Effects of voltage on AC induction motor 2

[gtf]

Could someone advise the actual effects on the ac induction motor when the supply voltage decreases and what happens when voltage increases. What happens to the motor currents and torque ?

Please provide formula if possible

Thank you.

 

[jbartos]

Suggestion: Reference:
1. Say M. G., "Alternating Current Machines," John Wiley & Sons, New York, 1978
Essentially, when
a. Voltage decreases, the current increases, under assumptions that shaft horsepower is constant and power supply frequency is constant.
b. Voltage increases, the current decreases, under assumptions that shaft horsepower is constant and power supply frequency is constant.
Torque=number of phases x square secondary current in primary terms x secondary resistance in primary terms / (slip x synchronous angular velocity), in newton-meters.
Parameters of the motor are often referred to the secondary (rotor side) and:
Secondary current=Secondary voltage (sE2)/(secondary impedance R2 + jsX2), when motor speeds up.
Visit

http://www.lmphotonics.com/

for more info.

 

[electricpete]

The following page has a graph of Full Load Amps vs voltage (see Figure 2 about 1/2 way down the page):

http://www.reliance.com/mtr/pwrspy.htm

The graph shows that F.L.A. is usually at a minimum at (or slightly above) nominal voltage.

As a first order model, the speed is approximately constant with voltage and load (since the speed changes only at most a few percent over normal ranges). If the speed doesn't change, then the torque cannot change, because the motor must match the load's torque-speed curve for the constant speed. Since power is the product of torque and speed, power is approximately constant. Neglecting power factor variations, the product of curent and voltage will also be constant so current must increase as voltage decreases below nominal.

But why does current also increase when voltage increases significantly above nominal? Because the assumption of constant power factor doesn't hold as we increase voltage significantly above nominal.... the core will go into saturation and draw extra inductive current (which doesn't contribute to real power).

Another refinement to the model... if you watch closely as you decrease voltage you would actually see speed decreasing slightly. The reason is that the torque speed curve in the operating range shifts down as voltage decreases. Accordingly, the running torque will likely be slightly less also.

All above discussion pertained to running behavior. For starting, starting current is proportional to voltage, and starting torque is proportional to voltage squared.

 

[rhatcher]

good one electricpete.

 

[jbartos]

Small encore:
1. The motor shaft load or shaft power P is usually considered constant during for small electrical terminal parameter intervals.
2. Generally, the Power P = Voltage V x Current I. If the power is kept constant and voltage increases, then the current must decrease. Conversely, if the P is kept constant, and voltage decreases, then the current must increase, to satisfy the above power P equation.
However, the above relationships represent very approximate reasoning behing much more complex relationships for which nowadays are software to cover them accurately and quickly.

 

[gtf]

Thank you for all the explanation

 

[SPI]

BETTER REFFER TO VECTOR REPRESTATION OF INDUCTION MOTOR.
TIPS
1)INDUCTION MOTOR OPERATION ON LAGGING POWER FACTOR
2) INDUCTION MOTOR BASICALLY INDUCTIVE CKT
3) ANY INDUCTIVE CKT CONSUME REACTIVE POWER i.e.
CAHNGE VOLTAGE PROFILE OF THE SYSTEM.