# Installing VFD on two speed motor



## erics37 (May 7, 2009)

Unless I'm missing something, you should be able to program a multitude of speed settings on the drive and just select the appropriate one whenever needed via your controls.


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## JRaef (Mar 23, 2009)

allenc said:


> Just curious to see what others think about this.
> 
> I have several compressors driven by two speed motors. We want to install VFDs on these for better capacity control. The issue is that these are two speed-constant horsepower motors. The first set that we want to do are 25 HP @ 1150 RPM and 25 HP @ 1750 RPM. These are HVAC compressors and they operate in low speed or high speed depending on the condition. The HP is approx 25 HP regardless of the speed (less torque requirement in high speed, greater torque requirement in low speed due to compression ratio)
> 
> ...


Your thoughts are correct, a VFD MAINTAINS torque at any speed, but your two speed motor, as a CHP device, INCREASES torque at the lower speed, so your 40HP idea satisfies that principal.

The problem however might be in the initial design that used a CHP 2 speed motor in the first place. The only type of compressor that might require MORE torque at a lower speed than at a higher speed would be a reciprocating type, and at 25HP you are within that range where a recip might be used. But here's the thing: in a recip, the REASON why they use a CHP to get hiogher torque is because of having to START the recip from a dead stop. In the beginning, you have high torque pulsations as the pistons try to compress immediately. One strategy is to unload the compressor head at start up, the other is to use a CHP 2 speed motor, and ALWAYS start it in the lower speed where you have more torque (and eliminate the need for the unload valve). So it sounds like maybe yours is that type. If so, then if you use a VFD with Sensorless Vector Control (SVC), and maybe over size just the DRIVE, it can make the 25HP motor pump out enough torque to start that compressor. Typical Starting Torque (Locked Rotor Torque) on a motor is 150-160% of FLT, but Break Down Torque (Peak Torque), can be as high as 250% of FLT. A VFD using SVC can make a motor produce BDT at any time for a few seconds, longer if the VFD is over sized. A common rule-of-thumb I have seen is to over size by 60-70%. Although I think that might be over kill, it comes from literature I have read on applying VFDs to recips, but made no mention of using SVC, so using SVC might make that kind of overshoot a bit much. Still, that means your 40HP idea would apply, but only to the DRIVE, not necessarily to the motor. Once the motor is at speed, it is a Constant Torque operation, so if it works on the torque of a 25HP motor at high speed, it will work on that at low speed. It is only the STARTING that is the problem.

the people to discuss this with would be the compressor mfr. Ask them what the necessary STARTING torque is for their compressor as well as the minimum acceleration time. Once ou know that, you can determine how much of a motor you need, then how much VFD you need to make it work. You may find that with the right VFD using SVC, you may be able to do this with a 25HP package.


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## allenc (Mar 3, 2011)

JRaef said:


> Your thoughts are correct, a VFD MAINTAINS torque at any speed, but your two speed motor, as a CHP device, INCREASES torque at the lower speed, so your 40HP idea satisfies that principal.
> 
> The problem however might be in the initial design that used a CHP 2 speed motor in the first place. The only type of compressor that might require MORE torque at a lower speed than at a higher speed would be a reciprocating type, and at 25HP you are within that range where a recip might be used. But here's the thing: in a recip, the REASON why they use a CHP to get hiogher torque is because of having to START the recip from a dead stop. In the beginning, you have high torque pulsations as the pistons try to compress immediately. One strategy is to unload the compressor head at start up, the other is to use a CHP 2 speed motor, and ALWAYS start it in the lower speed where you have more torque (and eliminate the need for the unload valve). So it sounds like maybe yours is that type. If so, then if you use a VFD with Sensorless Vector Control (SVC), and maybe over size just the DRIVE, it can make the 25HP motor pump out enough torque to start that compressor. Typical Starting Torque (Locked Rotor Torque) on a motor is 150-160% of FLT, but Break Down Torque (Peak Torque), can be as high as 250% of FLT. A VFD using SVC can make a motor produce BDT at any time for a few seconds, longer if the VFD is over sized. A common rule-of-thumb I have seen is to over size by 60-70%. Although I think that might be over kill, it comes from literature I have read on applying VFDs to recips, but made no mention of using SVC, so using SVC might make that kind of overshoot a bit much. Still, that means your 40HP idea would apply, but only to the DRIVE, not necessarily to the motor. Once the motor is at speed, it is a Constant Torque operation, so if it works on the torque of a 25HP motor at high speed, it will work on that at low speed. It is only the STARTING that is the problem.
> 
> the people to discuss this with would be the compressor mfr. Ask them what the necessary STARTING torque is for their compressor as well as the minimum acceleration time. Once ou know that, you can determine how much of a motor you need, then how much VFD you need to make it work. You may find that with the right VFD using SVC, you may be able to do this with a 25HP package.


 
JRaef, thanks very much for the reply. I had a chance to talk with my supervisor more extensively about this today and I also learned alot about the initial design.

Yes, these are recip comps. Open drive with unloaders. The particular ones I'm talking about that we want to modify first are 4 cylinder with 3 unloaders that start with only 1 cylinder loaded. I do not believe starting torque was of major concern because of the unloaded start strategy.

From what I am told, the original reason for selecting two speed CHP was for operating in "high temp" and "low temp" before VFDs were practical. These systems were designed and built around 1978 or so.. "High temp" would be subjecting the comp to a lower compression ratio and thus a higher torque requirement (use low speed.) "Low temp" would subject the compressor to a much higher compression ratio and thus a lower torque requirement (you lose mass flow at high comp ratios so we go to high speed in "low temp" to increase mass flow and capacity)

I also learned that this was done in the past, which kind of answers the question of my original post. We have similar systems with single speed motors and drives that are "oversized." I didn't realize that these were originally two speed CHP. A two speed CHP 50 HP system was converted to a single speed 75HP motor and drive. Two speed CHP 75 HP system converted to single speed 125 HP motor and drive. Apparently all of these selections were made with consulatation from the compressor mfr. I took the time to look at the compressor curves and the motor and drive selections that we now have are spot on.

The idea behind all this is to reduce speed at minimum load for better capacity control. (Reduce speed with only 1 cylinder loaded.) This also affords us the ability to even develop more full load capacity than we could before by slightly overspeeding the compressor above 1150 RPM in high temp since we now have more motor and drive. Of course, this is only if we don't exceed the operating envelope of the system...

The equipment is ordered, now we just need to wait until we can take the system down to make the changes which may be awhile.

Again, thanks for the reply.

-Allen


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