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Old 09-01-2014, 10:57 PM   #1072 (permalink)
MPaulHolmes
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So, following Edward Cheeseman's lead, a guess for the rotor time constant of my motor is as follows:

Zmotor = Rmotor + i*(2*pi*freq*Lmotor).
Now, ||Zmotor|| = 230Vrms / 20.5Amp = 11.2195 Ohm. (230Vac and 20.5Amp is from the nameplate.)

The power factor is 0.78, which is also from the nameplate. So, Rmotor = ||Zmotor|| * 0.78 = 11.2195 Ohm *0.78 = 8.7512 Ohm. The frequency on the nameplate is 50Hz.

If anybody remembers imaginary numbers in math class,
Z = a + b*i means
||Z|| = sqrt(a^2 + b^2)

So,
||Zmotor|| = sqrt(Rmotor^2 + (2*pi*freq*Lmotor)^2)
11.2195Ohm = sqrt((8.7512Ohm)^2 + (2*pi*50Hz*Lmotor)^2)

**mathy mathy math math**

Lmotor = 0.02431Henry

**It was assumed that Lrotor = 1/2 * Lmotor **

Lrotor =1/2 * Lmotor = 1/2 * 0.02431 Henry = 0.01216 Henry

************************************************** ****
To find Rrotor, Edward did the following:
Assume the motor runs at 92.5% efficiency at full load. The motor is rated at 5.5kW at 230V. The full load current is 20.5Amp. So,

PowerLossTotal = (1-0.925)*5500Watt = 412.5Watt
PowerLossPerPhase = PowerLossTotal / 3 = 137.5Watt
PowerLossRotor = PowerLossPerPhase / 3 = 45.83Watt **It was assumed that the rotor loss was 1/3 of total losses.**

PowerLossRotor = currentRotor*currentRotor*Rrotor, So
45.83W = 20.5Amp * 20.5Amp * Rrotor
So, Rrotor = 0.10906Ohm
************************************************** *****
So, the almighty rotor time constant is Lrotor / Rrotor = 0.111 seconds in my case!
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