Date: Sat, 21 Dec 2013 21:21:18 -0800
Reply-To: Rocket J Squirrel <camping.elliott@GMAIL.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Rocket J Squirrel <camping.elliott@GMAIL.COM>
Subject: Re: Van Dies while in motion
In-Reply-To: <1387688180.59738.YahooMailNeo@web162702.mail.bf1.yahoo.com>
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"Thus, when voltage gets low, the current must increase to provide the
same amount of power."
Correct -- to get the same amount of power at a lower voltage, the
current must increase. That doesn't mean that it will, it only means
that if you want the same power at a lower voltage, you'll need more
current. To do so requires that the motor be redesigned to draw that
increase in current.
I can buy a motor that provides x torque at 24 volts, and one that
provides the same torque at 12 volts. They ain't the same motor.
--
Jack "Rocket j Squirrel" Elliott
1984 Westfalia, auto trans,
Bend, Ore.
On 12/21/2013 08:56 PM, Stuart wrote:
> Ref: http://ecmweb.com/design/highs-and-lows-motor-voltage
>
> "To drive a fixed mechanical load connected to the shaft, a motor must draw a fixed amount of power from the line. The amount of power the motor draws has a rough correlation to the voltage 2current (amps). Thus, when voltage gets low, the current must increase to provide the same amount of power."
>
> "The Highs and Lows of Motor Voltage
> May 1, 2000 12:00 PM, By Ed Cowern, P.E.
> Operating a motor at the "outer limits" of its voltage requirements reduces its efficiency and causes premature failure.
> The economic loss from premature motor failure is devastating. In most cases, the price of the motor itself is trivial compared to the cost of unscheduled shutdowns of processes. Both high and low voltages can cause premature motor failure, as will voltage imbalance. Here, we'll look at the effects of low and high voltage on motors and the related performance changes you can expect when you use voltages other than those noted on the nameplate.
> Effects of low voltage. When you subject a motor to voltages below the nameplate rating, some of the motor's characteristics will change slightly and others will change dramatically. To drive a fixed mechanical load connected to the shaft, a motor must draw a fixed amount of power from the line. The amount of power the motor draws has a rough correlation to the voltage 2current (amps). Thus, when voltage gets low, the current must increase to provide the same amount of power. An increase in current is a danger to the motor only if that current exceeds the motor's nameplate current rating. When amps go above the nameplate rating, heat begins to build up in the motor. Without a timely correction, this heat will damage the motor. The more heat and the longer the exposure to it, the more damage to the motor."
>
> Another source from the same author...
>
> Ref: http://www.motorsanddrives.com/cowern/motorterms12.html
>
> "
> LOW VOLTAGE
> When electric motors are subjected to voltages, below the nameplate rating, some of the characteristics will change slightly and others will change more dramatically. A basic point is, to drive a fixed mechanical load connected to the shaft, a motor must draw a fixed amount of power from the power line. The amount of power the motor draws is roughly related to the voltage times current (amps). Thus, when voltage gets low, the current must get higher to provide the same amount of power. The fact that current gets higher is not alarming unless it exceeds the nameplate current rating of the motor. When amps go above the nameplate rating, it is safe to assume that the buildup of heat within the motor will become damaging if it is left unchecked. If a motor is lightly loaded and the voltage drops, the current will increase in roughly the same proportion that the voltage decreases.
> For example, a 10% voltage decrease would cause a 10% amperage increase. This would not be damaging if the motor current stays below the nameplate value. However, if a motor is heavily loaded and a voltage reduction occurs, the current would go up from a fairly high value to a new value which might be in excess of the full load rated amps. This could be damaging. It can be safely said that low voltage in itself is not a problem unless the motor amperage is pushed beyond the nameplate rating.
>
> Aside from the possibility of over-temperature and shortened life created by low voltage, some other important items need to be understood. The first is that the starting torque, pull-up torque, and pull-out torque of induction motors, all change based on the applied voltage squared . Thus, a 10% reduction from nameplate voltage (100% to 90%, 230 volts to 207 volts) would reduce the starting torque, pull-up torque, and pull-out torque by a factor of .9 x .9. The resulting values would be 81% of the full voltage values. At 80% voltage, the result would be .8 x .8, or a value of 64% of the full voltage value.
>
> In this case, it is easy to see why it would be difficult to start “hard-to-start” loads if the voltage happens to be low. Similarly the motor’s pull-out torque would be much lower than it would be under normal voltage conditions.
>
> To summarize the situation, low voltage can cause high currents and overheating which will subsequently shorten motor life. Low voltage can also reduce the motor’s ability to get started and its values of pull-up and pull-out torque. On lightly loaded motors with easy-to-start loads, reducing the voltage will not have any appreciable effect except that it might help reduce the light load losses and improve the efficiency under this condition. This is the principle that is used in the so-called Nola devices that are sold as efficiency improving add-on equipment to motors."
> Look at the digram in the article. You can see with a reduction in voltage, you get a corresponding increase in current. In the given case, I suspect that the battery and/or the alternator failed, with a resulting voltage drop. With several high power devices, such as fans and lights, the current demands on the wiring would have gone up. Perhaps this caused overheating of the switch and system failure. This will only be answered when the original poster tells what devices failed. State of the battery, alternator, switch and other wirng.
>
> What do you think happened? Your thoughts......
>
> Cheers!
> Stuart
>
> ________________________________
> From: Larry Alofs <lalofs@gmail.com>
> To: Stuart <ve3smf@yahoo.com>
> Cc: vanagonlist a <vanagon@gerry.vanagon.com>
> Sent: Saturday, December 21, 2013 10:33:59 PM
> Subject: Re: Van Dies while in motion
>
>
>
> Stuart,
> I'm afraid you are too far out on your limb. Your basic starting point that any device will use a fixed amount of power is incorrect.
> That makes the rest of your argument and conclusions false.
> I am wondering where you came up with that premise.
>
> Larry A.
>
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