Variable speed

[guylavoie]

Well...yes and no. Stepper motors are actually designed to be in a stalled position continuously. For that reason they are almost never used in situations where any kind of speed is required. Think of steppers as rudimentary open loop servo type systems. But I understand the similarity. The variable speed brushless DC motor would be another application of smartly driven coils around a permanent magnet rotor.

[billh]

They use permanent magnets on the rotor and a number of coils as the stator. A "computer" switches the current in the coils such that the rotor gets attracted to the next coil causing rotation. Since the circuit controls how quickly the different coils get energized you could loosely look at it as a "min vfd" but the motor itself is more complex than a simple 3-phase motor. Brushless DC motors use Hall effect sensors to determine the rotor position so coil switching can maximize torque.

Some lathe manufacturers offered brushless DC but then went to regular VFDs. Reliability???

Actually they have been around for a long time in the simple form of stepper motors.

billh

[iamtooler]

The current draw of a motor is usually indicated for it operating a full load (eg: requiring an actual 1 HP of work for a 1 HP motor). This also means that a certain amount of magnetic field "slip" is expected to occur. This is why a 60 Hz motor with a rotating magnetic field at 1800 RPM will normally end up rotating at a nominal speed of 1725 RPM.

I believe that the FLA on the tag is at the listed speed (say 1725) but an idling motor will be closer to it's synchronous speed of 1800 rpm. In this era of digital tachs that would be easy to check. With a universal motor the higher the slip the stronger the magnetic force rotating the armature. With an induction motor torque falls rapidly after 5% slip. Newer cordless tools have brushless motors, I have never had the opportunity to learn what principals they employ. I almost wonder if they are mini vfds!
Rob

[tool fan]

This means that the amperage draw goes up in line with the voltage (speed ) decrease because volts times amps = watts?

With the same load I believe that to be true. However when a load is applied, the amperage increases to the limit of the control board circuit, motor or circuit breaker.

Some of these smaller motors are rated as high as 18 amps and 130 volts or 2340 watts or just over 3 HP. This seems unrealistic. The better ones - the bigger ones - are rated around 20 amps and 90 volts or just under 2.5 HP which seems more realistic. My sense is that the little ones running at top speed under maximum load are on the verge of burning out. Conclusion: use the bigger motors and don't run them for long periods of time at top speed under load.

The other issue is cooling. When the motor runs faster, more heat is produced. However, the fan(s) turn faster presumably drawing heat away from the motor. Conclusion: either provide external cooling or do not run the motor for long periods of time at low speeds. I have yet to tear apart a treadmill and not find a motor with at least one fan. Someone wiser than me has determined that they are necessary.

When operating these motors, I think that there is probably a sweet spot somewhere between 70 and 90 volts where the motor is not over heating or over working and fan cooling is adequate.

[guylavoie]

This means that the amperage draw goes up in line with the voltage (speed ) decrease because volts times amps = watts?

The apparent "resistance" (ie: power usage, volts x amps) is more complex in a motor. It largely depends on the load. A motor with no load draws relatively little current, while a heavily loaded motor draws a lot of current. Lock the rotor and it draws maximum "locked rotor amps (LRA)". This is true for both AC and DC motors. Think about the following for a moment: The physical winding's resistance and impedance don't change whether the motor is under load or not, yet the current draw changes significantly. So what's happening?

The answer is that a motor also acts as a generator, and the spinning rotor is generating a current that counteracts the incoming current. This generated current is usually referred to as "counter emf (electro motive force)". At no load, the rotor is spinning at close to the theoretical rpm (eg: 1800 RPM for a 60 Hz AC induction motor) and most of the current is counteracting the incoming current. As load increases, the rotor slows down and the rotating magnetic field starts to "slip" more and more in relation to the incoming current's field. The generated current starts to get out of phase with the supply current and less resulting counter emf is generated...and the resulting current draw increases.

The current draw of a motor is usually indicated for it operating a full load (eg: requiring an actual 1 HP of work for a 1 HP motor). This also means that a certain amount of magnetic field "slip" is expected to occur. This is why a 60 Hz motor with a rotating magnetic field at 1800 RPM will normally end up rotating at a nominal speed of 1725 RPM.

[iamtooler]

Generally, a DC motor with brushes has a very linear power curve right down to very low RPMs, especially in a series wired configuration. Series wired is rare in a tool application though because they can attain insanely high RPMs if they have no load.

This means that the amperage draw goes up in line with the voltage (speed ) decrease because volts times amps = watts?

[tool fan]

"Last night turning a 15", 45 pound blank of wettish beech courtesy of hurricane Dorian, I roughed out a blank starting at ~200 rpm in low speed range. Set up worked well. Motor certainly didn't stall, however with a couple catches (one actually bent gouge tang) there was still slipping of 1/2" notched belt on smallest 2" jackshaft pulley. No slippage with 5/8" belt. Guess, one should look at slipping as a safety feature in this situation!"

The nice thing about your configuration, is that you can go up one step with the pulleys and turn the voltage down to attain the appropriate speed, hopefully providing sufficient grip. If you find that there is not enough torque, you can simply turn the dial to increase the voltage to compensate for the increased load.

[guylavoie]

Belts will certainly appreciate being left on larger diameter pulleys and that allows more torque to be transmitted if sufficient tension is available. I have no idea how linear the out put power of a DC motor is over it's speed range.
Rob

Generally, a DC motor with brushes has a very linear power curve right down to very low RPMs, especially in a series wired configuration. Series wired is rare in a tool application though because they can attain insanely high RPMs if they have no load.

[iamtooler]

Belts will certainly appreciate being left on larger diameter pulleys and that allows more torque to be transmitted if sufficient tension is available. I have no idea how linear the out put power of a DC motor is over it's speed range.
Rob

[billh]

Bent the gouge tang????? If you are using a spindle roughing gouge - don't. The tangs are not meant for work like that. Bowl gouge tangs are the same or close to the same diameter as the tool unless they are old cheapies. Yes, I know not everybody made their gouges the same.

Your photo appears to show that you are cutting on the outside circumference of the blank to round it. Much better and far less "clunking" to start near the center and basically start making the bowl shape. Your gouge will not be subjected to severe hits when the flat-spot ends and the wood starts again.

billh

[DougLA]

Yes iamtooler, belt slipping is a problem at slow speeds & large blanks with the 70 year old Craftmaster lathe as well.

In prior lathe set up with speed regulated by frequent belt changing (perhaps aggravated by running belts under some tension over next pulley edge) the belts would eventually stretch and start to slip frequently especially on smallest step pulley perpetuating problem by polishing inside of pulley surfaces as well as side of belts. CTC belts also tended to eventually come apart. This is now 3rd time in 2 years that I've replaced both belts. With present incarnation I have used a P. Auto Kevlar 5/8" belt from DC motor to jackshaft over larger diameter cast iron pulleys and have used a Gates notched 1/2" belt from jackshaft to headstock step pulleys with both adjusted to "perfect" tension (err... whatever that is!). I also roughed up inner polished surface of smaller pulleys with sandpaper and all pulleys look to be running true.

I think belt life is likely another advantage of electronic variable speed. With need for belt (speed range) changes much less frequent and future care to change belts with tension completely released, I'm hoping to avoid replacing belts anytime soon. Others experience?

Last night turning a 15", 45 pound blank of wettish beech courtesy of hurricane Dorian, I roughed out a blank starting at ~200 rpm in low speed range. Set up worked well. Motor certainly didn't stall, however with a couple catches (one actually bent gouge tang) there was still slipping of 1/2" notched belt on smallest 2" jackshaft pulley. No slippage with 5/8" belt. Guess, one should look at slipping as a safety feature in this situation!

Agree belt slipping also a potential problem with a drill press at slower speeds but with experimentation as suggested by Rory and attention to pulley quality/size, belt size/type and tension perhaps can overcome if go to electronic variable speed?

Good thing I enjoy eternal tinkering! Oh the joy of retirement!

[iamtooler]

Be aware that the problem with belt driven drill presses is transmitting the torque for low speed applications. It is hard to stall even a 1/2 hp motor because the belt slips below about 700 rpm. I have some English DPs that have a 2 speed final reduction to over come this.
Rob

[tool fan]

Hi Doug, I've seen the same motor as you have. There is no difference between them as far as I can tell. In my anecdotal opinion, the mass of these motors is the most important feature. If you tear apart one of those Weslo's you will probably find a much smaller motor with a label suggesting that it is 2.75 HP. Maybe just before it blows up at top speed at high load, but in reality, not meaningful.

Turning big forstner bits might be problematic with one of these DC motors. Although as I stated earlier, I had one on a Jet tabletop drill press that would blow the 15 amp circuit breaker before it stalled. I used a larger Leeson motor and a DC 51 speed control. I sold it. Since then, I have found that the DC 51 control boxes are not the same and will only output ~ 400 watts at full power.

I think that VFD's and 3 phase motors are better for drill presses and metal lathes and that would be my choice when I get around to converting another drill press.

If you find a free MC 60, hook it up and test its performance with your DC motor. Try it with the flywheel on and off and see what you think. Be mindful that you probably do not need the step pulley, so you might find that there is room with the flywheel on.

To turn down the flywheel I take it off and turn the drive stub down on my metal lathe.

I hope that this helps.

[DougLA]

Rory, you certainly have a lot of knowledge with treadmill motor/controllers conversions and great that pass on your experience. Nice job.

Interesting, I have that same motor down to motor model # but rated for 2HP for some unclear reason.

I have toyed with idea of putting in my tabletop Rockwel/Beaver drill press so as to slow down beyond 700rpm for drilling metal and perhaps large Forstner bits. Wondering about wisdom of using without flywheel as would be a lot easier and neater to install and quicker to shut down without. Drill press only gets occasional and short bursts of use for minutes only - nothing like turning for hours at a time. Wonder what you think? How do you turn down the flywheel - just with motor spinning or do you use a metal lathe?

Have seen a couple older Weslo Cadence (840 & DL40) treadmills with slider potentiometers. From what I could glean on line should have MC60 controllers? altho prices not right.

[tool fan]

The tach was from ebay and it's powered by a small dc transformer (wall wart) that I have strapped onto an electrical outlet that I wired into the AC circuit.