max gauss of a permanent magnet dc motor field?

[BobG]

I know there are some beefy servo motors of several HP, but at some
point the gauss of the strongest permanent magnet field can be passed
buy a series connected wound field and armature. Just wondering about
where this point of diminishing returns occurs. I assume armature
current and gauss should equal field current in the series motor and
gauss in the permanent magnet motor, so I guess my question is, how
many amps in the armature before the gauss in the armature is as strong
as the permanent magnet field? 100amps? 200? 400? They dont make a
permanent magnet motor strong enough to use in a fork lift, for
instance, right?

 

[John Popelish]

BobG wrote:

I know there are some beefy servo motors of several HP, but at some
point the gauss of the strongest permanent magnet field can be passed
buy a series connected wound field and armature. Just wondering about
where this point of diminishing returns occurs.

The limits involve saturation of the iron core of the coils and the
risk of demagnetizing the permanent magnets, and of course,
temperature rise due to resistance.

I assume armature
current and gauss should equal field current in the series motor and
gauss in the permanent magnet motor, so I guess my question is, how
many amps in the armature before the gauss in the armature is as strong
as the permanent magnet field? 100amps? 200? 400?

The limits are in ampere turns (total amperes circulating around the
core), and that depends on the geometry of the motor as well as what
it is made of.

They dont make a
permanent magnet motor strong enough to use in a fork lift, for
instance, right?

I think they do.
(excerpt from the following article)
"The actual motor design is a radial-flux construction, air or water
cooled, with a permanent-magnet rotor. Powers range from 22 to 670 hp
(17 to 500 kW) and include base speeds from 220 to 600 rpm with
voltages ranging from 380 to 690 Vac."

http://www.machinedesign.com/ASP/strArticleID/55641/strSite/MDSite/viewSelectedArticle.asp

This is talking about permanent magnet synchronous AC motors, but add
commutator and brushes and you have a DC motor.

Here is a permanent magnet DC motor designed for stuff like fork lifts:
http://www.lemcoltd.com/lem_200.htm

 

[BobG]

Hi John. Looks like the Lem motor is only a couple of HP. Evidently a
couple hundred amps is typical in forklifts, so now we know the amps.
How about turns? My goal here is to get a number for the strength of
the permanent magnet field in these 2 HP motors, so I can extrapolate
what it is in the 20 and 40 HP motors. Can you hazard an educated guess?

 

[John Popelish]

BobG wrote:

Hi John. Looks like the Lem motor is only a couple of HP. Evidently a
couple hundred amps is typical in forklifts, so now we know the amps.
How about turns? My goal here is to get a number for the strength of
the permanent magnet field in these 2 HP motors, so I can extrapolate
what it is in the 20 and 40 HP motors. Can you hazard an educated guess?

The field strength is limited by the saturation flux of iron:

http://www.oz.net/~coilgun/theory/materials.htm

The demagnetization resistance of the permanent magnets is also a
limit. Neodymium iron boron is especially good in this respect and
has really increased the power available from a given size of motor.
The resistance to demagnetization is probably best captured by the Hc
value:
http://www.oz.net/~coilgun/theory/materials.htm

The maximum field strength for these two reasons applies to any size
motor. If they are designed to the limit of the materials, the same
maximum flux occurs in all sizes.