25 HP 3 phase motor questions windings

[IntegrityTesting]

Hello all, and thanks for your help!

I'm running a 25 HP 3 phase converter on 230V/200A service and have some questions... Initially my motor 230/460 was permanently wired high. Unknown, it was connected to single phase 230 after being spun up by a pony motor. It ran fantastic, and only consumed 3A at idle, and was quickly determined not to have the load capacity to generate a significant 3rd leg because it must have been wired high...

The motor was dropped off at a shop, and I had the standard 9 leads led out from the windings... After install back into the converter system, my new idle current is 35A!?!?!?

I expected of course DOUBLE the idle current wired at 230, or 6A. It works fine for what I am loading it for, a 10HP 3ph compressor motor... BUT I think there must be something wrong with the lead ID's....??? My 10 HP pulls about 28A/leg when wired to normal 3 ph service, but current readings connected in this converter system are about 28A for the two passed on phases (wired direct by contactors) and 15A for the generated leg... My single phase load for the entire system under load is 60A/leg or 120A... I can see 60A for the passed through load plus 15A for the generated leg plus transient idle loads… Leaving my converter idler motor consuming 45A to generate the 15A 3rd leg…

Questions:

1. Isn’t 35A/leg idling a 25 hp @230V high???

2. There is some partial winding info on the plate, I’m thinking of dropping out some windings after the load motor inrush… What are anybody’s thoughts? Which leads to drop out?

Motor Plate pic attached…

Thanks for your help!

 

[ScottyUK]

From the nameplate 67A full load current the 35A magnetising current looks about right if the motor was connected to a 3-ph source. This current is highly reactive so although you still to size cables to handle the current it isn't actually drawing much power from the supply. It is occasionally called 'wattless current' for this reason.

I'm assuming that you're a domestic / small commercial customer who isn't metered separately for power and reactive power in the way that bigger industrial facilities are. You might want to google 'power factor' if you want to understand this a bit more - there are plenty of good descriptions already written which have diagrams included.

Now that variable speed drives have become a commodity item and prices have fallen so dramatically in the past decade or so, phase converters are increasingly hard to justify because of their high cost, limited capability and inflexibility, and indifferent quality of the synthesised phase. You might want to take a closer look at VFD technology.

 

[IntegrityTesting]

Hi ScottyUK,

Thanks for your help!

I'm of course running this 3 phase motor on 2 phase power with obvious 180 degree phases... I understand that hooked to a proper 3 phase source, it should draw the 67A under full load... It just seems like the 35A to turn this thing with absolutely no load on it should be far less than 35A ... It was running the same way wired for 460 hooked to 230 and only pulling 3A

By magnetising current, do you mean inductive current readings? If so, yes that's how I'm measuring...

You're right about being a small residential customer... I'm only familiar with power factors on motor nameplates... I'll do the suggested research! Thanks...
A VFD may be the way to go, as they are superior in many ways...

Thanks again!

 

[ScottyUK]

Hopefully you've read up a bit on power factor and the effect of phase shift between current and voltage.

The magnetising current is a concept used in a simple electrical model of the motor. Magentising current is required to establish the magnetic field in the motor. The windings are highly inductive, which means current flowing in them is displaced almost 90° relative to the voltage applied, and this results in the very low power consumption. Magnetising current is relatively constant if the supply voltage is constant in voltage and frequency. The other current used in the model is torque-producing current which is in phase with the voltage and this varies from a very low value at no-load up to a maximum at full load. The current you actually measure in the conductors in the vector sum of these two currents.

 

[IntegrityTesting]

I Scotty

I read up a little bit, and being an ME helped with my vectors! I'm getting this a bit more, but still have a problem with the idle current jumping by a factor of 10 when all I did was have the motor rewired for 1/2 the voltage.... Luckily, the power meter for the main is right next to this equipment, and I can easily shut down all other loads to this box...

I posted another nearly identical question with less fluff info...

Thanks!

Ed