Hello Everybody
The past friday i was talking with a co-worker and we where arguing about 3 phase motor.
So he says that if a configure the connections of a motor for work at 220VAC, and by making an arrange of connection, switch the configuration for working with 440VAC, and still supplying the 220VAC the motor RPMs will drop down...
what do you thing? i can be done??
Thanks in advance.
So to recap, based only on the highlighted text and foregoing all the side tracks...
The motor speed will not technically change. Speed in an AC motor is a function of the applied frequency and the number of poles in the motor winding configuration. Applying a lower voltage does nothing to change either of those.
BUT, when you reduce the voltage, you reduce the motor output torque capability by the square of the voltage reduction. So at 50% voltage, the torque drops to .50 x .50 = .25, so 25% of the available torque. And because motor HP is a function of speed and torque, and you have not technically changed the speed, you have reduced the motor HP to 1/4 of what it was. For example, if you had a 10HP motor and you applied 1/2 voltage to it, it effectively becomes a 2-1/2HP motor, but the speed is the same.
Now, if your load has not changed, and it took more than 2-1/2HP to move the load, then the motor will technically slow down. But as mentioned by others, it will stall completely and overload. That is the basics of what you need to know.
Now to the not-so-common occurrence.
When reducing the voltage AND THE LOAD but only to the point of not stalling, you will increase the slip, which causes an increase in current to probably around 300% of "normal". So if, from that 10HP example, you only needed maybe 2.6HP to move the load, it may do it. Because the voltage is reduced, the maximum current draw is equally reduced as well so that 300% is skewed by the voltage reduction; it is 300% of 25%, so it will really only draw about 75% of FLA and likely do so forever. But this is a very dangerous game. The 300% current draw comes from what is called the Breakdown Torque capability of the motor, defined as the
torque produced before the motor slip increases beyond maximum. If you fall behind that curve, even a little bit, the effective increase in current rises rapidly. If you can perceive a speed droop then, that means you are ALREADY behind that curve! If, for example, you drop even 10% more speed than that peak, your current increases to 112.5% FLA (450% of 25%) and the motor will overload.
Bottom line, don't do it unless your load is SIGNIFICANTLY LESS than 25% of what it was going to be with the correct voltage.