Attacking Motor Voltage Drop

Status
Not open for further replies.
Additional analysis brought to you on behalf of iwire

Additional analysis brought to you on behalf of iwire

iwire said:
Considering the very low mechanical start up load of a 3/4 HP sump pump is it really an issue?

It is not like we are trying to start a compressor or some such load like that.
Click to expand...
So let's go with 3-4X startup and that we should limit the drop to 15-20%.

Using 140?F, 14 A run, 56 A start, 85% p.f., 720 ft #6 CU:
Start voltage = 84.8 (29.4% drop)

Using 140?F, 14 A run, 42 A start, 85% p.f., 720 ft #6 CU:
Start voltage = 93.8 (21.9% drop)
 
mivey said:
Except we don't have a single fixed load because we have no motor, motor run, and motor start loads.
Click to expand...
What does that mean?

May not be a fixed load, but it appears to be the only load, which is the point I believe Don was attempting to make... i.e. not multiple loads which would cause an overvoltage condition on one when others aren't on.
 
Smart $ said:
What does that mean?

May not be a fixed load, but it appears to be the only load, which is the point I believe Don was attempting to make... i.e. not multiple loads which would cause an overvoltage condition on one when others aren't on.
Click to expand...
A buck/boost transformer transforms voltage as a % of what is there. We would like for the voltage at the load to be fairly stable so we can then boost it to what we want (say 208 to 240). With the voltage at the load varying drastically due to the various loads:
1) controls
2) controls + run amps
3) controls + start amps

The boost function does not keep us near the desired voltage. For example, if our voltage is at 120 with a 5% boost we get 126 volts. If our voltage experiences a 30% drop we only boost to 120*0.7*1.05 = 88.2 volts.
 
Actually it's a utility (steam pipe) pit and the closet power is 650'.
Rain water and the steam pipping don't care for each other and they are looking for options for taking a generator to the site.
 
mivey said:
A buck/boost transformer transforms voltage as a % of what is there. We would like for the voltage at the load to be fairly stable so we can then boost it to what we want (say 208 to 240). With the voltage at the load varying drastically due to the various loads:
1) controls
2) controls + run amps
3) controls + start amps

The boost function does not keep us near the desired voltage. For example, if our voltage is at 120 with a 5% boost we get 126 volts. If our voltage experiences a 30% drop we only boost to 120*0.7*1.05 = 88.2 volts.
Click to expand...
If the "controls" are just a float switch (which for a 120V, 14A sump pump, it likely is), there is no "controls" load.

As for boosting, the point is we only have a single load to deal with. If we start at 120V and take it up say 20% under no load condition, the run voltage drop would cause it to dip, if configured correctly, to say 110% or slightly less... within allowable tolerance. And also allowing for a slightly higher start voltage!
 
Last edited:
mivey said:
Except we don't have a single fixed load because we have no motor, motor run, and motor start loads.
Click to expand...
In this installation, I would not really be looking at the no load or starting current. I would be looking to get the votlage into the normal range for the running current. The voltage drop on start would be about the same as it would be on a circuit without excessive length and I don't see the somewhat higher than normal voltage as an issue when the motor is not running.
 
mivey said:
The regulation is poor and they do not play well to varying loads. Fix the voltage problem at start up and you will have overvoltage at normal/low loading. Fix the problem at normal voltage and you might not have adequate starting voltage. If you ran a little hot at normal/low load you might find a workable solution.

A 3/4 sump pump can probably take some abuse and if you shorten the life some, it is not like they are extremely expensive (~$200 bucks?). So maybe you could push the envelope a little.

I still like the 480 volt solution you proposed.
Click to expand...
3/4 single phase pump is not likely all that expensive. Even if pump doesn't fail maybe a control contact fails - so if failure of the pump is a big issue then some kind of monitoring is important because there is more to fail than just the motor itself. Pump could get stuck with foreign material or discharge line could be plugged and still cause a backup of water, even though the motor works.

hurk27 said:
yep but a flooded basement is:eek:

a BB transformer might bring the VD back up for the FLA but the LRA will still see a big drop, a 240 volt pump will cut the VD in half as well as the start up current.

If there are motor controls then they will see the full boosted voltage when the motor is not running
Click to expand...
It is also possible that if the voltage drops low enough the controls drop out during starting because of too much voltage drop.

Smart $ said:
Which makes me curious... why such a long run if it's in a basement???
Click to expand...
Gus cleared it up that no flooded basement is a problem with this install, but it easily could have been a situation though the pump is a long way away, the fact it fails could result in backing up of water and eventually flooding a basement or whatever is supplying the sump pump with its pumped media.

Gus, how about an battery, and an inverter sized to help with stabilizing voltage when starting, or run it until it fails and replace with at least 240 volt pump. If it is only needed when it rains it maybe doesn't see a lot of run time. It certainly would not have seen much run time around here this past year.
 
augie47 said:
Actually it's a utility (steam pipe) pit and the closet power is 650'.
Rain water and the steam pipping don't care for each other and they are looking for options for taking a generator to the site.
Click to expand...

It would be cheaper to change the pump to a 240v or 208v(depending on your service voltage) to further reduce the wire size needed.
 
Cow said:
It would be cheaper to change the pump to a 240v or 208v(depending on your service voltage) to further reduce the wire size needed.
Click to expand...
agreed and I think that will be the "game plan".
It appears that with a #6 Cu, there would be no need to add a buck/boost transformer.
I guess a part of my question was based on a fuzzy memory of the negative side of using b/b transformers, but with this "fixed" load it seems that such use, if needed, might be a viable option.
 
Smart $ said:
If the "controls" are just a float switch (which for a 120V, 14A sump pump, it likely is), there is no "controls" load.
Click to expand...
Quite possible. It might be prudent to install some type of water alarm or monitor.

Smart $ said:
As for boosting, the point is we only have a single load to deal with. If we start at 120V and take it up say 20% under no load condition
Click to expand...
Then you are no longer within an acceptable voltage. Standard procedure would be to correct a low voltage to just over nominal for no load conditions.
 
don_resqcapt19 said:
In this installation, I would not really be looking at the no load or starting current. I would be looking to get the votlage into the normal range for the running current. The voltage drop on start would be about the same as it would be on a circuit without excessive length and I don't see the somewhat higher than normal voltage as an issue when the motor is not running.
Click to expand...
Somewhat is ok. But designing for excursions outside the ANSI standard range might not be acceptable.
 
kwired said:
3/4 single phase pump is not likely all that expensive. Even if pump doesn't fail maybe a control contact fails - so if failure of the pump is a big issue then some kind of monitoring is important because there is more to fail than just the motor itself. Pump could get stuck with foreign material or discharge line could be plugged and still cause a backup of water, even though the motor works.
Click to expand...
Very true.

kwired said:
It is also possible that if the voltage drops low enough the controls drop out during starting because of too much voltage drop.
Click to expand...
Agreed.
 
Status
Not open for further replies.
Top