Step-down Transformer for 420 V motor

[overkill94]

I have a client who is asking if they can use an undersized transformer to supply their motor. It's a 30 HP motor rated for 420 Volts, but the transformer they want to buy is only 30 kVA (480 Volt primary, multi-tap secondary). I can't find anything in 508A or the NEC that would prevent them from being able to use an undersized transformer as long as the overcurrent protection and wiring are sized correctly although I'm guessing they might get some overheating and nuisance tripping issues. Am I missing something?

 

[GoldDigger]

I have a client who is asking if they can use an undersized transformer to supply their motor. It's a 30 HP motor rated for 420 Volts, but the transformer they want to buy is only 30 kVA (480 Volt primary, multi-tap secondary). I can't find anything in 508A or the NEC that would prevent them from being able to use an undersized transformer as long as the overcurrent protection and wiring are sized correctly although I'm guessing they might get some overheating and nuisance tripping issues. Am I missing something?

Is there some reason that they cannot or will not use buck transformers instead, which would be even smaller?

 

[overkill94]

Is there some reason that they cannot or will not use buck transformers instead, which would be even smaller?

They've been pinching pennies at every step so I'm guessing they found a good deal on this one.

 

[iceworm]

.... Am I missing something?

Only thing I can think of is 110.18 (2014). They may wish to enclose the transformer

ice

 

[iceworm]

I have a client who is asking if they can use an undersized transformer ...

Curiousity Q: Who is asking? their electrician? engineer? accountant?

ice

 

[overkill94]

Curiousity Q: Who is asking? their electrician? engineer? accountant?

ice

He calls himself a "technical solutions consultant" so he seems to be somewhere in between an electrician and an engineer. The company who owns the equipment hired him to facilitate their move to a new facility and I'm the third-party inspector who has to approve all the changes they made to get their equipment up to code.

 

[iceworm]

.... Am I missing something?

Okay - not being facetious this time. Just design issues - but only if they are interested in it running. If they are okay if it doesn't run then the circuit is okay.

For the following example I'm using 480V as a per-unit. On the 420V side, the currents will coorespondingly (is that a word?) scale higher.

Table 430.250 motor FLA = 40 A. LRA is ~ 240A

Transformer primary FLA = 36A. 125% x 36 = 45.1A round up to 50A. Normal motor CB is 250% x 40A = 100A

240A LRA on a 50A CB - hopefully it will trip regularly on startup.

Note on CB size: Yes, one can select a transformer Primary CB at 250% of Xfm FLA. If you do, then the secondary must be protected at 125%. Either way, the cb scales to 125% xfm primary FLA.​

Then there is the voltage drop through the transformer. Figuring the xfm %Z = 2% to 5%, and the LRC is 6.7 X xfm FLA, the VD has to be horrendous. I didn't figure it, but you can. Suggest using a pf of .3.

Maybe is it is lightly loaded, it will start and run.

One other question: Considering the really odd motor namplate voltage is 420V, is the motor also 50HZ? That could hurt - it will definitely try to overspeed.

He calls himself a "technical solutions consultant"... I'm the third-party inspector who has to approve all the changes they made to get their equipment up to code

seriously - heart felt condolences

So this word, "approve", is that:
1. You promise the installation will work?
or
2. You promise there is no legal consequence? But, you make no guarantees concerning it working, or eventually (sooner rather than later) letting the smoke out of the motor or transformer.

Good luck with this one.

ice

 

[overkill94]

Thanks ice!

I have to ensure that everything is safe and up to code, but I also have to test the equipment so if it can't run properly then I can't certify it.

I'm pretty sure the motor's not running to its full capacity most of the time since for some reason they have an undersized overload on the circuit (set at 18 Amps) that apparently hasn't been tripping. I still have my doubts that it will run properly with the undersized transformer, but I'm just wondering if I have any code reasons to disallow the installation.

 

[GoldDigger]

Thanks ice!

I have to ensure that everything is safe and up to code, but I also have to test the equipment so if it can't run properly then I can't certify it.

I'm pretty sure the motor's not running to its full capacity most of the time since for some reason they have an undersized overload on the circuit (set at 18 Amps) that apparently hasn't been tripping. I still have my doubts that it will run properly with the undersized transformer, but I'm just wondering if I have any code reasons to disallow the installation.

The NEC does not require that anything but a fire pump actually work, but it does put limits on what OCPD you can use for a particular size transformer. As long as the installation meets them, you can approve it.

@iceworm. There is such a word, correspondingly.
IMHO "coorespondingly" refers to the feedback you get when petting or feeding a pigeon. :angel:

 

[ron]

If the load will be less current than the transformer nameplate (because they are running the motor at a low electrical load on the curve), then it should be fine.

You can't use the transformer if it isn't within its nameplate ratings, which it was listed with.

 

[iceworm]

... IMHO "coorespondingly" refers to the feedback you get when petting or feeding a pigeon. :angel:

Ahhh.... Thancoo :slaphead:

ice

 

[iceworm]

... You can't use the transformer if it isn't within its nameplate ratings, which it was listed with.

I guess. It seems this should fall under the, " I don't need a code to tell me it's okay to build it so that it will run reliably"

However, considering some of the stories on here, it's apparent I am lucky enough to hang around a better class of dumpsters than most.

ice

 

[Jraef]

Never heard of a "420V" motor, more likely its a 460V motor, the 420 has to do with what took place right before he read the nameplate...

At best, it might possibly be a foreign source motor, in which case it is 420V 50Hz. If that's the case, do NOT buy a transformer, just hook it up at 480V. It will spin 20% faster, but that was going to happen no matter what if they bought a 50Hz motor.

No way a 30HP motor will start on a 30kVA transformer without using a VFD to power it. If it's Across-the-Line, the transformer needs to be at least 90kVA (so 112.5). With a soft starter, I'd say 75kVA, maybe a 50kVA if they can live with some voltage drop at start up. If they use a VFD, it could solve both problems, IF that really is a 420V 60Hz motor.

 

[Jraef]

A brief interlude with Google confirms that it is apparently common for IEC motors for OEM machines to be made as 380-420V 50Hz, likely for the same reason NEMA mfrs make 208-230V motors, because they have to deal with a wide range of systems available in their market.

So assuming the middling voltage design is 400V, that means the motor is going to produce design torque at a V/Hz ratio of 8:1 (400/50). If you use 480V 60Hz, it's the same, the motor will be fine, other than spinning 20% faster. You never said what it was for, but as long as it isn't a centrifugal pump or fan, you are probably OK. But again, if that is what the nameplate says, do NOT use a transformer. That will give you 420V 60Hz, with a V/Hz ratio of 7:1. The motor will not only spin faster anyway, it will also drop to about 76% of peak torque, which is more likely to be problematic and lead to higher slip, more current draw, higher temperature and earlier death.

 

[GoldDigger]

And that (Jraef's solution) will cost even less than the undersized transformer, namely zero! And be easier to install too!

 

[overkill94]

If the load will be less current than the transformer nameplate (because they are running the motor at a low electrical load on the curve), then it should be fine.

You can't use the transformer if it isn't within its nameplate ratings, which it was listed with.

Is that something I can just measure or do I have to go by the nameplate rating of the motor?

I guess the one worry with the transformer is overheating - I also take a temperature rise reading on all relevant components so I guess if it's running too hot I could ding them on that as well.

 

[overkill94]

A brief interlude with Google confirms that it is apparently common for IEC motors for OEM machines to be made as 380-420V 50Hz, likely for the same reason NEMA mfrs make 208-230V motors, because they have to deal with a wide range of systems available in their market.

So assuming the middling voltage design is 400V, that means the motor is going to produce design torque at a V/Hz ratio of 8:1 (400/50). If you use 480V 60Hz, it's the same, the motor will be fine, other than spinning 20% faster. You never said what it was for, but as long as it isn't a centrifugal pump or fan, you are probably OK. But again, if that is what the nameplate says, do NOT use a transformer. That will give you 420V 60Hz, with a V/Hz ratio of 7:1. The motor will not only spin faster anyway, it will also drop to about 76% of peak torque, which is more likely to be problematic and lead to higher slip, more current draw, higher temperature and earlier death.

Wouldn't this setup still be considered non-conformant since the component (motor) is outside the +/- 10% voltage range of the supply?

I figured they would just replace the motor but for whatever reason that's not an option.

 

[Jraef]

Wouldn't this setup still be considered non-conformant since the component (motor) is outside the +/- 10% voltage range of the supply?

I figured they would just replace the motor but for whatever reason that's not an option.

Only if the design frequency is the same as the applied frequency. What really counts is the V/Hz ratio being +-10%. That's how VFDs work too, they change voltage and frequency TOGETHER so as to maintain torque as speed changes. You are not trying to change the speed here, but if your motor design frequency is lower than your applied frequency, you increase the voltage to maintain it. It's no coincidence by the way that the ratios are the same, it allows IEC motor mfrs to sell their motors here, and NEMA mfrs to sell motors elsewhere in the world.