The Shop > Tools
DC motor for my lathe has low torque
vtsteam:
Thanks Andrew, no the problem is that there isn't enough torque to turn even 3" dia of iron reasonably quickly even if cleaned up already. The motor slows if I take more than a tiny cut.
I'd like to be able to shift more metal, and the new lathe definitely has the stiffness for it, the motor slowdown is greatly limiting depth of cut when running at slow speed. A conventional lathe has back gears for this. This direct drive variable speed method needs motor torque at low speeds to do the equivalent.
My question is -- is this DC motor DC/controller method inherently problematic at low speeds vs AC 3 phase motor VFD drive?
Or is my problem simply that my motor is adequate, but the DC controller is undersized/inadequate and I would get usable results by getting a higher rated one?
My question there is a result of not truly understanding how they work. Does the current rating of a DC controller mean that it will supply full rated current at any controlled voltage (speed)? Because if so, then it may well be that a higher rated controller would give me the torque I need.
This is the speed controller I have -- rated at "400 watts". My motor is actually rated at 900 watts.
https://www.amazon.com/24V-90V-Controller-Electric-Regulator-Permanent/dp/B076KPP92X
John Rudd:
Torque is proportional to speed which in the case of a dc motor is also a function of applied voltage.
So the lower the voltage the lower the speed and torque.
So to effectively cut a diameter of 3” you would need to speed the motor up but reduce the spindle speed to maximise the torque of the motor….( jack shaft?)
I fear that the controller is part of the problem too,in that there is no feedback to the controller of the motor’s speed or torque.
The KB electronics controllers ( Florida based..) are great for dc motor application once set up correctly,which is why they are favoured by the Chinese in their lathes and mills
vtsteam:
So John, what about a 3 phase motor and VFD? Would that be an advantage, or is it the same problem?
chipenter:
My voltage stays the same at all speeds , a controller chops the square wave and supplies shorter amounts the slower the speed .
John Rudd:
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....which in the case of many of the Chinese dc motors, is the reason for their demise.. Folk run them at low speeds, reducing what air flow is provided by the useless fan and inevitably killing the motor by its overheating...
3ph motors run at low speeds via a vfd have a similar problem, but rather than the armature windings overheating, its the stator of the 3 ph motor that suffers but to a lesser degree because they are afforded the cooling of the outer motor casing....
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