Attacking Motor Voltage Drop

[augie47]

I was presented a situation where a sump pump is required at the end of a rather long run and asked about using buck/boost transformers. My initial reaction was "no" based somewhat on previous reads here, but I could not qualify that answer.
The job consist of powering a sump pump rated 120v @ 14 amps. My first thoughts were: (a) a 2/0 AL branch circuit or (b) using standard transformers (120/480) to reduce the wire size. The electrician looking at this job is knowledgeable and had run calculations showing that the 14 amp load would lower the voltage to 111 using a #6 Cu and he asked about using a buck/boost transformer.
None of those scenarios take into account the starting load which I fear will be the main problem faced.
Any input is appreciated.

 

[Gregg Harris]

I was presented a situation where a sump pump is required at the end of a rather long run and asked about using buck/boost transformers. My initial reaction was "no" based somewhat on previous reads here, but I could not qualify that answer.
The job consist of powering a sump pump rated 120v @ 14 amps. My first thoughts were: (a) a 2/0 AL branch circuit or (b) using standard transformers (120/480) to reduce the wire size. The electrician looking at this job is knowledgeable and had run calculations showing that the 14 amp load would lower the voltage to 111 using a #6 Cu and he asked about using a buck/boost transformer.
None of those scenarios take into account the starting load which I fear will be the main problem faced.
Any input is appreciated.

If the motor is rated at 14 amps at 120 volts the LRA will be rather high and will probably trip on overload trying to start.

 

[don_resqcapt19]

I don't see a real problem using buck boost transformers to correct for voltage drop where there is a single fixed load.

 

[augie47]

Thats what I would expect but I'm unsure how to prevent that.

 

[mivey]

If this is about 720 ft like I'm estimating I don't think #6 CU and and a boost to 480 volts will quite get you there for a 6X start.

 

[Smart $]

If this is about 720 ft like I'm estimating I don't think #6 CU and and a boost to 480 volts will quite get you there for a 6X start.

I oftened wondered about voltage drop on motor startup... never researched it. Typical Vd is based on the conductor's resistance value at elevated-above-ambient conductor temperature due to current heating. Isn't there a lag in the elevating of the temperature, thus the resistance contributing to voltage drop on startup? Seems like there would be a curve involved...

 

[mivey]

I oftened wondered about voltage drop on motor startup... never researched it. Typical Vd is based on the conductor's resistance value at elevated-above-ambient conductor temperature due to current heating. Isn't there a lag in the elevating of the temperature, thus the resistance contributing to voltage drop on startup? Seems like there would be a curve involved...

That is true but what about re-starts where the conductor temp is already elevated? The conservative approach would be to use the elevated temperature.

 

[mivey]

I was presented a situation where a sump pump is required at the end of a rather long run and asked about using buck/boost transformers. My initial reaction was "no" based somewhat on previous reads here, but I could not qualify that answer.

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.

 

[mivey]

The regulation is poor and they do not play well to varying loads.
...
I still like the 480 volt solution you proposed.

For example, just making a simple calc without modeling the impedances and assuming a stiff source (and about 720 ft of #6):

With a buck/boost (it only boosts a % of the voltage at the load):
Unloaded: VS = 100%, VL=100%, VB=105%
Start: VS = 100%, VL=53%, VB=56%
Run: VS = 100%, VL=93%, VB=97%

With a 480 volt step-up/step-down:
Unloaded: VS = 100%, VL=100%
Start: VS = 100%, VL=88%
Run: VS = 100%, VL=98%

 

[Gregg Harris]

What is the actual distance?

I did 200 feet with #6 and had 2.7% at 14 amps

 

[Smart $]

That is true but what about re-starts where the conductor temp is already elevated? The conservative approach would be to use the elevated temperature.

That's sensible, but is it overly conservative for instances such as the OP?

 

[mivey]

What is the actual distance?

I did 200 feet with #6 and had 2.7% at 14 amps

I get just under that. As for the starting drop, one problem is not modeling the motor as the starting power factor is very low.

 

[mivey]

That's sensible, but is it overly conservative for instances such as the OP?

Probably. I would not expect a sump pump to have a short re-start unless there is a float problem or a serious water issue.

 

[hurk27]

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.

yep but a flooded basement is

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

 

[hurk27]

if the VD drop is only 6.9% that is well within the NEMA rating of a 120 volt motor.

The above is using:

Length: 650'
Conductor Size: #6 awg
Current: 14 amps
VD: 8.9v

Are you sure this is a 650' run one way? wow thats a long run


NEMA requires motors to be able to handle VD's as high as 10%

 

[ActionDave]

I think the pump in question will be fine. I also agree with Hurk- if you scarred to death of the voltage drop issue the solution is a 240V pump.

 

[mivey]

I don't see a real problem using buck boost transformers to correct for voltage drop where there is a single fixed load.

Except we don't have a single fixed load because we have no motor, motor run, and motor start loads.

 

[mivey]

if the VD drop is only 6.9% that is well within the NEMA rating of a 120 volt motor.

The above is using:

Length: 650'
Conductor Size: #6 awg
Current: 14 amps
VD: 8.9v

Are you sure this is a 650' run one way? wow thats a long run


NEMA requires motors to be able to handle VD's as high as 10%

But check the starting voltage.

Using 140?F, 14 A run, 84 A start, 85% p.f., 720 ft #6 CU:
Run voltage = 111.4 (7.17% drop)
Start voltage = 66.3 (44.8% drop)

Again, I have not accounted for the starting p.f. so that maybe that is more in the 25-30% volt drop range during starting. Still a lot.

 

[iwire]

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.

 

[mivey]

If it is worth analyzing it is worth over-analyzing, flooded basements and all that.