variable frequency drive conversions

[tonyfoale]

I have a variable frequency drive motor from a treadmill. Can I change my drill press to use it? The drill press is a 5 speed bench
mount unit. Is it useful? Any ideas on how to do it? I thought I read an article about it but can't find it. Any help is greatly appreciated.

Most if not all treadmill motors are DC commutated motors, speed variation is by varying the supply voltage.
Variable Frequency drive is a term used for 3 phase AC induction motors, speed variation is done by varying the supply frequency, the voltage is also changed in proportion to the frequency to avoid excessive motor current at low frequency.

Treadmill motors can easily drive a medium size drill press. I used one on a Bridgeport mill once as a temporary measure, it worked OK. http://www.homemadetools.net/forum/h...6661#post84115 Scroll down a little way to see the motor fitting and use.

[threesixesinarow]

I use smaller brushed permanent magnet dc treadmill motors on my bench top lathes and mill. Their speeds are different than the original motors, they’re rated ambitiously, and a couple were challenging to mount. I didn’t reuse the flywheel pulleys so I cool them using external fans - here’s my fanciest fan shroud.



This picture should give a sense of how their horsepower ratings compare - the motor on top is a regular but older 1-1/2hp 1750 rpm dc motor, and the bottom one is a 2.65hp 6700 rpm treadmill motor.

[DIYSwede]

Well - that PM DC motor sucks up max 18 Amps @ 130V, which gives 2,34 kW input & 2,65 hp max, intermittently
but it has only a CONTINOUS output of a mere 1,5 hp @ 95 VDC.
(Which sorta gives me 13,1 A and 1,3 kW IN, for 1,5 hp [or 1,1 kW] OUT at app 5000 rpm. And 200W of heat)

Reason it can be smaller for the same output is:
1) Power=Torque x Angular velocity (RPM), so the PM DC has near 4 times the revs,
but ONLY 1/4 of the torque of the AC motor.
2) Being a PM motor, it doesn't need the AC's bulky field windings, and (being hi-rev)
3) Delivering less torque, it can have a smaller rotor diameter.

Dia equals leverage: You can't loosen wheel bolts with a 6" wrench, you need bigger to get the necessary torque.

[Canobi]

Hand down, PWM (Pulse Width Modulation) is the best method for controlling DC motor speed as there is a relationsship between the motors torque and the voltage it receives. PWM speed control is achived by switching the motor on and off very quickly and shortening, or lengthening the gaps between the pulses.

Each "on" pulse is always at full voltage, so regardless of how slow the motor is actually turning, there will be no torque loss.

PWM speed control units are cheap as chips, power supplies not so much but I did find and purchase a unit that will power my 2.5HP 180v motor and has built in PWM speed control, as well as direction control for about £30 on eBay.

[tonyfoale]

Hand down, PWM (Pulse Width Modulation) is the best method for controlling DC motor speed as there is a relationsship between the motors torque and the voltage it receives. PWM speed control is achived by switching the motor on and off very quickly and shortening, or lengthening the gaps between the pulses.

Each "on" pulse is always at full voltage, so regardless of how slow the motor is actually turning, there will be no torque loss.

Not so.
In general terms unloaded speed is proportional to applied voltage, so applying your logic "there would be no speed loss", which we know is not the case.
The reality is that unloaded speed is proportional to AVERAGE voltage not the peak voltage of each pulse.
Torque is proportional to current, the characteristics of the motor will smooth out the pulsed nature of the input to give a steadier current than the pulsed voltage. The average current will vary as the average voltage, and so torque will not remain constant as speed is varied.
Over a limited range, torque can be boosted at a given speed by using some velocity feedback which boosts voltage and hence current when mechanical load tries to reduce speed.

[Canobi]

Fair comment, I was being a bit too "on paper" there but I'm under the impression that DC and PWM produce higher torque than an AC motor using frequency control.

[tonyfoale]

Fair comment, I was being a bit too "on paper" there but I'm under the impression that DC and PWM produce higher torque than an AC motor using frequency control.

Well, of course that depends on the relative sizes of the motors but as I pointed out before the torque of the DC motor will reduce with reduced voltage and speed. On the otherhand an AC motor with VFD will maintain torque as the speed reduces. The speed is controlled by the frequency, as that is reduced the current will increase leading to more heating. To avoid that, the VFD will reduce the voltage as well as the frequency to maintain the same current and torque. In fact some VFDs will allow a higher current at low speeds to boost torque but this boost will usually be less than 10%.

[Saltfever]

So Tony does that mean I can use a VFD to convert my 3-phase phase lathe to run on single phase service without significant torque loss when reducing rpm?

[tonyfoale]

So Tony does that mean I can use a VFD to convert my 3-phase phase lathe to run on single phase service without significant torque loss when reducing rpm?

If you have or acquire a VFD for single phase input, these are the most common type below 4 or 5 kW. You can get VFDs with
1 ph only input
3 ph only input
1 and 3 ph input selected by connections and programming the VFD.

All with 3 ph output.

So if you have a 3 ph motor you can easily run it on 1 ph with speed control with a 1 ph input VFD.
The MOTOR torque will remain reasonably constant throughout the speed range and you can probably boost low speed torque by 10% or so by programming the VFD

HOWEVER, that is MOTOR torque not SPINDLE torque. If you have a form of mechanical gearing to change spindle speed (gearbox or pulleys and belts) then you will multiply slow speed torque by the speed reduction ratio. That is, if you halve the spindle speed by mechanical means you will double the spindle torque. On the other hand if you reduce the speed of the MOTOR by a half and maintain the motor torque then the SPINDLE torque will stay the same, it will not double as it would with mechanical speed change. You cannot cheat physics. Many people only focus on the motor torque and are sometimes very disappointed with either DC or 3 ph motors with electronic speed control as a result.

Is there anything that you can do to keep good low spindle speed torque with the benefits of motor speed control? The answer is to use a grossly oversize motor as I did on my mill conversion and recently on my lathe. The mill motor went from 1.5 hp 3 ph to 5 hp 3 ph with VFD and the lathe from 2 hp 1 ph to 5.5 hp 3 ph with VFD. I will never need 5 hp on the mill nor 5.5 hp on the lathe but they give higher low speed torque than smaller motors.

Both the mill and lathe have reduced SPINDLE torque at speeds that would normally require back gears to obtain, and that could present a problem for certain types of work. I do not anticipate this being a problem for me because I am usually after higher speed rather than lower. I occasionally used to use back gears on the lathe for coil winding but that needs minimal torque and a test with the new lathe motor running very slowly was in fact better for coil winding because of the increased smoothness.

Click for full size.

Here are links to my mill and lathe conversions.
http://www.homemadetools.net/forum/i...515#post135903
http://www.homemadetools.net/forum/h...6661#post84115 this also discusses experience with a DC treadmill motor.

[suther51]

I just hooked up a cheapo vfd on my little mill. My understanding is that there is a loss of torque at lower rpm. At the moment I do not remember who posted it but recently there was a post about repowering a lathe (?) With a larger motor to have the necessary torque at lower speeds. With time I will find out for my self. What i have found out is the torque and rpm drops to neer zero at max frequency of 400 htz. I have been testing it with 40 to 80 htz and found this to work so far. This applies to my setup, I realize other setups may be different.
Eric
The mill is from 1950's and is 3 phase German made.