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 09-01-2014, 10:57 PM #1072 (permalink) MPaulHolmes PaulH     Join Date: Feb 2008 Location: Maricopa, AZ (sort of. Actually outside of town) Posts: 3,832 Michael's Electric Beetle - '71 Volkswagen Superbeetle 500000 Thanks: 1,368 Thanked 1,201 Times in 765 Posts 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! __________________ kits and boards
 The Following User Says Thank You to MPaulHolmes For This Useful Post: thingstodo (09-02-2014)