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Registered Member #1408
Joined: Fri Mar 21 2008, 03:49PM
Location: Oracle, AZ
Posts: 679
I have a wood lathe that would be perfect as a winding jig for secondary's but I am hesitant about connecting a varic to it for fear that I would harm the motor: is this possible? What configuration does a motor have to have to utilize such a diminishmet of power (or alternatives)? I have heard of alternatives and did (for a time) have a BBQ motor set up as a winding jig but the motor died & I really don't want to sink money into something I would only use on occasion. What methods aside from hand winding have people used in the past with success?
Registered Member #2463
Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546
The only motor you may use a variac to control the speed with is a UNIVERSAL motor. Such a motor will have a wound field and use brushes. If you use a brush motor with permanent magnet field, you must use a full wave bridge rectifier.
Most AC only drills are universal . Find one with a variable speed.
Registered Member #2028
Joined: Mon Mar 16 2009, 08:13PM
Location: Norway
Posts: 319
radiotech wrote ...
The only motor you may use a variac to control the speed with is a UNIVERSAL motor. Such a motor will have a wound field and use brushes. If you use a brush motor with permanent magnet field, you must use a full wave bridge rectifier.
Most AC only drills are universal . Find one with a variable speed.
A normal ac induction motor would also respond to a variac, and i see no reason for it to take harm from it, but you wouldnt control its speed directly, you would instead control the torque. To properly control an AC motor you need a Variable Frequency Drive, that would be a great solution for a lathe, albeit a bit expensive.
Registered Member #1408
Joined: Fri Mar 21 2008, 03:49PM
Location: Oracle, AZ
Posts: 679
Ahhhh. You guys give me a lot of food for thought. I'm glad to hear that I could experiment with little fear of damage. Pity that it would only affect torque. I had thought that if the magnetic field fell off the motor would spin slower (?) It seems unique that it would maintain the same speed but with less torque All things being equal, aren't BOTH speed & torque in a motor a product of the strength of the generated field?* I found a very old Sears Craftsman drill from the 60's that certainly does not use gearing to control speed (as I had thought many did). It's AC only and appears to use a type of pot in the form of the trigger switch, electrically to control speed. I am fascinated that the battery-type uses a motor that when turned manually will generate power though it's contacts (as a "crank-style" generator would).
* Edit (@ 10 minutes later): I experimented and found that a little of both existed. Using a variac did cut speed but only at the point of minimal power so that it was virtually useless and indeed the torque WAS cut. I am surprised that the field within maintained the motors speed even at half voltage. Interesting stuff.
Registered Member #2028
Joined: Mon Mar 16 2009, 08:13PM
Location: Norway
Posts: 319
quicksilver wrote ...
Ahhhh. You guys give me a lot of food for thought. I'm glad to hear that I could experiment with little fear of damage. Pity that it would only affect torque. I had thought that if the magnetic field fell off the motor would spin slower (?) It seems unique that it would maintain the same speed but with less torque All things being equal, aren't BOTH speed & torque in a motor a product of the strength of the generated field?* I found a very old Sears Craftsman drill from the 60's that certainly does not use gearing to control speed (as I had thought many did). It's AC only and appears to use a type of pot in the form of the trigger switch, electrically to control speed. I am fascinated that the battery-type uses a motor that when turned manually will generate power though it's contacts (as a "crank-style" generator would).
* Edit (@ 10 minutes later): I experimented and found that a little of both existed. Using a variac did cut speed but only at the point of minimal power so that it was virtually useless and indeed the torque WAS cut. I am surprised that the field within maintained the motors speed even at half voltage. Interesting stuff.
The speed of the rotating magnetic field inside these motors depend solely on the frequency at which they're driven at (and the number of pole pairs in the motors windings). With dropping voltage a synchronous ac motor would continue operating at the exact same speed until it finally looses syncronization and stops.
But the more common asynchronous motor sags, i.e. runs slightly slower than the rotating magnetic field. An asynchronous motor whith two pole pairs driven at 50 Hz will have a synchronous speed of 1500rpm, but because it sags it will idle at about 1450rpm. The closer the shaft speed is to the synchronous speed, the less torque the motor can produce, so when given a heavy load the shaft speed will drop slightly.
This sagging will be much more noticeable when the motor is driven at a reduced voltage, but as you said yourself, its idling speed wont be affected much. So a variac is a poor way to control ac motors.
Edit: The cheapest and best solution i can imagine for you is to get your hands on an old treadmill. They often have DC motors and either a variac with a rectifier bridge or some switchmode voltage regulator. DC motors dont have a rotating magnetic field like its AC counterpart, and their speed can be easily controlled by the operating voltage.
I took this rather shabby picture with my cell phone at a large papermill here in Norway. A papermill this size require probably a hundred large motors with accurate speed control. Surprizingly they decided not to use modern AC motors with expensive VFD's and used simple, old-fashioned DC motors instead. There are two of them visible in this picture, a blue one to the left and a smaller grey one in the middle of the picture.
Registered Member #2463
Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546
Renesis wrote "A normal ac induction motor would also respond to a variac, and i see no reason for it to take harm from it, "
The reason what a variac would be a bad choice is that an induction motor will respond to a lowering voltage by drawing more current in an attempt to maintain constant power output. (ie the same speed). There is also the issue of 'breakdown torque' That is the point where insufficient excitation causes the speed to drop further until it stalls. When that happens, current skyrockets, (locked rotor current can be 6X full load current).
An induction motor has no rotating field, it creates one only if its rotor turns at a lower speed than the syncronous speed. That difference is known as the slip. If you reduce the vol;tage, the slip increases, that is why the motor may slow a bit. However with high slip, speed is highly dependent on torque. Ceiling fan motors are high slip induction motors, and can be slowed by voltage reduction, however they also have locked rotor currents close to full load current, and are called "impedance protected"
If you want the theory on how a single phase induction motor works, good luck. They have two models, and they have been arguing about it ever since Nick tesla adapted one to work on single phase.
Registered Member #2028
Joined: Mon Mar 16 2009, 08:13PM
Location: Norway
Posts: 319
radiotech wrote ...
Renesis wrote "A normal ac induction motor would also respond to a variac, and i see no reason for it to take harm from it, "
The reason what a variac would be a bad choice is that an induction motor will respond to a lowering voltage by drawing more current in an attempt to maintain constant power output. (ie the same speed).
Yes you're right, i forgot that. Of course thermal protection relays are a must on lathes, whatever the voltage they're driven at.
An induction motor has no rotating field, it creates one only if its rotor turns at a lower speed than the syncronous speed.
If the rotor turns at the synchronous speed there wont be induced any current in the rotor, but the rotating field will still be there.
That difference is known as the slip.
Thats the word i was looking for. In norwegian this is known as "sakking", so i kinda guessed sagging would be a safe bet.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Radiotech: I've ruined an induction motor by trying to slow it down with a variac. It was a capacitor start/capacitor run type, and I slowed it down enough that the centrifugal switch closed and left the start capacitor engaged. After a few minutes, the start cap, which is an AC electrolytic not rated for continuous running, exploded in an evil smelling cloud of fumes.
Induction motors without a centrifugal start switch are less of an issue, but the speed control doesn't work very well. You don't get a very wide range of control before they just stall.
Renesis: I did some work at a paper mill here, where they upgraded those DC drives to state-of-the-art AC ones. They were struggling to get the new drives tuned, and a lot of paper was getting shredded. I wasn't involved in the drive tuning, but I heard about it from other people at the plant. When you want really fine control over speed and torque, it's hard to beat the old DC motors.
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Of Winding Jigs & Motors
It was a capacitor start/capacitor run type, and I slowed it down enough that the centrifugal switch closed and left the start capacitor engaged. After a few minutes, the start cap, which is an AC electrolytic not rated for continuous running, exploded in an evil smelling cloud of fumes.