High harmonic content and lightly loaded motors??

I know what harmonics are but how are they generated within the power system and why do they have a negative impact on power delivery efficiency?

Why is a motor running under a light load inefficient or cause the pf to dip?

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private
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in article snipped-for-privacy@private.com, private at snipped-for-privacy@private.com wrote on 10/17/03 2:21 PM:

If it is a typical induction motor, you are seeing the effect of leakage inductance and magnetizing inductance. After all, and induction motor is modelled well by a model of a transformer. Although the power factor may be small, the reactive power is small compared to what the real power will be at full load.

If you are looking for high efficiency, look at motors just large enough to do the job. They do not carry the high overhead of spining large rotors.

Bill

Reply to
Repeating Decimal

---------- There are harmonics in the supply voltage. These are usually very small. However many loads are non-linear (i.e. most electronic loads) and generate harmonics. Harmonic currents produce extra I^2R losses in the system and harmonics generated by one load may also have adverse effects on other loads.

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--------- A typical induction motor has a magnetising current which is quite large (i.e. no-load current approximately half the magnitude of full load current) and this is very inductive so the power factor at no load is poor. As the load increases, the "power" component of the total current increases faster than the reactive portion so the power factor is better at full load. In addition, the mechanical losses are the same at no load as at full load and at no load the power output is 0 giving an efficiency of 0 (efficiency =Pout/Pin =Pout/(Pout +Ploss and if Pout=0 then....) . Efficiency will rise as useful output (Pout) rises. until the I^R losses exceed the constant or no -load losses (Ploss =I^R loss + no-load loss). Design of the motor makes this typically occur near full load.

-- Don Kelly snipped-for-privacy@peeshaw.ca remove the urine to answer

Reply to
Don Kelly

in article snipped-for-privacy@private.com, private at snipped-for-privacy@private.com wrote on 10/17/03 2:21 PM:

I just realized that I did not answer your funcamental question. Magnetizin currents can be nonlinear.

Also, if you are thyristor control to reduce the speeed of a motor. That will introduce harmonics.

Bill

Reply to
Repeating Decimal

Distantly related: Consider a substation with AC in, and a 1000MW HVDC transmission line out. What do they do to keep the AC-DC conversion from generating harmonics? What kind of circuit do they actually use for this AC-DC conversion? (imagining a huge 6 leg bridge rectifier charging a HUGE capacitor!) I know the DC-AC conversion on the other end is usually more interesting, but ...

Reply to
Michael Moroney

___________ They do the same thing as is done for small power supplies. That is -put in filters. Yes- the filters are big and expensive but are more sophisticated than use of a fat capacitor as they generally are tuned for the harmonics which are the most troublesome. Also some harmonics are naturally eliminated . Such filters will be applied at both ends. check out (among otherrs):

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-- Don Kelly snipped-for-privacy@peeshaw.ca remove the urine to answer

Reply to
Don Kelly

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