Variable speed drive speed, torque and power

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 :

synchronous speed 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:

Torque formula

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.

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