The Shop > Tools

DC motor for my lathe has low torque

<< < (4/5) > >>

vtsteam:
Okay, thanks John, that puts it in perspective. I've never owned one of the mini-lathes or even seen one work, so I had no idea how much torque they had at low speeds. I just figured they must get better results than me with my homemade gear.

So the upshot is, I'm sending for another DC controller, in case this one is limiting the 1 hp motor in some way, and I'll see what needs to be done to add a jackshaft to change the present reduction from 5 to 1 to 10 to 1. The problem for me with that is it's sometimes problematic to find toothed belts in exactly the right length, plus pulleys, while I still need to maintain the 5 to1 ratio to encoder for the electronic feed screw.


Oh yes, and add a separate fan.

mc:
Most electric motors are constant torque devices (until they reach rated speed, at which point they become a constant power device), so the amount of torque they produce is consistent over their speed range.
Current is directly related to the torque (load) applied, and speed is directly related to voltage. (this also applies to AC motors/VFDs, but you have the added complication of frequency controlling speed, but the basic current/torque and voltage/speed rules still apply).

That is the theory behind motors, but the actual speed/torque curves will not be perfect straight lines due to other factors, but they'll be near enough for basic calculations.

A good motor speed controller will always limit current to the motors rated current. You can try driving more current through by increasing voltage at a given speed, but that is when you're likely to damage the motor, as you're essentially pushing more current through the motor than it's designed for, which will likely lead to saturation (aka the motor magnets aren't physically capable of producing any more torque), and overheating.


This means that the OP's 900W (90V x 10A) motor with the maximum spindle speed of 2000rpm, gives a constant torque of 4.3Nm at the spindle.
At 100RPM it should still produce 4.3Nm, but as it's only doing 5% of it's rated speed, it will only be capable of producing 45W of power.

To put it context, you can probably get more than 4.3Nm turning a typical screwdriver handle.

vtsteam:
Well mc you started me thinking....I'd given the motor specs in my second post only from (faulty) memory. Mainly because it had been quite a few years since it was added to the lathe, and the tag had been painted over. So I spent the last half hour looking for old pics of the lathe during construction, to try to see if there was one that included the motor label.

Finally I found one -- had to enlarge and enhance to read it, but I was fairly far off on the specs. Here it is:

And also note that there is a 5 to 1 reduction via toothed drive belt to the spindle as mentioned in the same post. An online motor calculator puts torque at 3.33 Nm direct drive, and I'm assuming that means 16.6 Nm after reduction. Is that correct?

vtsteam:
Also, the speed controller I have,as mentioned earlier is rated at 400 watts. The output is 90V not the 130 which the motor is rated for. I've also since read several purchasers saying that their measured output was under voltage (~70V).

If it's rated at 400 watts, and 90 V that must mean about 4.4 amps, is that correct? The actual motor is rated at 18 amps. So if the controller is limiting current to 4.4 and voltage to 90, seems like there's room for improvement with a better controller.

Right now I'm waiting to receive a treadmill DC motor controller, that would have been used with this motor, and I'm hoping that will give me better performance.

mc:
There is a fair amount of more power to be had from that motor with a suitable controller.
4.4A sounds about right, so even upping to 15A will more than triple the available torque.

Unless you're aiming for maximum speed, not having full voltage available will only limit maximum speed. It won't limit torque at lower speeds.

As you've only got 90VDC, that means the motor will likely max out around 4600rpm.

Navigation

[0] Message Index

[#] Next page

[*] Previous page