My son was using a 1.5HP 120V AC motor to power his loam screening machine. He was using two 50ft # 12 and one 75 ft # 14 extension cords to power th is up. After running a couple of hours he stopped the machine for awhile to do other things. When he attempted to get it going again about a half hour later he said it hummed loudly. He quickly unplugged it and noticed that t here was a big spark at the plug end and the motor was quite hot. A while l ater he tried it again and it ran fine. I suggested that he check the ends of the cords and just like I had expected he said that they were all quite warm. We replaced this mess with one continuous length of # 12 with 20 amp ends and all seems fine now so I suspect low voltage or rather excessive vo ltage drops were his problem. But I have a couple of questions. No one has ever really explained to me why low voltage can damage an AC motor. I had t hought that as per ohms law with less voltage there should be less of a cur rent draw and therefore less heat. My other question concerns the initial m otor failure. Wat kind of mechanism would cause a motor to shut down as it appeared to by drawing excessive current. It behaved just like a bad, (shor ted) motor initially and then resumed working perfectly later on. Thanks. Lenny
and it ran fine. I suggested that he check the ends of the cords and just like I had expected he said that they were all quite warm. We replaced this mess with one continuous length of # 12 with 20 amp ends and all seems fine now so I suspect low voltage or rather excessive voltage drops were his problem. But I have a couple of questions. No one has ever really explained to me why low voltage can damage an AC motor. I had thought that as per ohms law with less voltage there should be less of a current draw and therefore less heat. My other question concerns the initial motor failure. Wat kind of mechanism would cause a motor to shut down as it appeared to by drawing excessive current. It behaved just like a bad, (shorted) motor initially and then resumed working perfectly later on. Thanks. Lenny
You're correct about the voltage drop, it's not good for an AC motor. In your case however, you have a single phase motor and most likely a centrifugal switch and the initial start using this switch to connect a starting coil directly or via a capacitor, depending on the motor type. When it spins up the centre force will open the switch and disconnect that start coil. This process draws more current than normal and the switches are not that great to start with. The heat of the prior operation could have cause it to get stuck and the starting process didn't switch off, that causes a loud hum and excessive current.
As for running low voltage to a motor you need to be concerned about having enough to not cause the switch to engage once running. We operate 460V 3 phase motors with 230V 3 phase, the reason for this is to have a low torque motor. When load is not against it it'll spin up to speed otherwise it'll softly drop back with no damage and just stop if need be. This is good for driving acme screws that pick up heavy loads but you don't want the screw to get jammed. This seems to work better than using a inverter for torque mode because it responds faster to the load than the inverter can. We run 3 phase motors as tension motors with a 3 phase variac where the user can manually adjust the hold torque and the rotor does very little turning at all.
The biggest problems with reduced voltage to AC motors like this is you need cooling. These motors depend on air flowing through them otherwise you need a motor with an open frame to let it vent or a fan on the back. Those that drive the fan themselves depend on the motor being up to speed. Also effective induction decreases when the rotor slows down and is normally due to load, this causes the current to go up in the motor.
Insufficient voltage to the motor can also cause the current to go up because the rotor isn't cutting through fast enough to lower the effects that causes counter force in the field. Most of the time this isn't damaging, although it'll make the motor run a little hotter than normal and it'll also cost you more to operate it..
The motor starts out running by using a starting winding that draws maybe 3
0% more current than normal. Typically, if the heavier current causes the voltage to be low, the motor cannot get up to speed to disconnect the start ing winding as Jamie said.
Every plug/socket that is in the connection is a weak point for the overall system. Connections weaken with repeated heating and cooling, and may dev elope "crud" on the contacts that make the connection even worse. By discon necting the plug your son may have cleaned up the contacts just enough that the next time he tried to start the motor the connection was just a little better, but enough better that the mtor was able to get up to speed. Ther e may also have been a difference on the mechanical load on the motor when starting up, depending on how the loam screening is connected (or not) when the motor starts up.
ore less heat. My other question concerns the initial motor failure. Wat kind of mechanism would cause a motor to shut down as it appeared to by drawing excessive current. It behaved just like a bad, (shorted) motor initially and then resumed working perfectly later on. Thanks. Lenny
The speed of an AC motor is basically set by the frequency. If the load (HP) is the same, and the voltage is lower, the current has to be higher to supply the same HP. Higher current produces higher heat. (But the normal running temperature may be higher than you think.)
The NEC gives a running current for a 1.5 HP motor (at full mechanical load) as 20A (which is likely somewhat high). Typical starting currents for AC motors are about 6x the run current. Likely the motor did not start because of voltage drop at the starting current. Pulling the plug during a non-start you are interrupting something like 6x 20A. (The current would have been significantly lower because of voltage drop.)
No one has ever really explained to me why low voltage can
Ohm's law does NOT APPLY to AC induction motors. The excitation of the rotor shorting bars depends on applied magnetism from the stator coils. As the stator magnetic field decreases, the induced field on the rotor decreases, and the stator current must increase to provide the rotor with field. So, as the voltage goes DOWN, the current goes UP! Thus, the power input to the motor remains roughly constant.
Many motors have a thermal protector, sometimes self-resetting, sometimes manual reset. But, that should cut the motor completely off when overheated. Possibly it is wired wrong, so it only cuts out the starting circuit. Or, the windings and cord were so hot it couldn't draw enough current to start under load.
On Wednesday, June 25, 2014 9:54:40 PM UTC-4, Maynard A. Philbrook Jr. wrot e:
ine. He was using two 50ft # 12 and one 75 ft # 14 extension cords to powe r this up. After running a couple of hours he stopped the machine for awhil e to do other things. When he attempted to get it going again about a half hour later he said it hummed loudly. He quickly unplugged it and noticed th at there was a big spark at the plug end and the motor was quite hot. A whi le later he tried it again
like I had expected he said that they were all quite warm. We replaced thi s mess with one continuous length of # 12 with 20 amp ends and all seems fi ne now so I suspect low voltage or rather excessive voltage drops were his problem. But I have a couple of questions. No one has ever really explained to me why low voltage can damage an AC motor. I had thought that as per oh ms law with less voltage
question concerns the initial motor failure. Wat kind of mechanism would ca use a motor to shut down as it appeared to by drawing excessive current. It behaved just like a bad, (shorted) motor initially and then resumed workin g perfectly later on. Thanks. Lenny
I liked it anyways, but I know "squat" about motors.
What Jon says about AC induction motors is correct. It is the nature of these motors to draw more current as the voltage drops. And if the voltage is low enough the motor will not be able to start spinning and as a consequence will draw as much current as possible which can cook the motor and/or the wires supplying the power. Universal motors, like those in a vacuum cleaner, do not behave this way. Eric
To make the numbers easy to handle, assuming that your whole extension cord is a 14ga cord. Do the math on the extension cord; you'll find that a 175ft
14ga copper extension cord has a resistance of approximately 29 ohms (round trip). That means that for a running motor current of 15A, the wire will produce a voltage drop of about 13.25V. That's for a single conductor wire, not enclosed in a jacket or in a bundle. For wiring enclosed in a jacket and at elevated temperature, you must derate the ampacity of the wire by about half. In other words, a 14ga extension cord could cause the voltage at the motor to be reduced to about 100V. You could ease that loss by the inclusion of the 12ga cords, but then you have to assume some additional voltage drop caused by the plugs and sockets in the total length of the run. Additionally, the heat generated by the wire in the extension cords can cause the insulation to melt and cause a short circuit or fire. For a 175 ft. run, the wire property charts say that for a 15A load, you should be using a 6ga. extension cord. That would limit the voltage drop at
15A to about 4V, which the motor should be happy with.
If you measure the voltage that your motor is getting, you might be surprised to see how low it goes at startup and after running for a period of time. Low voltage to an induction motor can cause damage to the motor. Overheating at a minimum, burned windings in the worst case.
The NEC table gives the resistance of #14 as 3.14 ohms per thousand ft. For 350' round trip that should give about 1 ohm and a voltage drop of
15 volts. That is about what you came up with on voltage drop but not resistance.
The NEC would like a max voltage drop of 5%, but it is not a requirement. Motors often have a rating of 220V, which is the voltage at the motor on a nominal 240V circuit. That would be about an 8% drop.
When starting, the current drop would be a lot higher, and quite possible the motor wouldn't start.
Would be interesting what the actual current draw of the motor is.
For most cords with 2 current carrying conductors the NEC gives a current rating of 18A for #14 and 25A for #12. If run at their ratings I would expect them to be "quite warm".
Buy a Kill-a-Watt and you can see the voltage drop, current drawn, & watts.
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maybe 30% more current than normal. Typically, if the heavier current causes the voltage to be low, the motor cannot get up to speed to disconnect the starting winding as Jamie said.
30%, what a laugh! Induction motors pull up to 10X running current to start. 6X is quite normal. About the only case where this is not true is with reduced voltage starters (which are primarily used only with big motors (100 kVA and up).
overall system. Connections weaken with repeated heating and cooling, and may develope "crud" on the contacts that make the connection even worse. By disconnecting the plug your son may have cleaned up the contacts just enough that the next time he tried to start the motor the connection was just a little better, but enough better that the mtor was able to get up to speed. There may also have been a difference on the mechanical load on the motor when starting up, depending on how the loam screening is connected (or not) when the motor starts up.
It goes something like this. Renee Everhart was using a vibrator and the batteries were carbon zinc and they died rather quickly so Renee took out the vibrator and stomped on it and broke the motor. I know that batteries are DC and not AC but you get the picture.
The motor delivers power to its load. The motor tries hard to do that, even if the voltage is off spec.
Power_out = Power_in - losses in the motor
Power_in = Volts * Amps * Power Factor.
If power out stays the same, than power in does as well.
If Volts goes down, current must go up to maintain the same power in.
When the current goes up, so do the current-related losses, most importantly "I^2R losses" which [shockingly!] goes up as the SQUARE of the current. Those losses present as heat in the motor's windings.
IOW: something melts/burns outs.
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Also, when induction motors slow down they will draw much more current than when running at rated speed. A lower voltage will cause the induction motor to slow, the back EMF will drop, and the current will go way up. And since, as you point out, the heating from losses goes up with the square, it really doesn't take long for the magic smoke to escape. And if the speed drops low enough for the starting winding on a single phase motor to turn on then things will get hot really, really fast. Since corded drill motors, lawn mowers and weed whackers use universal motors they just slow down with a voltage drop and don't try to consume more current to get back up to speed. Eric
Keep in mind however, there are many applications of using induction motors that do not get up to speed and may even just sit there for tension operations. So if the motor has not been sized and vented properly, you'll have smoke!
We put 3 phase 480 motors in a 240 volt circuit so that we can get speed with very little load and when load hits the wall, the motor simple stalls. The motors have a self cooled frame so running them like this does no harm and it saves on dealing with inverters. Those that do not have self cooled frames will get on the warm side.
You don't want to do this with single phase starting aid type motors unless you externally control that part of the circuit. with split phase motors you can allow them to drag once they get turning and not allow the start switch to engage. I am sure you've seen many single phase motors with bad starters that you can spin start.
When a higher load slows the motor down the current goes up - higher power to the load results in higher power to the motor.
The motor speed is primarily set by the frequency in induction motors. If the speed dropped significantly with a voltage drop it would likely be advantageous - the motor would likely supply less power, so the current may not increase with lower voltage. It is particularly true for loads like fans, where the power is something like the 3rd power of the RPMs.
The problem is the speed does not drop much, so same power out, and then same power in, and higher current to compensate for the lower voltage.
Many motors are "thermally protected" (internal temperature sensor) so it disconnects until it cools. Larger motors are likely to have a "motor starter" that trips on high current.
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