Motor delta vs. Y

[NetNathan]

Actually, Depending on the machine load...you could run the complete machine or even maybe all the machines on a VFD.

 

[jraef]

Hi all - I'll get straight to the point:

Problem: Customer has Chinese machines with all motors rated for 220 or 380, but the ware house distribution system is 208.

Background: With the motors setup as delta (220), the motors seem to be running just fine. Some of the motors that are still configured as Y (380) seem to be running well except for the 'soft start' effect from the lower voltage.

Question: Can we leave the Y (380) connected motors on the 208 distribution system?

Any help will be welcomed.

Sure, as long as they are way underloaded. Anything over about 60% load with cause overheating and premature failure

So pay attention to what Dick says here (although I think he's being a bit generous at 60%).

What happens is, a reduction of the effective voltage (by virtue of the Wye connection) causes the motor torque to be reduced by the same amount. So by being connected in Wye, that effective voltage across the windings is reduced by the sq. rt. of 3 or 1/1.732 which is .58 (DickDVs 60%). The base synchronous speed remains the same because you have not changed the frequency or the number of poles, so unloaded it will "seem to be running well except for the 'soft start' effect from the lower voltage", but it's often a false sense of security. This is because worse than that direct loss of running torque, the PEAK torque, which is used by the motor to RECOVER from a step change on load, is reduced by the SQUARE of the voltage change, so the motor is now only capable of 33% (.58 squared) of its rated peak torque, and will either fail to accelerate under load, or if it manages to, will fall into very high slip under load and pull too much current, overloading the motor or often just stalling. Then if your OL relay is set too high because you use the incorrect values based on the nameplate and applied voltage, you often end up with a burned up motor. Until you have FULL LOAD on that motor, what you observe in initial testing unloaded means absolutely nothing.

The only SAFE rule of thumb then in connecting a motor designed for a Delta connection but using the Wye connection is to reduce the load on it to 33% of what it is rated for. If that's the case, you could have used a much smaller motor, so it's a fairly safe bet that it will not work.

By the way, there is no reason why you cannot use simple wire jumpers in place of those links. Just crimp some ring tongue or fork tongue terminals on little stubs of wire, that's what I do.

Secondary issue, not brought up until late: 50Hz vs 60Hz.
Motor design (rated) torque is directly related to the nameplate voltage and frequency, so the relationship can be expressed as a "V/Hz ratio". As long as you remain within +-10% of that ratio, the motor will operate within spec. So if you use a motor rated for 380V 50Hz, that ratio is 7.6 V/Hz. If you apply 460V to it, the ratio is 7.67 V/Hz, so no problem. But when connected for 230V operation (Wye), the design ratio is 4.6:1, but if you apply 208V 60Hz to it, that ratio is only 3.47:1, so you are severely under-fluxing the motor windings and they will produce less torque, slip will increase under load and the motor will over heat doing the same net work. So even if connected for the correct voltage, the increase in speed and DECREASE in torque means your motors are really capable of only at best, 80% of their rated HP, probably less. At the same time, if ANY of your motors are centrifugal pumps or fans, the increase in speed results in an increase in flow, which then, per the Afinity Laws, means that the POWER required by the load will increase at the CUBE of the speed change. So at a simple speed increase of 20% because of the 50Hz to 60Hz change, the pump or fan LOAD on that motor increases to 172% of the original design! So unless your Chinese mfr seriously over sized the motors for the load, either of these issues can present a significant risk of early failure of the motors. Given that they mis-connected the motors and appear to have mixed voltage rating willy-nilly, I would not bet on that forethought having much chance of having taken place.

Because 208V is not used anywhere but in areas that have 60Hz, it's likely that those motors are within tolerance if connected to 208V 60Hz.

Your only safe bet is to put a VFD on each and every motor (except the 208V ones), then connect them and run them at the nameplate designed voltage and frequency. Alternative 2 is to junk that Chinese junk and replace all the motors with ones rated for use here, then deal with the speed change issues separately. If there are pumps or fans, you will also have to find a way of reducing flow to the design rate.

Bottom line, it appears that your customer got what they paid for: a cheap machine designed by copycat incompetent engineers, if any engineers were even involved in the process.

Caveat Emptor!

 

[osmanmomin]

the most important follow the spec

spec ok process ok

 

[JRB]

Originally Posted by JRB
OL's should be set to 1.25% X FLA.

NO !
The overloads already have factored in some overload ability before they trip. Overloads must be set to the nameplate value. There are exceptions, for example using a standard overload for extreme heavy start conditions, but that is an exception.

Only motors rated more than 1-hp (without integral thermal protection) and motors 1-hp or less (automatically started) [430.32(C)],
must have an overload device sized per the motor nameplate current rating [430.6(A)]. You must size the overload devices no
larger than the requirements of 430.32. Motors with a nameplate service factor (S.F.) rating of 1.15 or more must have the
overload protection device sized no more than 125 percent of the motor nameplate current rating.


You also have to consider another factor: nameplate temperature rise. For motors with a nameplate temperature rise rating not
over 40°C, size the overload protection device no more than 125% of the motor nameplate current rating.


Motors that don't have a service factor rating of 1.15 or higher or a temperature rise rating of 40°C and less must have an overload
protection device sized at not more than 115% of the motor nameplate ampere rating (430.37).

 

[NetNathan]

Another consideration with OL's.....panel temperature.

 

[JesperMP]

@JRB.

I dont know the standards you mention. Is that direct quotes ?
I think you possibly mistake "sized" for "set to".
Different things.

 

[jraef]

Originally Posted by JRB
OL's should be set to 1.25% X FLA.




Only motors rated more than 1-hp (without integral thermal protection) and motors 1-hp or less (automatically started) [430.32(C)],
must have an overload device sized per the motor nameplate current rating [430.6(A)]. You must size the overload devices no
larger than the requirements of 430.32. Motors with a nameplate service factor (S.F.) rating of 1.15 or more must have the
overload protection device sized no more than 125 percent of the motor nameplate current rating.


You also have to consider another factor: nameplate temperature rise. For motors with a nameplate temperature rise rating not
over 40°C, size the overload protection device no more than 125% of the motor nameplate current rating.


Motors that don't have a service factor rating of 1.15 or higher or a temperature rise rating of 40°C and less must have an overload
protection device sized at not more than 115% of the motor nameplate ampere rating (430.37).

@JRB.

I dont know the standards you mention. Is that direct quotes ?
I think you possibly mistake "sized" for "set to".
Different things.

Jesper, this is out of our National Electric Code (NEC).

JRB,
He's right. The NEC rules have to do with SELECTION of the protection device, not how it's accomplished within a particular mfr., that differs from one mfr to another. So if you buy a eutectic melting alloy OL relay with heater elements, you follow a chart from the specific mfr that follows these guidelines, and IN that chart selection process, THEY tell you whether or not you use the nameplate FLA value itself, or a value calculated per the above. It all depends on whether or not the specific mfr. has ALREADY factored the pick-up point into that chart of not. You cannot assume yes or no.

But Jesper, being in IEC world, is used to the fact that for IEC bi-metal OL relays, they have ALL factored in the pick-up points in the design of the device already, so they all tell you to SET the dial to the motor nameplate FLA. Unfortunately, the vast vast majority of users in the US do not RTFM, so they read that section of the NEC and ASSume that they add the 1.25 factor to the setting of an IEC OL relay, and end up smoking their motors the first time there is a problem. Motor rewinders love that...

 

[rroze002]

Once again I would like to thank everyone for their input on this matter. Comments like those from jraef are very helpfull in understanding this situation.

I believe the brakedown of the problem is as follows:

- there is no uniformity in the motor ratings. Some are 380 Y, others are 220 DELTA, and most of them will be greatly derated if converted (based on my understanding).

- the difference in frequency may be responsible for the overload I mentioned earlier. The 19A motor is actually a blower and seems to be drawing too much current due to the increase in speed. I understand that a VFD would solve most of these issues, but in this case i think its more economical to replace the motors when they fail.

- many of these machines are poorly designed and the motors are underated. Today we tested using VFDs sized to the motor nameplate but ran into issues with the starting torque. The VFD would either overload or just clamp down the current.

I will try to keep this post going as more issues come up. I think that this is a good lesson and something that rearly comes up but is important to understand.

 

[JesperMP]

Yes a blower will be very sensitive to changes in frequency.
I bet it is driven via V-belts, yes ?
If so, get new V-belt pulley with a diameter to match the speed and frequency.
Dont go the VFD route.

Some are 380 Y, others are 220 DELTA

* sigh *
Comments like this makes me wonder if you understand the difference between Y and D, and how to read and understand motor name plates.
A motor marked 380 Y can be operated at 220V D no problem.

This is what you should do:
1. Register all data of all motor name plates. Take photos.
2. Register the layout of all motor terminal boxes. How many terminals are there, and how are they presently connected. Take photos.
3. Analyse all the motor/load combinations. Are some underdimensioned because of the 60Hz frequency (like the blower). If so, is there a simple fix (like the V-belt pulley). If not, you may have to change the motor or go the VFD route (for the individual motor, not the entire installation).
4. When all the loads and motor currents have been finally established, make sure that all overloads match the rated motor currents for the mode of connection (Y or D).