Category Archives: VFD

Variable speed drive speed, torque and power

VFD,

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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What is a Variable Frequency Drive or a Variable Speed Drive?

VFD, ,

VSD’s or VFD’s , also referred to as frequency converters or adjustable speed drives, are devices that convert fixed frequency supply voltage ( typically 50Hz or 60Hz) to a variable frequency voltage. The frequency of voltage supplied to a motor determines the speed at which that motor rotates.

VSI_Topology

By Cblambert (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons

What are the benefits of a drive (VFD) over a motor starter?

    • Drives protect motors from the in rush of current when starting.
    • In applications where full speed operation is not required, drive saves energy by facilitating operation at lower speeds.
    • Drives allow for speed regulation to maintain the set point of a process ( could be a pump motor speed for pressure and flow or fan speed for temperature)
    • In applications where a high torque is required at a low speed, drives are able regulate both speed and torque at its output to allow for continuous operation a low speed. An example could be a hoist where the load is suspended ( at zero speed)in the air without the engagement of brakes.
    • Drives are able to provide current and torque limiting functionality so as to prevent motor and other equipment damage.
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