Oversizing motor conductors

[don_resqcapt19]

But if only the size of its feeder conductors is increased, this would solve the lights dimming problem.

Often the dimming is from the votlage requlation of the supply transfomer and changing the wiring will not have any effect on the dimming lights.

But this may impact on the induction motor stability.

Assuming that the conductor has adaquat ampacity for the running load, the votlage drop on starting, assuming it is not so great to prevent the motor from starting, will not have any effet on the motor stability.

 

[affordable]

electrical maintenace guy

electrical maintenace guy

Thirty years experience with many other knowledable tricians and have never solved this problem.

 

[kwired]

But if only the size of its feeder conductors is increased, this would solve the lights dimming problem.

But this may impact on the induction motor stability.

If voltage is dropping at the transformer it does not matter how large the motor conductors are, the transformer is too small and the lights are also going to see the drop and will dim anyway.

 

[G._S._Ohm]

208 volt 46.2 amp air compressor motor
owner does not like the slight dimming of the lights when the motor starts.

Using a wall oven and a 4-1/2 digit DVM I measured the source impedance at my load center to be about 20 milliohms so at my house a 46A load would drop the voltage about a volt, and a half volt on the 120.
Incand brightness depends on the V^(3.5) so it's (119.5/120)^3.5 = 99% of the no load brightness.

The perceived brightness is not the same as this, but I still have to wonder why your source impedance is so high.

 

[don_resqcapt19]

Using a wall oven and a 4-1/2 digit DVM I measured the source impedance at my load center to be about 20 milliohms so at my house a 46A load would drop the voltage about a volt, and a half volt on the 120.
Incand brightness depends on the V^(3.5) so it's (119.5/120)^3.5 = 99% of the no load brightness.

The perceived brightness is not the same as this, but I still have to wonder why your source impedance is so high.

The voltage dip and dimming lights is on the starting of the motor...he would be looking at about 300 amps, not 46.

 

[petersonra]

Yes, I mentioned VFD's, but failed to mention that you can't just put one on any unit, as it can introduce other problems - the unit needs to be designed to be run at variable speeds in order to use one. If it is a reciprocating compressor, it likely is not going to last if run at speeds below its rated speed, short term acceleration excluded, which is cheaper to do with the soft starter.

It is not unheard of to use small VFDs as what amounts to soft starts. We have had customers that put 200HP VFDs on motors when they really just needed a soft start.

Happens a lot more than you might think.

 

[G._S._Ohm]

The voltage dip and dimming lights is on the starting of the motor...he would be looking at about 300 amps, not 46.

My bad.

300 x 0.02 = 6v.
(202/208)^3.5 = down to 90% brightness.

Soft start seems much easier to implement than trying to reduce the source impedance of PoCo. Maybe there is some pneumatic/mechanical way to ramp up the load on the motor: same effect.

 

[Haji]

Often the dimming is from the votlage requlation of the supply transfomer and changing the wiring will not have any effect on the dimming lights.

But still the possibility that the supply is strong and the feeder conductor size is not adequate exists and is worth being checked up.

Assuming that the conductor has adaquat ampacity for the running load, the votlage drop on starting, assuming it is not so great to prevent the motor from starting, will not have any effet on the motor stability.

Sorry, you are not correct. The induction motor characteristic has stable and unstable operating region and for variable torque load, the induction motor may operate in the unstable region when its supply conductors have high resistance.

 

[broadgage]

Or what about a bit of lateral thinking, and simply accept the voltage drop.
To avoid obtrusive dimming of lamps, use only flourescent lamps on electronic ballasts that tolerate voltage drop well.

To change all/most of the fixtures or ballasts will be an expensive job, but could well be justified on energy saving grounds, or if the existing lighting is near the end of its economic life.

 

[kwired]

I have seen more cases of voltage drop causing light dimming when starting motors to be because of either small capacity POCO transformers, long runs of service conductors or both, than I have seen undersized premises conductors to be the main cause, especially on residential and small commercial installations.

 

[don_resqcapt19]

...
Sorry, you are not correct. The induction motor characteristic has stable and unstable operating region and for variable torque load, the induction motor may operate in the unstable region when its supply conductors have high resistance.

We are not talking about a system that is so weak that it cannot supply the normal motor starting and running current. We are talking about a system that experiences a small voltage drop when the motor is started. That will not have any effect on stable or unstable motor operation.

 

[Haji]

We are not talking about a system that is so weak that it cannot supply the normal motor starting and running current. We are talking about a system that experiences a small voltage drop when the motor is started. That will not have any effect on stable or unstable motor operation.

If the system is so weak that it cannot supply the normal motor starting and running current, the induction motor woud not start at all.

The voltage drop may be small, but still it causes objectionable dimming of lights.

 

[iwire]

If the system is so weak that it cannot supply the normal motor starting and running current, the induction motor woud not start at all.

So you just told us if there is insufficient power to start the motor it will not start.

Thanks, but the motor is starting and running so why are we talking about things that are not an issue?

The voltage drop may be small, but still it causes objectionable dimming of lights.

You have just repeated what was clear in the opening post and as Don mentioned increasing the motor circuit conductors would only make the problem worse.

 

[Haji]

the motor is starting and running so why are we talking about things that are not an issue?

The issue is don appears to be suggesting to use supply conductors with sufficient resistance to facilitate motor starting and presumably to reduce dimming of lights on motor starting. One objection to such use is the impact on induction motor stability as already explained.

You have just repeated what was clear in the opening post and as Don mentioned increasing the motor circuit conductors would only make the problem worse.

Don talked about motor branch conductor and I talked about motor feeder conductor. The voltage drop in each has different effect.

 

[don_resqcapt19]

The issue is don appears to be suggesting to use supply conductors with sufficient resistance to facilitate motor starting and presumably to reduce dimming of lights on motor starting. One objection to such use is the impact on induction motor stability as already explained.

I said the use of a larger motor circuit conductor could make the dimming problem worse. I said that the votlage drop on the system acts like a reduced votlage starter and limits the motor current. I never suggested using conductor smaller than the code requires for the motor.

Don talked about motor branch conductor and I talked about motor feeder conductor. The voltage drop in each has different effect.

Yes they have different effects. The voltage drop on the motor circuit conductor only effects the motor. The voltage drop on the feeder and the supply system effects both the motor and everything else connected to the system. In this case, increasing the size of the motor circuit conductors will likely increase the starting current and the voltage drop on the rest of the system. Assuming the the feeder is substantially larger than the motor circuit conductor, an increase in size of the feeder will likely have little effect. It is my opinion that most of the voltage drop that is causing the lights to dim would be from the utility transformer and not the feeder conductors, unless there is some type of poor connection on the system.

 

[Haji]

I never suggested using conductor smaller than the code requires for the motor.

In earlier post you stated without reference to the code.

Smaller conductors actually create a limited form of reduced voltage starting.

But now you added code reference. The picture is now clear.

most of the voltage drop that is causing the lights to dim would be from the utility transformer and not the feeder conductors,

Replacing the transformer with a higher size one may not be practical. The most practical solution to the dimming of lights during motor starting is to reduce the starting current of the motor.

 

[iwire]

Replacing the transformer with a higher size one may not be practical. The most practical solution to the dimming of lights during motor starting is to reduce the starting current of the motor.

There is no way any of us can know which is more practical with the little info we have about the installation.

 

[Haji]

There is no way any of us can know which is more practical with the little info we have about the installation.

You may not be right, sorry, because my statement is a quote taken from a reliable book from CRC publishers 'Electric Motors' by lead editor Hamid.

 

[iwire]

You may not be right, sorry, because my statement is a quote taken from a reliable book from CRC publishers 'Electric Motors' by lead editor Hamid.

Interesting as that book would not know the specifics of the installation either.

 

[Haji]

Interesting as that book would not know the specifics of the installation either.

Yes. You are correct in that the book would not know the specifics of the installation either.
But it used the superlative 'The most' i.e the most practical solution is to reduce the motor starting current. In other other words a resistance or auto transformer transformer or star-delta starter instead of DOL starter is to be used.