RE: [CAD_CAM_EDM_DRO] Variable speed drives

AC induction motors need to slip in order to develop torque. This means
they will fall behind synchronous speed. For example, synchronous speed of
a standard motor on 60 hz is 1800 RPM, but the speed at rated torque is
normally 1725 RPM. This is explained better in the two references below....

Page 6 of this reference:
http://www.ab.com/support/abdrives/documentation/fb/1019.pdf

figure 2 of this reference
http://www.ecmweb.com/mag/electric_minimizing_ac_induction/

OK, now with this as background, understand that slip is not what most
people want. Normally you want it to go at the commanded speed with little
speed variation despite variation in load. This is done differently in
different types of AC inverter (VFD) drives.

Volts/HZ: Most simple drive. The stiffness in velocity is dependent on
the motor winding as noted in the graphs on the above mentioned references.
The drive creates a 3 phase AC output and the motor turns proportional to
frequency. The drive also reduces the voltage proportional to frequency so
it does not go over current at lower speeds. (lower frequency into a fixed
inductance would result in higher current unless the voltage is reduced).
Expect 5% speed variation as the load changes.

Sensorless Flux Vector, aka: sensorless vector: The drive senses the slip
through characteristics of the electrical load and increases the frequency
to adjust for the motor slip while under load. Expect 2% speed variation as
load changes.

Flux Vector: The drive has an encoder and knows exact motor speed as well as
rotor angle. Magnetic field angle is adjusted dynamically to deliver very
accurate speed or torque regulation. Expect 0.2 % speed variation or better
depending on the nature of the load.

I hope this helps.

By the way, I have a large number of 1.5 HP VFD drives for sale if anybody
needs one. These are standard volts per hz drives, new in the box,
warranted for 1 year. The drives will operate on either 230 V single phase
or 115 V single phase. They will operate 3 phase induction motors up to 1.5
HP and will run up to 120 hz (1725 motor will spin at 3450). Please contact
me off list if you're interested, I'll send you a PDF datasheet. The drives
are $165 to members of the list.

Greg

> -----Original Message-----
> From: turbulatordude [mailto:davemucha@...]
> Sent: Thursday, November 04, 2004 11:26 AM
> To: CAD_CAM_EDM_DRO@yahoogroups.com
> Subject: [CAD_CAM_EDM_DRO] Variable speed drives
>
>
>
>
> Hi all,
>
> I'm looking into putting a variable speed drive on my lathe and am
> wondering about the different type of drives.
>
> I have seen ones optimized for HVAC, some constant torque, others
> like sensorless vector control, flux vector....
>
>
> All the sales hype sound like anyone will run your motor and wash
> your dishes.
>
> Can anybody help with what the differences are ?
>
> Dave
>
>
>
>
>
>
>
> Addresses:
> FAQ: http://www.ktmarketing.com/faq.html
> FILES: http://groups.yahoo.com/group/CAD_CAM_EDM_DRO/files/
> Post Messages: CAD_CAM_EDM_DRO@yahoogroups.com
>
> Subscribe: CAD_CAM_EDM_DRO-subscribe@yahoogroups.com
> Unsubscribe: CAD_CAM_EDM_DRO-unsubscribe@yahoogroups.com
> List owner: CAD_CAM_EDM_DRO-owner@yahoogroups.com,
> wanliker@..., timg@...
> Moderator: pentam@... indigo_red@...
> davemucha@... [Moderators]
> URL to this group: http://groups.yahoo.com/group/CAD_CAM_EDM_DRO
>
> OFF Topic POSTS: General Machining
> If you wish to post on unlimited OT subjects goto:
> aol://5863:126/rec.crafts.metalworking or go thru Google.com to
> reach it if you have trouble.
> http://www.metalworking.com/news_servers.html
>
> http://groups.yahoo.com/group/jobshophomeshop I consider this
> to be a sister site to the CCED group, as many of the same
> members are there, for OT subjects, that are not allowed on the CCED list.
>
> NOTICE: ALL POSTINGS TO THIS GROUP BECOME PUBLIC DOMAIN BY
> POSTING THEM. DON'T POST IF YOU CAN NOT ACCEPT THIS.....NO
> EXCEPTIONS........
> bill
> List Mom
> List Owner
>
>
> Yahoo! Groups Links
>
>
>
>
>
>
>
>