3
A:
Have a question for the
experts? Contact us
at jim.bryan@emotors.com
This motor has very
good versatility and may be
used in several power supply
applications. It is a dual voltage
machine and may be used on
either voltage as defined on the
connection plate. It is designed
for use on a Wye Start, Delta
Run starter. This is a special
motor contactor which starts
the motor on its Wye connec-
tion to limit the inrush and
then switches to Delta for run-
ning. The motor must not be
run on the Wye connection for
more than 30 seconds as severe
winding damage may occur.
This motor may also be started
across the line and run on the
Delta connection. In addition,
the motor may be used on the
low voltage connection as a
part-winding start motor, also
to limit the inrush required.
After a brief period, it is
switched to the full winding.
continued on back >
by Ben Biondi
Winding Thermistors
A thermistor is a non-linear
resistance
temperature
detector, made from semi-
conductor material. Each
specific thermistor has its
own unique resistance vs.
temperature characteristic.
There are two general
types of thermistors. The
positive temperature coeffi-
cient (PTC) type has a resist-
ance that increases with
increasing temperature. The
PTC thermistor has a sharp
rise in resistance when the
temperature
reaches
the
'knee', or switching point.
The negative temperature
coefficient (NTC) type ther-
mistor has a resistance that
decreases with increasing
temperature. U.S. Motors
supplies the PTC-type thermistors.
As with RTDs, a control module must
be used with the thermistors. The con-
trol module is generally an externally
excited solid-state device that senses
the thermistor resistance and switches
contacts when the thermistor input
resistance rises above a set resistance
value.
The thermistor trip temperature
determines the point at which the ther-
mistor resistance rises to a very high
value, thus tripping the controller.
Thermistors are available with different
trip temperatures. Each thermistor has
one pre-set, non-adjustable switch
point.
Thermistors are normally installed in
the end turns of the motor. Depending
on the controller, they are either wired
in series, or in a special configuration
that can be referred to as a common
lead circuit (four leads out including a
common ground). The thermistors wired
in series can be either wired in series
internal to the motor with only two leads
brought out (usually for Canadian
orders), or wired in series in the outlet
box so that all six leads are brought out
of the motor. U.S. Motors will bring all
six leads out unless specified otherwise.
Thermistors have several advantages
over RTDs, thermocouples and thermo-
stats. Like thermostats, thermistors 2. Therma-Sentry
®
assembly - typi-
cally for NEMA frames 400 and smaller,
or FLA under 240 amps, mounted on the
motor in a special partitioned conduit
box with the main motor leads in the
second partition.
When specifying Therma-Sentry
®
at
time of order entry, the mounting loca-
tion of the controller must be specified.
Options are: Motor mounted or sepa-
rately mounted (customer control
panel). The most common specification
is motor mounted, separately excited
(MMSE).
VOLT SOURCE: U.S. Motors only pro-
vides separately excited control mod-
ules. The control module must be sepa-
rately excited by either a 115, 230, 460,
or 575 volt source, or 380 volt/ 50
hertz. If not specified, 115v will be sup-
plied. The control module must be excit-
ed before the motor can be started.
Excitation voltage must be sine wave.
Winding Thermocouples
A thermocouple is a pair of dissimilar
metal conductors so joined at one point
that an electromotive force (EMF) is
developed by the thermoelectric effects.
Any given set of thermocouple wires
have a known "EMF vs. Temperature"
characteristic.
Thermocouples can only generate a
low-voltage, low-power signal, that is in
the milli-volt range. Therefore the cus-
tomer must supply the electronic control
equipment to translate the thermocou-
ple voltage signal into a temperature
reading, or to operate the controls
alarm or shutdown devices.
There are many types of thermocou-
ples. The U.S. Motors standard types
are copper-constantan, chromel-con-
stantan and iron-constantan.
The standard installation procedure is
to install the thermocouples in the slot
portion of the winding. If required by the
customer, they can also be installed in
end turns of the winding. The standard
quantity of thermocouples is six,
installed two per phase.
Selection
The customers choice of monitoring
equipment dictates the type of thermo-
couples to be installed in the motor, so
orders need to specify the exact type of
thermocouples required. Monitoring
equipment is not supplied by U.S.
Motors.
For more information about winding
temperature protection, contact Ben
Biondi, Product Service Engineer with
US Motors, at 314-553-3875 or
ben.biondi@emotors.com.
product service BULLETIN
have a tamper-proof pre-set switch
point, with a reset only after the motor
cools down. Due to their small size and
heat sink construction, they have fast
response times. In mush-wound
motors, non-rotor limited designs,
thermistors can provide locked rotor
protection. Also, since the controller
trips at a high resistance value, resist-
ance variations due to long lead runs
can be tolerated. This feature allows
for one controller to be used for sever-
al temperature sensing locations.
Selection
Except for Therma-Sentry
®
, the cus-
tomer must advise the exact type of
thermistor required so it will match the
controller. All orders that do not speci-
fy the exact type of thermistor
required will be delayed. Unless speci-
fied otherwise on the face of the order,
three thermistors will be supplied.
Types of Thermistors
1. Power Control Corporation (PCC)
8000 series are usually installed in the
'common lead circuit' configuration.
They cannot be installed in series as
false tripping will occur. PCC makes
numerous controllers for customers to
select from
2. Texas Instruments (TI) 4BA series
thermistors are usually wired in series.
Up to three thermistors may be
installed in series without false trip-
ping the controller. The U.S. Motors
procedure is to bring out all six leads
and make the series connection in the
outlet box. The standard TI controller
is a model 50AA control module.
3. Siemens Q63100-P, PTC thermis-
tors must be wired in series. Up to six
Siemens thermistors may be wired in
series without false tripping the con-
troller. U.S. Motors standard procedure
is to install three thermistors in series
and bring all six leads out, making the
series connection in the outlet box. For
Canadian orders, the three thermistors
are wired in series internally in the
motor, and two leads are brought out.
We ask that this configuration be spec-
ified when ordering. The Siemens
standard controller is a model 3UN
tripping unit control module. The con-
trol module has one normally open and
one normally closed contact.
Therma-Sentry
®
Therma-Sentry
®
is a thermal protec-
tion system that is activated by exces-
sive winding temperature. Therma-
Sentry
®
protects against the most
common causes of motor failure
including: high or low supply voltage,
unbalanced line voltage, single phase
conditions, abnormally high ambient
temperatures, blocked ventilation,
starting overload and running over-
loads.
The system consists of three PTC
thermistors embedded in the motor
winding end turns, one per phase, with
the leads connected to a solid state
electronic controller. The solid state
controller can be supplied permanently
potted in the main or accessory outlet
box, or as a separate module for
mounting in a control panel.
Note: The Therma-Sentry
®
controller
uses output triacs, which are specifi-
cally designed to operate directly in
series with mainline contactors. If a
small interpose relay must be used,
the sealed VA rating of the interposing
relay must be greater than 10VA.
Difficulties may be encountered when
interfacing with solid state motor con-
trol equipment (VFDs, Soft Start,
etc.). Use with solid state starting
equipment may also result in false
tripping.
The leads from the three thermistors
are connected internally in the com-
mon lead circuit configuration.
The Therma-Sentry
®
system will only
provide locked rotor protection for cer-
tain motors. In form-wound motors,
the winding insulation is so heavy the
temperature gradient across the insu-
lation to the thermistor is too great to
give the rapid response required to
prevent rotor failure. Also, for mush
wound motors, only motors with a
winding safe stall time less than the
rotor safe stall time will have locked
rotor and starting overload protection.
Selection
The Therma-Sentry
®
controller has
one normally closed (N.C.) contact with