Soft starter or VFD

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.

'Cliff' is gone, and was reported to have died. Rich Grice is no longer starting lots of off topic, crossposted threads. If you killfile 'too_many_tools' and 'Hawke' a large chunk of political crap is gone. The rest is crossposts from the survival newsgroups.

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.

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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.)

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.

I think VFDs are fine if used on dual-voltage (240/480 V) motors where the power supply to the VFD is 240 V. In this condition, the motor's windings are never subjected to a voltage higher than the insulation is designed for. The square-wave nature of the VFD output does change the voltage distribution across the windings, the end turns have much more of the square edges across them. But, if the windings are rated for this voltage, they should be fine.

One place where there could be problems is if the VFD is very far (as in 50 - 100 feet or more) from the motor. The wires in conduit form a kind of unterminated waveguide, and could cause ringing of those square wave edges. That is reported in the literature, anyway. Inductors on the VFD output can remove those sharp edges.

I have been running a VFD on my mill for almost 20 years, and have added one to my lathe, now. I have not had any problem with this, both motors are quite old, before there was any such thing as inverter-rated motors. They are both rated for 240/480, however.

Jon

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The problem here is that you dont understand the problem (as usuall). Soft starters have a minimum starting current of around 3 times FLC. On a small motor with low inertia (like the op) this produces a sudden kick, smaller than DOL, which is what the op is trying to get rid of. To get true ramping from zero you need a 6 thyristor stack with individual gate control not the usuall 1 control per pair. You'll have noticed that the WEG is over twice the price of an inverter.

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There is some truth in that. The problem arises on motors where the shaft is insulated from earth except by the bearings. eg the shaft goes to a rubber flexible coupling. Long cable runs from inverter to the motor can cause large spikes to apear at the motor. Its also much more of a problem with higher voltage (380v + ) systems. The op is very unlikely to be effected by these issues.

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

?-)

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.

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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.

?-)

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.

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.

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VFD

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

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

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.

It depends ...

I've heard it both ways, but I don't do enough work with them to have accumulated my own statistics. Some people have problems with it, others do not. It may depend on the model of VFD, filtering the HF out of the output, etc.