R&M Materials Handling HOIST MONITORS - Manual (Page 23)

R&M Materials Handling, Inc.
4501 Gateway Boulevard
Springfield, Ohio 45502
P.: (937) 328-5100
FAX: (937) 325-5319
23/105
R&M Materials Handling, Inc. reserves the right to alter or amend the above information without notice.
5.3 Hoist motor overheating supervision
To prevent the hoisting motor from burning up, the hoist-monitoring unit will constantly measure the
internal temperature of the motor. The motor is therefore equipped with temperature sensors; PTC-
thermistors, NTC-thermistors or Bi-metal switches (Klixons). The hoist-monitoring unit can be
programmed to either prevent both hoisting and lowering (default), or only the hoisting motion.
PTC-thermistor connections
PTC-thermistors are commonly used in most of the motors
in the past. PTC-thermistors will rapidly increase their
resistance when the measured temperature exceeds the
limit. The maximum allowed temperature depends on the
motor and is determined by the used thermistors. The
hoist-monitoring unit will detect over temperature when the
resistance of the PTC exceeds 4.0kOhm.
Bi-metal switches are mounted on the motor stator
windings. Bi-metal switches operate as thermal-switches
and will open up the contact when the measured
temperature exceeds the limit. The maximum allowed
temperature depends on the motor and is determined by
the used Bi-metal switches. The hoist-monitoring unit will
detect overtemperature when the resistance exceeds
4.0kOhm (thus open contact).
In practice, PTC-thermistors and Bi-metal switches
operate in the same way. The hoist-monitoring unit stops
the motion when the resistance of the temperature-
measuring device exceeds 4.0kOhm.
NTC-thermistors connections
NTC-thermistors are used when motor torque calculation
is the load measurement method. NTC-thermistors will
decrease their resistance in an almost linear relation to
temperature increase. The actual motor temperature is
accurately measured with NTC-thermistors and the hoist-
monitoring unit will stop the motion in case the resistance
of the temperature-measuring device drops below the
preset value.