Electric motor question.

[catgate]

I have just bought a new portable compressor and on a label attached to the mains cable is says that if an extension lead is used (instead of sticking the plug directly into the mains socket) damage can be caused to the motor.

Now to my mind the socket on the wall can itself be the end of an extension lead which is hard wired into the ring main.. The motor is a claimed 1.5 HP and so should draw no more than 6 amps when running, and the start up load may be 15 to16 amps for but a few seconds .

I am at a loss to understand what this potential problem might be. Can anyone enlighten me please?

One clue might be the fact that the capacitor is marked 28mfd. whereas one would have expected something about three times this value for the normal cap. start motor.

Also the motor itself is much smaller than I would have expected. It is about the same size as the 1/4 hp motors of old.

[Mike567]

If it breaks they can say "oh you used a extension lead we won't replace it your loss buy a new one" :lol:.

[rong]

The portable compressor I bought had the warning about extension leads, but more to do with them not being able to cope with the power needed.

[catgate]

The portable compressor I bought had the warning about extension leads, but more to do with them not being able to cope with the power needed.

I can understand that, because some people do some silly things. When we first went into the old farmhouse we bought, about thirty years ago, the electrics were so desperate that I decided to rip them out and rewire it all. One fairly newly installed fifteen amp socket let into the skirting board of one bedroom was fed with a length of clear plastic covered twisted lighting flex.out of the back of another one ten feet away and that was fed by some old lead covered rubber insulated cable which ran back to the "consumer unit".

[Gandalph]

I have just bought a new portable compressor and on a label attached to the mains cable is says that if an extension lead is used (instead of sticking the plug directly into the mains socket) damage can be caused to the motor.

Now to my mind the socket on the wall can itself be the end of an extension lead which is hard wired into the ring main.. The motor is a claimed 1.5 HP and so should draw no more than 6 amps when running, and the start up load may be 15 to16 amps for but a few seconds .

I am at a loss to understand what this potential problem might be. Can anyone enlighten me please?

One clue might be the fact that the capacitor is marked 28mfd. whereas one would have expected something about three times this value for the normal cap. start motor.

Also the motor itself is much smaller than I would have expected. It is about the same size as the 1/4 hp motors of old.

Yes,

Take an 6 Meter coiled extension as an example. When the extension cable is in the coiled position it sets up and EMF (Electro Magnetic Field) and dissipates, this will only be capable of delivering 4.5 Amps / 1080 Watts as the EMF has dissipated the rest. The same Extension uncoiled will be capable of 6 Amps / 1440 Watts as there is far less dissipation.

[AlanHo]

George Simon Ohm, discovered an interesting mathematical relationship concerning electricity. The formula he discovered is very simple. It simply states that if you multiply volts times amps you get watts. For example, say you have 120 volts and a motor that draws 15 amps while running. 1,800 watts are used while this motor spins.

So what does this have to do with voltage drop and extension cords? I'll cut to the chase. Mathematical formulas dictate that if you adjust one value on one side of the equation up or down, you have to adjust another value on the same side so that the total stays the same.

This means that if there is a voltage drop, the amperage must go up to compensate for the voltage loss. When the amperage goes up in an electrical motor, it can cause the wires in the motor winding to overheat. This in turn causes the insulation on those wires to melt which leads to motor failure.

So if you power your new circular saw, table saw, hedge trimmer, etc. with a wimpy extension cord, you may be causing damage to the motor windings.

[Gandalph]

George Simon Ohm, discovered an interesting mathematical relationship concerning electricity. The formula he discovered is very simple. It simply states that if you multiply volts times amps you get watts. For example, say you have 120 volts and a motor that draws 15 amps while running. 1,800 watts are used while this motor spins.

So what does this have to do with voltage drop and extension cords? I'll cut to the chase. Mathematical formulas dictate that if you adjust one value on one side of the equation up or down, you have to adjust another value on the same side so that the total stays the same.

This means that if there is a voltage drop, the amperage must go up to compensate for the voltage loss. When the amperage goes up in an electrical motor, it can cause the wires in the motor winding to overheat. This in turn causes the insulation on those wires to melt which leads to motor failure.

So if you power your new circular saw, table saw, hedge trimmer, etc. with a wimpy extension cord, you may be causing damage to the motor windings.

Correct, but you emitted to mention the coiled extension which is one of the causes of loss set out in your explanation. The coiled extension would act the same as a primary winding. :D

[catgate]

Thank you for that chaps.

As far as Gandalph's coiled lead is concerned I was aware of that partially through the experience of making myself a "sensible" sized extension reel many years ago. I wound the wire on it and went in for my tea. In my excitement to use it I came out after tea and just unwound enough to get me from the socket to outside the garage door..and then struck up with an electric welder!!!!! What a sad waste of cable that was.

I was also aware of Alanho's Volts x Amps = Watts, but I was discounting this on the grounds that if this was the sole reason for the caution then a warning about ensuring that the extension cable was capable of carrying sufficient current would have been all that was required. It seems odd that a "portable" compressor's mobility (on wheels) should be restricted by a six foot length of cable.

Any thoughts on the size of the capacitor?

[Gandalph]

As far as the capacitor is concerned catgate it must be the right part for the job as it will be fitted by the manufacturers. I know that whenever I build a 12 volt 5amp power supply I always use a 2200 mfd cap just behind the rectifier bridge to smooth out the current before it hits any of the other components.

Just out of curiosity what voltage and current is there in front of the capacitor.

[catgate]

Just out of curiosity what voltage and current is there in front of the capacitor.

?

On the front of the capacitor it says:-

28mfd

450VAC 50/60 Hz

25/70/21 C Po

The 450V is not unusual, but 28mfd seems to me to be low if it is a capacity start 1.5 hp motor. Still that is what is fitted.

[Gandalph]

Just out of curiosity what voltage and current is there in front of the capacitor.

?

On the front of the capacitor it says:-

28mfd

450VAC 50/60 Hz

25/70/21 C Po

The 450V is not unusual, but 28mfd seems to me to be low if it is a capacity start 1.5 hp motor. Still that is what is fitted.

That's what I was thinking catgate, a bit on the low side. As you state, it must be OK, as that what is fitted by the manufacturer.