Over Temperature Conditions
Placing the motor into overload conditions is one cause of over-temperature.
High ambient temperatures and dirty or clogged air filters on the
machine or motor blowers also contribute to over-temperature failures.
High temperature inside the motor cause expansion stress in the wire
insulation, resulting in cracks, which in turn can cause contamination and
eventual wire failure. The shrinking and hardening of the wire lacquer
insulation is a cause for loss of insulation strength.
Ambient Temperature
Typical recommendations are for the motor ambient conditions not to
exceed 40oC (104oF). Most motors are designed for continuous operation
at this ambient temperature. However, motors that will continuously be
used in higher temperatures will typically be designed with a lower temperature
rise class of insulation.
DC motor insulation must have mechanical and dielectric strength. It must
withstand the normal handling necessary in the assembly of the motor, as
well as operation thereafter. The major insulation classes are A, B, F, and
H, and a brief description is as follows:
- Class A is the lowest grade, suitable for typical household appliances,
but not normally industrial applications.
- Class B is general purpose, used in many industrial applications.
More demanding duty requires Class F or Class H.
- Class H is the heavy-duty insulation, capable of withstanding high
ambient and internal motor temperatures.
Normal life expectancy of an insulation system is 10,000 to 15,000 hours
of operation, depending on temperature. Reducing the motor's winding
temperature by 10°C will double the insulation life. Conversely, increasing
the temperature by 10°C will cut the life expectancy in half.
If you need more information, contact a local motor distributor, NEMA
(National Electrical Manufacturers Association) or a local representative of
EASA (Electrical Apparatus Service Association).
If the ambient temperature is above 50°C, special consideration must also
be made for the bearing and shaft lubricants. The motor manufacturer
must always be consulted when continuous temperatures rise above this
value.
Vibration
Vibration causes problems such as shaft stress and eventual shorting of
conductors between winding turns or between layers of windings. Severe
vibration can cause cracks in the lacquer insulation, which exposes the
conductors to contamination. Commutation problems may also develop
from the "bouncing" of brushes on the commutator. Continuous vibrations
tend to cause metal fatigue, which may a cause for premature casting
(frame) or bearing failure.
Altitude
Standard motor ratings are based on operation at altitudes of up to 3300
feet (1000 m) ASL (above sea level). Many manufacturers recommend the
user to lower the motor rating by 1% for every 330 feet above 3300 feet
ASL. The reason is that the air is less dense at higher altitudes (less air molecules
to take the heat away from the motor frame). To reduce the need for lowering the rating a motor-mounted blower normally will be sufficient
to cool the motor and prevent overheating.
Protection
Most motor manufacturers encourage the purchase and use of a motor
thermostat. This device is typically a bi-metallic disk or strip that is sensitive
to temperature rise. When the temperature reaches a predetermined
level, the thermostat acts as a switch and opens a control circuit, which in
turn shuts down the motor. (When a drive is connected to a motor, this
thermostat is connected to an auxiliary circuit that shuts down the drive
when over-temperature conditions arise.)
The thermostat is mounted on a commutating coil inside the motor, which
means the device needs to be installed at the time of manufacture. Standard
configuration is a normally closed contact. However, normally open
configurations are also available.
This type of device usually retails for about $150 and is very reasonable
insurance against motor overheating. Once a motor overheats, insulation
damage can occur, causing thousands of dollars in repair costs and additional
costs in down time.
Ratings
Typical DC motors have ratings that are found on the nameplate. Figure 3-
16 indicates a typical DC motor nameplate.
- Frame: indicating the frame rating per specific horsepower and
torque capability.
- HP: available horsepower at the designated armature and field
voltage and current ratings.
- Amps/field amps: designations for armature and field winding
amps, respectively. These ratings are needed when programming
the protection features in a drive controller.
- Base/max. speed: indicates the rated speed in rpm, when operating
at rated armature and field amps, as well as rated load. The max
speed indication is the maximum safe operating rpm possible, while
remaining within the limitations of the motor.
Additional ratings include enclosure type, thermostat type, ambient temperature
rating, catalog and serial number, and tachometer type and rating.
These ratings have been previously discussed. Refer to Chapter 5
"Drive Control and Feedback Devices," for more information on tachometers.
Most DC motors also carry one of three duty ratings:
- Continuous duty: rating given to motors that will continuously
dissipate all the heat generated by internal losses without exceeding
the rated temperature rise.
- Intermittent duty (definite time): rating given to a motor that
carries a rated load for a specified time without exceeding the rated
temperature rise.
- Intermittent duty (indefinite time): rating given to a motor
that is usually associated with some RMS load of a duty-cycle operation.
- Peak torque: the peak torque that a DC motors can deliver is limited
by the load point at which damaging commutation begins.
Brush and commutator damage depends on the sparking severity
and duration. Peak torque is limited by the maximum current that
the power supply can deliver.
- Calculating torque: An easy means of calculating the available
torque from a DC motor is to use the following formula:
where:
Torque = torque available from the motor in lb-ft
HP = nameplate horsepower at base speed
Speed = rpm
As an example, assume a 10-HP DC motor has a 240-V armature, 39.2
amp with a speed of 1775/2750. We will insert the needed numbers into
the formula and determine the base speed (1775) torque:
The above formula will work for determining torque at any speed up to
base speed. (Again, remember that base speed in rated: armature voltage,
field current, and load.)
To determine the torque per amp ratio, simply divide 29.5 by 39.2, which
equals 0.75 lb-ft of torque per amp. Determining the torque per amp ratio
above base speed is also possible by calculating the torque, using the above
formula for the speed over base, then using the ratio of the calculated
torque and the amp meter reading at that speed. As expected, the amount
of torque developed is less, above base speed, compared with below base
speed.
Over Temperature Conditions
Placing the motor into overload conditions is one cause of over-temperature.
High ambient temperatures and dirty or clogged air filters on the
machine or motor blowers also contribute to over-temperature failures.
High temperature inside the motor cause expansion stress in the wire
insulation, resulting in cracks, which in turn can cause contamination and
eventual wire failure. The shrinking and hardening of the wire lacquer
insulation is a cause for loss of insulation strength.
Ambient Temperature
Typical recommendations are for the motor ambient conditions not to
exceed 40oC (104oF). Most motors are designed for continuous operation
at this ambient temperature. However, motors that will continuously be
used in higher temperatures will typically be designed with a lower temperature
rise class of insulation.
DC motor insulation must have mechanical and dielectric strength. It must
withstand the normal handling necessary in the assembly of the motor, as
well as operation thereafter. The major insulation classes are A, B, F, and
H, and a brief description is as follows:
- Class A is the lowest grade, suitable for typical household appliances,
but not normally industrial applications.
- Class B is general purpose, used in many industrial applications.
More demanding duty requires Class F or Class H.
- Class H is the...
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