Differences And Types Of Dc Motors

ironmonger said:
As I understand it, the runaway situation only presents itself with series wound motors, especially larger ones. The shunt motors are self limiting, as are the compound motors AFAIK.

My motor theory is in the distant past, it's something to do with back emf induced in the armature I think. And while I have no experience with DRO's (but I would like to ) but do have experience with RF interference, the brush noise is a definite consideration.

Most of the things that we do in our home shops that are not for profit ventures depend on the material on hand. So in that light, using a motor that you have is superior to not using a tool because you don't have the BEST motor.

I have found that large 3 phase motors are much easier to find than large single phase motors. And both of those are easier to find than large DC motors. A lot of the larger surplus DC motors were rated for fork lift service. In that use they drive a pump, which presents no radial load on the armature. Transferring 5 HP though anything that present a radial load would likely be a problem for long term survival of the motors bearings. I hate to build something around a part that will fail and then cause an major redesign...

paul
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Large shunt motors can overspeed when their fields are interrupted under full speed no load conditions because all of the energy stored in the magnetic fields gets converted to kinetic energy of the spinning armature. You don't see this with small motors because they have large losses relative to the amount of stored energy as well as a large margin between rated speed and destruct speed.
 
On the subject of DC motors. My Hardinge lathe uses Im assuming a shunt wound motor on the carrage. But they separated the field and armature windings, feeding full dc power to the fields while varying the voltage to the armature for speed control. Why? Better low speed torque? Its the first time in my limited experience I've seen this done.

Greg
 
f350ca said:
On the subject of DC motors. My Hardinge lathe uses Im assuming a shunt wound motor on the carrage. But they separated the field and armature windings, feeding full dc power to the fields while varying the voltage to the armature for speed control. Why? Better low speed torque? Its the first time in my limited experience I've seen this done.

Greg
Click to expand...
You want full field excitation for maximum torque and good speed regulation. Note that most small DC motors these days are PM and so have no field windings.
 
Out of curiosity, how do 3 phase 60 hz motors fare when when controlling with a VFD at frequencies much lower or higher than 60 hz?
 
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