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!