Speed control problem with Jet 1220VS

[jlsalter]

I purchased a Jet 1220vs lathe at 2008 AAW symposium in Richmond. When I got the machine home, I mounted a 7 inch square bowl blank and when I started cutting, I found the spindle speed would increase as a function of the force applied. I tried to ignore the problem until I was doing a demo with an out of balance blank. When I started the machine it rev’ed up to a dangerously high speed. I called the Jet service department (very nice people) and they sent me a new speed controller. I replaced the controller but the problem persists. Has anyone experienced an out-of-control speed problem with the Jet 1220VS?

 

[Rick M]

Following a hand spin to check clearance, I always apply power with the speed control knob of my 1220VS at the low end of the range for any of the pulley steps I'm using. I've also noticed it seems speed changes while turning the control knob are noticeably nonlinear when increasing, while fairly uniform when decreasing. As a result, I tend to run up the speed a little then back down to where I want to turn.

Once turning is under way at the desired speed, and especially noticeable at the higher speed (less torque) pulley steps, a few moments of tool-induced slowdown result in the controller applying more power and returning the speed to it's previous value. That return to speed may have some overshoot, but if so, I've adapted to this and no longer really "notice" it. I suppose any overshoot might also depend on whether there's a tool against the wood or not.

Keep in mind that the maximum speed attainable is dependent upon which step of the pulleys you're using. From your description, considering you had an unbalanced blank of unspecified mass, unless you

1. had the control knob at minimum
2. had the belt on the lowest (or maybe adjacent) speed range
3. hand spun the blank (if heavy) prior to hitting the power switch

I'd surmise the controls were responding as designed--trying to "return to set speed" and perhaps overshooting due to the mass of the blank and lack of opposing/braking tool force. The control's "rate of change" and delay before reacting may be trying to compensate for starting from zero, overcoming the unbalanced (possibly heavy) blank, then backing down from the amount of power "built up" to reach your "set point".

If you can, balance the blank as much as possible before applying power, turn at low speed until enough wood has been removed to achieve final balance, then change belt as needed to turn at final choice of speed range. But always start with control knob at minimum, and give a hand assist to startup for heavy pieces.

 

[Bill Boehme]

The thing about the controller used on this lathe is that it does not have true "velocity feedback" to maintain stable speed control under varying load conditions. The controller senses motor current and uses that as an indirect method of estimating the velocity (although the whole problem with this approach is that motor current is more closely related to load torque than it is to speed). This speed control method works more satisfactorily in a situation where the load torque is constant or linearly proportion to speed, but in the case of a woodturning lathe, the load torque can vary all over heck especially if the diameter of the turning is large since that directly affects load torque. In a situation where you would be turning a bowl, the speed controller basically loses any predictable correlation between motor current and motor speed which means that the controller basically loses its "mind".

The above description is applicable to many DC controllers and also to some AC controllers that are classified as "sensorless vector" type. Most likely, your's is a DC controller and there may not be any provisions for adjustment. In some cases, there is an internal trimer pot that is used for overspeed compensation or limit pots to restrict seed excursion. If using an AC sensorless vector controller, the options are much better. They have control panels that allow the user to adjust many parameters that can optimize performance. Another option with the sensorless vector controllers is that they can step back to simple V/F control. Using V/F control results in some slowing down during high torque loads, but might be a better option than overspeeding.

 

[Bill Boehme]

I'd surmise the controls were responding as designed--trying to "return to set speed" and perhaps overshooting due to the mass of the blank and lack of opposing/braking tool force. The control's "rate of change" and delay before reacting may be trying to compensate for starting from zero, overcoming the unbalanced (possibly heavy) blank, then backing down from the amount of power "built up" to reach your "set point".

You have given some very good advice, especially the above quote. The mass of the turning is the largest part of the total load inertia and has a very significant effect on overall system stability.

The goal is to minimize total system load inertia -- to do that, it is very important to begin turning with the pulley in the lowest speed range (the load inertia is proportional to the square of the drive ratio so the importance of using the lowest speed range is very significant). Once a significant amount of the wood has been removed (the basic outer shape done and some hollowing of the inside), then you can shift up a notch in the pulley speed range.

 

[Rick M]

. . . Most likely, your's is a DC controller and there may not be any provisions for adjustment. In some cases, there is an internal trimer pot that is used for overspeed compensation or limit pots to restrict seed excursion. If using an AC sensorless vector controller, the options are much better. They have control panels that allow the user to adjust many parameters that can optimize performance. Another option with the sensorless vector controllers is that they can step back to simple V/F control. Using V/F control results in some slowing down during high torque loads, but might be a better option than overspeeding.

Interesting (and compliment appreciated)... The motor is rated 3/4 hp, 115V, single phase. I gather that means the controller would be an AC sensorless vector type, lending hope that there might be some adjustment(s) available. Unless I can find a schematic, I just may have to go open mine up and get a good look at what's under the hood while waiting for JL to jump back in to the discussion.

 

[Bill Boehme]

Interesting (and compliment appreciated)... The motor is rated 3/4 hp, 115V, single phase. I gather that means the controller would be an AC sensorless vector type, lending hope that there might be some adjustment(s) available. Unless I can find a schematic, I just may have to go open mine up and get a good look at what's under the hood while waiting for JL to jump back in to the discussion.

There are two possibilities regarding DC controllers:

• It might be a universal motor which is basically a DC motor, but the commutation scheme allows it to run on AC as well. Exmples of this type of motor are drill and router motors.
• It really is a pure DC motor and the controller is converting AC to DC.

If either of the two above are true, then the motor can easily be identified as DC by the fact that it has brushes. If you see something on one end that looks like two large screw heads about the size of a nickel on opposite sides, they are the covers for accessing the brushes.

There are also two possible types of AC motors being used:

• For almost all variable speed systems, a three phase motor is used because it allows the motor to develop full horsepower at base speed while also allowing it to maintain full torque below base speed. This is the best option. A sensorless vector drive requires a three phase motor. It can also use a V/F controller which does not have quite as many options for tweaking its performance.
• For some systems such as air handlers where the starting load torque is zero, load torque is light and proportional to speed, and not much load inertia, then a single phase motor can be used with a controller dedicated for this purpose. They are basically not suitable for much besides air handlers in HVAC systems because of their limitations. They can't handle varying torque loads at low speed and maintain speed regulation and they stall easily. I seriously doubt that Jet uses this system, but if they do, they should be publicly caned. All single phase AC motor controllers use a simplified V/F scheme.

Even the "big boy" lathes can have stability problems when turning heavy chucks of wood with drive belt in the high speed range before the bulk of the wood has been turned away.

 

[jlsalter]

Rick and Bill, thanks for your responses to my posting. I have to admit that you are way beyond me in your knowledge of variable speed drives. I have the schematic in hand and yes the motor does have brushes. Also, I know reversing the lead wires to the motor reverses the direction of the motor, leading me to believe the motor is really DC. The printed circuit board has 3 variable resistors, but there is no indication of their function.

I’ve turned on many lathes in my career but I’ve never seen a situation where the speed increases with load. As I turn out toward the edge of the blank, the spindle increases in speed wildly. Two nights ago a student of mine was hollowing out a bowl and before I could correct her, she got a catch. During the catch process, she applied a great deal of load which resulted in a major speed increase and even blew out the lamp connected to 110v through the speed controller.

I can see how limiting the top speed by keeping the belt on the lowest speed configuration would help. Basically, physically limiting speed the spindle could obtain in high load situations. I would certainly prefer the spindle speed to slow under a load rather than speed up!

 

[rjones]

Is your lathe bolted to a solid stand or is it on a stand with rollers. If it is not good and solid it could be viberating too much and causing problems. A lot of people that I have seen with the small lathes (No Matter the Brand) try to keep them to portable and that is not good imo. The stand is as important as the lathe.The slow speed belt setting gives you the most torque so the motor thinks it is working harder than it is and the spindle can only run so fast no mater where the dial is setting.

 

[Bill Boehme]

The printed circuit board has 3 variable resistors, but there is no indication of their function.

I’ve turned on many lathes in my career but I’ve never seen a situation where the speed increases with load. As I turn out toward the edge of the blank, the spindle increases in speed wildly. Two nights ago a student of mine was hollowing out a bowl and before I could correct her, she got a catch. During the catch process, she applied a great deal of load which resulted in a major speed increase and even blew out the lamp connected to 110v through the speed controller.

I can see how limiting the top speed by keeping the belt on the lowest speed configuration would help. Basically, physically limiting speed the spindle could obtain in high load situations. I would certainly prefer the spindle speed to slow under a load rather than speed up!

A DC motor is what I expected since cost of the controller is far less than a variable frequency drive for an AC motor.

Can you verify for me that this problem occurs when the pulley is in the slowest speed range (assuming that there is two or more speed ranges available). The slowest speed range can be identified as the one that uses the smallest pulley on the motor arbor and the largest pulley on the lathe spindle. Making certain that the belt is set correctly is essential to validate any further troubleshooting. One of our club members who had a large Vicmarc lathe contacted me several years ago about a very similar stability problem when he mounted a heavy piece of wood and began turning at low speed. It turned out that he had the belt set in the high speed range and moving the belt to the low speed range fixed the problem.

It would take a brave man to tweak the trim pots without having more information like a technical manual so I won't suggest that. If it were me, I would probably try to reverse engineer the schematic to figure out what each trim pot does and that might give me more confidence in tweaking them. My guess is that it is probably a lot easier to make the performance worse than better by monkeying around with things that are not completely understood.

 

[jlsalter]

Bill, I’ll try reducing the speed range by moving the belt this evening and report back. The belt setting for the events described in this post so far has been the second highest speed setting (second largest pulley on the motor).

RJ, the lathe is very securely mounted. The lathe runs fine until I apply force to the piece with a gouge. The problem is worse when I cut at the edge of the piece, or if the blank is out of balance.

 

[MichaelMouse]

Bill, I’ll try reducing the speed range by moving the belt this evening and report back. The belt setting for the events described in this post so far has been the second highest speed setting (second largest pulley on the motor).

Since no one else has mentioned it, when your motor pulley is twice the size of the spindle pulley, any controller compensation will double the effect at the spindle. Which is one VERY good reason to keep the belt at low size, spindle at high size. Ratio being reversed, a correction at the motor will only have half effect. You might not even notice it.

 

[Bill Boehme]

Bill, I’ll try reducing the speed range by moving the belt this evening and report back. The belt setting for the events described in this post so far has been the second highest speed setting (second largest pulley on the motor).

RJ, the lathe is very securely mounted. The lathe runs fine until I apply force to the piece with a gouge. The problem is worse when I cut at the edge of the piece, or if the blank is out of balance.

Well, shoot me dead. If I can believe the dismal online owners manual for the lathe the illustrated part breakdown indicates that there are six steps on the cone pulley. I am not the least bit surprised that you are encountering speed regulation problems by running in the next to highest speed range. When you do that, for normal speed turning you will be operating the motor at the lowest voltages, which in turn means that it will be primarily regulating torque and forcing speed to take a back seat. You need to reverse this situation and make speed regulation the primary concern. The way to accomplish this is to move the belt to the the lowest speed range as I indicated in two of my previous posts.

Michael M. indicated in his latest post that the torque load from the spindle is being multiplied through the drive train ratio and increasing the torque load on the motor -- he is correct, but it is even worse than that -- in addition the the torque load, the moment of inertia of the spinning mass is multiplied by the square of the drive ratio when reflected back to the motor.

As an example, suppose that the speed ratio of the pulley step that you are using has the spindle turning four times faster than the motor -- as far as the motor is concerned, the inertial load that is hanging on its shaft is sixteen times the actual inertial load. So, lets assume that you start out turning a fairly hefty bowl blank and are running the lathe at a prudent speed for initial roughing. Initial cuts have a lot of torque ripple as you alternate between cutting air and some fairly heavy cuts in the wood. As the motor attempts to provide the required torque, speed will go to heck in a big way -- and with the high inertial load that the motor sees, the effect will be much worse -- the motor and controller will be completely unstable.

Now, lets assume the opposite situation where you move to the lowest speed range on the pulley so that the motor is turning four times faster than the spindle. Torque from the spinning mass as seen by the motor will only be a fourth of the spindle torque -- this is much easier for the motor to handle. Additionally, the inertial load of the bowl blank will appear to the motor to be only one-sixteenth of its actual inertia (that's only 1/256 of the inertial load of the previous example). This means that the only significant inertial load for the motor will be its own rotor and not much else. Further, the motor will be running at a much higher voltage and lower current -- this is much better for speed regulation as well as motor heating due to I²R loss.


I am very disappointed that the users manual did not really have any wothwhile user instructions -- especially with respect to setting the pulley speed ranges. Basically, it had the usual boilerplate stuff and IPB and that was it. It reminded me of the sorry Jet manuals from thirty years ago when WHM was strictly importing cheap Taiwanese machines with manuals also prepared in Taiwan in broken English.

 

[jlsalter]

Conclusion

Last night tested the speed control by turning a 5” blank with various pulley configurations. I began with a slow speed by moving the belt to a small pulley on the motor shaft. When I started cutting, I found the speed to be pretty consistent as I applied greater loads throughout the speed range of the motor.
I then moved the belt to the high speed range (1100-2700rpm) and set the motor speed toward the bottom of its range. As I applied a load with my gouge, the speed of the blank increased greatly and then return to normal when I removed the load.
I then moved the belt down one pulley step to the 760-1800 rpm range. Now I could work with the motor speed set almost to its maximum. In this pulley / motor speed configuration, I found the speed control of the blank to be much more constant.
What I’ve learned from all of this: For best speed consistency, when turning larger stock on this lathe, the belt and pulley configuration should be set such that you are working within the top 10% (or so) of the top speed of the motor.
Thank you to all who have participated in this thread.
--Jeff

 

[Bill Boehme]

.... What I’ve learned from all of this: For best speed consistency, when turning larger stock on this lathe, the belt and pulley configuration should be set such that you are working within the top 10% (or so) of the top speed of the motor....

You have summarized things very concisely.

With the motor running close to its full speed (which equates to operating near its rated voltage), it is able to deliver its full torque and power to the load. If it is forced to run at a low speed (which equates being limited to a lower voltage) then it can only deliver a fraction of its full rated power. It does better on delivering torque to the load, but at the expense of constant speed.

Hmmm … your way of summarizing things is a lot easier to remember. 😀

 

[John Hart]

Jet 1220vs

I had a similar problem with my 1220. Eventually the vs quit working all together I took it to the local serviceman and he replaced the circuit board. It hasn't done it since. I didn't understand half of what you guys were saying, but I have noticed the lathe tries to regulate the speed while under a load. But, that is not what I experienced. My lathe would go to full power limited only by what pulley I was using and then slowly wind back down. Until I could get it fixed I kept the variable speed on high and I didn't have the problem, until it broke🙂 The Jet rep my serviceman talked to said this was an "anomaly". So maybe that was a different problem.
John.
p.s. This experience made me install a kill switch on the toolrest side of the lathe.