Excessive Motor Load Factor In Freezer Space

Look-up (1) copper-resistance change vs. temperature and (2) motor "cooling' (heat-dissipation) vs. temperature...should account for much of what you're seeing.

a VFD o .5 hp? are you kidding?

I am not kidding (Not a very funny joke had i been either). A single VFD controlling a bank of (4) 0.5 HP motors.

I am unsure of how to edit a post so I am just replying to my own post. The calculation should be as follows (i accidentally referenced a separate spreadsheet that was calculating savings for a different evaporator):

• Voltage = 480 V
• Assumed Power Factor = 0.9
• Average Amps when on (A) = 4.76
• Average Load (kW)= (Average Amps)*(RMS Voltage)*(Power Factor)*(3^0.5)/1000 = 3.55
• Theoretical load - 4 motors @ 0.5 HP (kW) = 4*0.5*0.746 = 1.492
• Load Factor = Average Load / Theoretical Load = 3.55/1.492 = 238%

https://www1.eere.energy.gov/manufacturing/tech_assistance/pdfs/10097517.pdf

if speed control is the goal I'd switch to DC motorshttp://www.weg.net/ca/Products-Services/Drives/Payback-VFD-Calculator

I understand the concept of having overload and short circuit protection when controlling multiple motors on a single VFD. Thank you though

I always thought that electrical resistance went down with temperature....anyway starting these motors with a VFD will provide a gradual startup with low amp draw...Your equation seems to be starting all 4 motors on high speed....

http://www.automation.com/library/articles-white-papers/motor-drives-control/vfds-can-control-multiple-motors

http://www.ke2therm.com/files/W-2-1_More_Fans_or_Fewer.pdf

I am not sure but I think the VFD is thinking it is looking at almost a dead short.

The VFD is going to calculate for six motors in parallel not series.

R Total = 1 / 1/R1 + 1/R2....etc.

For fire safety and motor protection you should use 1 VFD per motor. If speed

isn't a factor take a look at a fractional HP soft start such as Saftronics.

Please verify if the individual motors have thermal overload protection. (Small motors like this often do, but it is not a given.) What is the nameplate FLA on these?

Compared to room temperature air, cold air imposes a greater load on the motors. However, the cold air also cools the motor better than warm air. The second factor often outweighs the first, so that a 0.5hp motor can produce, say, 0.65hp. This may entail larger wiring and overcurrent devices, allow higher overload settings, etc.

Each installation needs specific study, with full information on components and temperatures.

Where you have checked the current ( at motor terminal or supply end )

We installed the data logger at the disconnect located at the MCC panel (maybe up to a few hundred feet from the evaporator). I am confident there are no other equipment on this circuit.

So the location of current measured is after VFD . Then calculation based on supply voltage may lead to wrong results on the load calculations.You should have taken the voltage prevailed at the location of current measurement for power calculations

No, currently there is no VFD installed - see #19.

• Your electrical input power equals to Pin = U x I x sqrt (3) x Power factor = 480 x 4.76 x 1.73 x 0.9 = 3557 Watt.
• Your mechanical output power for 4 motors equals to P out = 4 x 0.5 x 746 = 1492 Watt
• Your efficiency is Pout / Pin = 1492/3557 = 42%

This means

• a) you have very low efficiency motors
• b) your motor has a larger mechanical loaded than the presumed 0.5 Hp. This is possible if the wind flow rate is much higher then it should be.
• Are all the air ducts closed? (no false draught ?)


• Can the fans work in serie? (the more flow rate, the more energy : P x V = m x R x T)
• c) your current measurement is not correct. The current that takes a VFD is not a sinus wave, it can only be measured with a True RMS Amp meter

I think you've got it there.

Also 0.9 power factor is high for a 0.5 HP motor, my data says PF 0.7 and efficiency ~ 65%. Using that PF gives calculated efficiency 54%, not that far short of published figure. The difference could be covered by the other points you made.

I hope these help. Resistance in parallel loads is critical to a VFD. You will

still have to solve the problem of getting the VFD to function properly.

NEC Multiple Motor Load Calculations

Motor Load Factors

When measuring the input current to the VFD, which is the only valid external measurement here, you do NOT factor in the motor power factor, because the VFD is correcting it to .95 or better as far as the line source is concerned. This means that the amp draw on the INPUT of the VFD will be INCLUSIVE of the reactive current being used inside of the motors, making it read higher than what you would calculate on the OUTPUT side based on shaft loading. The Full Load Amps of a 460V 1/2HP motor is 1.1A at the most, likely less. If you have 4 motors, each drawing 1.1A, and each running at a .8PF, the input current to the VFD would be higher than 4.97A total; 4.4 / .8 = 5.5A total, meaning your motors are not even fully loaded. So there is that.

Also, if you are using a cheap multimeter to read the input current the accuracy is immediately suspect, because the cheaper the meter, the more it is going to be affected by harmonics, and the VFD creates a significant amount of harmonics. If you were reading the VFD amp output however, that's different.

I don't believe i was clear on my description of the problem. There is currently no VFD installed on the fan motors. I am trying to measure baseline operating conditions of the motors so that i may estimate the energy savings once a VFD is installed.

Hi Max,

Maybe I can help a little more.

1. Copy all of the 4 or 6 motor name plate information. Each load could be slightly different, but probably not if you are just dealing with a fan motor.

2. Are these single or three phase motors? Are the compressors part of the fan circuit?

3. It is always a good practice to use Full Load Amps off the motor nameplate when calculating loads.

4. I seem to remember we used a recorder over a ten day period to get the best baseline demand readings. This should help smooth out voltage and temperature changes. Fluke makes a perfect high accuracy meter for this purpose.

5. There is several blogs on the Internet where others have used 1 VFD for multi-motor systems. You might invest some time in reading them.

Applying VFD's to Industrial Refrigeration

Hope this helps.

Hi Keith,

1. I unfortunately do not have nameplate data. I have serial numbers of the equipment, contacted the OEM, and they confirmed the HP of the motors (at 0.5). I am experiencing the same excessive load factor problem on (3) different evaporators all logged independently (and with three different current transformers too!) and all on their own separate circuit.

2. These are three phase motors. The compressor is not part of the circuit.

3. Again, i do not have the name plate data.

4. I logged the motors for (2) full weeks.

5. I am confident that i will be able to control all the motors with (1) VFD. I have already done research into this and talked with a VFD manufacturer (Danfoss).

Thanks!

I spoke with another engineer from the OEM and they said that motors will draw significantly higher amperage when in cold temperatures (-10F). The load factors of 180%-250% did not surprise him. He said that we would need to oversize the VFD to account for the higher amperage draw.

Thanks to all who commented!