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VFD derated for 1ph Air compressor use AND overdriven

86turbodsl

Cast Iron
Joined
Aug 12, 2004
Location
MI, USA
Can anyone critique this? -

Is there any reason i can't do this.

10HP air compressor.
Rated 500rpm to 800rpm. Pump is rated for 10HP and 15HP use.

Use a 480V VFD, supplied by a 240 to 480V step up transformer.

Use a say 7.5HP TEFC interter motor (surplus)

Setup the drive sheaves so the motor runs like 40Hz @ 500 rpm.
Variable speed up to pump maximum of 800rpm. I know torque falls off
on a normal motor when overdriven but since we'd be supplying the motor with 480V when it's wired for 230V, would that not allow the voltage to continue to increase past 60hz, allowing the motor to double it's hp and get away with a commesurate smaller drive AND motor?
 
since we'd be supplying the motor with 480V when it's wired for 230V, would that not allow the voltage to continue to increase past 60hz, allowing the motor to double it's hp and get away with a commesurate smaller drive AND motor?

So you want to feed the motor 480V while it's connected to run on 230V.
Not sure how that would work out with a VFD.

Give it a try and report back when the smoke clears....
 
That's why i'm asking. Don't most VFD's lose torque after 60hz because the voltage doesn't go up after 60hz?

I'm trying to see if it's possible to double horsepower with double the Hz. I think it is, but i may be wrong. i seem to remember on a previous job that we doubled hp running 120hz on motors, but i wasn't paying close enough attention to the details to know how.


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That is a legit use as long as you are feeding it with 3 phase. Also, understand that there is no free lunch, so the current rating of the VFD must reflect the motor current as a 230V motor at the equivalent HP when run at the extended speed.

That said, if you get 500RPM at 40Hz, and you want to run it at 800RPM, that means 160% of of the original speed, but from 40Hz that is only 64Hz. I doubt you will notice the loss of torque at 64Hz to be quite honest...
 
Ok, in this instance i would need a VFD capable of driving a 10HP motor if i was attempting to overdrive a 5HP motor and actually create a 10HP motor? And then derate again by half to run on single phase? So in that event, all i really gain is not having to purchase a 10hp motor. If i have this correct anyway.

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Overdrive the motor?
.
The. HP is 750 ish watts per.

Increasing the voltage will increase the current but the windings are only designed to carry a .certian. current.

Exceeding that they act like a fuse

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Can anyone critique this? -

Is there any reason i can't do this.

10HP air compressor.
Rated 500rpm to 800rpm. Pump is rated for 10HP and 15HP use.

Use a 480V VFD, supplied by a 240 to 480V step up transformer.

Use a say 7.5HP TEFC interter motor (surplus)

Setup the drive sheaves so the motor runs like 40Hz @ 500 rpm.
Variable speed up to pump maximum of 800rpm. I know torque falls off
on a normal motor when overdriven but since we'd be supplying the motor with 480V when it's wired for 230V, would that not allow the voltage to continue to increase past 60hz, allowing the motor to double it's hp and get away with a commesurate smaller drive AND motor?

It's all about the volts/Hz.

The motor needs to be a 480V capable motor, so that it is OK at the voltage.

Then, if you connect it for 240V, and run it at 480V and 120 Hz, it will produce 2x rated HP. AND it will not know the difference...... because it is operating at the same basic conditions that it would see at normal speed and rated volts. 240/60 = 4 and 480/120 = 4, so the volts/Hz are the same either way.

Motor current will be the normal motor current for the 230V condition, which is MORE than the current that motor would take if wired for 480V. The VFD has to be good for the required power. No free lunches, you have to actually supply the power to the motor.

I suppose there really is not any advantage to doing this if you have choices. The cases where it works out well are ones in which you pretty much have to use a given motor, due to some size restriction, special motor, etc, but you need more power.

There may be a little extra heating due to higher frequency.

Any variation of that in between also works, so 90Hz and 360V, 100Hz and 400V, etc all work, because they all have the same V/Hz ratio.

You set the motor rated speed and voltage for the top condition you want, whatever that is, and the VFD will take care of the V/Hz if you command lower speed.

BTW, rough rule of thumb for common size motors is to figure about 1kVA per HP. That will be close for most motors you are likely to see. It's overestimating for large motors, and underestimating for small motors, but fairly close.
 
So I can follow and be less fuzzy. The motor should be 480 or dual voltage. Is it better to use a 7.5 frame size at 120 hz or a 10-15 hp with a larger frame size or doesn't it matter? Will there be additional heat generated that might shorten the bearing life? I'm assuming a 1750 rpm motor doubled to 3500 so maybe not a big deal. Compressors work about as hard as any motor at higher rpm, so does using a 7.5hp motor doubled with a vfd make more economic sense than a larger motor at 800 rpm running less often? Dave
 
so does using a 7.5hp motor doubled with a vfd make more economic sense than a larger motor at 800 rpm running less often? Dave

It only makes economic sense because its a smaller motor, and that doesn't even mean it will cost less. or it makes economic sense because you already have it and don't need to buy another motor?
---------

If you want to get 10hp out of your 7.5 hp motor, if its nameplate volts is 230/60hz, then you need to supply it approximately 320v/80Hz. so no you can't use a 480v motor, connect the windings for 230v. at a 1.3:1 speed increase there is nothing to worry about regarding bearings, motor overheating, etc.

Now regardless what the rpm of that motor is, at 80hz it will be 2340 or 4680 rpm for a 4 or 2 pole motor, you need to size the belts so that you get 10 hp shaft loading at the motor at the intended rpm. this is why it may not make sense to use a 2 pole motor, the rpms may be too high for a practical motor pulley diameter.
 
Can anyone critique this? -

Is there any reason i can't do this.

10HP air compressor.
Rated 500rpm to 800rpm. Pump is rated for 10HP and 15HP use.

Use a 480V VFD, supplied by a 240 to 480V step up transformer.

Use a say 7.5HP TEFC interter motor (surplus)

Setup the drive sheaves so the motor runs like 40Hz @ 500 rpm.
Variable speed up to pump maximum of 800rpm. I know torque falls off
on a normal motor when overdriven but since we'd be supplying the motor with 480V when it's wired for 230V, would that not allow the voltage to continue to increase past 60hz, allowing the motor to double it's hp and get away with a commensurate smaller drive AND motor?

Air compressor pumps rated output is based on a given Hp/torque delivered at a rated pump speed, the air pumps are rated for a specific operating RPM range and Hp requirement for CFM output. Setting up the sheaves so the 7.5 Hp motor runs at 40Hz with a compressor speed of 500 RPM will only deliver 5 Hp to the pump, and the torque would be ~75% of a 10Hp motor which is specified for that compressor operating point. I would think the motor would have difficulties running the air pump and overheat at this operating point. If one were to over speed the motor in a constant V/Hz fashion given that the motor insulation is rated at 480V and run with the wingdings set at 240V at 60Hz, one would need to design the compressor to motor pulley ratio such that you are delivering 15Hp to the pump running at 800 RPM. So this would be a factor of both the motor output and the mechanical ratio of the motor to the compressor. The current ratio for a 1750 RPM motor is ~3.5:1. So there would be two factors at work, the increasing Hp as you ramp up the V/Hz and the mechanical ratio. If you where to change this to say 4:1 ratio, the 7.5 Hp motor operating at 70Hz would be delivering ~10 Hp to the compressor (V/Hz Hp and mechanical ratio), at a pump speed of 800 RPM the motor would be at ~110 Hz. Torque would be fairly constant over this range, I am not sure if it would be sufficient at the 800 RPM pump speed.

The practical/cost factors in this situation may make it a questionable proposition for this scenario.
1. There should not be a mechanical limitation of running a 4 pole motor to 2X its base speed, but a TEFC motor would have cooling limitations at 2X its base speed. This may be a significant issue as compressors often have a high duty cycle running their motor at close to its maximum designed operating load.
2. A 15 Hp VFD would be needed, and if I recall there was a Siemens article specific to this type of application and they recommended up-sizing the VFD to the next size, so a 20Hp VFD might be used.
3. The step-up transformer would need to be commensurate to the larger VFD size, and the power supply chain would need to be also up-sized accordingly.

It would seem from cost defectiveness scenario, that getting a properly sized motor for the desired compressor operating point would be far less expensive in this situation. The alternate, would be too just change the motor to compressor pulley ratio to something like a 4.5:1 so the 7.5Hp motor runs at a fixed speed something like 75Hz, the applied Hp based on the new mechanical ratio would run the pump at 500 RPM speed. The VFD would be a set at 240V 60Hz output, there would be no cooling issues, there may be an issue with applied compressor torque beyond this operating point.
 
Can anyone critique this? -

Is there any reason i can't do this.

No real reason other than it makes no sense.
What exactly are you trying to accomplish?
If you need 10HP to run the compressor, get a 10hp motor.
If 7.5 is enough, use that, and size the pulleys according to the speed the pump requires. Throwing a transformer into the mix will waste watts as heat.
Using a VFD to ramp the RPM could same some starting current, but the VFD will also waste some watts as heat.
There is no magic money saving hocus-pocus to be gained by adding components into the mix that are less than 100% efficient.
 
No real reason other than it makes no sense.
What exactly are you trying to accomplish?
If you need 10HP to run the compressor, get a 10hp motor.
If 7.5 is enough, use that, and size the pulleys according to the speed the pump requires. Throwing a transformer into the mix will waste watts as heat.
Using a VFD to ramp the RPM could same some starting current, but the VFD will also waste some watts as heat.
There is no magic money saving hocus-pocus to be gained by adding components into the mix that are less than 100% efficient.

The thought was to use one of my plc's and a drive and build a variable speed air compressor that can save some energy and save some money on the motor. I am an engineer and enjoy building complicated devices with questionable value... :P
 
What would make sense is to do some calculations to determine a torque curve based on air pressure.

If a 2 stage pump is does not matter much until second stage is actually working then load increases more as pressure rises above some point.

The motor draws less power when in these lower pressure areas so one could add a couple extra pressure gages and have one set to the point where the maximum pressure between stages is.

Then another one where motor load approaches full.

The pressure switches can change frequency to maybe gain some output.

In the low pressure area the rpm can be increased until the motor loading comes up resulting in faster recovery then rpm can drop some as pressure rises.

Trick is instead of trying to SAVE energy costs one gets the most air for the motor capacity or run motor at rated hp more of the time to get more air out for less overall hp.

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The thought was to use one of my plc's and a drive and build a variable speed air compressor that can save some energy and save some money on the motor. I am an engineer and enjoy building complicated devices with questionable value... :P
Ah, then there's the fallacy. Slowing down the motor will NOT save energy on something like a compressor. Common mistake, one that is often promoted by VFD sellers who have no idea what they are talking about.

Energy is power over time, work is power over time. So energy = work performed. At a slower speed, your motor draws less power, but also performs less work, meaning the time it takes to finish a task goes up proportionately, so no net energy saved.

The only time a VFD saves energy is when it's ability to change speed efficiently can be used IN LIEU OF some other less efficient means of changing speed. So for example a centrifugal pump where variable flow control is necessary. If you use a valve to control the flow, that valve represents losses in the system due to pressure drop, turbulence, friction etc. If you eliminate that valve and change the pump speed, the motor energy reduces at the cube of the speed change and results in much less energy expenditure to accomplish the SAME amount of work as the valve, because you are reducing the LOSSES in comparison. The same might hold true for something like a mechanical varidrive (variable pulley system) because those are very inefficient in some applications.

So bottom line if you put a VFD on a reciprocal compressor, you don't really save energy because what a compressor usually does is charge up a tank and recharge it as air is used. Slowing the compressor just increases the time it takes to recharge the tank, so the compressor uses less power, but runs longer. No net savings. People put VFDs on screw compressors when use it in continuous mode and DON'T put in a tank, but mostly because it saves wear and tear on the screw rather that running full bore and unloading it when the pressure need is lower.
 
So whats left is the phase conversion part, and the soft start needed to get that beast started on a 1Φ service.
I have some motors if you need one, 3Φ & 1Φ

SAF Ω
 
And single phase 10hp without the normal unreliable 10hp motor is desirable. There's still benefits to be had even if it isn't energy savings. Noise, load matching, soft start, etc.

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