Excessive Voltage Drop - Why?

Status
Not open for further replies.
L

lakee911

Senior Member
Location
Columbus, OH
Hey All,

I was at a site the other day and we started an across the line motor on a pump (size unknown, probably somewhere between 20 to 50HP) and the lights dimmed until it got up to speed. I measured the voltage drop at the line side of the starter of one phase. Without it running, voltage was 489.0V. When starting, it dipped down to 431.7V! I don't have a lot of details about the electrical system here. I did get a similar (i.e., in proportion) drop in voltage on the 120VAC lighting circuit.

On one other occasion, we tried to start it and it just sat there humming until we shut it down. A minute later it fired up and was just fine.

What's likely going on here?

Thx,
Jason
 
what makes you think this is excessive for a motor starting?

the starting current is 5-6X the running current.

I^2R says that if you have 5X the current draw you will have 25X the voltage drop during starting.

What is the voltage drop when the pump is running normally?
 
petersonra said:
what makes you think this is excessive for a motor starting?

the starting current is 5-6X the running current.

I^2R says that if you have 5X the current draw you will have 25X the voltage drop during starting.

What is the voltage drop when the pump is running normally?
Click to expand...

Actually, I take back what I said about where I measured the voltage. It was on the line side of another different motor starter that happened to be convenient.

I figured that if the voltage is dropping for the entire building, something is obviously wrong. I know some is inevitable, but no other sites have this problem. We're having problems with PLC locking up and I'm thinking it attributed to this.

I didn't measure voltage while it was running, but I'm assuming it went back to around upper 480-something.
 
It is a poor design if the lights dim during the starting of a motor. I would look into the size of the wire that serves the motor.

If the motor did not start at all on that one occasion, then there could be a significant problem with the design. It might be something on the mechanical side, something like rust or other cause of friction. But if the motor is not getting enough voltage to get it rolling, then that is an electrical problem.
 
charlie b said:
It is a poor design if the lights dim during the starting of a motor. I would look into the size of the wire that serves the motor.

If the motor did not start at all on that one occasion, then there could be a significant problem with the design. It might be something on the mechanical side, something like rust or other cause of friction. But if the motor is not getting enough voltage to get it rolling, then that is an electrical problem.
Click to expand...
In my experience a voltage drop that dims the lights is very rarely a function of the size of the conductor that feeds the load that is causing the dimming. The dimming is a function of the feeder and or service voltage drop and the transformer voltage regulation. In fact with a motor, a larger voltage drop on the motor conductors, acts as a reduced voltage starter lowering the starting current, of course it does increase the time it takes for the motor to get up to speed and may make the dimming more noticeable.
 
don_resqcapt19 said:
In my experience a voltage drop that dims the lights is very rarely a function of the size of the conductor that feeds the load that is causing the dimming. The dimming is a function of the feeder and or service voltage drop and the transformer voltage regulation. In fact with a motor, a larger voltage drop on the motor conductors, acts as a reduced voltage starter lowering the starting current, of course it does increase the time it takes for the motor to get up to speed and may make the dimming more noticeable.
Click to expand...
Bingo. May have a long run to the transformer, too small of a transformer, both, or some other inherent choke effect somehow involved.

If it were just too small of conductors to the individual load it wouldn't have as much of a chance of dragging voltage down in entire facility.
 
The motor that just hums with no acceleration probably has other issues - but would still cause voltage drop as it is still going to draw high current. If it is still getting over half rated voltage it should accelerate, it is likely single phasing for some reason when it doesn't accelerate. Probably a controller or switch gear problem more so then the motor causing single phasing.
 
Status
Not open for further replies.
Top