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Discussion Starter · #1 ·
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)

The first time we started talking about doing this most of the other guys said "We'll just wire the VFD to the high speed winding." If I am thinking about this right and doing my math right the 25 HP motor operating at full load in low speed @ 1150 RPM will satisfy a 114 lb-ft torque requirement and operating at 1750 RPM will only satisfy a 75 lb-ft torque requirement assuming 25 HP at both speeds.

Here's what I told them, I'd like to see what anybody here thinks: If I install a 25 HP VFD on the high speed winding and I attempt to run the motor at a reduced speed (1150 RPM) with the 114 lb-ft torque requirement I will overcurrent the high speed windings, correct?

I figure, the only real practical way I can get away with this is if I get a single speed oversized motor and VFD. If I need 114 lb-ft at approx 1150 RPM and 75 lb-ft at 1750 RPM from a single speed motor then I will need a single speed 1750 RPM 40 HP motor and VFD, correct? A single speed 40 HP 1750 RPM motor at full load will produce 120 lb-ft. If I run this motor at 1150 RPM with my 114 lb-ft torque requirement I will not overcurrent the windings. If I run this motor at 1750 RPM with my 75 lb-ft torque requirement, obviously I will not overcurrent the windings.

I guess the motor wouldn't exactly be "oversized" for this application. In high speed you could consider it oversized but in low speed the motor will be near full load current when driving a 114 lb-ft load at 1150 RPM.

We ended up submitting a request to purchase two 40 HP 1750 RPM motors and two 40 HP VFDs.

Any thoughts? I'm pretty sure this is necessary considering the application and the desired outcome. I'm sure that we could use the existing motor with a 25 HP VFD and use load contactors to switch between windings depending on what speed we want and what our load is but that's getting a little hairy and I don't know if I want to go that route. That would require a little more than "simple" control logic.

Thanks very much for any input.

-Allen
 

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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)

The first time we started talking about doing this most of the other guys said "We'll just wire the VFD to the high speed winding." If I am thinking about this right and doing my math right the 25 HP motor operating at full load in low speed @ 1150 RPM will satisfy a 114 lb-ft torque requirement and operating at 1750 RPM will only satisfy a 75 lb-ft torque requirement assuming 25 HP at both speeds.

Here's what I told them, I'd like to see what anybody here thinks: If I install a 25 HP VFD on the high speed winding and I attempt to run the motor at a reduced speed (1150 RPM) with the 114 lb-ft torque requirement I will overcurrent the high speed windings, correct?

I figure, the only real practical way I can get away with this is if I get a single speed oversized motor and VFD. If I need 114 lb-ft at approx 1150 RPM and 75 lb-ft at 1750 RPM from a single speed motor then I will need a single speed 1750 RPM 40 HP motor and VFD, correct? A single speed 40 HP 1750 RPM motor at full load will produce 120 lb-ft. If I run this motor at 1150 RPM with my 114 lb-ft torque requirement I will not overcurrent the windings. If I run this motor at 1750 RPM with my 75 lb-ft torque requirement, obviously I will not overcurrent the windings.

I guess the motor wouldn't exactly be "oversized" for this application. In high speed you could consider it oversized but in low speed the motor will be near full load current when driving a 114 lb-ft load at 1150 RPM.

We ended up submitting a request to purchase two 40 HP 1750 RPM motors and two 40 HP VFDs.

Any thoughts? I'm pretty sure this is necessary considering the application and the desired outcome. I'm sure that we could use the existing motor with a 25 HP VFD and use load contactors to switch between windings depending on what speed we want and what our load is but that's getting a little hairy and I don't know if I want to go that route. That would require a little more than "simple" control logic.

Thanks very much for any input.

-Allen
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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Discussion Starter · #4 ·
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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