Generator vs UPS vs A/C

[ceb58]

Details 240v 1ph system. 60kw generator, 12kw UPS .93pf input .8pf output, 4ton A/C unit.
What is happening is when running under gen power the UPS will lock on to the power and work as should with about a 3.5 kw load. when the A/C starts ( not on UPS circuits) the UPS will drop reg. power and switch to batt. in a short time it will switch back to reg. power then the entire time the A/C is running the ups keeps switching back and forth from reg. to batt. The A/C when running is pulling about 40amps. The full load of the building, the communication equipment, and the A/C is pulling around 60 amps. What could be going on? The gen seems to be more than adequate to handle all the loads. I can understand the UPS droping out under A/C start up but not after it levels out.

 

[iwire]

Is the UPS capable of displaying the reason it has gone off?

Under / Over voltage?

Low / High Frequency?

Me I would probably through my meter on it set for cycles and see if the generator is holding the cycles steady with the compressor running.

My WAG is that it is running just a bit slow and the UPS is dropping due to low frequency or unstable frequency.

 

[cadpoint]

U've been playing around in the southern swamps, U think you'd have a meter that measures frequency!

 

[ron]

How loaded is the 12kW UPS?
Is the 3.5kW load you mention on the output of the UPS or the output of the gen?

 

[ron]

Another question is, does the UPS have an integral input filter?

 

[cadpoint]

Another question is, does the UPS have an integral input filter?

Throw some numbers on it ! Let's all learn something!

 

[dbuckley]

I'm assuming the A/C is connected to the genset output, not the UPS.

When the A/C starts, it shifts the frequency of the genset. UPS doesn't like this, and drops to batteries.

The UPS is phase-locked to the input frequency, and so just the shift in phase as the genset leans on its regulator a bit harder when the A/C load comes on is enough to make the UPS think its input power is turning ghastly...

 

[ceb58]

U've been playing around in the southern swamps, U think you'd have a meter that measures frequency!

I did come out of the mud long enough to hook my lap top to the gen. set controller. Monitoring the volts and Hz the volts would stay in a range of 238-242 L-L and the Hz would run 58.2- 61.3. I can see the ups dropping out when A/C starts, it has a LRA of 197amps

Is the 3.5kW load you mention on the output of the UPS or the output of the gen?

The 3.5kw is the load on the ups output

I am just trying to figure out what is going on. We have similar setups same size UPS, same size UPS loads. Only difference is the gen. sets are between 25 & 40 kw and the A/C units are 3 ton. And they will work fine

 

[ron]

Maybe at the other facilities that work, there are different manufactured parts, like a different voltage regulator on the gen or different settings on the UPS regarding what is an acceptable input.

 

[iwire]

I did come out of the mud long enough to hook my lap top to the gen. set controller. Monitoring the volts and Hz the volts would stay in a range of 238-242 L-L and the Hz would run 58.2- 61.3. I can see the ups dropping out when A/C starts, it has a LRA of 197amps

I would check the settings on the UPS, IMO 58.2 Hz is likely out of the default range setting.

 

[kingpb]

UPS rating is somewhere around 67A output; based on 12kW with 0.93 pf input, and 0.8pf Output, i.e. 12.9KVA rated; output capability is 16.13KW, 7.74Kvar.

1-ton ac is approx 3.517KW so a 4-Ton is approx. 14Kw. According to the OP there is about 3.5KW additional load on the UPS, making it around 17.5kW load, which is probably over the 120% overload capability.

In general, the rule of thumb is that back-up generator needs to be 2.5X the KVA rating of the Inverter. Design can be altered for the inverter to help decrease this size, but requires a custom designed inverter logic. Looks like your ok on generator KVA rating, but wait, there's more to it.....

The generator is rated for 60kW, usually at 0.85pf so top end it?s 70.5KVA, @ 240V, so rated current is around 294A. The LRA is 197A for the 4-ton unit plus the 3.5kW additional load. Assume the pf factor of the additional load is 0.85, which means it is drawing around 17.2 amps, for a total of 214.2A, which, is within the generator rated current. However, generator power capability is limited to roughly 0.85 leading to 0.85 lagging, which means you can service loads up to 60kW and 37.2KVAR. The 4-Ton unit probably has around a 0.47pf when starting, therefore it will need around 45KVA worth of power (197A @ 240V) but that is based on 21.2 KW and 39.72KVAR. The gen does not have the capability to support he Var?s needed when starting, nor does the inverter even come close.

During operation the generator can supply the power to the inverter, but the 4 ton unit is drawing 14kW @ say 0.88 pf of which it needs 7.6KVAR. The additional load of 3.5KW @ 0.85 pf would be another 2.2KVAR, for a total of 9.8KVAR, which is over the inverter rating, even if it has a 120% overload capability.

KVA is KVA, but you have to also account for the components that make up the KVA rating.

 

[qcroanoke]

UPS rating is somewhere around 67A output; based on 12kW with 0.93 pf input, and 0.8pf Output, i.e. 12.9KVA rated; output capability is 16.13KW, 7.74Kvar.

1-ton ac is approx 3.517KW so a 4-Ton is approx. 14Kw. According to the OP there is about 3.5KW additional load on the UPS, making it around 17.5kW load, which is probably over the 120% overload capability.In general, the rule of thumb is that back-up generator needs to be 2.5X the KVA rating of the Inverter. Design can be altered for the inverter to help decrease this size, but requires a custom designed inverter logic. Looks like your ok on generator KVA rating, but wait, there's more to it.....

The generator is rated for 60kW, usually at 0.85pf so top end it?s 70.5KVA, @ 240V, so rated current is around 294A. The LRA is 197A for the 4-ton unit plus the 3.5kW additional load. Assume the pf factor of the additional load is 0.85, which means it is drawing around 17.2 amps, for a total of 214.2A, which, is within the generator rated current. However, generator power capability is limited to roughly 0.85 leading to 0.85 lagging, which means you can service loads up to 60kW and 37.2KVAR. The 4-Ton unit probably has around a 0.47pf when starting, therefore it will need around 45KVA worth of power (197A @ 240V) but that is based on 21.2 KW and 39.72KVAR. The gen does not have the capability to support he Var?s needed when starting, nor does the inverter even come close.

During operation the generator can supply the power to the inverter, but the 4 ton unit is drawing 14kW @ say 0.88 pf of which it needs 7.6KVAR. The additional load of 3.5KW @ 0.85 pf would be another 2.2KVAR, for a total of 9.8KVAR, which is over the inverter rating, even if it has a 120% overload capability.

KVA is KVA, but you have to also account for the components that make up the KVA rating.

OP stated ACH unit is not part of the UPS load.

I think Iwire may be right about the hertz being out of the parameter of the UPS. You may be able to open up that window but that is a UPS technician thing. I would also call in the generator technician he may have a problem too.

 

[kingpb]

If only I had read it more closely the 20th time.

 

[ceb58]

UPS rating is somewhere around 67A output; based on 12kW with 0.93 pf input, and 0.8pf Output, i.e. 12.9KVA rated; output capability is 16.13KW, 7.74Kvar.

1-ton ac is approx 3.517KW so a 4-Ton is approx. 14Kw. According to the OP there is about 3.5KW additional load on the UPS, making it around 17.5kW load, which is probably over the 120% overload capability.

In general, the rule of thumb is that back-up generator needs to be 2.5X the KVA rating of the Inverter. Design can be altered for the inverter to help decrease this size, but requires a custom designed inverter logic. Looks like your ok on generator KVA rating, but wait, there's more to it.....

The generator is rated for 60kW, usually at 0.85pf so top end it?s 70.5KVA, @ 240V, so rated current is around 294A. The LRA is 197A for the 4-ton unit plus the 3.5kW additional load. Assume the pf factor of the additional load is 0.85, which means it is drawing around 17.2 amps, for a total of 214.2A, which, is within the generator rated current. However, generator power capability is limited to roughly 0.85 leading to 0.85 lagging, which means you can service loads up to 60kW and 37.2KVAR. The 4-Ton unit probably has around a 0.47pf when starting, therefore it will need around 45KVA worth of power (197A @ 240V) but that is based on 21.2 KW and 39.72KVAR. The gen does not have the capability to support he Var?s needed when starting, nor does the inverter even come close.

During operation the generator can supply the power to the inverter, but the 4 ton unit is drawing 14kW @ say 0.88 pf of which it needs 7.6KVAR. The additional load of 3.5KW @ 0.85 pf would be another 2.2KVAR, for a total of 9.8KVAR, which is over the inverter rating, even if it has a 120% overload capability.

KVA is KVA, but you have to also account for the components that make up the KVA rating.

Love your math. But help me understand something on the UPS. The UPS has 4 modules each rated at 3kw. Now I understand that if there were no connected loads on the UPS it would still have a draw because of the inverter. Now I connect a load that totals 3.5kw which would be around 14.5 amps. What would the UPS be drawing in amps? The A/C unit is NOT on the UPS output. I will add that the parameters of the UPS have been expanded to help compensate for "dirty" generator power. When the full load of the building is on generator power. Lights, UPS and A/C I am only seeing a 52amp draw on the generator. By the way you are dead on on the generator math.
I will add more info I learned today. The A/C unit is not a 4ton but a 6ton. The building has 2 of these units to supply the one big room, Yesterday I did not have access to the other side of the room ( room divided by chain link fence) The unit that is in the other side has a locked up compressor. Yesterday we could turn off the A/C that is running and the UPS would lock on to gen. power and be happy. But with the A/C running it would not lock on and keep switching back and forth. Since this building and the equipment are only 2 years old I now wonder if the problem is not with the A/c unit it's self. This whole problem did not show it's self until a few weeks ago. The site is on top of a mountain and is remotely monitored. There have been very short outages in the past but not long enough to drain the batteries in the UPS all the way down until a few weeks ago a tree fell across the poco lines and it took several hours for the repair.

 

[kingpb]

A UPS are very sensitive to overload, so exceeding the output will cause it to switch over to the batteries.

In rehashing some numbers: Output is rated 12kW and 0.8pf, that equates to 15KVA (very common size), but the marketing guru's don't tell you that even if you improve the pf to 1.0(whereby KW=KVA), you will still only get 12kW out of the UPS which equates to 50A. So you size your output cable for 62.5A and only load to 50A (meets NEC). This is because the KVA rating is applicable to the AC section as the DC does not care about power factor. The DC section is based on output KW, and whichever is the lower rated will be the limiting factor in capability, typically the DC kW rating.

With a load of 3.5kW you say the current is 14.5A, but that is assuming the pf=1.0. Is this a measured value or calculated? If not measured, I would have to assume that the load has a typical pf=0.85, which means the current would actually be around 17.2A, based on 4.12KVA.

2 x 6 ton a/c units is roughly 42kW in load and assuming 0.85pf, at 240V that equates to almost 50KVA in load (206A) something is not adding up if your generator is only showing 52A (12.5KVA)

You say one of the a/c units has a locked up compressor? It could be cycling on and off trying to run, and when it comes on, the generator is trying to feed the LRC, which drops the generator terminal voltage below what the UPS can handle for input, and so it looks like a loss of power and it switches to batteries. When the a/c unit shuts off because it doesn't start, then the gen voltage recovers, and the UPS switches back to gen power. Just a guess.

 

[ceb58]

A UPS are very sensitive to overload, so exceeding the output will cause it to switch over to the batteries.

In rehashing some numbers: Output is rated 12kW and 0.8pf, that equates to 15KVA (very common size), but the marketing guru's don't tell you that even if you improve the pf to 1.0(whereby KW=KVA), you will still only get 12kW out of the UPS which equates to 50A. So you size your output cable for 62.5A and only load to 50A (meets NEC). This is because the KVA rating is applicable to the AC section as the DC does not care about power factor. The DC section is based on output KW, and whichever is the lower rated will be the limiting factor in capability, typically the DC kW rating.

With a load of 3.5kW you say the current is 14.5A, but that is assuming the pf=1.0. Is this a measured value or calculated? If not measured, I would have to assume that the load has a typical pf=0.85, which means the current would actually be around 17.2A, based on 4.12KVA.

2 x 6 ton a/c units is roughly 42kW in load and assuming 0.85pf, at 240V that equates to almost 50KVA in load (206A) something is not adding up if your generator is only showing 52A (12.5KVA)

You say one of the a/c units has a locked up compressor? It could be cycling on and off trying to run, and when it comes on, the generator is trying to feed the LRC, which drops the generator terminal voltage below what the UPS can handle for input, and so it looks like a loss of power and it switches to batteries. When the a/c unit shuts off because it doesn't start, then the gen voltage recovers, and the UPS switches back to gen power. Just a guess.

The locked up A/C unit is out of the equation breaker off,disco pulled.
With only the lights,which are not on UPS power, and the radio equipment which is on UPS power I am seeing a draw of 17-22 amp on the gen. It will lock onto gen. power and stay locked. When the A/C unit comes on the UPS will drop out, which I would expect, it will then lock back on to gen. power but as the A/C continuous to run the UPS will start switching back and forth. When the A/C is running I am seeing a 50-55 amp draw. The Hz do not go below 58 and the voltage stays with in +/- 3 volts of 240. The parameters of the UPS have been expanded as far as they can be. I am now leaning toward something being wrong with the A/C unit. High head pressure or the compressor is going out. The UPS may be seeing bad sine waves from the compressor that I cannot with the equipment I have.

 

[qcroanoke]

The locked up A/C unit is out of the equation breaker off,disco pulled.
With only the lights,which are not on UPS power, and the radio equipment which is on UPS power I am seeing a draw of 17-22 amp on the gen. It will lock onto gen. power and stay locked. When the A/C unit comes on the UPS will drop out, which I would expect, it will then lock back on to gen. power but as the A/C continuous to run the UPS will start switching back and forth. When the A/C is running I am seeing a 50-55 amp draw. The Hz do not go below 58 and the voltage stays with in +/- 3 volts of 240. The parameters of the UPS have been expanded as far as they can be. I am now leaning toward something being wrong with the A/C unit. High head pressure or the compressor is going out. The UPS may be seeing bad sine waves from the compressor that I cannot with the equipment I have.

To bad your other A/C is down. You could make it your lead unit and see if the problem continues when it is leading. If it does it is not just that one A/C.

 

[dbuckley]

A UPS are very sensitive to overload, so exceeding the output will cause it to switch over to the batteries.

Wrong way up - give a UPS an overload and it will drop to bypass.

Nothing added since my previous post makes me change my opinion of why this is happening...

 

[ceb58]

Wrong way up - give a UPS an overload and it will drop to bypass.

Nothing added since my previous post makes me change my opinion of why this is happening...

Your other post is correct. The UPS will drop out and go to batt. when the A/C starts. Which you would expect due to the start up draw or phase shift. But after the A/C reaches its running amps the UPS still switches back and forth from normal to battery.

 

[mcclary's electrical]

The system is so borderlined that a soft start for a/c would fix the problem.


That is ,,,if that was the actual problem.


However, I agree with I wire in that the frequency is too low and most likely the culprit