I have a 15 HP PowerFlex 525 VFD driving a 15 HP motor with a nameplate power rating of 11 kW and 18 FLA (at 460 V). Right now, the drive is telling me it's outputting 21.8 A at 331 V and 10 kW of power. My question is whether the motor windings are going to have issues long term running like this since it is exceeding the FLA, however in my mind the output power is the only thing that matters for the windings and that is within the 11 kW limit. Am I thinking about this correctly or is this motor going to burn out over time? Trying to decide whether to upsize to a 20 HP motor for this application.
Motor current on VFD
- Thread starter EC Dan
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kwired
Electron manager
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My guess is it is not correct. Is ok for temporary boosting to tweak that ratio but not for constant running condition.
OP output frequency should be about 43 Hz at if voltage is 331 if I calculated correctly. Current draw will be dependent on how loaded the motor is. Variable torque loads will draw less when slowed down, constant torque loads may not depending on other load characteristics. If OP does have correct V/Hz and is drawing this kind of current, then there is likely something going on with the load itself that is driving the current up. I have run into this issue before with VFD on a high pressure positive displacement type pump. You can still overload a motor driving such a pump if you trying to build too much pressure even though you are running the pump at a lower speed.
ptonsparky
Tom
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- EC - retired
I believe those would be constant torque. Not typical fan or pump.
I'm sure I will be corrected if wrong.
Stand by
winnie
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What the motor winding cares about is _current_, not power.
When you look at power, it is always the product of two factors: torque × speed, voltage × current, pressure × flow.... and very often you will find that the _losses_ in a system scale mostly with one of those terms.
Loss in a wire is mostly caused by current, so to when you deliver the same power at 1A and 1000V or 100A and 10V you need much fatter wire in the 100A case.
To a good approximation a motor requires the same current and generates the same heating to produce a given amount of torque. If the motor needs 18A to produce 45 lb-ft of torque at full speed, it will pretty much require 18A to produce 45 lb-ft at 70% speed, and 18A (at significantly reduced frequency and voltage) to produce 45 lb-ft at zero speed. In each of these cases the motor losses and heating will be roughly the same.
IMHO your motor is somewhat overloaded. How much of a problem will depend on the service factor capabilities of the motor and how kind the environment is. The motor might last a good long while if ambient conditions are cool.
kwired
Electron manager
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- NE Nebraska
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I'd say with no sort of bypass or other pressure regulating methods, they basically are a positive displacement pump. The load is sort of softened when pumping gases vs liquids simply because gases will compress.
I will go ahead and upsize the VFD and motor to 20 HP. Fortunately, in both cases, the form factor is identical for the 20 HP version compared to the 15 HP version, so I just need to pull bigger wire. If my application results in X amps to deliver Y torque, which is what is needed to deliver air at the specified flow and pressure, then I can assume this will also be the case for the 20 HP motor. Since X amps in this case exceeds the FLA of the 15 HP motor and is within the FLA of the 20 HP motor, this should solve the overload problem. I would expect the motor to run at lower voltage/frequency to deliver that same amount of amps/torque. Do I have all that correct?
ptonsparky
Tom
- Occupation
- EC - retired
Sort of softened...but they are positive displacement and current goes up as restrictions are added. My experiences were limited to issues on the output side.
Correct. At a given outlet pressure, HP is proportional to speed. At a given speed, HP is proportional to pressure.
winnie
Senior Member
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- Springfield, MA, USA
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- Electric motor research
Basically correct, if all other factors are equal.
Things to watch out for: are the motors rated for the same voltage? Do the motors have the same pole count/base speed.
You can have a 15 Hp and 25 Hp motor with the exact same frame size, but when you look closely you will see that one motor is rated for 1750 rpm, and the other motor rated for 3500 rpm, and that will make a big difference for your application.
Also watch out that the motors have the same 'service factor' if you want to compare apples to apples. If one motor is rated 15 Hp with a 1.5 service factor, and the other is rated 20 Hp with a 1.25 service factor, you essentially have the same motor with different nameplates.
As I said, your motor is being used above its rated current, and is thus 'overloaded'. But if the overload is less than the 'service factor' of the motor, then you might be just fine with your current setup.
ptonsparky
Tom
- Occupation
- EC - retired
I thought we weren't supposed use service factor with drives.
Guessing the motors can handle the load but don't use the SFA for FLA in drive parameters.
??
Guessing the motors can handle the load but don't use the SFA for FLA in drive parameters.
??
winnie
Senior Member
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- Springfield, MA, USA
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- Electric motor research
The service factor is about heating; it is saying that you can heat the motor a bit more than you might expect based on the horsepower ratings. (Or you could understand it as 'this motor has the over-all motor characteristics of an X Hp machine in terms of acceleration, peak torque, etc, but can handle a continuous load of SF * X, just don't expect it to actually be an SF * X motor'
VFDs produce an imperfect waveform that results in greater motor heating. You can think of the VFD as 'using up' some of that motor's service factor.
All I was trying to say is that if you have a nominal 15 Hp motor and a nominal 20 Hp motor, but the 15 Hp motor has a greater service factor than the 20 Hp motor, that the two motors might be equivalent in terms of how they handle the OP's load.
VFDs produce an imperfect waveform that results in greater motor heating. You can think of the VFD as 'using up' some of that motor's service factor.
All I was trying to say is that if you have a nominal 15 Hp motor and a nominal 20 Hp motor, but the 15 Hp motor has a greater service factor than the 20 Hp motor, that the two motors might be equivalent in terms of how they handle the OP's load.
Service factor is 1.25 but like @ptonsparky mentioned I was not planning on letting these operate in the SFA range due to the VFD factor. These motors operate in Southwest Florida outdoor ambient conditions (under cover at least), so I'm not taking any chances with them operating above FLA. Right now they definitely are operating above FLA, but I should be able to replace the 15 HP motor with a 20 HP before it burns out (assuming that takes longer than a few days to happen). PowerFlex drives have an input parameter for motor OL amps, which I set to be the SFA, so if it exceeds that it will trip. The Drive Temp (Heatsink Temperature) is also another factor I'm taking a close look at. The VFD panel has an air-to-air heat exchanger, which is able to maintain 50 deg C internal temp, right at the start of the de-rate range for PowerFlex drives. Despite that, the Drive Temp has exceeded the trip setting of 105 deg C twice in the last several weeks, which is what alerted me to this problem in the first place.
winnie
Senior Member
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- Springfield, MA, USA
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- Electric motor research
Ok, that makes it pretty clear that you need to upgrade the hardware.
Given the numbers you describe, I believe that you are in the operating hotter and will have a shortened life range of overload, not the 'the motor will melt soon range of overload.
One other option to consider: is this a direct coupled motor or belt connected? Another option is to change the gear ratio between motor and pump. I would do whichever was easier, either increase the gear ratio so the motor runs at higher speed, or change the motor/drive.
-Jonathan
Given the numbers you describe, I believe that you are in the operating hotter and will have a shortened life range of overload, not the 'the motor will melt soon range of overload.
One other option to consider: is this a direct coupled motor or belt connected? Another option is to change the gear ratio between motor and pump. I would do whichever was easier, either increase the gear ratio so the motor runs at higher speed, or change the motor/drive.
-Jonathan
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- San Francisco Bay Area, CA, USA
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- Electrical Engineer
Motors will have a SF of 1.0 when operating on an inverter drive. Most of them that say they are rated for using in inverters put that right on the nameplate. Those that don’t, should. This motor is definitely overloaded.
The fact that the V/Hz ratio is incorrect points to something not being set up correctly. An 11kW nameplate rating would imply this is an IEC motor, and although 11kW is roughly 15HP, a nameplate FLA of only 18A would not be consistent with a 460V motor. It should be closer to 21A. The 18A value would be more consistent with a 575V rated motor. So something seems off. Also, the default control mode on a PF525 is SVC and as such, requires that the drive be tuned to the motor, usually just by doing an Autotune procedure at setup. Getting odd erratic operation is often a sign that it was not tuned. If the nameplate is correct and the autotune was done, this motor is seriously overloaded mechanically.
winnie
Senior Member
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- Springfield, MA, USA
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- Electric motor research
IMHO a nameplate 18A FLA is in the realm of plausible for a 460V 15Hp motor, but it is lower than the FLA tables suggest.
The V/Hz ratio is on the high side, but in the tolerance range of 'normal' (8.16*60 = 490; too much but not crazy too much). A high V/Hz ratio will push the motor into saturation and increase current flow and motor heating, so this is plausibly the cause of the problem.
I'm going to stick with 'operation on an inverter drive eats into your SF'; but I agree that with a typical motor and typical SF, 'eating into the SF' results in assigning a SF of 1. I was thinking of specifically designed motors where the extra heating from the inverter is already baked into the normal rating.
What I am leaning toward: double checking the drive programming/tuning as @Jraef suggests, but think the real issue is mechanical overload. The numbers are off enough that the extra current is quite plausibly a programming issue, and if you eliminate the excess current then you've solved the problem.
The V/Hz ratio is on the high side, but in the tolerance range of 'normal' (8.16*60 = 490; too much but not crazy too much). A high V/Hz ratio will push the motor into saturation and increase current flow and motor heating, so this is plausibly the cause of the problem.
I'm going to stick with 'operation on an inverter drive eats into your SF'; but I agree that with a typical motor and typical SF, 'eating into the SF' results in assigning a SF of 1. I was thinking of specifically designed motors where the extra heating from the inverter is already baked into the normal rating.
What I am leaning toward: double checking the drive programming/tuning as @Jraef suggests, but think the real issue is mechanical overload. The numbers are off enough that the extra current is quite plausibly a programming issue, and if you eliminate the excess current then you've solved the problem.
kwired
Electron manager
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- NE Nebraska
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- EC
I agree, and do believe I've seen them with that rating. Possibly seen 19 amps more often than 18 though.
NEC tables usually factor in the highest FLA you will ever see on a nameplate for the applicable conditions like worst case efficiency, power factor or both would be involved.
The 18 amp FLA rating is definitely correct, see nameplate below. You are 100% correct on these being in SVC mode and they probably were never tuned. That is one thing I will correct when I install the new motor.
