1 HP, 120 volt motor. Supply to motor shows 125 volts. With motor running lifting load shows 114 volts. With motor running with no load shows 118 volts. This motor is on a boat lift, only operates for 2 or 3 minutes and not used very often.
Is this too much voltage drop? If so, considering the run time would this be a problem?
If the motor seems to work properly without smelling of smoke, I’d say it’s fine. The motor evidently draws a lot of current in operation, especially when under load. This is perfectly normal. The problem is the supply is not perfect, cannot supply a perfect 120 volts (or whatever) under all loads. The heavier the load, the more current used, the more current used, the lower the voltage will drop.
If motor supply voltage falls from 125 to 114 volts, that means you have 11 volts along a wire (or perhaps across a corroded connection.) If the motor draws ten amps, that means there will be 11 watts of heating in the place where the 11 volts falls. Maybe the 11 watts is spread out along the connecting wires. Or maybe there’s a connection somewhere that’s getting as hot as the filament in a 11watt light bulb!
Now voltage AT the motor… it’s probably an AC/DC motor (with brushes,) and if so, then the motor voltage only determines the motor speed. The motor will turn slower than it would if you used thick cables and brought the voltage up near 125V.
1 HP is 746 watts, which means that you’ll have about 6 amps, assuming a 100 percent efficient motor. A real motor is going to probably something more like 8 amps (figuring 80 percent efficiency). Hmm. I thought bbeaty’s 10 amp figure seemed a little high but I guess he’s pretty close.
That would have to be a really long wire to drop that much voltage normally. Since this is on a boat lift, which means it’s near water, I suspect you have a bad connection somewhere. Get thee to an electrician ASAP.
The no load voltage drop is fairly significant too. I suspect that most of that is also due to the bad connection, but some of it could be due to the motor starting to wear out. As rarely as you use it, you might want to just run it until it dies. However, I suspect that once you get your cruddy connections all sorted out that the no load voltage drop won’t be so bad, in which case your motor is fine.
Thanks for the help.
The motor is new. May be connections, but I feel like it is the distance/wire size which is around 200 ft with the first 100 ft being #10 wire and the remaining is #14 wire. The line which is underground was original put in for only a light at the boat dock. I recently had the boat lift installed. It would be a major job to replace the wire for some it goes under concrete. The line at the dock is in plastic conduit with GFI protection.
Not really. If we assume the motor draws 10 amps and the wire gage is 12 AWG, then we’re only talking about 346 feet between the motor and the breaker box. And this assumes no junctions & connections. If we assume there are junctions & connections, then it requires even less distance to get that kind of voltage drop.
If we assume:
100 ft. of 2-conductor 10 AWG cable
100 ft. of 2-conductor 14 AWG cable
0.2 Ω in hot path due to connectors & junctions
0.2 Ω in neutral path due to connectors & junctions
125 VAC no load voltage
The motor draws 10 A
Then there would be 114 VAC under load at the motor.