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Nova DVR catch then speed up.

J

Jeff Eckrich

Joined
Feb 17, 2012
Messages
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Location
West Allis, Wi
I am just getting use to my Nova DVR and noticed that when I am turning a rough bowl blank (200 rpm) and get a catch that stops the lathe momentarily after I pull the tool out of the way the lathe speeds up for a short while as if it is catching up.It then goes back to the correct rpm.
This never happened on my 1624.
Any ideas out there as to why? Do I have a problem, or is this just the nature of the DVR?
Jeff
 
It is just a peculiarity of stepper motors which is the type of motor used in the Nova DVR. They behave differently than AC induction motors which is the type of motor used in all of the other electronic variable speed lathe drives. With AC induction motors the electronic controller is commanding a speed to the motor which is determined by the output frequency from the controller. Steppers are an entirely different type of beast and their controllers can only command a shaft position and in order to synthesize a speed command, the controller creates a continuously rotating position vector that the stepper motor chases and, in effect, produces rotation at the desired speed. Everything is fine and dandy unless something happens to stop the motor as you discovered. In that case, all that the motor knows is that it has fallen way behind its commanded position so the response is to accelerate to its maximum speed until it catches up with the commanded position. Once it has almost caught up to the commanded position, it begins to slow down to the desired speed and resumes normal operation.
 
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Sort of like hitting "Resume" on the speed control of your car and having it downshift and hurry up to the preset.

Stay over the wood with your tool. On a convex surface, cut above center, on a concave, below. Then your only catch will come when you neglect the tool position as you reach for the ON/OFF switch. Keeping the toolrest as close as possible gives you best control, so don't be afraid (after clearing the tool) to stop and move the rest often.
 
Nova DVR

Jeff call the Teknatool rep in Florida. Bob G I think and have him walk you through the steps to reset some of the DVR settings. It is simple and will solve your problem. It has to do with the factory settings (from China) and is not difficult to change when you know how. Once the change is made it stays in the controller chip it does not have to be constantly reset. Darryl
 
Its like Bill said, the motor has a certain speed you've asked it to run = the setpoint. When you slow it down, it tries to return to the setpoint. There are tuning parameters that determine how quickly the motor responds and the faster it responds the more likely it is to "overshoot" the setpoint and speed up, as you observed. Without getting into control logic, there are parameters that determine the response speed can be changed and Nova may have a different set that are more optomized, but I wonder why they wouldnt have put them on at the factory, if they were better. I dont know what type of control is used, but generally it's a tradeoff between speed in returning to the setpoint and minimizing overshoot. Ie you may not speed up at all, but it may take 5-10 seconds to get back to your original turning speed.
 
DVR catch and speed uo

Thanks for the replys. I never thought that a lathe could be so confusing.
I will call Florida and make the adjustment mentioned.
Jeff
 
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