VFD's and Electrical Current
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petersonra
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The current draw is a function of how much work the fan is doing.
question
question
VFD powers a 4-pole motor at 480v-3ph = would be equal to 7200rpm at 60hz if you bump from 60-90 now the fan is spinning @ 10,800rpm what happens to the current as the frequency is increased? does this mean that a fan can provide additional capacity by changing the frequency and using no additional power?
question
VFD powers a 4-pole motor at 480v-3ph = would be equal to 7200rpm at 60hz if you bump from 60-90 now the fan is spinning @ 10,800rpm what happens to the current as the frequency is increased? does this mean that a fan can provide additional capacity by changing the frequency and using no additional power?
weressl
Esteemed Member
Mike01 said:
Pump and other centrifugal loads, such as the fan in question, operate based on the affinity law that is stated as follows: The horsepower(current) ratio changes in cubed realtionship to the speed change.
HP1/HP2=(n1/n2)^3.
Up to 60Hz or whatever base frequency the drive has, the Voltage AND Frequency changes to modulate the speed and kVA output of the drive. After the base frequency is reached, the voltage is at maximum, say 460V, so the current output will change and of course the frequency also.
In practice you need to make sure not only that motor can operate electrically at higher frequencies, but that its bearings and all mechanical components of the driven equipment is designed to operate whatever higher speed you desire.
fans
fans
the question comes up only because on a project the fans for a unit are specified to run off a vfd at 90hz possibly 120hz my question is what does this do to the current not only within the motor in regards to the insulation on the copper within the motor but what does this change in frequency do to the current supplied to the fans, assuming the fan has the capability to operate at 120hz the way I calculate it that is approx 15,000rpms, I am still trying to understand this so if I have mis-stated some numbers I apologize.
fans
the question comes up only because on a project the fans for a unit are specified to run off a vfd at 90hz possibly 120hz my question is what does this do to the current not only within the motor in regards to the insulation on the copper within the motor but what does this change in frequency do to the current supplied to the fans, assuming the fan has the capability to operate at 120hz the way I calculate it that is approx 15,000rpms, I am still trying to understand this so if I have mis-stated some numbers I apologize.
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Mike01 said:
Somebody should be looking at that application very very carefully. Running a motor over base frequency is not a great plan with a centrifugal load, because as weressl said above, the load HP increases at the cube of the speed change. But the MOTOR HP remains fixed since you can't increase the voltage with the frequency. So at 120Hz, the fan now needs 8 times the HP than it did at 60Hz (120/60=2, 2 cubed is 8). So if your fans originally needed 10HP at 60Hz, you will need 34HP to run them at 90Hz and 80HP motor to run them at 120Hz. Are they putting a 100HP motor on a 10HP fan then? I doubt it.
There is a trick you can do however; use a 230/460V motor and wire it for 230V, then apply 460 through the drive. That way, you reach 460V at 120Hz and it's fine because your V/Hz is still there. To apply that then to the above example, you could use a 40HP motor strapped for 230V and run up to 120Hz at 460V. At that point, your motor is putting out 80HP and your current will reflect that and the VFD had better be sized that way as well. Why they would want to make it that complicated compared to just using the right size motor is beyond me though.
petersonra
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- Semi-retired engineer
Mike01 said:
The currant will be whatever it has to be to do the amount of work required to move the air. There is no way to tell how much work that will be from the information you gave us.
The volts/Hz ratio is generally fixed, so as the frequency rises, the voltage also rises proportionally. It does not reach 460V until 60 Hz. Unfortunately, above 60 Hz, voltage cannot increase anymore, so you you end up with a maximum of 460 volts at whatever frequency you are running above 60 Hz.
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- San Francisco Bay Area, CA, USA
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Well theoretically it will be roughly the equivalent of 1.5^3 x the aggregate FLA. So assuming that a 7.5HP 460V motor FLA is 11A, that is 66A x 3.375 = 223A!Mike01 said:
But in reality it just will not work. You will have 45 total HP worth of motors to do a job that now requires 152HP. Where is the other 107HP going to come from? Magic? As I implied earlier someone is all wet with this, they don't understand the principals of the affinity laws and how VFDs work. You cannot get something for nothing. Unless they had put 7.5HP motors on fans that only needed about 2HP each, you have seriously inadequate power here and it will overload.
Your question then is like asking how many marshmallows you can toast in the fire from an atomic bomb. There is a technically correct answer, but what's the point?
winnie
Senior Member
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- Springfield, MA, USA
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- Electric motor research
Mike,
You seem to be asking the same question over and over again, and as you describe the installation it keeps sounding less and less standard.
As a pretty good approximation, if the motor is properly magnetized, to produce the same torque requires the same current.
Changing the frequency does not change this result. Double the frequency, and the motor goes twice as fast. But to produce the same torque will require the same current.
But there aint no such thing as a free lunch.
The _voltage_ necessarily to 'properly magnetize' the motor changes with frequency. When you double the frequency, the motor will go twice as fast. If it is producing the same torque, it will require the same current. But now it requires twice as much _voltage_. Same current and double the voltage means twice as much power in to the motor. But that's okay, because same torque at double the speed means twice as much mechanical power out.
So _in principal_ changing the frequency to the motor won't change the current consumed at all. However in the above we made two whopping big ASSUMPTIONS 1) proper magnetization and 2) same output torque. We are also ignoring magnetizing current, which changes the value of the approximation.
1) Falls apart when you operate the motor above 'base frequency' unless you make special effort. This is because most VSDs cannot supply a higher output voltage then their input voltage. Unless you have a special VSD, or a non-standard configuration such as a 230V motor supplied by a 480V VSD, when you operate at 90Hz the voltage will be too low to properly magnetize the motor. The current to produce a given torque will increase.
2) Falls apart because most loads have different torque requirements at different speeds. In particular, for 'fan loads' the torque required changes with the _square_ of the speed. Double the speed, and that fan will require 4x the torque. Additionally, in a design situation, the _load torque_ may be adjusted by the designer. The designer may be selecting fans that properly load these motors at 91Hz.
Without knowing more about the mechanical loads in question and the VSDs that you are using, there is no way to know what will really happen to the motors. If the fans have been reduced in size, so that they would be undersized at 60Hz, then they might produce an acceptable torque at 91Hz. The 'acceptable torque' might be reduced from the normal full load torque to compensate for reduced magnetization, or the motor voltage may have been selected to maintain proper magnetization.
Finally, when running multiple motors from a single VSD, there can be significant issues with motor protection. What protects a single motor from overload, and what protects the VSD if an overload situation causes motor(s) to disconnect?
-Jon
You seem to be asking the same question over and over again, and as you describe the installation it keeps sounding less and less standard.
As a pretty good approximation, if the motor is properly magnetized, to produce the same torque requires the same current.
Changing the frequency does not change this result. Double the frequency, and the motor goes twice as fast. But to produce the same torque will require the same current.
But there aint no such thing as a free lunch.
The _voltage_ necessarily to 'properly magnetize' the motor changes with frequency. When you double the frequency, the motor will go twice as fast. If it is producing the same torque, it will require the same current. But now it requires twice as much _voltage_. Same current and double the voltage means twice as much power in to the motor. But that's okay, because same torque at double the speed means twice as much mechanical power out.
So _in principal_ changing the frequency to the motor won't change the current consumed at all. However in the above we made two whopping big ASSUMPTIONS 1) proper magnetization and 2) same output torque. We are also ignoring magnetizing current, which changes the value of the approximation.
1) Falls apart when you operate the motor above 'base frequency' unless you make special effort. This is because most VSDs cannot supply a higher output voltage then their input voltage. Unless you have a special VSD, or a non-standard configuration such as a 230V motor supplied by a 480V VSD, when you operate at 90Hz the voltage will be too low to properly magnetize the motor. The current to produce a given torque will increase.
2) Falls apart because most loads have different torque requirements at different speeds. In particular, for 'fan loads' the torque required changes with the _square_ of the speed. Double the speed, and that fan will require 4x the torque. Additionally, in a design situation, the _load torque_ may be adjusted by the designer. The designer may be selecting fans that properly load these motors at 91Hz.
Without knowing more about the mechanical loads in question and the VSDs that you are using, there is no way to know what will really happen to the motors. If the fans have been reduced in size, so that they would be undersized at 60Hz, then they might produce an acceptable torque at 91Hz. The 'acceptable torque' might be reduced from the normal full load torque to compensate for reduced magnetization, or the motor voltage may have been selected to maintain proper magnetization.
Finally, when running multiple motors from a single VSD, there can be significant issues with motor protection. What protects a single motor from overload, and what protects the VSD if an overload situation causes motor(s) to disconnect?
-Jon
weressl
Esteemed Member
petersonra said:
This has already been explained with data in http://forums.mikeholt.com/showthread.php?p=793235#poststop
weressl
Esteemed Member
realolman said:
Yes there is and the maximum value of that is the drive's full load capacity.
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