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Induction Motor By Dc Source


eidali

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You cannot connect an induction motor directly to Dc, it will just freeze up.

 

You can get an electronic device called an "inverter" that takes the DC supply and creates a simulated AC output which can be used to run the motor.

 

An inverter cannot create voltage, therefore if you have a 380V motor, your DC voltage must be approximately 540VDC to feed into the inverter. The reason why the DC must be higher is that AC voltage is measured in what is called RMS voltage, a sort of average between the peak and zero as it alternates back and forth. The DC must be capable of supplying the peak in order to have the RMS average come out correct. The RMS voltage is 70.7% of the peak voltage, so if your motor needs 380V, the peak must be 380/.707 = 537.48V.

"He's not dead, he's just pinin' for the fjords!"
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Welcome eidali,

please explain better your idea, adding more data like power and application type.

 

Probably your ac motor is a 220/380 V motor, using the 220 V windings you can use a DC voltage of about 300 VDC or something less, if you don't need to get the full power.

 

If you can accept also a speed lower that nominal, you can reduce further your DC Voltage. For example, you could supply about 100 VDC to get about 15 Hz at the output.

 

Regards

Mario

Mario Maggi - Italy - http://www.evlist.it - https://www.axu.it

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If you can accept also a speed lower that nominal, you can reduce further your DC Voltage. For example, you could supply about 100 VDC to get about 15 Hz at the output.

 

Mario

 

Mario,

 

Perhaps you have forgotten the consequence of UnderVoltage Trip function ? which typically monitors the d-c bus voltage, and when it falls below a preset value (based upon 10-15% low mains) ... initiates a TRIP fault that will not reset until the bus voltage is restored to proper value.

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Yes, typically the range of acceptable DC bus voltage on a VFD is -30 to -35%, although I have seen a few new models that now go as low as -50%. But I have never seen one lower than that. If it is indeed a 220/380V motor, then that makes the necessary DC input to attain 220VAC output only 311VDC, although it will require twice as much current. You will still need the same amount of battery power regardless of how you configure it.

 

However, Mario has a point. If you can live with a lower output speed, you can supply a lower voltage, i.e. if you only had 250VDC, you could run that motor through a VFD at 40Hz. The trick is, the motor's V/Hz ratio must be maintained. So on your motor, that ratio is 380/50=7.6V/Hz. If, as Mario says, your motor can be reconfigured for 220V, then the ratio is 220/50=4.4V/Hz. So if you were to program a 220V VFD to never deliver more than 40Hz, the voltage required would then be 40*4.4=176VAC, which means 176/.707=250VDC input.

 

Don't know how useful that is, but there you go.

"He's not dead, he's just pinin' for the fjords!"
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Dear All,

 

Can a three phase UPS not full fill our requirement? I think “Eidali” just want to accelerate the motor and a UPS, which may accept the available battery voltage, will fulfill his requirement.

Is it correct or a goofy idea?

 

"Don't assume any thing, always check/ask and clear yourself".

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Dear All,

 

Can a three phase UPS not full fill our requirement? I think “Eidali” just want to accelerate the motor and a UPS, which may accept the available battery voltage, will fulfill his requirement.

Is it correct or a goofy idea?

 

Hi AB2005, good to see thinking outside the square but a UPS is actually a very similar device to a VSD in that it converts AC to DC and then DC to AC. The simplistic difference is that the UPS stores energy by some method (batteries, supercaps, flywheel etc) and the VSD has motor control capabilities.

All the same issues apply in addition to the problem that the UPS cannot supply the variable frequency that the VSD can to allow a ramped start for the motor or to operate at the lower voltage as described earlier. The voltage source will always need to match the required output.

 

Ken

An expert is one who knows more and more about less and less until he knows absolutely everything about nothing
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Corrrect. The UPS will have a very low inrush capability and lacks the ability to ramp the motor, so the size of the UPS will need to be much larger than a VFD. The batteries would be the same either way.
"He's not dead, he's just pinin' for the fjords!"
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eidali,

can you supply more info on your application like power and so on?

 

If your appplication is for a three-phase 1 kW motor, it is a non-sense to discuss about use of UPS. There are no three-phase UPSs of so small power.

 

jOmega,

about undervoltage trip protection, I never wrote to use a standard inverter.

Regards

Mario

Mario Maggi - Italy - http://www.evlist.it - https://www.axu.it

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jOmega,

about undervoltage trip protection, I never wrote to use a standard inverter.

 

So Mario, what non-standard" inverter did you have in mind ? Is such an item commercially available ? Can you supply the name of a designer / manufacturer of such hardware that Edaili and others might consider ?

 

Grazie

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However, Mario has a point. If you can live with a lower output speed, you can supply a lower voltage, i.e. if you only had 250VDC, you could run that motor through a VFD at 40Hz. The trick is, the motor's V/Hz ratio must be maintained. So on your motor, that ratio is 380/50=7.6V/Hz. If, as Mario says, your motor can be reconfigured for 220V, then the ratio is 220/50=4.4V/Hz. So if you were to program a 220V VFD to never deliver more than 40Hz, the voltage required would then be 40*4.4=176VAC, which means 176/.707=250VDC input.

 

 

Jeff, it would seem that your argument neglects the SMPS that is run off the dc bus ..... the dc supplies for running the logic, gating, I/O, etc. are dependent upon the SMPS .... The threshold at which the SMPS crapsout must also be considered.

 

Also, consequent to the V/Hz falling off..... the amp draw for a given load would increase, possibly resulting in an overcurrent trip .... or an EOL trip. Increased ripple current on the dc bus could also be a consequence .... which could induce a phase loss trip ... You see, supplying the dc bus from a battery source or an external dc source, doesn't automatically disable some of the designed-in protective functions/features.

 

And how do you accomplish the coordination required for Fault Mode; i.e., when there is a short circuit on the output or if there is a failure of an output device. How does the external dc supply get protected; fusing ? disconnect ? ... etc ... Without which a catastrophic melt-down is virtually a certainty under output short-circuit fault state.

 

Have you considered how many 12v or 24v DC batteries would be required for such a task ?

 

How about going hi-tech ..... and use a fuel cell to supply the dc ....

 

Won't be long before technology makes it feasible to replace the batteries in UPS systems with fuel cells.

 

And won't that be a welcome change.

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A mind like a steel trap. Nothing escapes you!

 

hoinestly, the closest I have ever come to doing this was to add abattery bank to Toshiba G3 VFDs for oil well rod pumps. The batteries were added to provide braking in the event of a power failure (the application was too complicated to go into here). It worked, but it ended up impractical for other reasons anyway. Now that you mention it, I never considered the fault mode at that time, so maybe it's a good thing it never flew!

"He's not dead, he's just pinin' for the fjords!"
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Dear jOmega,

it seems that you are considering only standard VFD, not other inverters for UPSs or wind generators.

All technical problems have a solution, first to all we have to know what are the needs.

 

Special inverters are very different in power and performances, at this point we don't know enough details for this specific application. Probably for only one piece (I think that he need only one piece, not sure) will be not convenient to buy a special inverter.

 

Regards

Mario

Mario Maggi - Italy - http://www.evlist.it - https://www.axu.it

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