Protection and Ratings
Motor Protection
The typical method of starting a three-phase induction motor is by connecting
the motor directly across the power line. Line starting a motor is
done with a three-phase contactor. To adequately protect the motor from
prolonged overload conditions, motor overloads are installed, typically in
the same enclosure as the three-phase contactor. These overloads (OLs)
operate as heater elements-heating to the point of opening the circuit,
and mechanically disconnecting the circuit (Figure 3-31).
Overloads can be purchased with a specific time designed into the element.
Classes 10, 20, and 30 are the usual ratings for industrial use. A class
10 overload indicates that the overload will allow 600% inrush current for
10 s before opening the circuit. Class 20 overloads would allow 600%
inrush current for 20 s, and a class 30 would allow 30 s of operation. The
current draw from a typical induction motor, as well as the torque produced
can be seen in Figure 3-32.
Line starting an induction motor, as shown in Figure 3-32, would allow
the motor to develop rated torque, as soon as the motor starter button is
pressed. This is because across the line, the motor has the benefit of full
voltage, current, and frequency (Hz). As long as the input power is of
rated value, the motor would develop the torque as seen in Figure 3-32,
from zero to base speed.
If the ratio of voltage to hertz is maintained, then the motor will develop
the rated torque that it was designed to produce. This relationship can be
seen in Figure 3-33 and is designated the volts per hertz ratio (V/Hz).
As seen in Figure 3-33, the V/Hz ratio is calculated by simply dividing the
input voltage by the hertz. This characteristic is an important ingredient of
AC drive design, which will be covered in the next chapter.
There may be applications where full torque is not desirable when the
motor is started: a conveyor application in a bottling line, for example. If
the feed conveyor has uncapped full bottles on the conveyor, full torque
when the conveyor is started would be a not-so-good situation. (The bottles
would spill all of their contents.) In cases like that, a reduced torque
type of start would be required. There are also cases where full voltage and hertz, which causes 600% inrush current, would cause a serious power
dip on the utility system. High-horsepower motors connected to compressors
would be an example. In these cases, a reduced voltage start would be
required. If the voltage is less than rated value, the motor would not
develop rated torque (according to the V/Hz ratio listed in Figure 3-32).
Reducing the V/Hz ratio also reduces the starting current, which means
there is less of a power dip.
Reducing the starting current may be accomplished in any one of the following
ways.
Primary Resistor or Reactance
The primary resistor or reactance method uses series reactance or resistance
to reduce the current during the first seconds. After a preset time
interval, the motor is connected directly across the line. This method can
be used with any standard induction motor.
Auto Transformer
The auto transformer method uses an auto transformer to directly reduce
voltage and the current for the first few seconds. After a preset time interval,
the motor is connected directly across the line. This method can also
be used with any standard induction motor.
Wye-Delta
The wye-delta method applies the voltage across the Y connection to
reduce the current during the first few seconds. After a preset time interval,
the motor is connected in delta mode permitting full current. This type
of induction motor must be constructed with wye-delta winding connections.
Part-Winding
The part-winding method uses a motor design that has two separate winding
circuits. Upon starting, only one winding circuit is engaged and current
is reduced. After a preset time interval, the full winding of the motor is
connected directly across the line. This type of motor must have two separate
winding circuits. To avoid winding overheating and damage, the time
between first and second winding connections is limited to 4 seconds maximum.
Motor Ratings
When reviewing ratings, it is also necessary to review several design features
of the induction motor. Induction motor design classifications, characteristics,
and ratings will now be reviewed in detail.
Because of the variety of torque requirements, NEMA has established different "designs" to cover almost every application. These designs take into
consideration starting current and slip, as well as torque. These motor
design classes should not be confused with the various classes of wire insulation,
which are also designated by letter.
Table 3-3 indicates the various NEMA design classifications and suitable
applications.
Figure 3-34 indicates the relative differences in torque, given a specific
motor NEMA design class. The motors indicated are all line started.
As seen in Figure 3-34, the major differences are in the starting torque and
peak or breakdown torque capabilities.
Efficiency
The efficiency of a motor is simply the ratio of the power "out" to the
power "in," expressed in percentage.
Figure 3-35 illustrates the general relationship between current, slip, efficiency,
and power factor.
Generally, motor efficiency is relatively flat from rated load to 50% of
rated load. Some motors exhibit peak efficiency near 75% of rated load.
Power Factor
Power Factor (P.F.) is the ratio of real power to apparent power, or kW/
kVA. Kilowatts (kW) are measured with a wattmeter, and kilovoltamperes
(kVA) are measured with a voltmeter and ammeter. A power factor
of one (1.0) or unity is ideal. Power factor is highest near rated load, as
seen in Figure 3-35. Power factor at 50% load is considerably less and continues
to dramatically decrease until zero speed.
Current Draw
Current draw in amperes is proportional to the actual load on the motor in
the area of rated load. At other loads, current draw tends to be more nonlinear
(Figure 3-35).
Locked Rotor (kVA/HP)
Another rating specified on motor nameplates is locked rotor kVA per horsepower.
(Some manufacturers use the designation locked rotor amps.) A letter
appears on the nameplate corresponding to various kVA/HP ratings. Refer
to Table 3-4 for letter designations.
The nameplate codes are a good indicator of the starting current in
amperes. A lower code letter indicates a low starting current and a high
code letter indicates a high starting current for a specific motor horsepower
rating. Calculating the starting current can be accomplished using
the following formula:
Example: What is the approximate starting current of a 10-HP, 208-V
motor with a nameplate code letter of "K"?
Solution: From Table 3-4, the kVA/HP for a code letter of "K" is 8.0 to 8.9.
Taking a number approximately halfway in-between and substituting in
the formula, we get:
Therefore, the starting current is approximately 236 amperes. The starting
current is important because the purchaser of the motor must know what
kind of protection (overload) to provide. The installation must also include
power lines of sufficient size to carry the required currents and properly
sized fuses.
Insulation Systems
An insulation system is a group of insulating materials in association with
conductors and the supporting structure of a motor. Insulation systems are
divided into classes according to the thermal rating of the system. Four
classes of insulation systems are used in motors: class A, B, F, and H. Do
not confuse these insulation classes with motor designs previously discussed.
Those design classes are also designated by letter.
Another confusion factor is the voltage insulation system classes of the stator
windings. Those classes are also designated by class B, F, and H, for
example. NEMA, standard MG1, part 31 indicates the voltage insulation
classes, relative to use on AC drives. More review of motor voltage insulation
characteristics will be done in Chapter 4.
At this point, we will review the temperature insulation classes, common
in standard industrial induction motors operated across the line.
Class A. Class A insulation is one in which tests have shown suitable thermal
endurance exists when operated at a temperature of 105°C. Typical
materials used include cotton, paper, cellulous acetate films, enamelcoated
wire, and similar organic materials impregnated with suitable substances.
Class B. Class B insulation is one in which tests have shown suitable thermal
endurance exists when operated at a temperature of 130°C. Typical
materials include mica, glass fiber, asbestos, and other materials, not necessarily
inorganic, with compatible bonding substances having suitable
thermal stability.
Class F. Class F insulation is one in which tests have shown suitable thermal
endurance exists when operated at a temperature of 155°C. Typical
materials include mica, glass fiber, asbestos, and other materials, not necessarily
inorganic, with compatible bonding substances having suitable
thermal stability.
Class H. Class H insulation is one in which tests have shown suitable thermal
endurance exists when operated at a temperature of 180°C. Typical
materials used include mica, glass fiber, asbestos, silicone elastomer, and
other materials, not necessarily inorganic, with compatible bonding substances,
such as silicone resins, having suitable thermal stability.
Usual Service Conditions
When operated within the limits of the NEMA-specified "usual service
conditions," standard motors will perform in accordance with their ratings.
For service conditions other than usual, the precautions listed below must
be considered.
Ambient or room temperature not over 40°C. If the ambient temperature
is over 40°C (104°F), the motor service factor must be reduced or a
higher horsepower motor used. The larger motor will be loaded below full
capacity so the temperature rise will be less and overheating reduced.
(Note: Service factor refers to rated motor power and indicates permissible
power loading that may carried by the motor. For example, a 1.15 service
factor would allow 15% overload power to be drawn by the motor.)
Altitude does not exceed 3300 feet (1000 meters). Motors having
class A or B insulation systems and temperature rises according to NEMA
can operate satisfactorily at altitudes above 3300 feet. However, in locations
above 3300 feet, a decrease in ambient temperature must compensate
for the increase in temperature rise, as seen in Table 3-5.
Motors having a service factor of 1.15 or higher will operate satisfactorily
at unity service factor and an ambient temperature of 40°C at altitudes
above 3300 feet up to 9000 feet.
Voltage Variations. A voltage variation of not more than 10% of nameplate
voltage:
Operation outside these limits or unbalanced voltage conditions can result
in overheating or loss of torque and may require using a larger-horsepower
motor.
Frequency Variations. A frequency variation of not more than 5% of
nameplate frequency: Operation outside of these limits results in substantial
speed variation and causes overheating and reduced torque.
A combination of 10% variation in voltage and frequency provided the
frequency variation does not exceed 5%.
Mounting Surface and Location. The mounting surface must be rigid
and in accordance with NEMA specifications. Location of supplementary
enclosures must not seriously interfere with the ventilation of the motor.
