9x20 motor control

[nightowl499]

wondering if i can use a ac control like a reostat control for a router on my 9 x 20 lathe
or if i gotta go the dc motor route

 

[twstoerzinger]

Hello Nightowl,

First a little background on router motors.
Any router I have ever seen has a "universal" motor. This is a series wound, brush commutated motor that will run on either AC or DC. They are also used in power hand drills, "Skil" saws and the like. They are used where you need to pack a lot of power into a small space without much weight. They are also used where you need very high RPM. In fact, an unloaded universal motor will wind up to very high speeds - being limited only by bearing drag and air drag on the armature.

I am guessing that when you say "reostat" you mean SCR (silicon controlled rectifier). SCRs have been standard equipment on power hand drills for 25 years or more. Reostats went out of favor a long time ago when SCRs became available at low cost. The SCR generates a pulsed DC current on which a universal motor is able to run. So your variable speed power hand drill is actually operating on DC made out of AC. An SCR large enough to run a 1/4 hp drill, is about the size of a book of matches.

I suspect that your 9 x 20 lathe has a standard AC induction motor, probably 4 pole, running at a constant 1750 (or so) RPM, and that spindle speed changes are accomplished by switching the belt among various sized sheaves, with perhaps some gearing options. The AC induction motor will not run on pulsed DC - it will only hum and get hot. The AC induction motor requires a continuous AC voltage to operate.There are not brushes - current is "induced" into the rotor to generate the torque.

As you mention, one option is to change out the motor for a DC motor with DC drive to obtain variable motor speed. This is the classical solution for larger machines. The DC drive allows 0 to rated RPM speeds with full torque down to 0 speed. Requires a new motor and a new electronic drive.

You might first want to consider another option:
If your motor is indeed a constant speed induction motor, you could add VFD (variable frequency drive) to vary the speed of the motor. Induction motors try to spin at the speed of the frequency being supplied to the motor. When you reduce the frequency, the motor slows down accordingly. VFDs have come a long way in the last few years. You can get one for up to 3 HP for around $200. A 3 HP drive is about 1/2 the size of a shoe box. The only drawback to VFDs is that as speed is reduced, the output torque is also reduced. Also, very low speeds (say less than 15 Hz) you tend to get unstable motor speed. However, if you are going to run very low speeds, you still have the option of switching belt positions or gear changes to keep the motor in a more friendly speed range. If you are not a electronics fanatic, a VFD may look a little daunting to set up, but for a lathe application, you can use most of the "factory default" settings. There are lots of posts on this forum about setting up VFDs on various machines.

I have a Clausing 6900 lathe with a VFD installed to generate the 3 phase power needed for the factory motor. The Clausing has a mechanical variable speed system using variable pulleys on one of the drive belts. Yet, I find myself tweeking the VFD sometimes to get around speeds where the mechanical drive likes vibrate.

Sorry for the long dissertation. Hope it sheds some light.
Terry S.

 

[twstoerzinger]

Hello nightowl - again,

After I submitted the previous post, it dawned on me that your motor is almost certainly a single phase motor (dah).
VFDs only work on 3 phase motors.
So all that rant about VFDs is only of academic interest to you. Guess I been spending too much time in the 3 phase world lately.

Anyways, its probably not cost effective to change out your motor to 3 phase just so you can use a VFD.

Another member has published an article on using a industrial duty sewing machine DC motor on smaller lathes.
http://www.hobby-machinist.com/cont...dc-motor-with-control-from-an-untapped-source

Take a look and see if this might be a cost-effective path for you to get to variable speed.

Sorry for the confusion,
Terry S.

 

[nightowl499]

thanks for your time and info , looks like the dc motor and control is the way to go
for me glad you came back with the second note before i started studyin up on the vfd haha
again thanks for the help
Gary

Hello nightowl - again,

After I submitted the previous post, it dawned on me that your motor is almost certainly a single phase motor (dah).
VFDs only work on 3 phase motors.
So all that rant about VFDs is only of academic interest to you. Guess I been spending too much time in the 3 phase world lately.

Anyways, its probably not cost effective to change out your motor to 3 phase just so you can use a VFD.

Another member has published an article on using a industrial duty sewing machine DC motor on smaller lathes.
http://www.hobby-machinist.com/cont...dc-motor-with-control-from-an-untapped-source

Take a look and see if this might be a cost-effective path for you to get to variable speed.

Sorry for the confusion,
Terry S.

 

[Hysiderafter]

You might want to check out a VFD package like this one: http://dealerselectric.com/item.asp?cID=0&PID=6899