VFD course · Lesson 1 of 12

Why a VFD?

On its own, a 3-phase motor runs at roughly one speed and pulls a huge current while it starts. A VFD (variable frequency drive, also called an inverter) solves both problems: it starts the motor gently and lets you choose its speed. In this lesson you compare starting methods yourself and see how a small cut in speed saves a lot of electricity.

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1The problem: a motor while it starts

When you connect a motor straight to the supply (DOL, Direct On Line), the motor is stopped and the rotor has no back EMF, so it pulls 6–8 times its nameplate current until it gets up to speed. This causes:

A dip in the supply voltage: lights flicker and other equipment may trip.
A mechanical shock: torque jumps all at once onto the coupling, gearbox and belts.
No speed control: the motor is either at full speed or stopped.

A VFD = a device that takes power from the supply (fixed frequency, 50 Hz) and gives the motor power at variable frequency and voltage. Hence the name: Variable Frequency Drive.

2Try it: 4 starting methods

Same motor (15 kW) and same load. Pick a method, press «Start», and compare the red current curve.

Comparing starting methods

Method	Starting current	Speed control	When we use it
DOL	6–8 ×	No	Small motors on a strong supply
Star / delta	About ⅓ of DOL	No	Medium motors that start without a big load
Soft starter	2.5–4 × (you set it)	No, soft start and stop only	Fixed-speed pumps and fans
VFD	Close to rated	Yes, from 0 to any speed	Anything that needs variable speed or energy savings

3Speed = frequency

The speed of an induction motor is set by the supply frequency and the number of poles:

n_s = 120 × f ÷ P

Example: a 4-pole motor on 50 Hz: synchronous speed 120 × 50 ÷ 4 = 1500 rpm, and the actual nameplate speed is a little lower (1450 rpm, for example) because of slip. If the drive outputs 25 Hz ⇒ the speed is roughly half.

A VFD also gives you: soft start and stop on a ramp, reversing without contactors, motor protection (overload, short circuit, phase loss), and current and power readings on the display.

4Savings in pumps and fans

Centrifugal pumps and fans follow 3 rules (the affinity laws):

Quantity	Proportional to	At 80 % speed
Flow	Speed	80 %
Pressure (head)	Speed squared	64 %
Power	Speed cubed	51 %

So instead of closing a valve and leaving the motor at full speed, we lower the speed itself. Move the slider:

Savings calculator

Drill: pump laws

5From the field

From the field: a plant had a 55 kW cooling water pump running 24 hours a day with its valve half closed. After they fitted a VFD and ran the pump at a lower speed instead of throttling with the valve, the electricity bill for that pump dropped a lot, and the drive paid for itself in less than a year. This is the most common reason a VFD gets installed.

A VFD is not always the answer: if the motor runs at a fixed speed all the time and only needs a soft start, a soft starter is cheaper and simpler. A VFD also produces harmonics and needs a proper installation (we will see this in lesson 11).

?Lesson quiz

Get at least 5 of 6 right to complete the lesson.

The numbers and examples here are common training values, and parameter and terminal names differ from brand to brand and model to model: your drive's manual is the reference. Your progress is saved on this device.
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