Cheating a VFD

I have been working on retrofitting a skid to accomodate european voltage and make the skid CE compliant. I have been doing well so far until now. The total power on this skid is < 13 kVA, so I have been able to get away with 60 amp disconnect. I didn't put much thought into the VFDs, I assumed that when the PFlex 4/40 manual stated 380-480 VAC input 0-460 V output that I was in the clear. All the motors are rated 230/460 3 phase. My rep recommends going from single phase to three phase before adjusting the voltage on the VFD, however this doubles my amperage on all motors and puts me over the 60 amp limit. Changing the disconnect would be major surgery. I have been looking through previous threads and bus voltage seems to be the issue. Is the motor seeing this 380*2^.5 or the VFD? Is there something to setting the VFD to 87 Hz? please help.

THX

I don't understand what your problem is. If you connect the motors in Star configuration they will accept 460 Volts.
What is the new input Voltage?

I am working with 380 V 3 phase.

You can't get higher voltage out of the VFD than the input line voltage. If you need to get the motor back to 460/3/60 to keep the current down you will need to put a transformer ahead of the VFD. I am assumingthat your 380 is three phase.

I'm not sure what the rep was trying to accomplish with going to single phase. Did they explain?

Sorry if this isn't too clear. I am not trying to raise the voltage with the VFD, I am trying to drop the voltage from 380 to 220. The motors are 230/460, the available mains is 380 3 phase 5-wire. What the rep was suggesting was that if I run 1 leg w/ neutral of the 380 to the VFD, then use a vfd that allows single phase input/3phase output this would be a better option than trying to run a 220 motor via a vfd with 380 input.

You can run a 220 volt motor on a 480 volt PF40 (as long as the motor winding insulation can handle the peaks) just set parameter A88 to the maximum nameplate voltage of the motor as you intend to wire it.

Yes, the true peaks are going to be determined by the DC bus, but the "average" voltage felt by the motor is determined by the pulse width ratio and that is controllable.

Note, that to get the same amount of power available to these motors, the current is going to go up, so this may affect sizing of components between the drive and motor, and may affect the required drive size.

The motor should take the 3 phase 380 volts from the drive, the rpm will just be reduced to 1400 rpm at 50 hz. If you need more speed, then just bump the frequency up.

I think the OP thinks he has a tougher situation than most of us have picked up on. With a 380V 50hz supply and the motors all wired for 230V 60hz, the system will function but he is concerned that the 380V amperage will be over 60amps because the motor amps wired at 230V will be double that of 460V.

But, the good news here is that the VFD's will only draw from the 380V source the same kva as the motors draw out of the drive at 230V plus a couple of percent for losses. So, I think your 60amp breaker is safe.

This can be a handy little feature of VFD's. They basically convert excess voltage on the input to excess amps on the output when the output voltage is lower than the input.

The concern about motor insulation is invalid too because these motors have insulation good for 460V. It is true that the PWM pulses going to the motor will be at the 380V rectified level rather than the 230V rectified level but both are a lot lower than the 460V rectified level which the motor insulation can withstand.

So, I think the skid will be ok with the 60amp disconnect and the motors and drives will be ok too. The drive programming will be a bit tricky but otherwise no issues. And, where that single phase stuff came from, I don't know.

In the UK most new motors are "universal" and are rated for both 50Hz & 60Hz. AT 60hz they are rated for 460v nominal and at 50 Hz are 400V. The motors should be connected in star. The motor amps will be higher by 5-10 % however the total amps at 13kvA should be around 32A.
P.S. Don,t forget to change any transformer primaries to the lower voltage.
Paul

OkiePC said:

You can run a 220 volt motor on a 480 volt PF40 (as long as the motor winding insulation can handle the peaks) just set parameter A88 to the maximum nameplate voltage of the motor as you intend to wire it.

Yes, the true peaks are going to be determined by the DC bus, but the "average" voltage felt by the motor is determined by the pulse width ratio and that is controllable.

Note, that to get the same amount of power available to these motors, the current is going to go up, so this may affect sizing of components between the drive and motor, and may affect the required drive size.

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Ditto. Motor drive often run in Volt per Hz mode. In V/H mode, the output voltage is "PWMed" so that it is porportional to the output frequency. For example, when the output voltage is 480VAC, the frequency is 60Hz. You can reconfigure the motor drive so that the output voltage is 220VAC instead of 480VAC when the frequency is 60hz. This will give you full RPM without burning up your motor.

However, motor driver manufacturers do not recommend you do this because the output voltage peaks could prematurely wear out the motor wiring insulation.

What Dick said:

So, I think the skid will be ok with the 60amp disconnect and the motors and drives will be ok too. The drive programming will be a bit tricky but otherwise no issues. And, where that single phase stuff came from, I don't know.

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