Basic speed problems on a new VEVOR MX-7x14lathe

J

JustSomeHuman

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Hello,
I have a good friend who has a brand new VEVOR MX-7x14 lathe. When he turns the speed all the way up he gets 2250 RPM. But if he sets it for 1000 RPM and tries to take a light cut, the machine slows down to almost nothing. A couple of questions, if I may.
1. What type of motor is used in this lathe? Brushed or Brushless? Does the motor operate with any feedback to the controller?
2. How does the controller adjust the speed? Is it voltage, PWM or something else?

I am a retired electrical engineer. I would appreciate any thoughts about this problem.

Thanks!
 
I'm not familiar with that lathe but my guess would be brushed DC with PWM drive. Without some means of monitoring load or rpm, the speed will be highly sensitive to load. It will get worse as the no load speed decreases. I converted my lathe to a brushed DC motor with PWM drivel and experienced severe drop off in torque at lower rpm. Adding an encoder and feedback solved the problem.
 
If you run the lathe at its max speed, the motor will deliver its max power. If the speed drops severe, that the turned diameter could be very large. Some of these mini lathes have a low/high gear option activated by a lever at the back of the lathe.
Brushed motors and spindle bearings, take some time to run in. If there is nothing said in the manual, let it run for 5 min at 500 RPM. Increase the RPM every 5 minutes by 500 RPM until the spindle is running at max RPM. This way, the grease in the bearing gets warm and will spread. The brushes will worn a bit and make better contact to the motor collector.
Than try to turn a 10 mm bar and see how the lathe performs. This hould be no problem at all.
Is the tool at center height and is the lathe spinning in the right direction.
 
The controller on these lathes should keep the speed fairly constant with load if it is not broken and adjusted properly. I'm starting to wonder though if the factory even bothers to make these adjustments based on your post and a couple others I have read recently.
The factory should provide more info on these speed controls but they don't- and no schematics either unfortunately, which is a shame.
I have some familiarity with the older style SCR speed controllers but very little with the newer mosfet and brushless types.
If it has a SCR controller there should be an adjustment pot for torque compensation. SCR controls can often be identified by the row of 5 power tab devices mounted on a heatsink- they vary the speed by changing the firing angle of the rectified AC waveform- like a light dimmer but with feedback.
The motor will be brush type with 2 leads coming out, often red and black.
-Mark
ps the advertisements for these lathes usually mention "brushless motor" if it has one- if the motor has more than 2 leads coming out (or two cable bundles) it's most likely brushless
There's a guy named Pete who works on these systems:
www.olduhfguy.com
 
Last edited:
RJSakowski said:
I converted my lathe to a brushed DC motor with PWM drivel and experienced severe drop off in torque at lower rpm. Adding an encoder and feedback solved the problem.
Click to expand...

Can you give me any information about the motor and encoder and the controller you used? Thanks, it was I thinking about but have yet to find the pieces.


Sent from my iPhone using Tapatalk
 
If you run the lathe at its max speed, the motor will deliver its max power. If the speed drops severe, that the turned diameter could be very large. Some of these mini lathes have a low/high gear option activated by a lever at the back of the lathe.
Brushed motors and spindle bearings, take some time to run in. If there is nothing said in the manual, let it run for 5 min at 500 RPM. Increase the RPM every 5 minutes by 500 RPM until the spindle is running at max RPM. This way, the grease in the bearing gets warm and will spread. The brushes will worn a bit and make better contact to the motor collector.
Than try to turn a 10 mm bar and see how the lathe performs. This hould be no problem at all.
Is the tool at center height and is the lathe spinning in the right direction.


Sent from my iPhone using Tapatalk
 
I have some familiarity with the older style SCR speed controllers but very little with the newer mosfet and brushless types.
If it has a SCR controller there should be an adjustment pot for torque compensation. SCR controls can often be identified by the row of 5 power tab devices mounted on a heatsink


Sent from my iPhone using Tapatalk
 
JustSomeHuman said:
I have some familiarity with the older style SCR speed controllers but very little with the newer mosfet and brushless types.
If it has a SCR controller there should be an adjustment pot for torque compensation. SCR controls can often be identified by the row of 5 power tab devices mounted on a heatsink


Sent from my iPhone using Tapatalk
Click to expand...

I’ll take a careful look at the controller board. I “think” it’s a brushed motor.

Thanks!


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