Using a 440V rated motor in a 415V power supply 1

[PRincez]

I've been wondering why there's so much emphasis on using only a 415V rated motors in our plant - our power generation is 415V @ 50HZ. Many Suppliers always offer 440V as standard so we're having to knock them back. Just thought someone could enlighten me on the principle behind doing this.

Thanks

 

[jraef]

Either you ate buying your motor exclusively from Jamaican motor suppliers, or you are not asking the right questions and using suppliers with the mental agility of gnats. Motors must be bought based on voltage AND frequency together. The only place I know of that uses 440V 50Hz is Jamaica. So most likely you are simply asking for 415V motors and they offer you 440V motors, those are 440V 60 Hz motors, and they are not explaining to you why they are doing that.

Motors produce torque, speed is determined by the applied frequency. So a motor is designed to provide a certain torque value AT a certain frequency, which can be expressed as a V/Hz ratio. As long as you maintain that V/Hz ratio, or close to it, the motor produces its rated torque regardless of frequencies. That is how a VFD works by the way.

So your supply is 415V 50Hz, that means your V/Hz ratio is 8.3:1 so if you use a motor with a V/Hz ratio within 10% of that value, it will produce rated torque without detrimental over heating. A very common motor design rating that falls into that category is a 460V 60Hz motor, which has a V/Hz design ratio of 7.67:1 and therefor is less than 10% different from your supply.

Your gnat brained motor suppliers however are trying to sell you the wrong thing if their motors are truly 440V 60Hz design, because that would be too low and that means a loss of torque, which then means at the same load, an increase in slip, which means higher current and more heat. If your motors are significantly over sized for the load it may be OK, but it will take some investigation on your part.

However, a LOT of people are just plain sloppy about this. In the US, there used to be numerous voltage levels used in industry in that general range; 440V, 450V, 460V, 480V etc. each utility had their own ideas. Long ago, there was a movement to standardize and the compromised motor utilization voltage is now 460V 60Hz, because it falls within 10% of all of those. But sloppy engineers sometimes don't look into that and "hear" from someone that we use 440V, so they design their motors for that. There are some countries that still use it as well, which adds to the problems. So if your suppliers are really selling you true 440V 60Hz motors, they are idiots. But it might just be an issue of that sloppiness in description, when in fact those motors are 460V 60Hz design. If they are, they will work fine for you.

Or you have found a Jamaican motor supply enclave mahn.

"Will work for (the memory of) salami"

 

[ScottyUK]

433V and 440V were fairly common busbar voltages in the UK for the old 415V utilisation voltage. That's officially a thing of history now we're harmonised with Europe.

 

[PRincez]

Jraef: Sorry I think I mixed it up. We are mostly offered 400V/50Hz by our European Vendors and 440V 60Hz by American ones. I know this because we recently received a 440V 60Hz motor data sheet for review to be used within a compressor package. Will the same principle apply as you described above?

 

[jraef]

440V 60Hz is an old old spec that is no longer used in the US. Some people may still make it as a special, in order to satisfy legacy systems or other sloppy engineers, but the official ANSI motor utilization voltage for motors has been 460V for decades now. If someone is offering 440V 60Hz to you, it might be because someone at your end is ASKING for it, not knowing, and the suppliers are responding with oddball motors. Either that or there are some marginal motor mfrs out there where their engineers dont use modern records of specs like ANSI, NEMA etc., who have been TOLD we use "440" here, so they respond in kind. I've seen that lately, again I can only attribute that to sloppy engineering.

There are also some small countries that still have 440V 60Hz as their official distribution voltage, and although 460V motors would work for them too, again some people insist on sticking to numbers. So suppliers who serve those countries may also be stocking 440V rated motors based on that, I don't know. But if I pick up the phone here and order a standard off-the-shelf motor from a decent supplier, it will be 460V 60hz.

"Will work for (the memory of) salami"

 

[micalbrch]

Can't a 460 V, 60 Hz motor be connected to a 400 V, 50 Hz net? And a motor for 400 V, 50 Hz works at 415 V, too. Or am I completely wrong?

 

[jraef]

Can't a 460 V, 60 Hz motor be connected to a 400 V, 50 Hz net? And a motor for 400 V, 50 Hz works at 415 V, too. Or am I completely wrong?

No you are not wrong. It's just that these issues work within the margins of acceptability, but when he is offered a motor that is truly 440V 60Hz, it then falls outside of those margins. That's why I was going on about whether that really is the case or not, it seems odd to me that anyone is really quoting a 440V 60hz design as an off-the-shelf motor.

"Will work for (the memory of) salami"