Voltage Drop??

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jahilliard

Senior Member
My question is...when testing an active circuit (under load) for Voltage Drop, are you able to test the voltage at any part along the circuit and it accurately represent any voltage drop present. I imagine there are a million variables so I will provide a realistic example....3 phase 230V motor fed from a 3 phase 240V OD circuit. If voltage is tested at 250' of a total 500' long circuit, will the voltage shown accurately represent the voltage at the motor?
 

jahilliard

Senior Member
Thank you for the response. My understanding is that its "like" the flow of water in a conduit..the water flow will be consistent throughout, although that flow may be effected by the overall length of the conduit and the pressure at the source. So if the length slowed the flow due to excessive distance, it would consistently effect it throughout...in other words the water (electrons) wouldn't slow down in the middle and speed back up at the source. Is that not an accurate understanding?
 
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Thank you for the response. My understanding is that its "like" the flow of water in a conduit..the water flow will be consistent throughout, although that flow may be effected by the overall length of the conduit and the pressure at the source. So if the length slowed the flow due to excessive distance, it would consistently effect it throughout...in other words the water (electrons) wouldn't slow down in the middle and speed back up at the source. Is that not an accurate understanding?

You have 250 @ start and 225 in the center, you will not have 240 at the end. More likely the 200 as Drc suggests.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
Thank you for the response. My understanding is that its "like" the flow of water in a conduit..the water flow will be consistent throughout, although that flow may be effected by the overall length of the conduit and the pressure at the source. So if the length slowed the flow due to excessive distance, it would consistently effect it throughout...in other words the water (electrons) wouldn't slow down in the middle and speed back up at the source. Is that not an accurate understanding?
There's a difference between voltage and current. To use the water analogy, you have to think of voltage as pressure and current as flow rate. Voltage, like pressure, drops as you move away from the source, but the current, like flow, is consistent throughout the system.

To stick with electrical theory, think of the conductors as long, low-value resistors. As noted above, with consistent conductor conductivity throughout the circuit, the voltage drops linearly along the conductor length. The voltage drop is a product of resistance and current.

Simple Ohm's Law applies.
 

broadgage

Senior Member
Location
London, England
Or to put it another way, the electrical current, not the voltage, is somewhat comparable to the flow of water along a pipe.

If the electric motor draws 10 amps, then this 10 amps may be measured at the breaker, at the motor, or at any point in between.
The voltage will reduce along the cable and is not constant. But the current IS constant.

(to be pedantically accurate, the current at the beginning of the circuit will be very slightly greater than the current at the load. This is due to leakage and capacitive coupling along the cable. The difference is normally minute and is much too small to detect with most electricians test equipment.
For a very long 10 amp circuit, you might get 10.001 amps at the origin and 10.000 amps at the load. Since a typical meter that reads "10 amps" in fact means at best anything between 9.98 amps and 10.02 amps, a difference of 0.001 amps is irrelevant.)
 

jahilliard

Senior Member
With the knowledge that amperage is consistent, would you be able to determine excessive voltage drop on a motor by evaluating the current? For example if I have the 230V 15Hp motor and the amperage reads 38 Amps (42A Nameplate FLA), would that be a fair indicator that there is not likely a voltage drop significant enough to damage the motor?
 
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
EC - retired
With the knowledge that amperage is consistent, would you be able to determine excessive voltage drop on a motor by evaluating the current? For example if I have the 230V 15Hp motor and the amperage reads 38 Amps (42A Nameplate FLA), would that be a fair indicator that there is not likely a voltage drop significant enough to damage the motor?
Is the motor setting there idling with no load, or at full load? How long does it take to get the motor up to speed with it fully loaded? Is the time similar to a motor 20’ from the source?
The amperage of a motor is not consistent. You have to deal with starting current.

My opinion is NO, but the # of times I’m wrong is significant.

I asked at a CE class what amperage I should use for a motor when determining VD. Their answer was 1.25 * FLA (of the tables). Seems small to me when typical startup can be six times that.
 

jahilliard

Senior Member
Assume the motor has been running at a consistent full load for more than four hours. Assume the load does not fluctuate, it is consistent.
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
181109-0759 EST

jahilliard:

If everything is constant, load, source voltage, line impedance, temperature, motor is at speed, motor current is below rating, and steady state conditions are reached, then average motor life time will not be hurt by voltage drop no matter how great the drop is. How you got the motor started, and how long it took to get there is a different matter.

If you are at your consistent conditions and below full load current, then power dissipation in the motor is less than the design value and temperature rise in the motor is less than design value. If ambient is below design value and expected thermal transfer is per design, then no problem.

Power loss (temperature rise) is from I^2*R.

.
 

jahilliard

Senior Member
All aspects are consistent with very minimal variables. In this case there are two other motors present at about one quarter of the distance to the furthest motor and all appear to start and reach full load/RPM within the same amount of time. What would be considered a marginal difference to start and get the motor to full RPM? Is the time exposed to the start in rush extremely minimal...say .5 seconds etc? Motors and motor loads seem to have so many variables that it's interesting to learn and understand more. I appreciate every ones time and I encourage any additional feedback or recommendations in regards to this thread.
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
191109-1014 EST

jahilliard:

You need to study a lot of basic theory --- physics and electrical.

I suggest that motors for the most part do not exhibit inrush current, but rather what many call inrush is really motor starting current. Motor starting current can last for a substantial amount of time, like many seconds. Its duration is a function of available torque and mechanical load. Inrush for the most part would occur for about 1/2 cycle.

.
 

retirede

Senior Member
Location
Illinois
191109-1014 EST

jahilliard:

You need to study a lot of basic theory --- physics and electrical.

I suggest that motors for the most part do not exhibit inrush current, but rather what many call inrush is really motor starting current. Motor starting current can last for a substantial amount of time, like many seconds. Its duration is a function of available torque and mechanical load. Inrush for the most part would occur for about 1/2 cycle.

.

I would suggest that the instantaneous starting current at T=0 is indeed inrush and is independent of load.
Once rotation begins, assuming it does begin to rotate, I would term as starting current and it is very load-dependent both in magnitude and duration.
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
181109-1535 EST

retirede:

Can you instantaneously change the current in an inductor? No. If the current in a motor at t=0- is zero, then is there an inrush current at t=0+? Where t=0 is when voltage is applied to the motor?

In the first half cycle of applied power to a motor is there any large peak current into the motor other than defined by the torque load? I have not observed any. Residual flux in a motor core is not generally much, and thus core saturation at startup isn't much, and thus, not much inrush current.

See my thread at
http://forums.mikeholt.com/showthread.php?t=174880&highlight=motor+startup+current
for plots of motor starting currents.

.
 

retirede

Senior Member
Location
Illinois
Voltage Drop??

181109-1535 EST

retirede:

Can you instantaneously change the current in an inductor? No. If the current in a motor at t=0- is zero, then is there an inrush current at t=0+? Where t=0 is when voltage is applied to the motor?

In the first half cycle of applied power to a motor is there any large peak current into the motor other than defined by the torque load? I have not observed any. Residual flux in a motor core is not generally much, and thus core saturation at startup isn't much, and thus, not much inrush current.

See my thread at
http://forums.mikeholt.com/showthread.php?t=174880&highlight=motor+startup+current
for plots of motor starting currents.

.

I’m good with that....

What I should have said was that until the motor starts rotating, it draws locked rotor current regardless of the shaft load.
 
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