 30% more efficient than 3 phase ac motor. It also has less resistance in the electrical circuit so less waste heat to the motor, so supposedly less risk of damage to winding insulation.
 
trlvn said:
If blowing them starts to be a regular occurrence, maybe I'm go up to 15.
Click to expand...

Please don't do that Craig. I got a 120k tractor for 40k cuz the guy before me replaced a 10A fuse with a 20A fuse cuz it kept blowing. The dealer couldn't fix it. It took me several months to find everything that little stunt caused. I ended up redoing most of the main harness.

The original problem was a drooping headlamp wire that touched the engine every so often. He never did find that. He just kept replacing the 10A fuse. When he got tired of doing that, he eventually replaced the fuse with a 20A one. That caused a MUCH BIGGER prioblem.
 
Electric motors are actually a fascinating topic and a lot more complicated than they seem. In the first ten minutes of motor theory class they explain how the rotor (magnet#1) chases the rotating magnetic field of the stator, and your thoughts are that's pretty cool and so simple. Then over the next 100 and counting years countless clever engineers figured out elegant ways to improve the various characteristics; torque, efficiency, speed regulations, cost, etc. When you dig into this detailed motor theory, crack out the high level Physics and Calculus because motor theory is anything but simple. So depending on the application two near identical looking motors (same HP, RPM, etc) can have completely different characteristics. A good VFD is able to measure without sensors and then in real time calculate and manipulate the three phase waveform to offer many of the benefits associated with a specifically wound motor type without introducing the downsides associated with that motor type.
 
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