Why servomotor has such low torque compared with stepper motors?

Question

It is easy to found stepper motors with torque over 2NM for a few coins, however, only expensive industrial servomotors achieve this torque.

Examples:

• Stepper motor: 10usd for 1.3nm with 2.8V at 3.5A rpm 0.6kg
• Servo AC motor: 278usd for 2.39nm with 220V~ at 3A 3000rpm 6kg
• Servo DC motor: 67usd for about 0.4nm 24V at 7.8A 3000rpm 1Kg

Torque is not power, and closed loop has many benefits, but I am still confused:I believe many industrial purpose require quite high torque (CNCs, robotic, etc.) and it would be preferable (to my understanding) a lower rpm with higher torque. Additionally, it seem to me that any purpose for a servomotor require more than a few 0.4nm

Why is this torque that different?

What is the benefit / purpose of those non-hybrid servo-motors?

What am I missing?

Answer 1

Operating principle is different. In essence if you buy just a motor it will be just that. You can now add gearing that will reduce speed and increase torque. Since torque is directly proportional to gear ratio and mechanical advantage;

$$ MA = \frac{v_{input}}{v_{output}}. $$

You get a hundred times more torque if you gear 3000 rpm down to 30 rpm. So now that DC motor you listed would be 40Nm and that AC servo a 239Nm monster. So indeed those are way more powerful.

Further there may be gearing inside your stepper motor, its quite common. You can get geared servos. It is just more uncommon as designing the drive train needed is kind of the point of using normal motors. As the drive train is the part where you optimize things for your application. That or manufacture/design the motor yourself.