# Time Taken To Start Rotor Resistance Starter

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Hi,

I am currently looking into designing a rotor resistance starter for a 640kW wound rotor IM, now ive managed to suss out the relevant resistances ill need for my starter to keep the current nice and low and have plenty of nice speed torque and speed current curves and load curves on my desk. However i am unsure how to determine the time that it will take for each resistance step to happen.

I assume that there is some sort of acceleration calculation with respect to load and motor, or perhaps energy transferred. But im not a very mechanically minded person and as a result am struggling a fair bit.

If you could point me in the right direction or suggest how i might solve my problem itd be greatly appreciated.

Regards,

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Hello tetbit

Welcome to the forum.

The time taken to accelerate is a function of the load inertia and the torque produced. As you increase the inertia, you reduce the rate of acceleration. As you increase the torque, you increase the rate of acceleration.

You can break the start up into steps of speed and calculate the time for each segment based on the average torque available with the resistors in the rotor.

Best regards,

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cool thanks heaps for that, ive been digging around on the internet and i found this following formula, is this what you were getting at? if there are any other sites or books you could recommend id be appreciative.

"

Calculating the time to accelerate a direct coupled load can be determined quite easily by utilizing the following formula:

t = (WR2 x N)/308T

T = AVERAGE ACCELERATING TORQUE IN LB. FT.

N = REQUIRED CHANGE IN SPEED

WR2 = INERTIA IN LB. FT.2

t = TIME IN SECONDS

"

Regards.

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That will work as long as you keep the units correct.

Most problems occur where the units are incorrectly mixed/scaled.

Best regards,

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