Electical Q's

Sounds like too much of a voltage drop.

Check this out

Resistance of 150' (round trip) of 12 gage is about 0.28 ohms. Locked rotor current on a 1.5 HP 115-volt induction motor can be around 150 amps, which would be a 42 volt drop just in the 12 gage wire.

Check the connections on the new start cap.

You're probably go> What would the voltage reading be when measuring between the hot and

I think a more useful check would be to find out what kind of current the motor is drawing. Then you can find out if you have bad wiring or not. Your long wire runs will drop a normal amount if the current is high, but if the drop doesn't match the current, then you need to go check the connections (or possibly corrosion/damage in the buried wires. Been there, done that.)

If as you say, the motor is fed from a 15 amp breaker in the panel, then it can't be drawing more than 15 amps for very long or the breaker wou9ld pop, no? So, taking Don's word that the round trip resistance of the wire between the panel and the motor is 0.28 ohms, 15 amps would give you only 5 volts of drop.

When checking the voltage, it was 240 volts

That last bit makes it sound like the conductivity of the neutral going from the panel to the pole isn't what it should be. Try measuring from the neutral to an independent ground, like a buried water pipe, with the motor load connected.

If you read more than a few volts there, then start looking for "loose disconnection" problems heading back towards the pole. (I assume you already tried cleaning/tightening those three heavy cables where they connect to the panel.) Those cables might also have problems at the pole end, and depending on whether its your pole or the utility's, you may have to call your power supplier to check that for you.

Try also measuring between the unloaded hot in the panel and neutral there with the motor load connected. I'm betting you'll measure near 140 volts which will cinch it as being a problem with the neutral feed.

Let us know what it turns out to be.

Happy New Year,

Jeff

Jeff, that's what I was thinking. Double the voltage drop on the neutral means the neutral has twice the impedance. The other thing is I can't imagine a 1.5 HP motor on a 15 amp120 V circuit sustaining a locked rotor amperage for long if the breaker is good.

Aluminum wire? Outside? My bet is on corrosion in one or more connectors, even if they look OK.

I would be tempted to simply unbolt, mechanically clean, and reassemble all connections, using the gray grease (zinc dust in silicone grease) liberally in all connections.

Joe Gwinn

Aside from the likely corrosion of some connection of the aluminum wiring (I'm suspecting the neutral line given the voltage readings under load, especially the fact that hot to hot stays 230V), I have to wonder if this motor can, as many can, be rewired to 230V instead of 115V. Cuts the amperage, which cuts the voltage drop, and it also happens to avoid the neutral line, though you should still track down where the high-resistance connection is, if I'm not wrong about there being one.

On Dec 31 2006, 5:53=A0pm, "Wayne S" in,

Wayne Sippola

Time to go to 220 volts and maybe up a wire size. Also you have wiring connection problems. Take it all apart, do the 220 and redo all the connections.

Bob AZ

Better yet, crimp on Mac adapters which will form a gas-tight seal around the aluminum. That's assuming there's enough space, which might be a problem in an older panel.

Current = Volts/ resistance I = [100V - 87V] / [[150'][0.001588 Ohms/foot]] I = 54 Amps

I would expect the 15 Amp breaker to trip in a second at 54 Amps.

But if the breaker is not tripping, that would make me suspect that there is more resistance than the wire length between panel and motor.

Take off the screws and clean them. Take apart any wire splices and clean them. Try helping the motor start with a pull rope.

First, I'm sure you know to keep the time you have the motor powered under locked rotor conditions to the absolute minimum while you make measurements to avoid degradation or burnout. Sounds like you have been handling that OK, but damage can occur even without noticeable or catastrophic failure due to excess insulation temperature deep inside the windings. Set up for your measurement, turn on power and make your observation quickly (preferably with a DVM with a "memory" function that stores the reading when you push a button), then turn off power promptly. Additionally, let the motor cool for a long time between starting attempts. Large motors are only intended to go through NORMAL starting surges a few times per hour and your locked rotor tests are much more stressful than normal starting.

I think your best bet is to measure voltage from connector clamp to conductor at each junction along the power loop. The voltage drop at each connection should be negligible.

Since you are using aluminum conductors to the sub-panel, are you using anti-oxidant goo on each connector clamp and are you using connectors specifically rated for aluminum wire? The major problem with aluminum wiring has been that when it gets hot at a junction the aluminum expands and flows so that when it cools there is less clamping pressure than before the heating cycle. This can continue and, along with corrosion, results in even more heat next time until something fails. This is why the connector has to be rated for aluminum conductors and protected with anti-oxidant goo.

If ALL connectors and switches show negligible voltage drop, then you probably have either the corrosion problem described by another poster or a defective motor. If the aluminum conductors are truly 1/2" diameter, they will have less voltage drop than the 12 gauge copper to the motor if they are in good condition. A general rule of thumb is that you have to go up 1 step (i.e., from # 12 copper to #10 aluminum) to maintain about the same ampacity of your conductors. This is not strictly true if you analyze the resistivity of the conductors. In reality the aluminum 1 step larger in gauge has slightly higher ampacity (lower resistance per 1000 feet) than the 1 step smaller copper, so in the absence of corrosion or weak connections, your 1/2" aluminum conductors are better than your 12 gauge copper conductors. The 1/2" diameter stranded aluminum is 6 or more gauge steps larger than the #12 copper, so it should be fine in the absence of corrosion.

It would help a lot to have a clamp-on ammeter for this type of problem analysis.

Good luck.

Jeff, I was kind of suspecting the neutral because of the same voltage readings from either hot. I didn't think of checking the non used hot to neutral with the load on - that would have cinched it. Dad is currently working on putting on a repulsion start induction motor, so that may be done before we figure out what the real problem is. This is 150 miles from where I am.

Thanks to all for all the responses. The breaker trips in about 3 seconds. I was up and down power poles all Saturday morning checking connections right back to the meter. There is one set of splices about 20' from the pole with this panel which have not been checked. Assuming it is a poor neutral splice, I figured the ground at the panel would take care of it (neutral and ground is tied at the panel).

My question is more of the theoretical "what voltage should I see at the motor plug with a locked rotor?" Since the volt meter reads the difference between the hot and the neutral, would there be a voltage on the neutral line? I'm thinking that there is probably some due the distance to the ground at the panel, something like 75' of 12 gauge wire. I'm thinking that of the 87 volts at the motor plug, half the drop is voltage drop on the hot, and half the drop is due increased voltage on the neutral.

Happy New Year, and thanks again for all the inputs,

Wayne

To verify the line loss put a known load on the motor end of the line. A 500 watt bulb will work fine. Meaasure the voltage at the bulb and at the branch ckt box. It sure looks like you might have a corroded cable or splice.

John

If you can change the motor to the 220 V setting ang rewire the cable for 220, it will reduce the % drop by a factor of 4. (Supplied voltage up by factor of 2, current draw drops by factor of two.) This will probably fix the trouble. If the motor cannot be rewired for 220, then you need to use heavier gauge wire for such a long run. I might suggest #8, roughly.

Jon

Wayne S wrote: The breaker trips in about 3

Breakers can get more sensitive when they get old.

Machinery can get harder to start. Can it start the motor at the end of the long wires with no load? Can you oil up the load and make it easier to start?