600V Inverter and 220V Ac motor

Customer wants to put 3 phase transformer beetwen 600V VFD and 220v motor. Is it going to work?

I would be more inclined to put the transformer upstream of the VFD.

karlag said:

Customer wants to put 3 phase transformer beetwen 600V VFD and 220v motor. Is it going to work?

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Why do they wish to do this? What's the reasoning behind this idea?

Personally if I'm correct in assuming (hate to do that) their intentions then the transformer would be better in front of the VFD.

That is a problem. He already bought 600v inverter and only after found the right gearmotor but this motor 220v.... I advised him to changhed inverter to 220v, but no way to convince

How big is the motor? The real question is how much power loss would you have and how will the VFD behave feeding the transformer? Not even going to mention the circuit protection you would want.

Typically VFDs can feed any 3-phase motor but what limits the VFD is how much current to output. For example a motor that is 10HP 230V 3 phase would need an equivalent 20HP 460V VFD minimum to supply enough current.

I see more headaches going to a transformer. What will your incoming power be? Either change the gear motor or the VFD IMO.

LJBMatt said:

How big is the motor? The real question is how much power loss would you have and how will the VFD behave feeding the transformer? Not even going to mention the circuit protection you would want.

Typically VFDs can feed any 3-phase motor but what limits the VFD is how much current to output. For example a motor that is 10HP 230V 3 phase would need an equivalent 20HP 460V VFD minimum to supply enough current.

I see more headaches going to a transformer. What will your incoming power be? Either change the gear motor or the VFD IMO.

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The incoming power is 600V 3ph Inverter rated 575V 3HP 3ph The motor is 2Hp 220v 3ph

I'd certainly be inclined to go with a transformer upstream of a 220V 3HP rated VFD. I'm sure those with a lot more expertise on drives can give you better information but I'd be worried about how the VFD would behave feeding a transformer, power loss, transformer protection, and speed response.
There are so many headaches that could arise if your client wants to go down this path.

Thank you! That is exactly what I am thinking about it.

Yes, it will work. We can't know if the VFD will be happy about it. It expects feedback from the motor, not a transformer. Some of the feedback info will be attenuated by the transformer.
For the most part, it should work okay. You won't know until you try. We can't give you any absolutes.
I agree with others that upstream is better.

The key question is what kind of load is the motor driving. Or, to say it another way, does the motor have to develop full torque at low speeds, say, under 20hz. If the answer to this is yes, then a load-side transformer is not going to work.

On the other hand, if the speed range is limited to, say, 30 to 60hz, and the load is a centrifugal fan or pump, the transformer idea would work ok.

But, really, at 2hp, why struggle like this. A 230VAC drive at 2 hp is probably less than $US 400 and the input side transformer would be cheaper than an output side transformer.

I'd save the 600V drive for my next 600V project and go with a 230V unit instead.

Dick,

Presuming the drive has the ampacity and the motor wire is of good quality. Couldn't he just use a custom volts/hertz curve and be done with it?

AKA set up the drive for 240 volts at 60 hz and 480 volts at 120 hz. Then do what ever speed limiting was needed?

milldrone said:

Dick,

Presuming the drive has the ampacity and the motor wire is of good quality. Couldn't he just use a custom volts/hertz curve and be done with it?

AKA set up the drive for 240 volts at 60 hz and 480 volts at 120 hz. Then do what ever speed limiting was needed?

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I have done that before to run a 240v motor with a 480v drive. We did not have much other choice and it never seemed to cause any problem.

I guess if the insulation rating is not high enough, it could cause serious harm to the windings though.

The feed is 575V to a 220V motor, as stated. So the curve would have to be larger, plus he stated the VFD was 3HP so depending on the power pull of the 2HP motor I would have to think the drive might struggle to supply enough power unless the motor doesn't pull much load.

Hey guys you are forgetting something

From wiki "

Operation of a transformer at its designed voltage but at a higher frequency than intended will lead to reduced magnetising current; at lower frequency, the magnetising current will increase. Operation of a transformer at other than its design frequency may require assessment of voltages, losses, and cooling to establish if safe operation is practical. For example, transformers may need to be equipped with "volts per hertz" over-excitation relays to protect the transformer from overvoltage at higher than rated frequency.

Knowledge of natural frequencies of transformer windings is of importance for the determination of the transient response of the windings to impulse and switching surge voltages.

gas, that's partly why the limitations on load and frequency I mentioned earlier regarding use of a transformer on the drive output.

milldrone, theoretically you can take a 600V drive and set the volts per hz curve up so the output reaches 230V at 60hz, 460V at 120hz, and 600v at 150hz. As long as the drive is speed limited to 60hz, the motor runs in its intended voltage range. As long as you weree running V/Hz and not sensorless vector, the drive would not object.

Two things make this undesireable or impossible to do. First, some drives do not have the frequency and voltage setting range needed to do the above configuration. And, second, even if you could, the input voltage determines the voltage of the drive's internal DC bus and, therefore, the height of the output pulses to the motor.

Since the peak voltage of a 600V rms wave is 600 x 1.414 = 848.4V, you can expect the DC bus and the output pulses to run at this level. Motors wound for 230VAC are not going to withstand these pulse levels without rapid insulation failure. At these low hp's, output pulse ringing is going to result in double the 848V's with leads only 100 feet long. Bottom line = rapid motor insulation failure.

The 230V motor may well be wound for 460/230V which requires that it have a better insulation system. But double 848V is 1700V which is even higher than the test voltage for a MG1 P31 inverter-rated motor so you could expect short motor life due to insulation failure under these conditions, as well.

You could, in either case above, install a sine filter in the motor leads which would take the 848+ V pulses and integrate them down from ringing square pulses to something closer to the equivalent sine wave voltage. The motor would be fully protected and you could expect normal motor life. Unfortunately, a sine filter would likely cost more than a proper 600V motor and/or a transformer combined.

So, after all is considered, at 2hp it just doesn't make much sense to cobble together a system that is not going to be reliable. Do the right thing and buy the recommended pieces. Now, if we were talking 300hp, things would be a lot different but, not this time!