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Old 04-05-2011, 09:19 AM   #1
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In general what is the recomended minimum frequency for a V F D

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Old 04-05-2011, 09:32 AM   #2
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Zero hertz.

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Old 04-05-2011, 12:05 PM   #3
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Depends if you want the motor to stop, when the reference signal is at zero. Generally like Eric said, zero hz should stop the motor.
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Old 04-05-2011, 01:34 PM   #4
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it is not for the vfd but for the motor, some motors may overheat if run too slow with no additonnal cooling fans, some equipement (pumps, compressors,...) can't be run very slow and it may generates overcurrent fault at very low speeds
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Old 04-05-2011, 02:40 PM   #5
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Quote:
Originally Posted by oliquir View Post
it is not for the vfd but for the motor, some motors may overheat if run too slow with no additonnal cooling fans, some equipement (pumps, compressors,...) can't be run very slow and it may generates overcurrent fault at very low speeds
Bingo. To add to that, most centrifugal loads such as pumps and fans will cease to move their load (water or air) at some speed anyway, so there is no point in running the motor below that speed.

So the answer is....

It depends on the use.
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Old 04-05-2011, 05:30 PM   #6
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Originally Posted by Calectrical View Post
In general what is the recomended minimum frequency for a V F D
The motor is going to be your biggest concern. If you spend the money on an "Inverter Duty" motor, you will be fine. These come with thicker insulation on the windings. This keeps the voltage spikes from the IGBTs from burning through the first few windings like it would on a normal duty motor. These also come with bigger, more efficient cooling fans, and more then likely internal thermal over-loads that will save the motor in case it does over heat.

A regular duty motor will do the same thing, it just will not last as long and will probably overheat sooner.

Then the use matters, and how the drive is set-up comes in.

If the drive is set up for straight V/Hz, the motor will not burn up at low speed because you have kept the V/Hz ratio the same through the entire speed range. The problem with this comes at about 6 Hz or less, you lose torque very quickly if you have any at all at that point. This is because you can no longer keep the magnetizing current at where the motor needs it to be. So yes, an impeller may begin to turn on its own causing the motor to become a generator- and then trip the drive on an "over-voltage" fault. Some drives come with braking resistors, or even DC injection for braking to clear this up.


Add an encoder, or a drive with "sensor-less vector" control. this gives the drive more feedback, and you can safely lower the Hz even more. After a "motor tune" has been done, the drive will supply that motor with the full magnetizing current all the way down to zero Hz. So as soon as you start to ramp up the Hz from zero, you are starting to cause slip creating torque.


So long story short, whatever Hz that motor will run at without overheating and it still has enough torque to drive the load, that is the recommended minimum frequency for that AC dive and motor.
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Old 05-09-2011, 03:53 AM   #7
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Quote:
Originally Posted by Calectrical View Post
In general what is the recomended minimum frequency for a V F D

It depends. It's different for V/Hz control or vector VFD.

For V/Hz control VFD, it may be 20 hz.
For vector VFD, it will be smaller! 5Hz, even 0 HZ. ( but in fact, 0 hz is not a reality for most VFDs).

Hope it helps.
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Old 05-09-2011, 03:59 AM   #8
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Originally Posted by lefleuron View Post
The motor is going to be your biggest concern. If you spend the money on an "Inverter Duty" motor, you will be fine. These come with thicker insulation on the windings. This keeps the voltage spikes from the IGBTs from burning through the first few windings like it would on a normal duty motor. These also come with bigger, more efficient cooling fans, and more then likely internal thermal over-loads that will save the motor in case it does over heat.

A regular duty motor will do the same thing, it just will not last as long and will probably overheat sooner.

Then the use matters, and how the drive is set-up comes in.

If the drive is set up for straight V/Hz, the motor will not burn up at low speed because you have kept the V/Hz ratio the same through the entire speed range. The problem with this comes at about 6 Hz or less, you lose torque very quickly if you have any at all at that point. This is because you can no longer keep the magnetizing current at where the motor needs it to be. So yes, an impeller may begin to turn on its own causing the motor to become a generator- and then trip the drive on an "over-voltage" fault. Some drives come with braking resistors, or even DC injection for braking to clear this up.


Add an encoder, or a drive with "sensor-less vector" control. this gives the drive more feedback, and you can safely lower the Hz even more. After a "motor tune" has been done, the drive will supply that motor with the full magnetizing current all the way down to zero Hz. So as soon as you start to ramp up the Hz from zero, you are starting to cause slip creating torque.


So long story short, whatever Hz that motor will run at without overheating and it still has enough torque to drive the load, that is the recommended minimum frequency for that AC dive and motor.
I agree.

But it depends on the exact products and manufacturers, for example, some products for straight V/F control, the minimum frequency inverter may be 5 hz.
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Old 05-09-2011, 09:08 PM   #9
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When a motor is controlled by a VFD, and you attempt to stop it, the motor will try to act as a generator because of the field external to it's windings. Other considerations must be thought about because it causes the drive to possibly fault out.

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