The Shop > Tools
DC motor for my lathe has low torque
John Rudd:
--- Quote from: chipenter on February 17, 2022, 11:53:38 AM ---My voltage stays the same at all speeds , a controller chops the square wave and supplies shorter amounts the slower the speed .
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That is PWM control....the DC supply is chopped into 'segments', whose width varies to provide the different speed....
philf:
--- Quote from: John Rudd on February 17, 2022, 11:55:48 AM ---Similar kind of issue, but a 3 phase motor controlled by a vfd is frequency controlled rather than voltage....
Secondary to the lack of torque at low speeds is the lack of cooling too....
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For that very reason I fitted a mains powered fan to the 3ph motor on my CNC mill. It also has the advantage of being a lot quieter when the motor is running at 5,000 rpm.
awemawson:
But all these issues can be solved with a counter shaft / back gear arrangement - can that not be retro-fitted Steve?
vtsteam:
Thanks John for the cooling issues explanation. So there's no advantage then to going to 3ph/vfd motor combo re. low speed torque?
Yes Andrew, of course a back gear or jackshaft reduction can be added. I'm just trying to understand whether it is possible to get reasonable torque at low speeds via a motor, rather than going to gearing.
To those with them, how do the Asian mini-lathes stack up when trying to machine let's say a 4" diameter piece of cast iron with a carbide tool? Are they able to do that fairly well with their DC controllers and motors? Or do they have to take very light cuts?
John Rudd:
--- Quote from: vtsteam on February 17, 2022, 01:42:30 PM ---
To those with them, how do the Asian mini-lathes stack up when trying to machine let's say a 4" diameter piece of cast iron with a carbide tool? Are they able to do that fairly well with their DC controllers and motors? Or do they have to take very light cuts?
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I remember the days of when I had a mini mill....that had a dc motor/controller....I found that I could only take light cuts when using a fly cutter...let alone carbide tipped tool....
The controller was set up such that if the motor was overloaded, the controller would just 'fold back' ultimately reducing the power to the motor which would stop...( my doing after reading the tech manual for the KB controller...) hence the machine had a better chance of survival without breaking anything...
Those folk with less experience that I've come across have unfortunately suffered with either a burnt out motor or they simply broke the controller because I suspect the controller wasnt set up properly at the factory....
My big lathe ( Warco WM290 lookalike...) was fitted with a 1.5kw dc motor and corresponding controller, but that didnt stay very long, I swapped it out for a 3ph 1.5kw motor, with a shaft speed of 2800 rpm ( a 2 pole motor rather than a 4 pole), winding the max frequency up on the vfd to around 72Hz gets me a spindle speed of around 1000rpm on the slow pulleys which is similar to the dc setup ( so that I got the same speed readout as before the changes..)
As it is now, I rarely use speeds in excess of the 1000rpm and have had no issues with slower speed for larger material...I made a riser block for my VMC mill which was 6 inches in diameter...No bother facing the ends with a carbide tool at slow speeds...
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