Micromill transformer speed controle

[hobby]

Hi,
I'm converting my micromill from gear driven to belt and pulley drive.

I built the 2 pulleys, I gave myself 4 step pulley speeds, now to eliminate the need to keep bying speed controle boards, I decided to build a DC supply with 3 transformers in series, but these transformers can be switched in in steps using a selector switch, so I can get up to 5 voltage steps, using all the tappings of the last 2 transformers. From around 32v. up to 100v. in 16v. increments, this is the DC values after rectification and filtering.
{Vout =~ 1.33 X Vin.} using a bridge rectifier.



With a 4 step pulley system and 5 speed voltages, gives a good variable range of speeds, without the need for continuous variable controle, using a potentiometer, and PWM circuitry, as in commercial speed controles.

 

[Herbiev]

Just wondering. With DC motors isn't the power output proportional to the voltage input. Ohms law sort of thing P=E x I ?

 

[jct842]

I got ticked at the price of the board for my micro mill so I just happened to have an isolation transformer and a small auto transfromer. I built a crude box to house same in and made a diode bridge rectifier. I have had mine running all the way up to 140 volts and could go to 160 with that set up. much better power than the board ever got. my board kept tripping out and was pretty much that way from new. Before I could easily trip it out with a 3/16 drill going easy in soft steel. Now I have never yet had the motor even get past luke warm. I have taken out a couple 3 am fuses experimenting with it. should I burn up the motor I will just get a larger one on ebay. I have had the LMS belt drive kit on it. john

 

[hobby]

Herbiev :

I think there is probably more to it then the usual power equationt, when it comes to motors efficiency, I really don't know all the factors involved, maybe someone more knowledgable in this area could answer your question better.

John:

I know what you mean I went through 3 boards, the first board was free from harbor freight, due to the 1 year warranty, the other 2 boards meeded replaced due to inconsistent speed controle, this last board kicks out when the motor is not loaded when first turned on, these boards are not reliable at all, foer the micromill, the 7x10 lathe board is still the original, it has excellent speed controle.

 

[Tin Falcon]

the dc motors on the mini mills ,lathes and for that matter tread mills. are controlled with a pulse width modulator.
at full speed and hence full power the voltage is constant. at lower speeds the board feeds square wave DC pulses. the lower the speed the shorter the pulse the higher the speed the longer the pulse. I am not an electrician or electrical engineer but do not think rationing voltage is the way to regulate speed.
Tin

 

[hobby]

Hi,
TinFalcon,

I agree with what your saying to a certain extent.

I'm not a professional in the electronics field, I only design and build circuits as a hobby in electronics, so I have no professional opinion on this subject, but I do a lot of searching out when I design a circuit, to try to understand the functions as best as possible.

Iv'e been prototyping PWM circuits, for the last few months, and I have one breadboarded and it works real well, just need to build the mosfet circuitry next, and some torque feedback controle,
that's the reason for this 3 transformer setup.

Originally I had it delivering around 100VDC. to the motor, so I could use it to build the rest of the PWM system, when as I was thinking it through, I thought about utilizing the taps on all 3 transformers to pick off certain voltages, thast way there I could do away with buiulding this PWM system, and make it very simple speed controle, that would not have many circuit bugs to have to contend with all thie time.

Here is why I agree with what your saying to a certain extent,
From what I have learned through experimenting
PWM is used to vary the voltage that a output device will see.

The signal is full power output, however the output device still only recieves the voltage according to the duty cycle.

Something like the ACrms value, where the peak voltage is multiplied by 0.707 to give the effective voltage across the device, as if it were DC value.

But since a PWM is a squarewave and not a sinewave, thern instead of multiplying peak values, the duty cycle (on to off times) is used to determine the effective voltage across the device.

It is the same thing as using a rheoastat to drop a voltage for running a motor, however with a rheostat, the power consumed in it is so great, that it is better efficiency to vary the on-off times,
to lower voltages to a device.

If I was using a resiastor in series with the motor, than I think that would be rationing voltage to it, but since my motor is getting the full DC voltage from the transformer rectifier circuit, than it's recieving full power that can be given at that particular voltage.

I think that is the key, that the motor is getting full power output from the supply, there is no considerable series dropping resistance, built into the circuit branch to lower the voltage.

This is just my understanding of PWM as a hobbyist point of view,in electronics.

If I'm wrong in this understanding please feel free to reply, so I can have a right understanding on this, as I'm building this project.

Thanks..

 

[jct842]

I believe a motor runs best at full constant voltage what ever that may be. a transformer that can supply a varying voltage is an example. actually as far as I can see if the motor does what you want and does not get hot it is right. john

 

[lordedmond]

Hi,
TinFalcon,

I agree with what your saying to a certain extent.

I'm not a professional in the electronics field, I only design and build circuits as a hobby in electronics, so I have no professional opinion on this subject, but I do a lot of searching out when I design a circuit, to try to understand the functions as best as possible.

Iv'e been prototyping PWM circuits, for the last few months, and I have one breadboarded and it works real well, just need to build the mosfet circuitry next, and some torque feedback controle,
that's the reason for this 3 transformer setup.

Originally I had it delivering around 100VDC. to the motor, so I could use it to build the rest of the PWM system, when as I was thinking it through, I thought about utilizing the taps on all 3 transformers to pick off certain voltages, thast way there I could do away with buiulding this PWM system, and make it very simple speed controle, that would not have many circuit bugs to have to contend with all thie time.

Here is why I agree with what your saying to a certain extent,
From what I have learned through experimenting
PWM is used to vary the voltage that a output device will see.

The signal is full power output, however the output device still only recieves the voltage according to the duty cycle.

Something like the ACrms value, where the peak voltage is multiplied by 0.707 to give the effective voltage across the device, as if it were DC value.

But since a PWM is a squarewave and not a sinewave, thern instead of multiplying peak values, the duty cycle (on to off times) is used to determine the effective voltage across the device.

It is the same thing as using a rheoastat to drop a voltage for running a motor, however with a rheostat, the power consumed in it is so great, that it is better efficiency to vary the on-off times,
to lower voltages to a device.

If I was using a resiastor in series with the motor, than I think that would be rationing voltage to it, but since my motor is getting the full DC voltage from the transformer rectifier circuit, than it's recieving full power that can be given at that particular voltage.

I think that is the key, that the motor is getting full power output from the supply, there is no considerable series dropping resistance, built into the circuit branch to lower the voltage.

This is just my understanding of PWM as a hobbyist point of view,in electronics.

If I'm wrong in this understanding please feel free to reply, so I can have a right understanding on this, as I'm building this project.

Thanks..

without the correct pulse width control logic with the emf feed back cct. the motor will behave as a simple series wound motor with poor speed control ( speed / load ) not what you want on a mill , it will bog down when the load is applied .

 

[hobby]

Hi,

Yeh, with the PWM boards there is the advantage of torque feedback, I was in the process of designing that part for my PWM circuit as well, when I came upon this method of using tapped transformers.

I'm hoping that it won't bog down as much for the kind of operations I do with it, if it does bog to much I will then try one more method before continuation of my original PWM circuit, and that is to build a voltage regulator.
I may have to build 5 different ones one for each voltage output, but I'll see how it works without voltage regulation first.

Thanks for your input..

 

[lordedmond]

if you want to go super duper, then fit a chopper ring to the spindle with a opto sensor and use that to feed back your control logic with a closed loop feed back

that should keep the speed Ok and should keep the torque up as well


but it seems that you are reinventing the wheel when very good PWM modules are available ( not from the manu. of the mill ) but else where just match to the mill motor

 

[Ken I]

wrt Tin's reservations about "chopping" DC into square waves to feed a DC motor.

I see this objection frequently - the output is supposed to go through and inductive / capacitive filter - this removes the pulses.

I have a 5A PWM drive (using an LM338) and have run my scope on it - it is "flatline" DC until you overload it - then some distortion starts to creep in.

The PWM driver chip normally has a pin for a resistor / capacitor to set the output frequency (typically 10-20 KHz.)

Whilst the method of chopping might seem barbaric it will produce 100% smooth DC if properly filtered.

Ken

 

[Kermit]

... my board kept tripping out and was pretty much that way from new. Before I could easily trip it out with a 3/16 drill going easy in soft steel. ... john

Did you ever check out the spindle bearings? Having them too tight can cause that exact problem. Perhaps a new board and close look/adjustment of the bearing will remedy the entire situation?

 

[jct842]

the bearings seemed free. a new and supposed to be improved set of bearings came with the belt drive kit which I intend to install some time in the future. I have finally been scheduled for a knee replacement at the VA in a couple weeks so it may be late summer before I can get to it. I may also make a jack shaft to slow down the motor some. I have never measured it but the belt drive from LMS has a high speed that is probably 5k or more in the highest of the 3 speeds.

I milled 6 spokes into a 6 1/2" X 1 1/8" flywheel out of cast iron a couple months ago and the motor never got past warm and there was several pounds of chips that came out of it.