Soft starter or VFD

[Oppie]

I have a milling machine that's starting to show its age. It has a 5 Hp,
208V, 3 Ph spindle motor that's currently on a full voltage Fwd/Rev starter.
Motor really slams when turned on and has damaged the variable speed sheaves
and keyways.

Was thinking of adding a soft start system in series with the original
starter. Considered a VFD but have heard that using one with a motor not
designed for such makes problems. Even adding a line reactor to remove
harmonic content may be required. All told, a soft starter appears to be the
best choice.

Does anybody have a favorite unit to recommend?
Ones I've seen so far have a SCR in series with two of the three legs and
control starting voltage and ramp-up time. If you keep them powered, they
can also control ramp-down time. A contactor bypasses the reduced voltage
system after the ramp time. All fine and well but only some of the vendors
seem to rate # of starts per hour. I would assume that this is due to heat
generation when the SCRs are carrying the load and the need to dissipate it.

Thanks in advance for any answers
Oppie

 

[Oppie]

This is a group of folks who can design you a VFD or a soft starter.

For folks that actually _use_ them, go to rec.crafts.metalworking. Once
you sort through all the political crap, there'll be some good advice for
you there.

Yeah, I could probably design one myself too but with time constraints as
they are, better to buy this off the shelf. I used to visit r.c.m. often
when I was heavily into blacksmithing. Now its more the occasional welding
repair with a torch or MIG rather than working over a coal forge...
I'll give a post there, thanks.

 

[Oppie]

Soft starters aren't really made for small motors like yours, vfd
should be fine.

I see several for 5 Hp (14 amp minimum, 208/220V, 3 Ph.)

http://www.wolfautomation.com/products/31923/soft-starters-5-hp-to-200-hpbr-weg-ssw07-series

http://www.galco.com/buy/ABB/PSR16-600-70

http://www.temcoindustrialpower.com/products/Soft_Starters/S60002.html
(Duty cycle restriction 10 starts/hour)

http://www.alliedelec.com/search/productdetail.aspx?SKU=70007395

http://www.grainger.com/Grainger/soft-starts/motor-supplies/motors/ecatalog/N-gqmZ1z0r2lh

http://products.schneider-electric....tDetail&countryCode=us&partNumber=ATS01N222LU
So far, I like this one best.

http://www.automationdirect.com/adc...ft_Starters_(SR33_Series,_22A_-_482A)/SR33-22
(10 starts/hour)

Many choices but as usual, devil's in the details about which will work best
in our mill and at a reasonable cost.

Am I mistaken or are VFDs best applied only to "inverter rated" motors? I've
heard all sorts of stories about how high frequency harmonics cause
overheating of the motor and circulating currents that cause current flow
through the bearings that eats them up. Figured ask the opinion of folks
that have already successfully applied one.

 

[josephkk]

Soft starters aren't really made for small motors like yours, vfd
should be fine.

Maybe. They normally take inverter rated motors to work best.

?-)

 

[[email protected]]

I have a milling machine that's starting to show its age. It has a 5 Hp,
208V, 3 Ph spindle motor that's currently on a full voltage Fwd/Rev starter.
Motor really slams when turned on and has damaged the variable speed sheaves
and keyways.

Was thinking of adding a soft start system in series with the original
starter.

How does your original starter operate ? Is it an ordinary Wye/Delta
starter, i.e. first starts in Wye configuration with 120 V at each
winding and after gaining some speed switch to Delta with 208 V on
each winding ? Is this starter working properly or does it slam the
full voltage Delta configuration immediately, thus the motor would
take up to six times the nominal current at startup.

 

[Oppie]

How does your original starter operate ? Is it an ordinary Wye/Delta
starter, i.e. first starts in Wye configuration with 120 V at each
winding and after gaining some speed switch to Delta with 208 V on
each winding ? Is this starter working properly or does it slam the
full voltage Delta configuration immediately, thus the motor would
take up to six times the nominal current at startup.

The spindle motor itself is 220/440 (or something like that - don't recall
exactly) and is operated on 208, 3 Ph directly from a manual reversing
switch. So it's essentially a full voltage start/run. Would be possible to
add a Wye/Delta starter but that's old technology and is only a two step
sequence. A SCR soft starter or better yet a VFD is far better for
controlling starting and stopping.

While looking up what is available, found everything from Wye/Delta, 3ph
autotransformer voltage reducers with bypass relay to SCR and VFD
controllers.

 

[josephkk]

The spindle motor itself is 220/440 (or something like that - don't recall
exactly) and is operated on 208, 3 Ph directly from a manual reversing
switch. So it's essentially a full voltage start/run. Would be possible to
add a Wye/Delta starter but that's old technology and is only a two step
sequence. A SCR soft starter or better yet a VFD is far better for
controlling starting and stopping.

While looking up what is available, found everything from Wye/Delta, 3ph
autotransformer voltage reducers with bypass relay to SCR and VFD
controllers.

Sooner or later it comes down to this, all older tech soft starters just
moderate the startup current and do nothing to affect the frequency. VFD
can continuously adapt the frequency and voltage to the load. However, to
use VFD on an older machine it is worthwhile to add load inductors, and it
is always worthwhile to add line inductors.

?-)

 

[[email protected]]

Sooner or later it comes down to this, all older tech soft starters just
moderate the startup current and do nothing to affect the frequency. VFD
can continuously adapt the frequency and voltage to the load.

Unless you are using a synchronous motor or need to exactly control
the speed up ramp, why would you want to alter the frequency ?

In ordinary motor starting applications, reduce the current and
voltage available to the motor and the slip will do the rest to avoid
excessive torque.

 

[josephkk]

Unless you are using a synchronous motor or need to exactly control
the speed up ramp, why would you want to alter the frequency ?

In ordinary motor starting applications, reduce the current and
voltage available to the motor and the slip will do the rest to avoid
excessive torque.

If you run that motor a lot, common in industrial situations, the energy
savings becomes noticeable to important. Plus VFD intrinsically adds
variable speed drive to whatever you add it to. And more control
possibilities.

?-)

 

[Jamie]

If you run that motor a lot, common in industrial situations, the energy
savings becomes noticeable to important. Plus VFD intrinsically adds
variable speed drive to whatever you add it to. And more control
possibilities.

?-)

You must remember that fitting a VFD drive on a conventional squirrel
cage motor may not be such a good idea when operating it much less than
80% speed.

In many cases, the mechanical design of the rotor have formations in
it to help move air around in side, in hopes that it will suck in fresh
air from the out side via the vent ports. Others have a fan on the back
and are totally enclosed, like in the case of a shop machine where you
may not want foreign debris to fall in.

THen you have the case where the windings of such motors are not
potted or lined to prevent vibration effects which in turn can hurt
the insulation on the wire over time. Most of these motors are designed
to operate around 50/60hz and mechanically are designed to not vibrate
around there. Running them at a lower frequency rate is gambling but for
soft starts you should be able to get an average life cycle from a motor
before that takes place. The bearings will most likely die first or
mother nature will make that determination for you

If you look at a inverter duty motor, you'll see some difference in
design. First off is the case. Heat radiating fins and lots of potted
metal is part of the design. The rotor many times is larger, this is
done for various reasons of course, not just life extending reasons.
And the windings and most everything the windings sit on are normally
laminated. Many of the inverter duty motors have designs in the rotor
to help reduce effects of currents passing through the bearings but it
still can happen.

A lot of hack installers will cut corners by using wire for the motor
feeds at the smallest gauge possible. This actually will cause the wire
to operate a little on the warm side but will induce some R in the feed
which lowers the Q of the motor windings. Less noise in the line with
out using a reactor or transformer on the output side. What a way to
save money!

Jamie

 

[Oppie]

choice.

The two major problems you will encounter retrofitting a VFD to a constant
speed (line operated) motor are cooling and rotor grounding.

Cooling is a problem if you intend to use the VFD to run the motor
underspeed and loaded for any length of time. The internal fan on the
motor
shaft will not provide sufficient airflow under these conditions. But just
using the VFD to start/stop a motor should not be a problem.

Rotor grounding may be a problem due to high frequency components being
coupled to the rotor. There must be a path to ground provided, or these
currents will be conducted to ground through the motor bearings, resulting
in possible damage to these bearings. There are slip ring kits that can be
fitted to older motors to provide rotor grounding.

Had read theory about this but wondered how it translated to real world
situations.
I've also been trying to get a budget line to replace our two 10 Hp air
compressors (big starting surge and often) with a Kaeser rotary screw type
driven by a VFD. They run according to demand...
Thank you.