The IMC program calculates the steady-state running conditions for one three-phase induction motor at a time. The induction motor equivalent circuit is used in the calculations.
Here are some questions that we may have about a motor installation:
Question 1 : A 25 HP, 460 Volts, 1500 RPM three-phase induction motor has a mechanical loading equal to 80 % of motor rated power when measured at 102 % of motor rated voltage. The motor has no speed control. The load torque is assumed proportional to the square of the speed. What are all the steady-state running conditions at 102 % voltage (the speed and the slip are unknown before the calculations) ?
Question 2 : Same motor and same mechanical load. No speed control. What are the conditions of the motor if stator voltage amplitude falls to 90 % of motor rated voltage (again both the steady-state speed and the slip are unknown before the calculations) ?
Question 3 : Same motor and same mechanical load. If we add a variable frequency drive (VFD) speed control, what are motor conditions and the powers seen by the network if the motor speed is set equal to 1100 rpm ?
The Induction Motor Calculations (IMC) Program can help us answer these questions.
The IMC Program has 4 options :
- Option 1 : Calculations of motor starting conditions at zero speed.
When the motor has a variable frequency drive (VFD) speed control, the user must specify the motor starting current that he wants (see parameter CC1 on IMC Start-Up Dialog Box (next image)).
- Option 2 : Calculations of motor conditions at maximum torque.
- Option 3 : Calculations of motor steady-state conditions when motor speed or motor slip is known before the calculations
User must specify stator voltage amplitude, stator voltage frequency and motor speed on IMC Start-Up Dialog.
- Option 4 : Calculations of motor steady-state conditions when motor speed and motor slip are both unknown before the calculations
When motor has no speed control : User must specify stator voltage amplitude and stator voltage frequency on IMC Start-Up Dialog.
When motor has a speed control : Nothing to specify. IMC program does all the calculations.
Options 1 and 4 can do calculations for three-phase induction motors with or without variable frequency drive VFD speed controls.
Options 2 and 3 cannot do calculations for motors with VFD speed controls.
Option 3 corresponds to conventional calculations we usually see in the technical literature.