speed of vfd drive reduce as the load increses, i don't know why this happen??

[piyush_bhatt]

hello friends,

i am trying to build variable frequency based ac drives, and i got success at 50-60%. in no load condition the drive works fine and motor runs normally and i can vary speed too. but now my problem is when i attached 1A resistive load motor speed reduced to half level from its maximum. i don't know why this happen. i have design my ckt as follows.

single phase i/p - rectifier - pic controller(for generating pwm) - inverter - 3 phase ac motor.

my readings are....

single phase i/p dc volt o/p line-line volt-curent speed condition

270v(rms),1.5A 340vdc 200v(rms), 2.2A 1470rpm no load
270v(rms),4.5A 340vdc 216v(rms), 9.8A 660rpm load-1A_resistive


i guess anyone could help me to figure out this problem,

thanks
best regards

 

[BradtheRad]

When you apply 50 percent duty cycle, you create 50 percent average current through your load.

However you create 25 percent average power.

 

[piyush_bhatt]

ok, i found that my supply frequency to motor is 50hz at 200vrms so i have to reduce it to 25hz for 200vrms, beacause 50hz is for 440vrms voltage,

and i feel that beacause of reduce speed upon appling load the reactance of motor reduce so that current is flowing more then expected.

am i correct???

 

[BradtheRad]

I have incomplete knowledge about the forces working inside motors.

beacause of reduce speed upon appling load the reactance of motor reduce so that current is flowing more then expected.

Yes, certain types of AC and DC motors try to draw more current as their speed slows.

I am used to seeing current draw go down when the motor runs fast with no load attached.

However I do not know how the factors of load, reactance, and duty cycle combine and interact.

There are people who work with motors full-time, who could give better answers than me.

It might help if you were to run a number of experiments, and plot the results. Various duty cycles versus various loads. See what the speed turns out to be in each case.

I believe speed will not be linear according to duty cycle. You need to go above a certain duty cycle, for the motor to turn at all.