phvac Dec 2006.qxd - Plumbing & HVAC

Refrigeration
Working with induction motors
By Ed Gravelle
An induction motor
(IM) is a type of
alternating current
(AC) motor where
power is supplied to the
rotating device by
means of electromagnetic induction.
The other commonly used name is
“squirrel cage motor,” due to the fact
that the rotor bars with short circuit
rings resemble a squirrel cage (hamster
wheel).
An electric motor converts electrical
power to mechanical power in its
rotor (rotating part). There are several
ways to supply power to the rotor.
In a direct current (DC) motor, this
power is supplied to the armature
directly from a DC source, while in an
AC motor this power is induced in the
rotating device.
An induction motor is sometimes
called a rotating transformer because
the stator (stationary part) is essentially
the primary side of the transformer and
the rotor (rotating part) is the secondary
side. Induction motors are widely
used, especially polyphase induction
motors, which are frequently used in
industrial drives. (Wikepedia)
Rotating magnetic field
Fig 1 shows how 60-cycle AC power
generates a rotating magnetc field
(RMF) in a single phase motor. A threephase
motor winding has the equivalent
of three single phase motor windings
spaced around the stator, with each
phase rise following one after the other
to provide running and starting torque.
A complete study of motors can be
found at www.allaboutcircuits.com
(Vol. II, AC motors).
Motors used with VFD speed drives
are no different than across-the-line
starter motors except that they have to
have winding insulation rated to 2000
volts or higher. Most premium motors
are wound with 2000-volt rated windings.
The energy to run the motor
comes from a series of square wave DC
volt pulses (see fig 2) produced by the
inverter at pulses up to 900 per second
in each direction to simulate an a/c sine
wave. The frequency of the change controls
motor speed.
Induction motors are used for fan
coil air handlers, pumps and many
other applications. Up to the time that
inverter drives were introduced, they
were only capable of running at fixed
speeds – the speed being determined by
the number of poles in the motor (two
When motors are required to
start heavy loads, starters and
motor conductors may need
to be oversized.
pole = 3600 RPM; four-pole = 1750
RPM; six pole = 1150 RPM). The adoption
of dual windings allowed twospeed
motors to be created.
Each motor circuit is required (by
code) to have overload and over-current
protection. A starter with an overload
relay, or motor with built in overload
devices, will satisfy the requirement
for overload. Fuses or breakers
have to be supplied to provide for overcurrent
protection. Table D12
(Canadian Electrical Code) shows the
proper breaker or fuse to use for induction
motor circuits.
WAVE FORM 3 PHASE IN RED
DC BUS VOLTAGE IN GREEN
PULSE MODULATION SQUARE WAVE IN BLACK
Fig 2 - sub 30
(Danfoss Drives)
Fig. 2: The energy to run the motor comes from a series of square wave DC volt
pulses produced by the inverter.
φ−1
a
φ−1
φ−2
φ−2
a a’ b c d
Heavy starting loads
Induction motors may be required to
handle heavy starting loads. However,
larger across-the-line starters and fusing
may have to be used for high torque
and long start times. For example,
motors driving heavy backward
inclined fans (or similar loads) may take
15 to 20 seconds to reach full speed,
with current draw starting close to
locked rotor amps with amp draw dropping
off as speed increases. Starters,
when required to carry prolonged high
starting currents, need class 20 or 30
overload relays. Class 20 or 30 are slower
to react and will allow higher amp
flow over longer start times.
Starters may also have to be sized for
the starting load rather than the motor
HP as a “HP-rated starter” may not be
rated for the current flow required for
long start times. When motors are
required to start heavy loads, starters
and motor conductors may need to be
oversized, selected for starting amps
rather than motor HP.
A guide for selecting conductors and
starter ampacity can be taken from
Table D12 (Canadian Electrical Code).
Wire size and starter capacity are selected
to match the ratings shown for time
delay fuses, rather than the motor name
plate amps or motor HP rating.
VFD driven motors
Input fuses or breakers for motors driven
by a variable frequency drive (VFD)
can be selected from code table D12 or
sized as specified for the VFD being
used.
Motor and VFD fuses are designed to
protect the complete power circuit,
beginning at the alley pole transformer
(or vault transformer), main power distribution
center, and any sub (breaker)
circuit all the way to the drive or motor,
from short circuit faults.
The building or pole transformer is
supplied with a fused primary power
a’ b
c
Fig. 1: Sixty cycle a/c power generates a rotating magnetic field in a singlephase
motor. (Toni R Kuphaldt - allaboutcircuits.com)
d
supply, with voltages as high as 25,000
to 125,000 volts. The transformer
capacity in a building vault or
on a pole outside is usually sized for 125
to 300 percent of the maximum size of
the building’s main breaker.
For example, a building distribution
center with a 1000 amp main breaker
may be supplied with a transformer
capable of providing 1250 to 3000
amps. Hydro usually allows for future
loads, or shares the same transformer
with other buildings. Vault or pole
transformers do not have overload protection,
just over-current protection (a
fuse on the primary side)
A 1250-amp transformer on a pole
with 25,000 to 100,000 volts on the primary
side is capable of producing short
circuit amps up to 200,000 before the
primary fuse blows. Each circuit in the
building has a breaker or fuses to interrupt
high amp flows if there is a short to
ground or phase to phase. They are
rated to react in two ways – circuit over
amps and high amp faults (short to
ground or phase to phase, in other
words). For example, a 40 amp Class T
fuse will fail if it’s subject to over amps
for a longer period that it is rated for
(time - amp factor of the fuse). It will
also safely interrupt a short circuit fault
current up to 200KA (200,000 amps).
Breakers are rated the same way.
If there wasn’t a breaker or fuse capable
of opening the circuit under fault
conditions, the motor or drive could go
up in flames if there was a fault to
ground or from phase to phase, or virtually
explode in a similar way to a
tree or structure when hit by lightning
In the next issue we will go into more
detail on how induction motors work
with inverters.
Ed Gravelle is a consultant on refrigeration,
air conditioning and heating
systems based in Brentwood Bay, B.C. He
can be reached at epgco.09@shaw.ca.
22 Plumbing & HVAC Product News – April 2009 www.plumbingandhvac.ca