You can talk all the theory in the world, our resident psycho said he was burning out motors on a regular basis, and then a large diameter lead solved the problem. I'm always happy to give weight to empirical evidence.

Select to expand quote

Harrow said..
You can talk all the theory in the world, our resident psycho said he was burning out motors on a regular basis, and then a large diameter lead solved the problem. I'm always happy to give weight to empirical evidence.

Don't doubt that the problem's disappearance coincided with the fitting of larger cables. But needing a theory to explain observations is a human trait that got to where we are. Google voltage tolerances of electric motors and figures of +- 10% are bandied about. When we put our multimeters on 15 metres of 10 amp cable we get less than an ohm out and back. Doesn't quite add up. Have we overlooked something?

Select to expand quote

Ian K said..
Don't doubt that the problem's disappearance coincided with the fitting of larger cables. But needing a theory to explain observations is a human trait that got to where we are. Google voltage tolerances of electric motors and figures of +- 10% are bandied about. When we put our multimeters on 15 metres of 10 amp cable we get less than an ohm out and back. Doesn't quite add up. Have we overlooked something?

Is it because of the starting current? A compressor does a lot of start and stopping, so while the voltage drop while running isn't a lot, is the voltage drop during starting causing a more severe problem?

Maybe ? But the copper windings would have a good thermal mass. So you'd think that even if the current is way high it would take a bit longer than the start up time for the windings to get too hot? Maybe there's another component in there that's sensitive to current. Are over current problems always associated with over heating something? How does a slow blow fuse work? Maybe the supply end of the cable voltage was down anyway and the cable just tripped it over the edge?

(Though I just see that Paddles has seen 13 times the steady state full current at start up! Power is I^2 R so that's 169 times the rate of heating in the windings even if only for a short time. Maybe a restart before a full cool down could melt the insulation? Surely you'd throw a breaker somewhere before you put 130 amps down the line?)

Select to expand quote

Ian K said..


(Though I just see that Paddles has seen 13 times the steady state full current at start up! Power is I^2 R so that's 169 times the rate of heating in the windings even if only for a short time. Maybe a restart before a full cool down could melt the insulation? Surely you'd throw a breaker somewhere before you put 130 amps down the line?)

For fear of upsetting people in the know, I think most circuit breakers do allow a surge current through greater than the figure on the nameplate. They have curves that can tell you how quick they will trip based on the fault currents.

I thought Paddles said P=VI ? Where did the i^2 come from?

Select to expand quote

FormulaNova said..

Ian K said..


(Though I just see that Paddles has seen 13 times the steady state full current at start up! Power is I^2 R so that's 169 times the rate of heating in the windings even if only for a short time. Maybe a restart before a full cool down could melt the insulation? Surely you'd throw a breaker somewhere before you put 130 amps down the line?)

For fear of upsetting people in the know, I think most circuit breakers do allow a surge current through greater than the figure on the nameplate. They have curves that can tell you how quick they will trip based on the fault currents.

I thought Paddles said P=VI ? Where did the i^2 come from?

substitute V = IR in power = VI.

All the discussion about higher current at low voltage is for a synchronous motor turning a fixed load at steady state. At start up the current is determined by stationary resistance and inductance of the coil. If anything, lower voltage due to resistance in the supply cable might lower the peak current during this brief phase. I was recently checking the windings on a motor that draws 6 amps. From memory they were only about 6 ohms. If we ignore the static inductance that'd be 40 amps on start up before back emf kicks in. But if the cable is one ohm that'd straight away knock peak current back to 34 amps. ( Pretty sure it doesn't do either so inductive resistance even on the motor at rest, must be significant at 50hz.)

No most likely it was an asynchronous induction motor. (Yes I've been googling) But still they only slip a couple of percent of the synchronous speed. They're not all that happy slowing down too much, so if the torque load remains constant the power requirement essentially stays the same and hence with lower voltage current must go up.

en.wikipedia.org/wiki/Induction_motor

Select to expand quote

Harrow said..
You can talk all the theory in the world, our resident psycho said he was burning out motors on a regular basis, and then a large diameter lead solved the problem. I'm always happy to give weight to empirical evidence.

It depends if he means he was burning out motors on a whole range of machines or on just one machine; i.e burnt out three motors in succession on the same machine or burnt out a motor on three different machines.

I am assuming it was an asynchronous squirrel cage induction motor on a machine requiring a high start up torque and that is not a good combination to begin with if the voltage is low, such as on a long low rating extension cord.
So why do they so commonly use this type of motor?
They are cheap and efficient when running and are much more quiet than a universal motor like in your electric drills and routers etc.
They are commonly used on lots of things such as your refrigerator, air conditioner, air compressor etc .
Unfortunately some of these do present a high start up load to the motor. You sometimes see the lights dim when your fridge starts up, even though the normal run consumption of the fridge might only be 200 watts. The start consumption for about half a second can be ten times that.
If it is on the end of a long extension cord it can be that this sort of power is just not available at the full voltage required for starting. Thus the motor remains in a stalled state and will burn out, even though the input voltage to the motor is lower than normal.

Something which can contribute to the failure can be the Start capacitor and/or the Run capacitor which this type of motor will probably have. It is usually external to the motor and they usually slowly degrade in capacity so that the value can be anywhere between 100% when new to 0% of the original value when they get older, say as low as three to five years.
Many years ago in better times, the electrolytes used in these start capacitors could be as toxic as you like because nobody actually ate them so nobody cared. The good thing about them was they lasted forever. They did not degrade over time so it was very rare to find one had gone soggy and diminished in value.
However, in these more modern times when we all care about things which we never cared about before, the people who have authority over such matters outlawed using PCB's as electrolytes in capacitors insisted that to be salable in Australia they had to be more environmentally friendly, so we ended up in the situation where salad oil is used as the electrolyte ! Yes, I kid thee not! Salad oil, peanut oil and similar. It's written on the device for all to see.
The problem with salad oil, as many would know, is it suffers degradation and goes rancid in a few years even when left in a bottle in the pantry.
If it goes rancid sitting in a jar in the pantry you can well imagine how rancid it goes in a capacitor canister with 240 volts ac buzzing through it at 50 cycles per second.
Now, fresh peanut oil does a half decent job as an electrolyte, but stale rancid peanut oil does a rotten job as an electrolyte and the capacitor ceases to function., just a little bit at first but eventually a whole lot. At that point your air conditioner or air compressor needs more power than is available to start up so it doesn't. So then the motor burns out because it sits there stalled with 240 volts being dissipated in it through a resistive process rather than an inductive process, which turns it into a 1 kilowatt electric heater,.. for a short time.

The point of all this is that to the end user, they will see the problem as the extension lead not being up to the job, or the mains power from the power station a hundred miles away not being up to the job, when in fact it is the start capacitor no longer being up to the job.

In the early stages a heavier extension cord will help start it up because it can deliver more power during starting, to compensate for the lower capacitor value, but it is quite possible the problem is in the start capacitor or the run capacitor or both. That's if it has both, many smaller motors do not and have only one. I wont go on with an explanation of why thye have only one other than to say it depends on how the motor is made.

Hmm,.. now how can I make this relevant to water sports?
Oh yes,.. in the event that the motor catches fire do not hose it down with water because water from the tap can be conductive so if you are standing in it you will be electroluxed.
Yeah,..close enough. That should do it.

Of course! Then start capacitor. Why didn't we think of that? The motor just sits there motionless drawing the start current until it cooks. Don't blame the extension lead. A 10 amp motor should be designed to run on a 10 amp extension lead. Thanks Pweedas.

Select to expand quote

pweedas said..
It depends if he means he was burning out motors on a whole range of machines or on just one machine; i.e burnt out three motors in succession on the same machine or burnt out a motor on three different machines.

I am assuming it was an asynchronous squirrel cage induction motor on a machine requiring a high start up torque and that is not a good combination to begin with if the voltage is low, such as on a long low rating extension cord.
So why do they so commonly use this type of motor?
They are cheap and efficient when running and are much more quiet than a universal motor like in your electric drills and routers etc.
They are commonly used on lots of things such as your refrigerator, air conditioner, air compressor etc .
Unfortunately some of these do present a high start up load to the motor. You sometimes see the lights dim when your fridge starts up, even though the normal run consumption of the fridge might only be 200 watts. The start consumption for about half a second can be ten times that.
If it is on the end of a long extension cord it can be that this sort of power is just not available at the full voltage required for starting. Thus the motor remains in a stalled state and will burn out, even though the input voltage to the motor is lower than normal.

Something which can contribute to the failure can be the Start capacitor and/or the Run capacitor which this type of motor will probably have. It is usually external to the motor and they usually slowly degrade in capacity so that the value can be anywhere between 100% when new to 0% of the original value when they get older, say as low as three to five years.
Many years ago in better times, the electrolytes used in these start capacitors could be as toxic as you like because nobody actually ate them so nobody cared. The good thing about them was they lasted forever. They did not degrade over time so it was very rare to find one had gone soggy and diminished in value.
However, in these more modern times when we all care about things which we never cared about before, the people who have authority over such matters outlawed using PCB's as electrolytes in capacitors insisted that to be salable in Australia they had to be more environmentally friendly, so we ended up in the situation where salad oil is used as the electrolyte ! Yes, I kid thee not! Salad oil, peanut oil and similar. It's written on the device for all to see.
The problem with salad oil, as many would know, is it suffers degradation and goes rancid in a few years even when left in a bottle in the pantry.
If it goes rancid sitting in a jar in the pantry you can well imagine how rancid it goes in a capacitor canister with 240 volts ac buzzing through it at 50 cycles per second.
Now, fresh peanut oil does a half decent job as an electrolyte, but stale rancid peanut oil does a rotten job as an electrolyte and the capacitor ceases to function., just a little bit at first but eventually a whole lot. At that point your air conditioner or air compressor needs more power than is available to start up so it doesn't. So then the motor burns out because it sits there stalled with 240 volts being dissipated in it through a resistive process rather than an inductive process, which turns it into a 1 kilowatt electric heater,.. for a short time.

The point of all this is that to the end user, they will see the problem as the extension lead not being up to the job, or the mains power from the power station a hundred miles away not being up to the job, when in fact it is the start capacitor no longer being up to the job.

In the early stages a heavier extension cord will help start it up because it can deliver more power during starting, to compensate for the lower capacitor value, but it is quite possible the problem is in the start capacitor or the run capacitor or both. That's if it has both, many smaller motors do not and have only one. I wont go on with an explanation of why thye have only one other than to say it depends on how the motor is made.

Hmm,.. now how can I make this relevant to water sports?
Oh yes,.. in the event that the motor catches fire do not hose it down with water because water from the tap can be conductive so if you are standing in it you will be electroluxed.
Yeah,..close enough. That should do it.

That's exactly what I was going to say, but I was too lazy to type it all out.

Select to expand quote

pweedas said..

Harrow said..
You can talk all the theory in the world, our resident psycho said he was burning out motors on a regular basis, and then a large diameter lead solved the problem. I'm always happy to give weight to empirical evidence.

It depends if he means he was burning out motors on a whole range of machines or on just one machine; i.e burnt out three motors in succession on the same machine or burnt out a motor on three different machines.

I am assuming it was an asynchronous squirrel cage induction motor on a machine requiring a high start up torque and that is not a good combination to begin with if the voltage is low, such as on a long low rating extension cord.
So why do they so commonly use this type of motor?
They are cheap and efficient when running and are much more quiet than a universal motor like in your electric drills and routers etc.
They are commonly used on lots of things such as your refrigerator, air conditioner, air compressor etc .
Unfortunately some of these do present a high start up load to the motor. You sometimes see the lights dim when your fridge starts up, even though the normal run consumption of the fridge might only be 200 watts. The start consumption for about half a second can be ten times that.
If it is on the end of a long extension cord it can be that this sort of power is just not available at the full voltage required for starting. Thus the motor remains in a stalled state and will burn out, even though the input voltage to the motor is lower than normal.

Something which can contribute to the failure can be the Start capacitor and/or the Run capacitor which this type of motor will probably have. It is usually external to the motor and they usually slowly degrade in capacity so that the value can be anywhere between 100% when new to 0% of the original value when they get older, say as low as three to five years.
Many years ago in better times, the electrolytes used in these start capacitors could be as toxic as you like because nobody actually ate them so nobody cared. The good thing about them was they lasted forever. They did not degrade over time so it was very rare to find one had gone soggy and diminished in value.
However, in these more modern times when we all care about things which we never cared about before, the people who have authority over such matters outlawed using PCB's as electrolytes in capacitors insisted that to be salable in Australia they had to be more environmentally friendly, so we ended up in the situation where salad oil is used as the electrolyte ! Yes, I kid thee not! Salad oil, peanut oil and similar. It's written on the device for all to see.
The problem with salad oil, as many would know, is it suffers degradation and goes rancid in a few years even when left in a bottle in the pantry.
If it goes rancid sitting in a jar in the pantry you can well imagine how rancid it goes in a capacitor canister with 240 volts ac buzzing through it at 50 cycles per second.
Now, fresh peanut oil does a half decent job as an electrolyte, but stale rancid peanut oil does a rotten job as an electrolyte and the capacitor ceases to function., just a little bit at first but eventually a whole lot. At that point your air conditioner or air compressor needs more power than is available to start up so it doesn't. So then the motor burns out because it sits there stalled with 240 volts being dissipated in it through a resistive process rather than an inductive process, which turns it into a 1 kilowatt electric heater,.. for a short time.

The point of all this is that to the end user, they will see the problem as the extension lead not being up to the job, or the mains power from the power station a hundred miles away not being up to the job, when in fact it is the start capacitor no longer being up to the job.

In the early stages a heavier extension cord will help start it up because it can deliver more power during starting, to compensate for the lower capacitor value, but it is quite possible the problem is in the start capacitor or the run capacitor or both. That's if it has both, many smaller motors do not and have only one. I wont go on with an explanation of why thye have only one other than to say it depends on how the motor is made.

Hmm,.. now how can I make this relevant to water sports?
Oh yes,.. in the event that the motor catches fire do not hose it down with water because water from the tap can be conductive so if you are standing in it you will be electroluxed.
Yeah,..close enough. That should do it.

A well written logical description of what goes wrong and why? Are you crazy!

Laurie, stop this sort of thing happening please!

Select to expand quote

Ian K said..
Of course! Then start capacitor. Why didn't we think of that? The motor just sits there motionless drawing the start current until it cooks. Don't blame the extension lead. A 10 amp motor should be designed to run on a 10 amp extension lead. Thanks Pweedas.

Despite Pweedas' excellent description, is a 2.2HP motor okay on a 10amp connection, as per Mark's original setup?

Of course someone seems to make it, so it 'must' be okay, but isn't 2.2HP something like 3.2 - 3.6KW? Which should convert back to at least 15 amps? No?

Is this 'Chinese' optimistic rating of 2.2HP?

www.rapidtables.com/convert/power/kw-to-hp.html

2.2 hp looks like 1.7 kW according to the internet. But is that electrical power in or mechanical power out?

Select to expand quote

Ian K said..
www.rapidtables.com/convert/power/kw-to-hp.html

2.2 hp looks like 1.7 kW according to the internet. But is that electrical power in or mechanical power out?

Ahh, now I realise how stupid I am. It has been so long since I have thought about HP to kW, that I went the wrong way

I must remember that a 200HP car is not as good when you rate it in kW.

As you were, nothing to see here