# Starting Torque At Lowest Frequency

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hi friends it is my first topic;

i am preparing a project about " induction motor speed control using plc and implementing pwm signals with microcontroller"

plc calculates the error speed signal (in hz) with PI algorithm and transmit it to the pwm inverter to obtain required signal by using the constant v/f algorithm. pwm inverter uses the error frequency and produces the required voltage according to constant v/f.

i wrote the PI but at starting time i must have a starting torque. namely a min voltage and a min frequency values. how can i obtain this values. the graph for constant V/f, look the vmin and fmin point;

http://img146.imageshack.us/img146/4518/vfsc7.th.jpg

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Hello Emrah

Welcome to the forum.

In an ideal motor, you would start at zero voltage and zero Hz.

In a practical motor, it is often a good idea to give a small voltage boost at very low frequencies because the stator winding is not purely inductive, it has resistance as well.

The idea is to try to keep the flux in the iron constant and with a purely inductive stator, you need to keep the v/Hz constant. When you have resistance as well, the series resistance begins to become dominant at low frequencies and you need to add some extra voltage to compensate for that. The "correct" amount of boost varies from motor to motor and also changes with size.

For a motor with no load, it is not an issue. It is only a problem where you need to increase the torque at very low speeds.

Best regards,

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thank you marke;

so in my program i have to define a boost voltage and frequency and implement my program according to this values.

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• 3 weeks later...

Now i implement my project in my brain and so, i want to have a constant speed with changes load. The same logic wtih the escalators.

I think i must have a constant torque, isnt it? I will give a starting frequency and a starting voltage to the induction motor (0,6 kw). And for example i want to have 900 rpm in both no load and full load condition. The motor speed never be changed.

And also for example at full load condition my load torque is "A" N-m and at no load condition my load torque is "B" N-m. A>B and i adjust my motor torque to "T" N-m. At starting and running conditions the torque states will be T>A>B. T,A,B are constant. Is it the right solution?

T is motor constant torque

A is the max load torque

B is the min load torque

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