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Old 11-05-2011, 05:49 PM   #1
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Why can't some generators run appliances that use an induction motor?


Why can't some generators run appliances that use an induction motor? I've heard of that and recently I read a review on a generator I was considering buying that says:

Quote:
As for starting air conditioner with induction motors, I can't help. I will say it runs power tools very well including a 15 amp planer. But it will not start my radial arm saw rated at 12 amps. My guess is the radial is an induction motor and the others are universal (have armature brushes).
I am not an electrician so please bear with me. Thanks
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Old 11-05-2011, 05:57 PM   #2
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Why can't some generators run appliances that use an induction motor?


Mainly because induction motors have a huge start up surge !
Depending on the load it can be easily 3 to 4 times normal
running currant.
Now a normal old style genny can tolerate short term overloads,
so they tolerate it ! even thou it is technically wrong.
But some of the newer style gennys,
,
especially the invertor types,
have much more sensitive electronic regulators,
and overload circuits,
so they wont tolerate the overload even if it is breif.
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Old 11-05-2011, 07:35 PM   #3
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Why can't some generators run appliances that use an induction motor?


Induction motors typically require 6 times the running Amps to start. Your generator is too small for such applications.
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Old 11-05-2011, 11:30 PM   #4
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Why can't some generators run appliances that use an induction motor?


Generally speaking, a small generator will start an induction motor of about 1/3HP per KW.

In other words, a 3KW generator will usually start a 1HP motor, a 5KW will start a 1 - 1/2HP motor.

This is only a generalization, if the motor starts fully loaded, you'll need more generator; if it starts completely unloaded, a smaller gen will do.

With larger 3 phase generators (over 50KW), the figure is more like 1/2HP per KW.

Rob
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Old 11-06-2011, 01:48 AM   #5
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Why can't some generators run appliances that use an induction motor?


There are some inverter generator will not able handle standard AC motour due the modifed sinewave that is the key issue why some of the AC motour will not start or run very well at all.

The non invertered generators aka plain jane generators can start the AC motour if the surge wattage is large engough to spin the motour up to the running speed.

Just one major quirk the surge wattage the voltage and HZ will be lower for few seconds until the generator unit get back up to the running speed. ( that is why you have to watch out on this with motour load )

So that one thing with inverter generators and also you have to watch out the rating on both running and starting or surge wattage that is the key item you have to look for.

Merci,
Marc
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Old 11-06-2011, 10:32 PM   #6
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Why can't some generators run appliances that use an induction motor?


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Originally Posted by frenchelectrican View Post
There are some inverter generator will not able handle standard AC motour due the modifed sinewave that is the key issue why some of the AC motour will not start or run very well at all.
Don't the modified sine wave inverters have larger swings? I am surprised to learn that motors cannot handle those swings. I figured they would be the most robust, and sensitive electronics would be the least.
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Old 11-06-2011, 10:39 PM   #7
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Why can't some generators run appliances that use an induction motor?


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Don't the modified sine wave inverters have larger swings? I am surprised to learn that motors cannot handle those swings. I figured they would be the most robust, and sensitive electronics would be the least.
If that was the case, the peak voltage would be higher.....that is what starts to hurt electronics....

The simple fact is, the generator goes into current limit when you try to start a motor that is more than about 1/5th the current of the generator.

For example...say you have a 2400W generator (that is 20A at 120Vac)

If you try to start a motor that draws 5 amps at normal run speed....it might start up....but if you try to run a motor that pulls 10A....at startup it's going to try to pull around 50A or more....the generator will go into current limit.
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Old 11-06-2011, 11:48 PM   #8
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Why can't some generators run appliances that use an induction motor?


Here the photo it will clear up the details little more why.,



This will clear up the question why.

The red line is true sine wave while the green line is square wave then the bleu one is modifed sine wave.

You can see the differnce on voltage and it way it raise the voltage more like harsh on / off waveformat that why quite few electric motours do not work very well with it ditto with any transfomers { it can work but will reduce the performace or make alot of noise one of few can cause the issue }

Merci.
Marc
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Old 11-07-2011, 12:22 AM   #9
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Why can't some generators run appliances that use an induction motor?


Quote:
Originally Posted by frenchelectrican View Post
Here the photo it will clear up the details little more why.,



This will clear up the question why.

The red line is true sine wave while the green line is square wave then the bleu one is modifed sine wave.

You can see the differnce on voltage and it way it raise the voltage more like harsh on / off waveformat that why quite few electric motours do not work very well with it ditto with any transfomers { it can work but will reduce the performace or make alot of noise one of few can cause the issue }

Merci.
Marc
Good diagram.....

Just to clarify for those that may question the duty cycle....this shows a 50Hz waveform. In the US, the duty cycle would be 16.66666 ms.

As french noted....the square wave generates more noise.....he was being gentle.....in reality, a square wave generates an infinent number of frequencies.....noise is an understatement.

Also notice that the peaks are the same for the sine wave and modified? However, the widths are different....but, the RMS voltage would most likely be pretty much the same. RMS (Root Mean Square is a way of measuring the real power in an AC waveform. If my old memory serves me correctly....the peak voltage of a 120Vac sinewave would be 170 v peak.

What does all this mean?

Transformers and motors are very inductive devices....the higher the frequency, the less current that will flow through them....so, when you have a source that has a lot of harmonics....they don't go through the motor or transformer....translation, less power.

So...if you have an inverter that generates a 'modified' sine wave....you have less power available for inductive loads.
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Old 11-07-2011, 01:41 AM   #10
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Why can't some generators run appliances that use an induction motor?


Ddawg16.,

You are correct the RMS on 120 volt 60 HZ supply is peaked out at 168-170 volts ( 50 HZ is little lower but not much )

Basically it the same issue I have ran into with some of the dimmers they can make the light bulb filment sing as well { that can happend to any wattage bulb espcally with cheap verison }

on Induction AC motours if run on square wave or modifed square wave it will actually produce knocking noise { try that on large 25 HZ motour it will sound like diesel engine or hevey pinging sound }

Also not all the electronic devices will work with modifed sine wave source I have one portable tool battery charger went down the drain as soon I plug into inverter generator the other type of charger it charge very little or just don't do anything at all.

So to the OP.,

If you want inverter generator make sure you get one that is frenidly with computer or electronique loads.

Merci,
Marc
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Old 11-07-2011, 02:51 PM   #11
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Why can't some generators run appliances that use an induction motor?


Thanks for all the help guys!
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