Move it with a VFD - Part 1

A VFD is a Variable Frequency Drive. It works with AC motors and sets
them into motion. It does so by varying the frequency of the voltage
supplied to them. There you go. That's it. Simple... but now let's see
some details.

The VFD controls two factors of the motor... speed and torque. As you
may remember from your school days, torque is a rotational force and
speed is ... well... speed. Combine the 2 and you get into horsepower
but that's a topic for another issue.

An induction motor (real commonly used by folks in the industrial
factories of the world) would rotate at a fixed speed that is
determined by the frequency of the voltage supplied to it.

Alternating current (i.e. AC)applied to the motor windings in the
'stator' produces a magnetic field that turns the motor shaft. We vary
the speed of the motor shaft by either changing:
1- The frequency of the AC applied to the windings
2- The number of magnetic poles in the windings

So, using a little math... and I mean a 'little math'... we can
calculate the speed of the motor. Just use the formula:
(120 * frequency) / number of poles = speed

If we supply a 50Hz voltage to a 4 pole motor and use the formula
above we can see that we will be turning the motor shaft:
(120 * frequency) / number of poles = speed
(120 * 50) / 4 = speed
(6000) / 4 = 1500rpm (Revolutions Per Minute)

So, if we examine the formula above a little closer, we can see
that if we want the motor to go faster we can increase the frequency
of the AC voltage applied to the windings. If, for example, we change
from 50Hz to 60Hz in the example above we get:

(120 * frequency) / number of poles = speed
(120 * 60) / 4 = speed
(7200) / 4 = 1800rpm (Revolutions Per Minute)

So, now instead of turning at 1500rpm we're flying at 1800rpm.

How else can we change the speed? Well as we read above we can also
change the number of poles our motor has. So, if for example our
motor had 8 poles in the original example we'd be moving at:
(120 * frequency) / number of poles = speed
(120 * 50) / 8 = speed
(6000) / 8 = 750rpm (Revolutions Per Minute)

So, now instead of turning at 1500rpm we're crawling at 750rpm.

Trouble is, however, that the poles are a physical 'fact' of the motor
so we can't just change them on the fly. We'd have to change the whole
motor! What good is that if we just want the speed to vary?
No good...

So, you can see that the best way for us to change the speed of the
motor is to 'vary the frequency' of the voltage being applied to it.
Can we do that? Sure we can... with a Variable Frequency Drive (VFD)!

Stay tuned as next time we'll see how the VFD actually works along
with it's relationship to our powerful friend 'torque'.

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