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Operation and speed control of induction motors

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shermaine

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Hello,

May i know where can i find the information for operation and speed control of induction motors like a 3-phase squirrel-cage induction motor, determine the performance characteristics of an induction motor under load condition and no-load condition.
I have got some questions too.May i know where to get those help?
Pls advise.
(1) The caculated no-load input current and power of the motor as a percentage of their full-load(rated) values are considerably hiher than those of a transformer.Why is this so?

(2) Estimate the starting current of the motor(as percentage of its rated value) when it is started at rated voltage. How can the starting current be reduced to an acceptable value in practice?

(3) What's the relationship between the no-load speed and frequency?

Pls advise.
Thanks.
 
Those sound a lot like homework questions.

Let's play "Jeopardy"

1) I don't know the answer for sure, but I'd guess it has something to do with the motor spinning and the transformer not.

2) Alex, what is "six times"? Alex, what is an "Inverter, a softstarter, or an autotransformer"?

3) Alex, what is "slip"?

j.
 
1. Are two reasons for that:
A. The internal friction of rotor when it spin request additional current from main 3Ph.
B. The freq of current induced in rotor is much low than the main 3Ph freq because of spining, so the motor will not work at same conditions as a transformer. The main 3Ph feeding the static coils with the main freq and these coils create a rotating magnetic field which spin with main freq. Now if the rotor spin with the same freq as rot mag field will result no induction and no energy to shaft. So the rotor must spin with a freq smaller than freq of mag field for motor work and the induced currents have a freq equal with diference between mag field freq and rotor spin freq. The freq of induced currents will be equal with main 3Ph freq only when rotor not spin.

2. One practic method for reduce the value of starting, in case of freq controlled sistems, is to reduce the voltage and freq at start and increase them gradually with motor spining.

3. I don't know the exact math relationship between main freq and rotor spin freq but ca be deducted from working principle of motor.
 
Where can i find great report writing on operation and speed control of induction motors?
Any one here can advise me.
Thanks.
 
I too am toying with the idea of speed control for an induction motor.
I am looking at altering the frequency of the A.C. input.
For a 50Hz motor, an increase or decrease of 10 Hz should result in a +/- 20% change in motor speed.
This project is aimed at controlling a telescope and hopefully should overcome the tremors inherent in using a stepper motor. Also, as steppers are limited in speed, they require more complicated gearing.
I am looking at an oscillator, variable over the range of 30Hz to 70Hz, feeding a 100W amplifier which has a reversed power transformer as the load.
Anyone ever tried this?
 
Useing a normal audio amplifer would give a horrible efficiency.

Aslo, if you reduce the speed, you need to reduce the voltage proportionally, otherwise core saturation will occur and cause lots of smoke.
 
Due to the massive gear ratio required (1500:1 down), the motors required are less than 10W and the amplifier would not be audio quality. Probably pushpull 3055/2955 combination into a centre tapped 12V o/p transformer, the wave form is not that critical.
The proposed speed variation need be no more than +/- 20%. It is only to fine tune the final speed to 1 revolution in 23 hrs 56 min, (sidereal time).
 
Centretek said:
I too am toying with the idea of speed control for an induction motor.
I am looking at altering the frequency of the A.C. input.
For a 50Hz motor, an increase or decrease of 10 Hz should result in a +/- 20% change in motor speed.
This project is aimed at controlling a telescope and hopefully should overcome the tremors inherent in using a stepper motor. Also, as steppers are limited in speed, they require more complicated gearing.
I am looking at an oscillator, variable over the range of 30Hz to 70Hz, feeding a 100W amplifier which has a reversed power transformer as the load.
Anyone ever tried this?


be carefull when selecting your motor. some induction motors have a centrifugal switch which disengages the start winding after it reaches its operating speed. if you run the motor on the startup winding for long periods, you may burn the start winding.
 
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