The following are formulae used to calculate some important aspects of a variable speed drive application such as torque and speed.
Synchronous speed of an ac induction motor = 120 x frequency / number of poles,remembered easier with the formula :
Where : f= frequency
p= number of poles
n = speed in rpm
Torque is defined as a rotating force or work in a rotary motion. When calculating work, we use the formula force x distance. As such, the formula for torque is force x radius. When converted into electrical terms:
3-phase Power calculation
Power (HP) = Voltage x Current x Power Factor x 1.73/ 746
DC bus voltage
To calculate the dc bus voltage of an ac drive, for a 3 phase rectifier on the drive input, the DC bus voltage is input ac voltage x √2
Another popular calculation related to motor loads in general is power factor.
Power factor is defined as the ratio of Real/Active Power (kW) over Apparent Power (kVA). The description and calculations related to power factor probably deserves a post of its own, to be included later.
