VFD running a air compressor and regenerative effects causing over-current faults?

[rons]

Motion, Jst,

Good advice.

 

[Mark Rand]

In addition to the other comments, do you really need 130-150psi? 90 is usually more than enough for air tools and general use. At the higher pressure, your load is probably 70% higher than it need be.

 

[mksj]

You use a 2 stage compressor ideally so that it cycles less often, so you pump it up to 175PSI and the it will cycle back on at between 125-135 PSI depending on the settings. I run my air system with a primary high flow regulator at 125 PSI and step down regulator at each outlet location, that way I can maintain a high air flow when needed and not have the line pressure dropping with high air flow demand. I can also have the compressor set in a continuous run mode with an unloader valve. I still do not necessarily agree that the motor load amperage is pretty much the same when running an empty air tank, my compressor the load increases as it gets to 175 PSI where the motor is maxing out based on my pulley ratio. My air pump is suitable for 5-10 Hp motor, the pulley ratio is changed depending on the motor. It sounds like in this case you may have been pushing the edge of the current that the VFD could deliver. It may be your line voltage is a bit lower or something else.

I did have a recent VFD install issue where the motor would cog in SVC mode, and auto-tune did not help. It ran fine in V/Hz mode with torque boost/HD mode. I had a previous install with the same lathe and vector motor and did not have this issue, but it was a different VFD. Recommendation from tech. support for the HItachi VFD was to change the default motor base speed from 60 to 70Hz and run in V/Hz. You loose some performance at lower speeds going that route.

 

[rons]

Ron, I did a little homework for you to see if I could assist. Your pump looks to be an earlier Kellog American type from the photo you provided, based on the head style, crankcase shape, centrifugal unloader and port layout. They used a few different corporate names over the years. From the looks of it, you have a 5HP pump that is over-sped based on your motor sheave size causing your hard start problems.

Get the motor sheave size corrected and keep the ramp up time as short as possible, like 3 sec to keep the overload period from tripping out your drive. As far as motor selection, newer energy efficient motor designs have a much higher inrush current than the older design motors, so you may want to try an older model before the energy efficiency models were mandated.

SAF,
Is the higher inrush due to less iron? The big difference between a modern and old motor is the weight.

 

[johansen]

SAF,
Is the higher inrush due to less iron? The big difference between a modern and old motor is the weight.

No it's due to lower air gap, results in higher power factor, lower running amps.

 

[SAF]

SAF,
Is the higher inrush due to less iron? The big difference between a modern and old motor is the weight.

Ron, the differences are due to several factors, but mainly due to better and more materials. Higher efficiency requires lower winding resistance, better steel lamination's for better magnetic permeability and closer manufacturing tolerances. I don't think that you can make a judgement based just on weight, The housing material, cast iron with cooling fins versus a rolled steel would make a large difference in weight, irrespective of other factors. Here is an article that covers some of the differences.
https://electrical-engineering-port...ess-up-motor-efficiency-and-how-to-improve-it

As a case example that I experienced years back when energy efficient motors first came on the scene.

A large bump shop that I serviced at the time had some existing Ingersoll T30 reciprocating compressors that had been in operation for many years previously. These had 7.5 HP motors that were started across the line. The feeders supplying these compressors were switchboard supplied, underground from across the building.

When the government motor energy efficiency standard changed, the local utility started offering rebates for facility motor upgrades. A contractor sold the dealership a complete upgrade package to change out all of the facility motors. As soon as the compressor motors were changed I received a service call of the compressors going down.

The replacement motors were blowing the 250V FRN-60 time delay fuses in the switchboard. Installing new fuses got them back running temporarily, but the problem persisted in a short amount of time. Checking the running currents, measured all good. There was nothing changed with the pumps other than the motors. The inrush currents during the starting periods were melting the fuses after repeated loaded starts. The feeder conduits were under the shop floor and were not large enough to install larger conductors. Similarly the switchboard had a twin 60A switch, and no extra room to install a 100A twin switch, for upgrading the circuits supply.

It was decided to install standard efficiency motors back on the compressors. The old ones were already scrapped by the installing contractor. At that time the older efficiency motors were still available as new. We replaced the motors and never had a problem with melting fuses again.

Several years later I used one of those 7.5HP motors on a compressor I was building out of used components. I had a Quincy oil pressure lubed model compressor, that could be run at 5 - 7.5HP. I belted it for a 5HP load and wired it up to a 30A 3Φ circuit. It has never been turned off and never blown a 30A fuse or breaker in more than 20 years of operation. The Quincy pump has an oil pressure driven unloader system. Pumping does not start until the oil pressure gets to the predetermined setting. It has disc actuated unloader valves on the top of the head. pneumatically operated. I'm fanatical about maintaining my piping, hose reels and hoses to keep my system free of leaks. If I don't use any air in a full day's time, the pump will cycle one time to make up for the losses. But air is always available without waiting for the pump to cycle and build pressure.

For your application with a VFD, I would suggest that you set your sheave ratio just slightly less than 5HP for easier starts, but not so low that you don't immediately get to the minimum recommended speed for proper lubrication. If you get the start problem cured, then you could set the VFD run frequency a bit higher to make up for the reduction in speed due to down sizing the motor sheave. Should result in less stressful starts, but full capacity run performance, without overloading the motor.

 

[rons]

Ah, then to make my newer efficient motor with all the newer designed steel less responsive...
I shall mount the rotor on a lathe and take a few thou off the diameter.

 

[markz528]

Ah, then to make my newer efficient motor with all the newer designed steel less responsive...
I shall mount the rotor on a lathe and take a few thou off the diameter.

Pretty sure I already covered that inrush has nothing to do with a VFD because VFDs always run to the right side of the speed torque curve...........

 

[johansen]

Ah, then to make my newer efficient motor with all the newer designed steel less responsive...
I shall mount the rotor on a lathe and take a few thou off the diameter.

So potentially that would make it worse.

The air gap is entirely inductance and is a significant limit for the maximum power factor the motor can operate at. Less air is better, always.

But it's the shape of the rotor bars determines the torque rpm curve. Shaving the rotor down might actually increase starting amps.

It's a good experiment for an old motor you are going to scrap.

Now shaving down the end rings will make starting easier and less amps because it increases rotor resistance.

 

[JST]

Shaving the end rings does reduce starting amps. It also increases "slip", so it is a trade-off.

The best rotor is a dual cage.... thinner conductors at the surface for low start amps, and larger buried conductors for low slip. But that is a choice the manufacturer had to make. Not very practical to add later (although it "can be done" in some cases)

 

[rons]

I saw that inside a Phase-a-matic motor made by Baldor. Very light.

 

[DDoug]

Shaving the end rings does reduce starting amps. It also increases "slip", so it is a trade-off.

The best rotor is a dual cage.... thinner conductors at the surface for low start amps, and larger buried conductors for low slip. But that is a choice the manufacturer had to make. Not very practical to add later (although it "can be done" in some cases)

Yes, this guy wrote a article on the pro/con of hi-slip/hi-resistance rotors.

Home - Smith Electric Motorworks

Where is this block?

gwm4-3phase.com

Why not a wound rotor from a synchronous motor, and vary the slip with the field voltage control ?

 

[JST]

That used to be done. Wound rotors on induction motors, and external resistors. Yep, set to high resistance, for low start current on your 100HP motor, and reduce resistance as it speeds up.

Not done any more. Costs a lot more than putting a VFD on it, and doesn't give as much benefit.

 

[DDoug]

That used to be done. Wound rotors on induction motors, and external resistors. Yep, set to high resistance, for low start current on your 100HP motor, and reduce resistance as it speeds up.

Not done any more. Costs a lot more than putting a VFD on it, and doesn't give as much benefit.

I was suggesting it for making a RPC.
You could apply field voltage as needed thru a regulator to keep the generated leg balanced.
And yes, lower it for starting.
Several members on here are capable of stripping out the bars of a squirrel cage rotor, and making windings.

 

[JST]

The low resistance bars, or low resistance added externally would have lower slip, and also better output from the generated leg.

So you could get both good effects. You would essentially have a synchronous motor started up as an induction motor. Synchronous motors are good generators, but you would still need to account for the back EMF having to be lower than the applied voltage.

 

[rons]

In addition to the other comments, do you really need 130-150psi? 90 is usually more than enough for air tools and general use. At the higher pressure, your load is probably 70% higher than it need be.

It was originally designed for 150-175 PSI. At full throttle I could blow a medium heavy rubber door mat off the floor several feet.

 

[rons]

Starting to sound like the old lady who swallowed a fly, yes!

Wondering what sort of hobby shop activity NEEDs half that much air?
That part, you've not explained?

But I don' picture you doing large or long duration media blasting?

Money ahead to buy a new, smaller, commodity 1-P C-H unit from Pig Box every few years, maybe?

Nobody cares about what the fly on the wall is wondering...