Tek-Tips is the largest IT community on the Internet today!
Members share and learn making Tek-Tips Forums the best source of peer-reviewed technical information on the Internet!
-
Congratulations MintJulep on being selected by the Eng-Tips community for having the most helpful posts in the forums last week. Way to Go!
stupid question?
- Thread starter susanp
- Start date
- Moderator
- #4
Some installatioms have a 3 phase compressor with single phase fans, some have a three phase compressor with single phase fans at the evaporator, and three phase fans at the condensor.
buzzp's post gives you the basics.
Check each motor for three phase or single phase.
Power factor; The power factor depends largely on the load on the motor.
Single phase motors with start run capacitors will run close to 100%
Motors without power factor correction will usually run at 80% to 90%. (There are serious exceptions to this rule depending on the motor loading, but refrigeration equipment tends to be well loaded and power factors are moderate.)
I did a power usage study for a supermarket a few years ago. The major load was refrigeration compressors and the overall power factor was over 90%. (No penalties.)
Short answer. Use 100% power factor for motors with start-run capacitors. Use 90% power factor for other motors.
yours
buzzp's post gives you the basics.
Check each motor for three phase or single phase.
Power factor; The power factor depends largely on the load on the motor.
Single phase motors with start run capacitors will run close to 100%
Motors without power factor correction will usually run at 80% to 90%. (There are serious exceptions to this rule depending on the motor loading, but refrigeration equipment tends to be well loaded and power factors are moderate.)
I did a power usage study for a supermarket a few years ago. The major load was refrigeration compressors and the overall power factor was over 90%. (No penalties.)
Short answer. Use 100% power factor for motors with start-run capacitors. Use 90% power factor for other motors.
yours
Susanp,
The voltage is needed to give you the wattage consumed. Note that the loads are generally expressed in Volt-Amperes, VA, and the wattage is a function of this and the power factor noted by the other posts. But looking at the values given, lets say the feed to this unit is at 208V/3phase. The compressor would be 3-ph and the condenser fans would be single phase in most cases. The locked rotor amparage you gave of 266 amps is not for loading, it is for calculating the starting current requirements. So at 208V, you have:
63.6A x 208V x SQRT3 = 22896 VA or 22.9 kVA
2 @ 4.3A x 208V/1ph = 1789 VA or 1.8 kVA
1 @ 3.7A x 208V/1ph = 770 VA or 0.8 kVA
_______
Total Load = 25.5 kVA
At a reasonable Power Factor of 0.85= 21.7 kW
Other numbers you will see are the MCA, minimum circuit amps which is 125% of the largest load plus the rest, in this case this would be 79.5A + 7.2A = 86.7A. Also the MOCP, which is the Max OverCurrent Protection allowed which adds the largest load on top of this figure which would be 150.3 Amps. So for this unit, I would select a breaker and feeder of between 100A and 125A, depending on load profile, start stop requirements, etc....
If of course your system is 480V or some other voltage, the numbers would adjust accordingly.
The voltage is needed to give you the wattage consumed. Note that the loads are generally expressed in Volt-Amperes, VA, and the wattage is a function of this and the power factor noted by the other posts. But looking at the values given, lets say the feed to this unit is at 208V/3phase. The compressor would be 3-ph and the condenser fans would be single phase in most cases. The locked rotor amparage you gave of 266 amps is not for loading, it is for calculating the starting current requirements. So at 208V, you have:
63.6A x 208V x SQRT3 = 22896 VA or 22.9 kVA
2 @ 4.3A x 208V/1ph = 1789 VA or 1.8 kVA
1 @ 3.7A x 208V/1ph = 770 VA or 0.8 kVA
_______
Total Load = 25.5 kVA
At a reasonable Power Factor of 0.85= 21.7 kW
Other numbers you will see are the MCA, minimum circuit amps which is 125% of the largest load plus the rest, in this case this would be 79.5A + 7.2A = 86.7A. Also the MOCP, which is the Max OverCurrent Protection allowed which adds the largest load on top of this figure which would be 150.3 Amps. So for this unit, I would select a breaker and feeder of between 100A and 125A, depending on load profile, start stop requirements, etc....
If of course your system is 480V or some other voltage, the numbers would adjust accordingly.
About the only thing that is practical is to connect a 3-phase power analyzer to each machine and read off how many watts and vars it is using. This is the easiest way. You should take readings when the load is reasonable on a mild day and also the maximum load on a very hot day.
I have done this with some industrial machines that are good candidates for power factor correction. In a lot of cases the motor is only running at 50% load because there is some other constraint, such as starting torque, poor ventilation, or how much could be placed on it that dictate a larger motor. For instance, in the case of a vibratory finisher there is the case that you could be using steel shot instead of stones and steel shot presents more load. In the case of a punch press the motor has to be rated to run the heftiest die that can be installed. In all of these cases a 3-phase power analyzer will tell you more information faster than you can if you have an amps versus load table from the motor manufacturer.
I have done this with some industrial machines that are good candidates for power factor correction. In a lot of cases the motor is only running at 50% load because there is some other constraint, such as starting torque, poor ventilation, or how much could be placed on it that dictate a larger motor. For instance, in the case of a vibratory finisher there is the case that you could be using steel shot instead of stones and steel shot presents more load. In the case of a punch press the motor has to be rated to run the heftiest die that can be installed. In all of these cases a 3-phase power analyzer will tell you more information faster than you can if you have an amps versus load table from the motor manufacturer.
Susanp
Be aware that refrigeration compressors are rated at a specified load conditions. Higher the load=higher amp draw and lower the load=lower amp draw. Compressor manufactures rate and label the compressor at a nominal load YOU need to know where the system is operating in order understand the amperage draw of the compressor. Fan amp draws can be taken off of the data tag not true with the compressor.
Talk to the specifier and the supplier of the equipment.
Best regards
pennpoint
Be aware that refrigeration compressors are rated at a specified load conditions. Higher the load=higher amp draw and lower the load=lower amp draw. Compressor manufactures rate and label the compressor at a nominal load YOU need to know where the system is operating in order understand the amperage draw of the compressor. Fan amp draws can be taken off of the data tag not true with the compressor.
Talk to the specifier and the supplier of the equipment.
Best regards
pennpoint
- Moderator
- #8
Why do you need to know the kilowatts susanp? The reason for the question often has a lot of information in it. Do you need Kilowatts, Kilowatt Hours or KiloVoltAmps? Are you estimating operating costs, (Kilowatt Hours), sizing an electrical installation (KiloVoltAmps) or sizing a standby generator (Kilowatts)
If your unit is for a comercial application in a large supermarket or a shopping mall, it will probably be three phase. If it's residential or "Mom and Pop" comercial it is more likely to be single phase. Is this an existing installation or a proposed installation?
If your unit is for a comercial application in a large supermarket or a shopping mall, it will probably be three phase. If it's residential or "Mom and Pop" comercial it is more likely to be single phase. Is this an existing installation or a proposed installation?
- Status
Similar threads
- Locked
- Question
- Locked
- Question
- Locked
- Question