Adjustable Speed Drive Technologies
“Wave Chopping” vs. Inverters and the role of Slip
The two main benefits of slowing motor speed are process control and energy
conservation. Two standard methods for achieving this are through inverters
or by “Wave Chopping” (phase cutting).
Each of these processes will be explained in more detail below, but it is
important to first take a quick refresher course in motor basics.
The synchronous speed (RPM) of an A.C. induction motor is:
where Ns = the synchronous speed and P= the number of poles.
Therefore, based on this relation, the synchronous speed of a typical 60Hz 6-
pole motor is 1200RPM.
Ns=120 x (60Hz) = 7200 = 1200RPM
However, due to slip in electrical induction motors, the actual achieved RPM
will be slightly less. Slip is speed differential between the rotor bars and the
stators’ rotating magnetic field when current is applied. Slip can be defined as:
S=Ns-N or N=(1-S) Ns
Ns=synchronous speed of magnetic field (rev/min, rpm)
N=actual shaft rotating speed (rev/min, rpm)
Therefore, as a result of slip, a 60Hz 6-pole motor is rated at 1075 to 1175 RPM.
Slip is necessary for torque generation. When a motor first starts running,
the slip is at 100% and voltage is at maximum. As the motor picks up speed,
slip and voltage are reduced. Motor frequency will also decrease as the slip
During this initial start-up period, a motor will draw more current to help
overcome higher rotor impedance (the phase sum of the constant resistance
and the variable inductive reactance). With more current flowing through the
motor, excess heat is generated. As a motor approaches its optimal running
speed, the inductive reactance goes down equaling the resistance, thereby
drawing less current to operate and allowing the motor to run “cooler”.
“Wave Chopping” or phase cutting looks to reduce motor speed by reducing
voltage and torque. This is accomplished by a reduction in voltage at line
frequency. The result is an increase in slip.
As the motor slows and approaches stall, the back electromotive force (emf)
or “back torque” of the motor decreases and the motor current remains
essentially constant. As the RPM is decreased and subsequently the air over
is reduced, the motor temperature will rise as the motor speed decreases.
While effective and economical, a “slip controlled drive” results in excess heat
gain that makes this process less efficient. It can also adversely impact the
motor and/or system unless certain precautions are taken to minimize the
risk of bearing wear and/or motor burnout due to the temperature rise in the
Inverters, or variable speed drives (VSD), accomplish motor speed control by
changing the frequency AND the voltage of the motor. Therefore, speed is
attributable to a change in frequency and the corresponding adjustment
made to motor voltage.
Using this motor control method, the slip differential remains low and the
motor maintains torque at a reduced speed. With less current required, the
motor windings remaining cool. The net result is greater energy savings with
minimal risk to the motor and/or system.
To better understand the energy savings potential of an inverter process, we
can look at the relationship below:
Using as an example:
If a motor operating at 60Hz with power = 1hp,
Then the motor power of a motor running at 30 Hz will =1/8 hp.
From the above example, one can deduce that at theoretical half speed,
motor power can be as little as 1/8 that of the motor at full speed. For fan
blower speeds, this means that if a motor consumes 1000 watts of power at
60Hz, that same motor operating at as little as 125 watts of power can move
½ the amount of air. This huge power reduction translates into greater motor
efficiencies and a huge energy savings potential.
Even when typical inverter losses that will lessen the efficiency gains are
factored into the equation above, half the airflow at 1/5th the power is very
While both ASD technologies will achieve the same results – motor slowdown
– inverters are clearly the preferred, more efficient and more beneficial
process to use in most applications.