Leather burnisher, blueprints anyone?

[Northmount]

When using a light dimmer (triac) or rheostat to control the speed of an induction motor, the motor will tend to run hot. And the slower it runs, the lower the cooling air flow through it, and it runs even hotter.  The motor wants to run at a little lower than synchronous speed (for a 2 pole motor on 60 Hz, that is 3600 RPM, typical running speed would be about 3450 RPM under normal voltage and load).  If not supplied with adequate voltage (and current), it can't build up normal speed, therefore can't build up the normal back EMF and will draw higher current, run less efficiently, and run hotter.  Without adequate cooling air flow, it then runs even hotter.  So could run at moderate speeds with a light load.  If it is getting too hot, increase the speed or reduce the load.  If you let the smoke out, you will have to replace the motor.

Fan speed controllers (triac) will last longer than light dimmers on an induction motor.  Always pay attention to the electrical ratings of the triac and the motor.

Tom

[480volt]

It’s very bad ju-ju to try and regulate the speed of an AC induction motor by varying the voltage, regardless of the means. The speed of the motor is determine by the number of poles and the frequency of the source, hence the need to regulate speed by changing the frequency. You lower the voltage, the motor will still try to run at it’s rated speed and it will begin to draw more current. Torque will drop off as well. AC motors not rated for variable speed (inverter duty) will also not cool themselves properly if run at less than design speed. I’m not saying that there aren’t AC motors with enough margin designed into the windings that they can tolerate what is essentially a continuous overload situation, but I am surprised that someone identifying as an electrical engineer would suggest it. And good luck trying to explain it to your insurance adjuster.

Some home shop machinist types have adapted DC motors and controllers from treadmills, these motors are designed to operate under load at a wide range of speeds and can sometimes be had for the effort of hauling it away.

 

[dikman]

1 hour ago, 480volt said:

Some home shop machinist types have adapted DC motors and controllers from treadmills, these motors are designed to operate under load at a wide range of speeds and can sometimes be had for the effort of hauling it away.

 

Yep, a commonly suggested mod for small lathes. Many years ago I built one of those speed controllers and to play a joke on someone we connected it to a motor they were using. The joke was on us as it burned the tracks on the board out in nothing flat. I've since learned that "ordinary" single phase induction motors are tricky things if you want to make them variable speed. The common advice is to replace them with a 3-phase motor and use a VFD.

[chrisash]

Is not the speed of rotation irrelevant, as its the pressure applied that is the key factor, you can use high speed with little force or low speed with a touch more force, hence you can hand burnish or machine burnish

Maybe we should also have a round edge tool to cut of the corners rather than a flat edge tool

Edited September 26, 2018 by chrisash

[Matt S]

1 hour ago, chrisash said:

Is not the speed of rotation irrelevant, as its the pressure applied that is the key factor, you can use high speed with little force or low speed with a touch more force, hence you can hand burnish or machine burnish

Maybe we should also have a round edge tool to cut of the corners rather than a flat edge tool

In my experience speed, combined with pressure, is a crucial element, even when hand burnishing. Getting the right combination of surface speed, pressure, dwell time and moisture for that particular piece of leather is the key.

There are round bottomed edgers available and they make burnishing an edge a little easier, especially when you get above say 6mm thickness. However they can be tricky to sharpen compared with the normal flat bottomed type and having flat facets before I start burnishing isn't that much of a problem -- the burnishing compound (water/soap/gum/etc.) combined with the pressure from the burnishing tool, forces the edge round anyway. I've never got on with Dixon style hollow edgers though Ivan bisonette edgers aren't bad once you get them well stropped.

[koreric75]

12 hours ago, dikman said:

I'm still not convinced that a light dimmer can be used to vary the speed of a squirrel cage AC motor.

You're correct, what you can do is burn up a good motor.  To control the speed on an ac induction motor you have to adjust/change the frequency and it needs to be a 3 phase motor paired with a vfd, then your potentiometer will work, but seems like that would end up costing more in the long run.