Servo Motor Rated Speeds, Max Speed & Gearing

[BigBrand]

The servo motors im looking at (as im sure all do) have a rated speed and a maximum speed for argument sake 2k rpm rated and 3k rpm max . I dont think the manufacturer (Delta) have produced a torque vs speed graph.

Do i understand correctly that the motor can generate the rated torque at any speed up until the rated rated speed quite happily all the time. And above this speed the motor loses torque generally in a linear fashion down to its maximum speed at which point it no longer generates any torque and presumably starts to shake itself to pieces (pointless)?

My question is do i want to calculate my gearing including pinion (assume 37.5mm pitch diameter) and theoretically only operate the motor in the 0 - 2k rpm window - say 5:1 thus giving me ~50metres per min feed rate at 2k rpm with maximum torque.

Or

is it advisable to theoretically operate the motor up to its maximum speed between 2-3krpm to gain some resolution perhaps? If i modified the gearing to say 7:1 i could still reach 50m/m. The torque would be tailing off but i guess at these higher speeds i wouldnt be cutting anyway at a guess it would just be rapids so would it matter?

I keep using the word theoretically there as im sure there will be many factors during implementation that probably stand in the way of achieving this i just thought it would be better practise to design like this and accept limitations when implementing rather than designing in limitations at this stage?

[pippin88]

Delta do have torque graphs. They have a good manual which is partly why they are worth paying for.

You will get 100% torque to rated speed then it will drop off to max speed.

You then have continuous vs intermittent duty zone.

The servo will be happy all day in the continuous zone. The time in the intermittent zone depends on how much over the continuous zone you are. E.g. 110% will be ages where as 300% willintermittent
This info is also in the manual but slightly harder to find.

[pippin88]

One approach is to use rated speed for cutting and max speed to rapids.

Be aware that acceleration of the system is usually the greatest load, more so than cutting.

[BigBrand]

Good to know they have manuals thank you.. i've searched on their web and not found anything as yet but ill take another look

Rated speed for cutting and then max speed for rapids made more sense in my head after i thought about it. However, i wonder if i should just keep it simple at 2krpm gear it down to a sensible speed (still more than i could ever really need) while giving me more torque than ill probably ever need and just be done with it.

[Kitwn]

I'm not the expert on motors of any kind but in all types there are factors that limit the maximum torque the motor can deliver and others that limit the power. Since power is equal to torque times speed there will be a range of speeds where the torque is at it's maximum and the power produced rises with speed. Once you hit the maximum power possible from the motor and/or driver the torque must fall as the speed rises. Eventually all the power is used just to turn the motor itself and you've maxed out.

Kit

[pippin88]

Good to know they have manuals thank you.. i've searched on their web and not found anything as yet but ill take another look

Rated speed for cutting and then max speed for rapids made more sense in my head after i thought about it. However, i wonder if i should just keep it simple at 2krpm gear it down to a sensible speed (still more than i could ever really need) while giving me more torque than ill probably ever need and just be done with it.

Can be a little hard to find on their website. I found easier to Google:
Delta B3 servo manual
Delta A3 servo manual
Delta B2 servo manual

[JAZZCNC]

Good to know they have manuals thank you.. i've searched on their web and not found anything as yet but ill take another look

Rated speed for cutting and then max speed for rapids made more sense in my head after i thought about it. However, i wonder if i should just keep it simple at 2krpm gear it down to a sensible speed (still more than i could ever really need) while giving me more torque than ill probably ever need and just be done with it.

Thinking in speed terms is the wrong approach because after your rolling it takes very little power to keep it rolling, slowing down, and changing direction is completely different and this is where rotor inertia, Mass, torque, etc come into play.
The fact you can travel at 50mtr.min might seem great until you come to stoping the bloody thing and it overshoots because the inertia of the moving mass and rotor together run out of brakes.
Likewise hitting the throttle hard is the same thing in reverse, so high rotor inertia, heavy mass requires lots of power to get things moving, after which hardly any is needed to keep it moving, then the cycle starts again to slow down/change direction.

It's these things, among others, which can make tuning the motors difficult, achieving high feeds is easy but not always a good thing.