Using Option 4 of the IMC program (examples)

SimPhase Induction Motor Calculations (IMC) program has 4 options for three-phase induction motors.

Option 1 calculates the starting zero-speed conditions, option 2 calculates the maximum torque conditions while options 3 and 4 calculate the steady-state running conditions of a motor using its per-phase equivalent circuit.

Option 3 corresponds to conventional induction motor calculations. The terminal voltage and the slip must be known before the calculations (the slip may be calculated using the known speed value). This is the type of motor calculations we see 99% of the time when we learn about induction motors.

With option 4 we only need to know the voltage before the calculations.

Sometimes we may not be able to use option 3 but we know enough to use option 4.

It is important to mention that the speed (and thereby the slip) of an induction motor changes when the terminal voltage changes. An induction motor is not a constant impedance machine.

Example : Three situations. The same 25 HP, 460 Volts, 1500 RPM three-phase induction motor and the same mechanical load are used in all 3 situations. The load torque is assumed to be proportional to the square of the speed (the load could be a water pump).

Simulation 1 : What are the running conditions of the motor if it is 80% loaded with the terminal voltage equal to 100% of its rated value ? The motor has no speed control. Both the slip and the speed are unknown before the calculations.

We cannot use option 3.

Image 1 shows the running conditions calculated by option 4.

Image 1 Simulation 1 results

Simulation 2 : Same motor and same mechanical load. No speed control. What are the conditions of the motor if the terminal voltage amplitude falls to 90 % of its rated value ?

We cannot use option 3 (again both the slip and the speed are unknown).

Image 2 shows the running conditions calculated by option 4.

Image 2 Simulation 2 results

Simulation 3 : Same motor and same mechanical load. But now the motor has a variable frequency drive (VFD) speed control. What are the steady-state running conditions if the motor speed is set equal to 1000 rpm ?

We cannot use option 3 because we do not know the terminal voltage amplitude and frequency.

Image 3 has the conditions calculated by option 4.

Image 3 Simulation 3 results