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Motor Accelerating Under Load

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Kajunbee

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Will post question later.

IMG_1335.jpg

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In this setup I found that the motor would speed up when I connected gen. 2 to input of b-2 ( 3 phase bridge rectifier). Without gen. 2 connected the .25 hp motor draws about 360 watts. 195 watts from gen.1 and 165 watts from variac. When I connect gen. 2 in circuit motor speed increases and motor only consumes 180 watts. 80 watts from gen. 2 and 100 watts from variac. I also noticed that the windings of gen. 2 don't seem to heat up but gen. 1 does.
The generators are bldc motors with commutation modules removed that I am using as a basic 3 phase generator. Gen. 2 does not have a motor turning it and since it has no module the rotor just locks up in one position.
Both gens are identical.
Long story short why doesn't gen. 2 load down gen. 2 even more. Also why don't gen.2 windings heat up. The frequency of gen.1 is 195 hertz at 88 volts when gen. 2 is not connected. 200hz and 86 volts when gen. 2 is connected. I know it seems pointless but please indulge me.

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Revision: why doesn't gen. 2 load down gen. 1 even more.

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Revision: 80 watts from gen. 1 and 100 watts from variac
 

How are you measuring power?
By taking a voltage and a current measurement then multiplying ?

There may be an issue with AC voltage current phase shifting that is not taken into your power calculations.
 

Thanks for the reply. I am taking voltage and current measurements then multiplying. I take measurement with leads of DMM between c-2 and motor of 2 amps 90 vdc. I measure .8 amps 90vdc between b-2 and c-1. Then measure 1.2 amps 90 vdc between b-1 and c-2.
The question I should have started is if phase relationship matters once the voltage is rectified. Can this give me false current readings.

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I am also taking current measurement with clamp on amprobe on a/c side coming from variac. It shows 1.49 amps at 70 vac. These measurements are taken when gen2 is connected and motor is using 180 watts. To me it seems that the numbers add up.
 

Yes you will get current / voltage phase shifting when you have inductors and capacitors.

With the right values of a capacitor , an inductor, and a resistor in series connected to a AC signal source. You can measure the AC voltages across the capacitor which can exceed the source voltage, then measure across the inductor and also see voltages exceeding the source voltage. But the AC voltage across the resistor will be less then the AC source voltage. If you uses the calculated current from V/R and try to use that to calculate power on the L and C, you would be making a classic mistake that 'greater then unity' power generator inventors often make.
 

I was hoping that working with DC would make power measurement simpler. You said that voltage across the resistor will be less than the source voltage. I thought when measuring across a resistor the reading is always lower than source voltage. Could you maybe clarify for me.
 

Hi,

I'm confused. You say motor power. So you measure motor voltage and current.
I assumed this is AC. But now you say it is DC...

Klaus
 

The power measurements I describe are DC side after rectifiers. On the AC output of gen.1 I measure 88 vac @ 2.2 amps when I do not have gen. 2 connected. In this configuration the total amp draw at motor is 4 amps @ 90vdc.
I then connect gen. 2 and gen.1 to the input terminals of bridge rectifier together. When I do this the motor accelerates and its amp draw drops to 2 amps @ 90 vdc. The AC output of gen. 1 now shows 86 vac @ 1.75 amps. Current through gen. 2 is 1 amp @ 86 vac.
Keep in mind that gen. 2 has no prime mover and doesn't provide any power. When I connected gen. 2 I expected it to increase the load on generator 1 but instead it decreased.

Motor turning gen.1 no load- .81amps 90 vdc @ motor. 2060 Rpm. Only variac supply power.

Motor turning gen. 1 with output looped back to motor plus variac - 4 amps 90 vdc @ motor. 1750 rpm.

Motor turning gen. 1 with gen.2 connected plus variac - 2 amps @ 90 vdc. 1935 rpm
 

The power measurements I describe are DC side after rectifiers. On the AC output of gen.1 I measure 88 vac @ 2.2 amps when I do not have gen. 2 connected. In this configuration the total amp draw at motor is 4 amps @ 90vdc.
I then connect gen. 2 and gen.1 to the input terminals of bridge rectifier together. When I do this the motor accelerates and its amp draw drops to 2 amps @ 90 vdc. The AC output of gen. 1 now shows 86 vac @ 1.75 amps. Current through gen. 2 is 1 amp @ 86 vac.
Keep in mind that gen. 2 has no prime mover and doesn't provide any power. When I connected gen. 2 I expected it to increase the load on generator 1 but instead it decreased.

Motor turning gen.1 no load- .81amps 90 vdc @ motor. 2060 Rpm. Only variac supply power.

Motor turning gen. 1 with output looped back to motor plus variac - 4 amps 90 vdc @ motor. 1750 rpm.

Motor turning gen. 1 with gen.2 connected plus variac - 2 amps @ 90 vdc. 1935 rpm

I think there is something wrong with the bridge , might have it wired wrong or that it is not rated for the voltage.
 

The bridge I'm using is rated 100 amps 1000 volts and wired correctly and is working properly. It seems to me that we when I connect gen. 2 it cancels or bucks the output of gen. 1. which has and unloading effect therefore the motor speeds up. Since it doesn't cancel the generator output completely it still delivers 70-80 watts to the motor. Do you think I am on the right track with this assumption or is there something about my measurements that makes you think otherwise.
 

A motor driving a generator, which feeds back (via a bridge rectifier) into the same motor. And the oher generator does no have a prime mover............

This doesn't make sense at all.
-or-
This has the smell of an over unity project.
 

Came across a website this morning about power in resistive and reactive circuits. One paragraph in particular seems to answer both my questions. Why motor speeds up and why windings of gen. 2 do not get hot. This is how it reads.

Note that the power alternates equally between positive and negative.this means that power is being alternately absorbed from and returned to the source. If the source were a mechanical generator, it would take ( practically ) no net mechanical energy to turn the shaft, because no power would be used by the load. The generator shaft would be easy to spin, and the inductor would not become warm as a resistor would.

So it seems that gen. 2 is acting as a purely reactive load.
 
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I bought a new DMM yesterday. When I checked the AC voltage input of B2 ( bridge rectifier 2) I read 101 vac. with new meter. My other DMM reads 90.8 vac. When I measure voltage with them at 120vac receptacle they are within .1 volts of each other. The frequency I measure is around 210 hz. Any ideas what is causing this 10 volt difference in meters.
 

Compare DMM specifications. Most likely one meter is displaying true RMS, the other averaged rectified value, scaled as RMS. Difference occurs only with non-sinusoidal waveforms.
 

The new meter is cen-tech 61593.
Other meter Klein mm400
 


The cen-tech manual says it will measure up to 700 volts rms AC sine. Like you say it's not really very explicit though. Would you expect the rms reading to be the higher of the two. Or would it depend on other factors.
Also when taking measurements on non sinusoidal source what meter would be preferred. Does it effect the outcome of power measurements. Will the math work out the same as long as I use the same meter to take all the measurements.
 

I would check the waveform and also measure the voltage with an oscilloscope with an oscilloscope. The relation of averaged rectified value and true RMS depends on the form factor. See https://en.wikipedia.org/wiki/Form_factor_(electronics)

Does it effect the outcome of power measurements.
In the general case, you can't derive AC power from separate voltage and current measurements. A dedicated power meter is needed.
 

I've been debating whether to buy a scope are a power analyzer first. The analyzer would be great but I haven't found one at a price I can afford right now. I ordered a instek gos 622 g analog scope today. Bought on eBay for 150 us$ brand new in the box (supposedly). Trying to make AC power measurements accurately with a DMM and amprobe is and exercise in futility.
I found a method online for measuring power factor with DMM. Google " three voltmeter method for measuring power factor". This method is only good for ballpark measurements. The slightest difference in your voltage measurements will effect the outcome greatly. I thought about buying one of those cheap plug in kill a watt meters but I don't know if it would be accurate for anything other than pure 60 hertz ac.
 

three voltmeter method for measuring power factor
Apart from possible accuracy problems, the method is only accessing displacement power factor and doesn't work for non sinusoidal waveforms.

In your test setup, the motor and generator power can be best determined by measuring DC currents, under the condition that DC voltage ripple is relative low.
 

Received my scope today. You couldn't knock the smile off my face with a baseball bat right now. Was hoping I could get a little help in setting it up on my project. I would like to monitor voltage and current phase shift. I felt comfortable enough hooking the scope to a CT on my generator output. But I'm not sure safest way to monitor voltage. From what little I know you can cause a dead short through case ground if not attached properly. Any advice would be much appreciated. Kajunbee
 

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