How Do AC Drives Work?
As you learned in Chapter 1, AC motor speed is controlled by frequency. An AC drive is a
device for controlling the speed of an AC motor by controlling the frequency of the
voltage supplied to the motor. It does this by first converting 3 phase 60 Hz AC power to DC
power. Then, by various switching mechanisms, it inverts this DC power into a pseudo sine wave
3 phase adjustable frequency alternating current for the connected motor. Because of this, some
people call AC drives “inverters,” although this is technically incorrect.
The frequency coming in to the converter has a fixed frequency of 60 Hz. However, the
adjustable frequency coming out of the inverter and going to the motor can be varied to suit the
There are two general types of solid state frequency control systems available: six step and
pulse width modulated (PWM) control. All of Square D AC drives use the pulse width modulation
(PWM) method of frequency control, and that is the one we will concentrate on here.
Let’s look at how an AC drive functions in a little more detail. The two main sections of a PWM
drive are the converter and the inverter.
Three phase 60 Hz AC power is coming into the converter. The converter typically uses a
rectifier (which is a solid state device that changes AC to DC) to change the incoming 60 Hz AC
into a rectified DC voltage.

The DC voltage coming out of the converter is rather rough. Different types of filtering can be
used to smooth out the rectified DC so that it is of a more or less constant voltage value. This
filtering takes place between the converter and inverter stages. This “smoothed” DC is then sent
on to the inverter.
The inverter section produces an AC output which is fed to the motor. Positive and negative
switching occurs within the Inverter which produces groups of voltage pulses. The output
frequency of an PWM drive is controlled by applying positive pulses in one half cycle, and
negative pulses in the next half cycle. The pulses within each group have varying widths that
correspond to voltage values. Notice on the output side of the inverter that the narrow voltage
pulses represent the lower voltage values on the sine wave and that the wider voltage pulses
represent higher voltage sine wave values. The varying of the pulse widths gives this method its
name of Pulse Width Modulation (PWM).
This diagram is only showing 6 pulses per half cycle. For each specific frequency, there is an
optimum number of pulses and pulse widths that will closely simulate a pure sine wave.

Volts per Hertz Ratio

When current is applied to an induction motor it generates magnetic flux in its rotating field and
torque is produced. This magnetic flux must remain constant in order to produce full-load torque.
This is most important when running a motor at less than full speed. And since AC drives are
used to provide slower running speeds, there must be a means of maintaining a constant
magnetic flux in the motor. This method of magnetic flux control is called the volts-per-hertz
ratio. With this method, the frequency and voltage must increase in the same proportion to
maintain good torque production at the motor.
For example, if the frequency is 60 Hz and the voltage is 460 V, then the volts per Hertz ratio
(460 divided by 60) would be 7.6 V/Hz. So, at half speed on a 460 V supplied system, the
frequency would be 30 Hertz and the voltage applied to the motor would be 230 V and the ratio
would still be maintained at 7.6 V/Hz.
This ratio pattern saves energy going to the motor, but it is also very critical to performance. The
variable-frequency drive tries to maintain this ratio because if the ratio increases or decreases as
motor speed changes, motor current can become unstable and torque can diminish. On the other
hand, excessive current could damage or destroy the motor.
In a PWM drive the voltage change required to maintain a constant Volts-per-Hertz ratio as the
frequency is changed is controlled by increasing or decreasing the widths of the pulses created
by the inverter. And, a PWM drive can develop rated torque in the range of about 0.5 Hz and up.
Multiple motors can be operated within the amperage rating of the drive (All motors will operate
at the same frequency). This can be an advantage because all of the motors will change speed
together and the control will be greater.

The type of load that a motor drives is one of the most important application considerations when
applying any type of AC drive. For some types of loads, the application considerations may be
minimal. For other types of loads, extensive review may be required. Generally, loads can be
grouped into three different categories:
·  Constant Torque Loads - conveyors, hoists, drill presses, extruders, positive displacement
pumps (torque of these pumps may be reduced at low speeds).
·  Variable Torque Loads - fans, blower, propellers, centrifugal pumps.
·  Constant Horsepower Loads - grinders, turret lathes, coil winders.

Constant Torque Loads
Constant torque loads are where applications call for the same amount of driving torque
throughout the entire operating speed range. In other words, as the speed changes the load
torque remains the same.

The chart shows speed on the bottom and torque on the left. The torque remains the same as
the speed changes. Horsepower is effected, and varies proportionately with speed. Constant
torque applications include everything that are not variable torque applications. In fact, almost
everything but centrifugal fans and pumps are constant torque.

Variable Torque Loads
As was just mentioned, there are only two kinds of variable torque loads: centrifugal pumps and
fans. With a variable torque load, the loading is a function of the speed. Variable torque loads
generally require low torque at low speeds and higher torque at higher speeds.
Fans and pumps are designed to make air or water flow. As the rate of flow increases, the water
or air has a greater change in speed put into it by the fan or pump, increasing its inertia. In
addition to the inertia change, increased flow means increased friction from the pipes or ducts.
An increase in friction requires more force (or torque) to make the air or water flow at that rate.
The effects that reduced speed control has on a variable torque fan or pump are summarized by
a set of rules known as the Affinity Laws. The basic interpretation of these laws is quite simple:
1. Flow produced by the device is proportional to the motor speed.
2. Pressure produced by the device is proportional to the motor speed squared.
3. Horsepower required by the device is proportional to the motor speed cubed.

The cube law (third item) load is at the heart of energy savings. The change in speed is equal to
the horsepower cubed. For example, you might expect a 50% change in speed would produce a
50% change in volume, and would require 50% of the horsepower. Luckily for us, this 50%
change in speed must be cubed, representing only 12.5% of the horsepower required to run it at
100% speed. The reduction of horsepower means that it costs less to run the motor. When these
savings are applied over yearly hours of operation, significant savings accumulate.
This table will help show these relationships:
% Speed   % Torque    % HP
    100            100            100
     90              81             72.9
     80              64             51.2
     70              49             34.3
     60              36             21.6
     50              25             12.5

 Constant Horsepower

A constant horsepower load is when the motor torque required is above the motor’s base speed
(60 Hz). With a constant horsepower type of load, the torque loading is a function of the
changing physical dimensions of the load. These types of applications would include grinders,
turret lathes and winding reels. Constant horsepower loads require high torque at low
speeds and low torque at high speed. While the torque and speed changes the horsepower
remains constant.

For example, an empty reel winding a coil will require the least amount of torque, initially, and
will be accelerated to the highest speed. As the coil builds up on the reel, the torque required will
increase and the speed will be decreased.
An electric motor moves its load and demands whatever amount of power is required to get the
load moving and keep it moving. Once the load is in motion it has inertia and will tend to want to
stay in motion. So, while we must add energy to get the load into motion, we must somehow
remove energy to stop it.
Some large motor loads develop high inertial forces when they are operating at high speed. If
voltage is simply disconnected from the motor, the load may coast for several minutes before
the shaft comes to a full stop. This is true in applications such as those involving large saw
blades and grinding wheels. It’s important for safety reasons to bring these loads to a smooth
stop quickly.
In other load applications, such as elevators and cranes, the location where the load stops is as
important as moving the load. This means that the motor shaft must stop moving at a precise
time to place the load at its proper location.
There are several different types of braking techniques used, however we are only going to
mention the two types used on Square D drives: DC braking and Dynamic braking.
In DC braking, DC current is applied to the stationary field of an AC motor when the stop button
is depressed. Since the field is fixed and it replaces the rotating stator field, the rotor is quickly
stopped by the alignment of the unlike magnetic fields between the rotating and stationary
windings. The attraction between the rotating and stationary fields is so strong that the rotor is
stopped quickly. This method of stopping is only effective at 10 Hz and below. The energy
created with this type of braking will be dissipated in the rotor and care needs to be used when
applying this type of braking. Square D uses this method to a limited degree.

Dynamic braking is used by Square D in its Altivar 66 drives. When the voltage is removed
from a motor and the inertia of the load continues on in motion, the motor is being driven by the
load until it coasts to a stop. Since the rotor will continue to spin, it will produce voltage and
current in a manner similar to a generator. This generator action can be used to bring the rotor to
a quick stop by sending the generated energy out to a resistor. There the energy is dissipated as
heat through the resistor. The resistor will cause the rotor to generate very high levels of current,
which produces magnetic forces on the shaft and causes it to stop quickly.