VFD to reduce HVAC compressor starting current?
- Thread starter Todd0x1
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A soft starter would be a much less expensive option.
Almost true.
First off below around 25-30 HP the prices are very similar especially against a cheap VFD.
In my experience soft starts are a mixed bag depending on application. The issue is that (ignoring cycloconverter mode) as voltage is reduced, so is torque. So with a true variable torque load like a fan or pump, you will never notice. With a relatively fixed torque load like a conveyor you can trim some starting current off (about 20-25%) but that’s it. Compressors fall in between. Some can go as low as 350% of FLA and still start, others 450-500% is all you can get. That’s still knocking 25% off LRA. With a VFD though you can reduce frequency AND voltage so that torque remains relatively constant and you can easily start with say 150% current limiting even if it takes 5 minutes to come up to speed. So it’s highly application dependent.
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First off below around 25-30 HP the prices are very similar especially against a cheap VFD.
In my experience soft starts are a mixed bag depending on application. The issue is that (ignoring cycloconverter mode) as voltage is reduced, so is torque. So with a true variable torque load like a fan or pump, you will never notice. With a relatively fixed torque load like a conveyor you can trim some starting current off (about 20-25%) but that’s it. Compressors fall in between. Some can go as low as 350% of FLA and still start, others 450-500% is all you can get. That’s still knocking 25% off LRA. With a VFD though you can reduce frequency AND voltage so that torque remains relatively constant and you can easily start with say 150% current limiting even if it takes 5 minutes to come up to speed. So it’s highly application dependent.
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That would surprise me. A three-phase VFD needs 12 semiconductor devices. A soft start can be done with four and can be bypassed once the motor is up to speed to reduce heat losses.
But I take your point about it being application dependent.
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- San Francisco Bay Area, CA, USA
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- Electrical Engineer
Is your HVAC compressor single phase or 3 phase? If it is single phase, then no. There are a couple of VFDs that will work for PSC single phase motors only, but most single phase A/C compressor motors are not PSC and I seriously doubt any hermetically sealed compressor mfr is going to tell you this is OK.
The same might be true of Soft Starters.
Is your system giving you problems, or is this theoretical? The high starting current should only be there for a second or so. If it’s longer, there might be something wrong.
The same might be true of Soft Starters.
Is your system giving you problems, or is this theoretical? The high starting current should only be there for a second or so. If it’s longer, there might be something wrong.
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Jraef: The OP stated 3ph. You might have misread that as 3 hp?
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- San Francisco Bay Area, CA, USA
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- Electrical Engineer
Right you are. Never mind then on all the single phase stuff.
The upside if using a soft starter is in not having to use a special motor or wiring or methods as you would with a VFD. Also if you use a soft starter with a bypass, there are no added losses when running. With a VFD running at full speed, 3% of your power will be wasted as heat. True, it’s not much on a small motor, but it adds up at the end of the year. VFDs make sense on things that need their speed controlled, but that’s their downside as soft starters with no speed change.
Theoretical fix for an issue experienced in the past. Was trying to run a 3 or 5 ton hvac -I forget the exact size, off a 40kva generator rated at a bit over 100 amps per phase and the generator could barely start the compressor. Looking for a solution other than drastically increasing the generator size.
drcampbell
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- The Motor City, Michigan USA
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- Registered Professional Engineer
If there are large interruptible loads, (water heater, range?) you might interlock them so they're turned off when the compressor wants to start.
You might also look at the mechanical side of the HVAC system.
For example, installing a timer to assure that the compressor remains off for a minimum of five or ten minutes, the system has a chance to equalize high-side and low-side pressures, and the compressor is not asked to start with a large pressure differential imposed on it. Or a solenoid valve that remains open, shunting the compressor discharge directly to its inlet and eliminating any pressure differential, for maybe 10 or 20 seconds until the compressor motor's up to full speed. (if you do that, be sure to get a "slow closing" valve)
You might also look at the mechanical side of the HVAC system.
For example, installing a timer to assure that the compressor remains off for a minimum of five or ten minutes, the system has a chance to equalize high-side and low-side pressures, and the compressor is not asked to start with a large pressure differential imposed on it. Or a solenoid valve that remains open, shunting the compressor discharge directly to its inlet and eliminating any pressure differential, for maybe 10 or 20 seconds until the compressor motor's up to full speed. (if you do that, be sure to get a "slow closing" valve)
Thanks. I had considered the valve, similar in theory to an unloader on a large air compressor. I was looking for a purely electrical solution that could be applied to a variety of hvac units. The application is portable, no other substantial loads. Maybe ill get ahold of a junk but working ac unit and a cheapo vfd and do some experimenting.
StarCat
Industrial Engineering Tech
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- Moab, UT USA
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- Imdustrial Engineering Technician - HVACR Electrical and Mechanical Systems
Campbell has stated some valid approaches, although " Hot Gas Bypass " is generally not configured for this purpose. There are certain types of expansion devices that do not equalize as well as others on the off cycle. Even with equalization, compressors can be hard to start. 3 Phase is of course much better, and 240V I would expect is better than 208 on a 208-240 machine based on experience.
The minimum OFF CYCLE time for any typical HVACR compressor should be 5 min, and this is easily done with modern thermostats. I would ask the COMPRESSOR OEM, if you could install a drive on said motor. Worst thing that could happen is you lose a compressor, but I'd expect it may have already been stressed under the conditions you describe. The internal design differences of said windings in a hermetic pot, I am not versed on. Possibly Jraef could comment. You are dealing with a gas cooled motor.
Interesting edit on this post. I wonder if anyone will edit the edit?
DRGREG
Member
"Theoretically" you could add a secondary rotary load (a second motor), with a slightly lower LRA, wired in parallel, and start it prior to starting the compressor. Not too practical, but it would do the trick.
kwired
Electron manager
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- NE Nebraska
40kVA should be well over what is needed for 5 ton of AC, could be a pretty noticeable voltage sag while trying to start though.
What other load was already running may be an issue?
I stated under 25 HP pricing is nearly the same from experience. First off a VFD does require “12” devices but only 6 are transistors and at that size they can be MOSFETs too instead of more expensive IGBTs. But realistically it will be an MPM or IPM these days which means it comes as a big plastic block that you bolt to a heat sink. Everything is integrated into one module. I have not seen SCRs come this way. It’s possible but have not seen one. True you can get away with just 4 SCRs but realistically 6 works far better. The control board, terminals, etc., is realistically about the same. Capacitors are very cheap. But the argument is the same as the old one about how AC drives are so expensive compared to DC drives because you have twice as many components...these days on small drives the power stack is not the major cost driver. Price out a few low end soft starts and VFDs and you’ll see the pattern. It used to be that at 10 HP or less VFDs beat starters too but low cost IEC contactors brought the prices way down.
Locked rotor Amos is independent of load. On across the line starting at stall (start) torque goes to max of whatever the motor can put out. If the load is less it comes up to speed faster and current drops down below FLA. But locked rotor current is locked rotor current. Only way to change it is change the voltage (soft start, Star-delta, part winding) or voltage and frequency (VFD) or change motors say NEMA C.
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The LRA is just over the max amps the generator can supply, so that coupled with the instantaneous jump in load the generator couldnt respond fast enough to the high inrush.
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