alfonso1900
Member
I have a 480 V VFD to run a 208 V pump. Sugestions??
480 v vfd, 208 v pump
- Thread starter alfonso1900
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petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- Semi-retired engineer
buy a new vfd
buy a new pump with a 480v motor
replace the pump motor with a 480v motor
if you have 208v coming into the vfd you are probably out of luck using what you have short of a transformer to boost the voltage upto 480v, although some vfds will allow you to set the incoming voltage as low as you want, but they are not real common.
if it has 480V coming into the vfd, you might be able to tweak the vfd settings so it peaks out at 208v with the right v/hz setting.
buy a new pump with a 480v motor
replace the pump motor with a 480v motor
if you have 208v coming into the vfd you are probably out of luck using what you have short of a transformer to boost the voltage upto 480v, although some vfds will allow you to set the incoming voltage as low as you want, but they are not real common.
if it has 480V coming into the vfd, you might be able to tweak the vfd settings so it peaks out at 208v with the right v/hz setting.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Technically it is possible to use a transformer between a VFD and a motor, however I do not recommend if either the motor or the VFD can be replaced instead.
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
Just as a point of experience, the widest range of input voltage I have ever seen is 2:1, making it 240V for a 480V VFD. Anything less and the VFD will trip out on UV.
All other points are well founded, but you (OP) have not stated what your incoming voltage is. As others have said, if you have a 208V or even a 240V supply, replace the VFD if you can, it s not worth the effort to try to use it. By the way, we are assuming you know that the VFD would have to be rated fr the proper CURRENT at 208V, not HP.
If you have 480V input and a 480V drive, but a 208V motor that cannot be strapped for a higher voltage, do not succumb to the temptation to program the VFD output to max out at 208V. Although you can technically do that, it is REALLY REALLY bad for the motor windings. If the supply is 480V, the DC bus level is 679V, which means each pulse going to the motor is at that level. The VFD can adjust the PWM pattern to deliver only 208V RMS, but that will still be made up of short little 679V DC pulses, probably exceeding the insulation level of the motor windings and causing premature failure.
- Location
- Massachusetts
So am I understanding this correctly?
A VFD controls the RMS output voltage by just changing the length of time the DC bus is connected to the output?
winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
Pretty much.
Generally the DC bus is always connected to the output. But remember that the 'DC bus' consists of both the + bus and the - bus. If all of the output terminals are connected to the + bus, then there is no voltage between them. Similarly if they are all connected to the - bus. Voltage is seen _between_ the output terminals when some are + and others are -.
The drive is continually switching the outputs between the + and - bus. When the drive is actively synthesizing '0V', what really is happening is that the output is continuously switching between + and -, but all terminals are switching at the same time and in phase with each other, so there is no net voltage _between_ the terminals.
When the drive is synthesizing a different voltage, then the transitions between + and - are adjusted on the different phases to get the desired average phase to phase voltage.
This means that the insulation stress placed on the winding relative to frame ground is always going to be related to the DC rail voltage, no matter what the synthesized output voltage.
(Note: I am describing the most common technology. There are approaches being researched that have more 'DC bus' voltages and different switching approaches.)
-Jon
- Location
- Massachusetts
Thanks Jon, I now know more about how VFDs work then before. :smile:
Pretty much.
The on to off ratio is also varied over a half cycle to get close to sinewave current in the motor.
Provided the VFD is rated at the motor current, there is no problem.
You would need to program the drive to get the right V/f ratio but that generally isn't too complicated.
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
To help illustrate the point:
Notice that in both cases, the magnitude of the voltage pulses is the same.
This is what that becomes in the inductive environment of the motor.
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