Sizing a Transformer...

[cfordyce]

I need to size a TX for a piece of equipment.

1)The manufacturers spec indicates a MAX KVA of 207KVA for the machine.

2)I add up all the motors on the machine and I calc a full load KVA draw of;
152 1KW motors
2 1.4KW Air conditioners
1 5.0KW motor
1 5.5KW motor
plus some controls, electronics, displays, etc.
TOTAL CALC FULL LOAD= 165KW

3) I put a power monitor on an identical machine, only with less stations (each station has 1KW motor);

132 station reading: 75kVA, 70KW, PF:0.93
est. 152 station reading: 75KVA*(152/132)=86.6KVA

Question 1) Does anything seem unreasonable with the above information?

Question 2) Could I use a 175KVA Transformer safely? The TX is speced to handle 200% of rated load for 30sec. and 150% of rated load for 60sec. Duty cycle of one start every two hours is permitted.

*Additional info: The machine has the ability to start all stations (1KW motors) at the same time but it is not our practice to do so. We start each station one after the other.

Thanks already.

 

[dpc]

Since a standard 225 kVA transformer would meet manufacturer's requirement, I'd ask myself if the savings between a 175 kVA and a 225 kVA transformer would be worth the slight increase in risk of a premature transformer failure?

Also, use of smaller transformer would give the machine supplier a place to point his finger if there are problems in the future. The calculated full load of 165 kW agrees with his max load of 207 kVA.

Based on your analysis, I suspect a smaller transformer would work, (175 kVA doesn't sound like a standard size to me), but I'd put in a 225 kVA.

 

[busbar]

Aside: Expect 150°C-rise drytypes to run to quite hot at rated load. 115- and 80°C-rise transformers are made.

 

[cfordyce]

The reason that I would prefer the 175KVA TX is not the cost savings. 175KVA is the largest TX the manufacturer has that is wall/column mountable. If I go with 225KVA I need to use up floor space to house the transformer.

These small transformers are only a short term solution for power distribution to these new machines. Once a few more go in we will invest in a larger 1000MVA transformer outside at the substation. I may need to get a reliable 1 to 2 yrs out of these small TXs.

I am asking - Is there any forseeable risk in going with 175KVA given the above information of the equipment?

Remember this is not a standard distribution TX, this is a 'drive isolation TX' with ratings for 200% loading up to 30secs, 150% loading up to 60sec.

Thanks.

 

[cuky2000]

If not new load is added and the system is operated within the transformer rated temperature rise and and less than the limits environmental condition your estimation should be OK.


<sup>NOTE: If the transformer is manufactured per ANSI Std, verify the foollowing condition are not exceeded.

TEMPERATURE RISE
- Secondary & primary voltage.......< 5 %
- Ratio primary volts/hertz.........< 10 %
- Ratio secondary volts/hertz.......< 5 %
- Load Power Factor.................> 80 %
- Frequency.........................> 95%

ENVIROMENT CONDITIONS:
- Ambient temp........................<40 oC
- Average 24 hr ambient temp..........<30 oC
- Load current harmonic factor....... <0.05 pu
- Altitude........................... < 1000 m
- Sec. voltage and volts/hertz....... <1.05 pu.
-</sup>

 

[jbartos]

Suggestion: The power distribution including the power source, transformers, cables, motors, buses and loads should be modeled by commercial electrical computer software

 

[cfordyce]

Which software would you suggest? Preferably one with a demo or shareware version available to try out.

CAF

 

[cuky2000]

Software for complex applications including large motor stating could be a good tool to simplify the transformer sizing process and increase the confidence level sizing smaller transformer if will be doing by simplified hand methods.

However, for the case presented with well defined connected load with the largest motor load less than 5% of the OA transformer rating, I can not see how a software could estimate any result dramatically different than the one performed by hand calculation.

If you have the software in house and know how to use it, could be a good idea to validate your hand calc. However, If you need to search, purchase a specialize software and learn how to use it, probably will be difficult to justify the cost/benefit for this investment.

Beware that could be a frustrating experience for the new user of a software the amount of unknown or assumed data require to build a fairly simple electrical system that not only include each type of transformer, motor, feeder but also other electrical components connected to the power system.

_{SUGGESTION:
Refine your calculation with diversity factor for each motor. Stating motor may not be a major problem for 5.5 kW motor. Evaluate any future load connected to the system.}

 

[moturcu]

Mechanical loading of the motors may play an important role in estimation. I mean, can you estimate how much the motors are overloaded and how many of them are subjected overload simultaneously.

The other concern if there is any inverter driven motor exist in the system. If there is, does your power monitoring system consider the harmonic power?