Re: Servo Question

Mark,

The short answers are: (1) It all has to do with heat. (2) Always
figure your pulley ratio using the smaller torque rating.

(1) Long answer:

Current is proportional to torque.
Heat is proportional to the square of the current.

The rated continuous torque (and thus current) limit is set by the
maximum wattage (heat) the motor can safely dissipate. This is the
old W = I^2 * R thing.

Brush-type motors are miserable things from a thermal viewpoint. The
armature contains the windings that generate heat but there is no
path for this heat to escape. A small amount gets conducted out thru
the motor shaft but steel is a terrible heat conductor. The rest
heats the air inside the motor (another terrible heat conductor)
which then heats the case of the motor.

A typical ratio between continuous and stall torque may be 1:8,
(double check your numbers, you may have slipped a decimal point).

This means at stall the motor is dissipating 64 times more heat than
it can safely get rid of. The motor doesn't have long to live if
operated at that torque continuously.

(2) Long answer:

Think in terms of power, not torque.

Power (Watts) is what gets things done. Power is RPM times torque. By
definition stall torque is at zero speed. That means the motor is
generating zero power.

Your motor's continuous torque is 53 in-oz (16 times 3.313) but you
didn't mention its no-load RPM, so I'll guess it to be 6000 RPM. I
will also guess the stall torque is 424 in-oz (8 times 53 in-oz).
Let's use the following equations to generate the table below:

1) Output power (Watts) = RPM times in-oz / 1351
2) Load RPM = (Stall - Load) / Stall times no-load RPM
3) Efficiency = (Stall - Load) / Stall
4) Heat (W) = (load torque)^2 * (no-load RPM) / 1351 * (stall torque)


# LOAD RPM Eff Power Heat

0) 0 6000 ??% 0W 0W
1) 53 5250 87% 206W 29W
2) 106 4500 75% 353W 118W
3) 159 3750 62% 441W 265W
4) 212 3000 50% 471W 471W
5) 265 2250 37% 441W 736W
6) 318 1500 25% 353W 1059W
7) 371 750 12% 206W 1442W
8) 424 0 00% 0W 1884W

Couple of things to note: In (1), the motor is putting its rated
power of 206W and is dissipating 29W of heat, the max. it's rated
for. In (4), the motor's output power peaks at 471W but it is
dissipating 16 times more heat than it can safely get rid of. In (8)
the motor's output power zero but it is heating up to the tune of
1884 Watts!

Mariss

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "mmurray701" <gmurray@n...>
wrote:

> I'm considering converting from steppers to servos (DC). After
> looking at some specs it seems that continious stall torque is far
> lower than peak torque. One motor I was looking at has a continious
> stall torque of 3.313 lb-in, and peak torque is 300 lb-in! Why is
> there such a huge difference? Can somebody give me a defenition of
> each?
>
>
> I know approximatly how much torque I want on the leadscrew, which
> motor torque value do I use to figure out pulley ratios? Please

help

> me out. Thanks.
>
> Mark