All Kinds of Electrical

Drive Variable

Why use a variable frequency drive along with an electric motor?

The three most important reasons are:-
1. Allows a wide range of speed control.
2. May be more economical and reliable than the alternatives.
2. Allows much HIGHER shaft speed than available using mains frequency.
. How is this?

. Motor rotating magnetic field RPM
= Frequency of applied voltage x 60 / No of pairs of poles.

. This shows max RPM occours with one pair of poles.
There cannot be less than one pair.
. The 60 converts RPSec to RPM.
Mains frequency is fixed at 60Hz for USA, 50Hz Europe.
. When a 50Hz supply is connected to a motor,
with one pair of poles, then the rotating field RPM
= 50 x 60 / 1 = 3000 RPM.
This the motors max speed!

The actual rotor or shaft RPM, will be about 3.5% less than this, due to necessary slip.
Modern woodworking cutters run in excess of 24,000 RPM.
What frequency is required to run a two pole motor at this speed?
Note two pole = one pair of poles.
Re-arranging; Freq
= Field RPM x Pole pairs / 60
= 24,000 x 1 /60 = 400 Hz.
Much aircraft equipment requires a 400Hz AC supply.
Electronic circuits can easily convert 50Hz/60Hz mains
to any other higher or lower frequency.

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