DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING
ENEL280: ELECTRICAL SYSTEMS: Laboratory
AC MOTOR CONTROL
1. INTRODUCTION
This laboratory introduces power electronic variable speed AC motor control. The
principles of induction motor speed control are shown, and it is demonstrated how the
physical aspects of induction motors are incorporated into the control algorithms.
Industrial applications often require variable speed electric motor drives, e.g. cranes and
conveyor belts. Traditionally, these needs have been met with DC motors. However, DC
motors are complicated and require commutators and brush-gear. Such complexity
results in extra maintenance, extra expense and less plant production time.
More recently, three-phase induction motors have become the most common rotating
electrical machine found in industrial applications. Such motors are relatively
inexpensive, reliable and are available in a wide variety of power (kW) ratings.
Power electronic technology has been applied to create variable voltage and frequency
AC supplies. These supplies can be matched to rotating electrical machine requirements
to create variable speed induction motor drives. The SEW "Eurodrive Movitrac" AC
motor control unit is an example of a variable speed induction motor drive.
A basic motor speed drive circuit is
A 3 phase, fixed frequency voltage supply is rectified to dc and then converted to a
variable 3 phase ac voltage and frequency supply to the motor.
2. THREE-PHASE INDUCTION MOTOR CHARACTERISTICS
Within a three-phase induction motor a rotating magnetic field is established by the
stator windings. The speed (called the synchronous speed) of this stator field is given by
120 f s
ns = (rpm)
p
where fs is the three-phase supply frequency
p is the number of rotor poles
• For the laboratory induction motor, calculate the speed of the stator field at a
supply frequency of 50 Hz. Use the information written on the induction motor
name-plate for this calculation.
If the rotor motion is steady at a speed nr (rpm), the speed of the rotor field relative to the
rotor must be
n = n s − nr (rpm)
The induced currents within the induction motor rotor have a frequency
⎛ n − nr ⎞
f r = ⎜⎜ s ⎟⎟ f s (Hz)
⎝ ns ⎠
• Calculate the frequency of the induced rotor currents at the motor rated speed of
1400 rpm.
One characteristic of this induction principle is that as the induction motor is loaded, the
speed of its rotor slows with respect to the stator rotating field, and the frequency of the
currents in the rotor increases. This characteristic is often referred to as "per-unit slip",
where
n s − nr
s=
ns
Thus fr = sfs
The induction motor direct on-line experimental set-up is
• Connect the circuit as shown. The lab supervisor must check the
circuit before you switch it on.
• Run the induction motor directly connected to the variable three phase ac supply.
Set the voltage at 400V line to line. Leave this set for this part of the lab and use
the three phase circuit breaker to switch the supply on and off.
• Vary the torque from 0 to 2 Nm in steps of 0.2 Nm and measure the speed. Do
not run the motor for long at high torques as it will overheat and fail.
• Calculate the per unit slip.
Torque (Nm) Speed (rpm) Per unit slip
• Graph the speed and slip versus torque.
• What happens to the motor per unit slip as the mechanical load on the motor
increases?
• How do you change the direction of rotation?
3. MATCHING INDUCTION MOTOR CHARACTERISTICS TO THE AC
MOTOR DRIVE
WARNING: Switching the motor drive off will not instantly isolate the motor
terminals. Wait for the capacitor to discharge prior to approaching the circuits.
• Connect the induction motor to the Eurodrive. The Eurodrive runs from a three-
phase 400V, 10A supply. Don't forget to earth the motor and the drive.
• Input the induction motor name-plate parameters into the Eurodrive. Use the
up/down buttons until the small motor symbol is illuminated and press ‘enter’.
Choose non-SEW motor in the Motor menu, VF (Voltage-Frequency) in the
Mode menu and the motor parameters from the nameplate after this. Each setting
is saved by pressing ‘enter’ again and then ‘out’. The motor is star connected to a
400V supply. Record what you enter.
• Set the motor maximum speed to 1600 rpm in the nmax menu.
• Set zero torque on the dynamometer.
• Start the induction motor and check that the set speed is 1500 rpm. Record the
motor speed from the dynamometer and the frequency displayed on the
Eurodrive.
• Calculate the synchronous speed ns from the frequency. Compare it with the
motor name-plate speed and the Eurodrive speed. Why are all three speeds
different?
4. FREQUENCY/SPEED CHARACTERISTIC
With the motor speed set at 1500 rpm, gradually increase the torque on the motor, from 0
to 2 Nm in steps of 0.2 Nm) and measure the actual motor speed (from the
dynamometer) and the frequency generated by the motor speed controller. When the
motor speed significantly slows down, stop taking measurements or it will overheat.
Calculate the synchronous speed and slip.
Torque Rotor Applied Synchronous Slip
Speed Frequency Speed
(Nm) (rpm) (Hz) (rpm)
Graph the rotor speed, synchronous speed and slip versus torque.
5. DISCUSSION
The three-phase induction motor volt-ampere rating S is given by
S = 3VI (VA)
where V and I are the motor name-plate voltage and current respectively.
• Calculate S and compare it with the induction motor rated real power output.
• Deduce whether the 0.18 kW rating is the electrical input rating or the mechanical
output rating.
• Why is the induction motor VA rating larger than its real power output?
• The motor speed controller attempts to hold a steady motor speed under all load
conditions. How is this done?
