thinking about servo options

[senor J.]

does anybody know the what the difference between a dc servo motor and just a regular dc motor is. It seems like you could stick an encoder on a regular dc motor and it would work. I know there is a difference in the heat dissipation because a dc servo is longer but beyond that whats the difference?

[ESjaavik]

A DC servo have very smooth delivery of torque.
A "regular" DC motor is not built to deliver torque at zero RPM. So the ratio between current and torque will be very dependent upon the position of the motor. The servo drive does a lot of calculations internally, and they assume that when given a certain current, the motor can be expected to deliver a torque in proportion to that. With a "regular" DC motor you can keep on tuning the parameters until your shrink comes.

A "proper servo" have more commutation points (segments in commutator) and more than one pair of brushes. Sometimes also other "tricks" are used to achieve a torque which is depending only on current and not on the position of the rotor.

There are servo motors with only one set of brushes, and they'll do fine if they are used in applications where they are not expected to be used close to zero speed. Like a conveyor belt or a spindle. For positioning, like the axes of a machine, they must perform well down to a full standstill.

There are other factors too, like the rotating inertia, which for positioning should be low to get a good dynamic behaviour (acceleration, retardation). For a fan motor and most other uses, that's not very interesting, and they don't put production cost into it to achieve it.

So producers of servo motors does not get more golden noses than others, they simply spend more money on producing the motor. And pass on the cost to the user.

Some earn their golden noses by taking a low-spec servo motor and sell it on eBay implying they are eminently suitable in positioning drives.

[ESjaavik]

Oh, you can check it:
Connect a "proper DC servo motor" to an adjustable power supply. Increase the voltage slowly until it starts turning. If it turns veeery slowly and without hesitations, it's a good sign. A standard DC motor will race away after it have started moving. If lowering the voltage, it will stop at a "deadpoint" long before it gets down to barely rotating.