Applying Generator Sets to Motor Loads
By Robert A. Barr, Product Specialist, Kohler Co., Generator Division
© 1983 by KOHLER CO.
Motor starting is one of the major applications of both prime power and standby generator sets.
Years of experience in
successfully applying generator sets to motor loads leads us to the conclusion
that the subject is often complicated beyond necessity.
The basic characteristic of motor loads that causes difficulty is that motors
draw high starting currents when started at full voltage. A typical motor, if
there is such a thing, draws approximately six times its rated full load current
when starting, and continues to do so with only a slight reduction until it
reaches about 75% of rated speed.
Figure 1: Starting vs Running Loads
||Kilovolt-Amperes per Horsepower with Locked Rotor |
Table 1: Locked-Rotor Indicating Code Letters
The KVA required by a motor
starting at full voltage is quite close to the "locked rotor KVA" requirement
of the motor, which is usually fairly easily determined either from the actual
motor's nameplate or from the manufacturer. The National Electrical Manufacturer's
Association (NEMA) sets design standards for motors, and has established a NEMA
Code letter designation to classify motors by the ratio of locked rotor KVA
per horsepower. These code letters range from A to V, covering motors with a
locked rotor KVA per horsepower of 3.14 or less to 22.4 KVA per horsepower or
more. Small motors commonly have a higher NEMA Code letter and corresponding
higher locked rotor KVA per horsepower requirement than large motors. Typical
motor sizes and codes are:
||Locked Rotor KVA/HP
||L or M
|15 HP and up
Table 2: Typical Code Letters
for Various HP Motors
Fractional horsepower motors
usually have even higher locked rotor KVA requirements.
In trying to identify motors,
the NEMA Code letter should not be confused with the NEMA Design letter which
refers to the motor's torque curve.
Figure 2: National Electrical Manufacturer's Association Motor
Since the KVA requirements
of a motor running at full load and rated speed are normally somewhat less than
one KVA per horsepower except for small motors, it would obviously be very inefficient
to size a generator set by matching its rated continuous or standby KVA to the
motor's locked rotor or starting KVA rating. Several factors combine to make
this conservative approach unnecessary.
One of these is power
factor. Three phase generator sets are usually rated in KVA at 0.8 power
factor. The actual power factor of a given motor load during starting is difficult,
if not impossible to measure since it is changing constantly. Various sources
put it in the range of 0.3 to 0.5. A typical generator set will produce somewhere
near twice its continuous rated KVA at 0.4 Power Factor for the time required
to accelerate a motor to the speed at which its KVA requirement drops sharply.
The engine does not stall even though the generator set is delivering more than
rated KVA, because only slightly more horsepower is required for reduced generator
efficiency on low power factor loads (see Figure 3). This generator characteristic
allows us to obtain satisfactory motor starting results with generators half
the size which would theoretically be required using the most conservative approach
of matching the generator 0.8 Power Factor KVA rating to the motor locked rotor
.4 Power Factor = 50% HP Factor
Figure 3: Engine Power Required
for Low Power Factor Load
The other factor which can
substantially reduce the size generator set needed for a particular motor load
is voltage dip. Values for motor locked rotor current or KVA are based on full
voltage starting. In practice, there is always some voltage dip even when starting
on utility power. When voltage drops, current also is reduced proportionately
so that starting KVA is reduced as the square of the voltage dip. A 30% voltage
dip reduces KVA by about 50% (0.7 volts x 0.7 amps = 0.49 KVA).
The voltage dip for a given
motor load on a particular generator set does not change without modification
to one or the other. At least for the first few cycles, it is determined by
the size of the load (locked rotor KVA draw) and the amount of copper and iron
in the generator. The problem of selecting a generator for a particular load
can be considered as a problem of determining what voltage dip will be acceptable,
considering its effect on all components in the system.
Voltage dip has important
effects on motor loads themselves, aside from any possible effects on other
equipment in an emergency system. One is that excessive voltage dip will cause
control relays or magnetically held motor starting contactors to drop out. If
this happens, the contactor or relay opening immediately removes the load from
the generator, causing voltage to rise, and the cycle repeats rapidly. This
can damage contactors if allowed to continue. In our experience, most control
relays and motor starting contactors will stand a 40% voltage dip. There are
exceptions however, and we have seen cases where relays or contactors would
chatter if subjected to more than a 20% dip. To assure satisfactory operation
the voltage limitations of control components should be obtained from their
manufacturers or suppliers.
The second effect of voltage
dip which may determine whether a generator will successfully start a motor
is that it reduces the torque available from the motor. A common NEMA Design
B motor will develop 150% of rated full load torque during starting. Torque
is proportional to the KVA delivered to the motor, so the same 30% voltage dip
that reduces KVA to 49% of the rated locked rotor value reduces torque to 49%
of its rating at any given speed and 100% voltage. If the motor starts unloaded
as most fans, centrifugal pumps and motor-generators used with elevators do,
this produces no problem other than a somewhat longer acceleration time. Other
types of loads such as positive displacement pumps may require more torque than
the motor can develop at reduced voltage, and the motor will not come up to
speed. To be sure of a successful start on these applications, it is necessary
to compare the torque curves of the pump and the motor, correcting the latter
for the effect of the expected voltage dip.
Reduced Voltage Motor Starters
The high inrush current
and high starting torque associated with across-the-line starting of motors
may create problems with the equipment driven by the motor or objections from
the power company. Reduced voltage starters are used to solve these problems.
There are various types of reduced voltage starters, and their use sometimes allows successful starting
of a given motor on a smaller generator set than would otherwise be the case.
Most such starters involve applying the load to the power source in two or more
steps. The starters are "open" or "closed" transition type depending on whether
the load is momentarily disconnected from the power source between steps. Open
transition starters are of no value in reducing voltage dip when the power source
is a generator set. The maximum dip at the transition can be higher than the
maximum starting the same motor across-the-line.
Figure 4: Open Transition Vs. Closed Transition Starters
This is because the motor
acts as a generator for a few cycles after power is removed, until the residual
voltage decays. When the motor is reconnected to the power source the effect
is the same as connecting two generators out-of-phase—high inrush current. Closed
transition starters do reduce the maximum voltage dip, but the amount of reduction
depends on the speed of the motor at the time of transition. To keep the voltage
dip to a minimum, it is important to allow as much time for the motor to accelerate
as possible before making the transition. The starters usually have an adjustable
time delay for this purpose.
Some Types of Reduced Voltage Starters
Primary Resistor Starting
In this type of starter,
resistors are connected in the motor lines to produce a voltage drop due to
the motor starting current. A timing relay is connected to short out the resistors
after motor acceleration. The motor starts at reduced voltage and operates at
full voltage. These starters provide very smooth starting due to increasing
voltage across the motor terminals as the motor accelerates. Since motor current
decreases as speed increases, the voltage drop across the resistor is less as
the motor approaches rated speed. When the resistor is shorted out when the
motor is nearly at rated speed, there is little increase in current or torque.
Standard primary resistor starters provide one step of resistance with about
70% of line voltage at the motor terminals at the time the starting contacts
close. Line current is also reduced to about 70% of rated locked rotor current
and about 49% of rated locked rotor torque is available.
Maximum inrush KVA is also
limited to about 50% of the motor's rated locked rotor KVA. Because this type
of starter increases the power factor of the motor starting load, we find that
it frequently overloads the engine on the generator set. We recommend that resistance
starters not be used on generator sets.
Figure 5: Primary Resistor Starting
Primary Reactor Starting
This type of starter uses
reactors connected in the motor lines to reduce starting power requirements.
These starters provide a low power factor load, but must be designed to match
the characteristics of the particular motor being started.
Primary reactor starters
reduce the starting KVA load by 35-50% and reduce the starting torque available
from the motor by 60-75%. When properly applied, these starters work well with
Some motors have 6 leads
which allow them to be connected in either wye or delta. When wye connected,
the motor has a voltage rating 173% of its delta voltage rating. By connecting
the motor windings in the wye configuration and using a voltage source corresponding
to the delta rating, starting current and torque are reduced. The starter is
not truly a reduced voltage type, but the effect is the same. Wye-delta starters
are usually open transition type. On the initial start, two contactors close:
one to connect the windings in wye and one to energize them. After a time delay
the wye contactor opens and a third contactor closes to reconnect the motor
in delta. With the motor connected in wye, both starting torque and current
are 33% of the delta connection values.
Figure 6: Wye-Delta Starting
Because of the high current
inherent with the open transition the maximum voltage dip on a generator set
is not improved over normal full voltage starting. Special closed transition
starters are available which will reduce the maximum inrush current and KVA
to approximately 60% of the maximum starting the same motor across-the-line,
delta connected. Because of the very low starting torque, the time to the transition
point must be relatively long to avoid high currents at that point.
Part Winding Starting
Part Winding starters are
used with motors having two identical windings intended to be connected in parallel.
These windings can be energized in sequence to provide reduced starting current
and torque. When one winding is energized, the torque is about 50% of "both
winding" rating, and line current is 60-70%, depending on motor design.
Part winding starters are
not truly reduced voltage starters, but the effect is similar. Since they are
inherently closed transition, the maximum inrush current occurs at the moment
the first winding is energized and the maximum inrush KVA load on a generator
set will be 60-70% of the normal across-the-line rating of the same motor.
Figure 7: Part Winding Starting
This type of starter generally gives the best results with generator
sets. The starter provides reduced voltage at the motor terminals from a tapped
3-phase auto-transformer. Taps on the transformer provide selection of 80, 65
or 50 percent of line voltage to the motor terminals initially. Starting torque
is reduced to 64%, 42% or 25% of the full voltage value but because of transformer
action line current and KVA are reduced in the same amounts, depending on the
transformer tap selected.
Figure 8: Autotransformer Starting
Table 3 shows the various types
of reduced voltage starters and summarizes their relative advantages and disadvantages.
Figure 9 shows the inrush current characteristics of the various types of starters
on a single graph for comparison.
Figure 9: Part Winding, Wye-Delta, Primary Resistor or Autotransformer Starting
|Type of Starter
Percent of Full Voltage Values
|Voltage at Motor
highest torque per ampere of line current.
2. Taps on autotransformer permit adjustment of starting voltage.
3. Suitable for long starting periods.
4. Closed transition starting.
5. While starting, motor current is greater than line current.
6. Low power factor.
|1. In lower
HP ratings, is most expensive design.
acceleration--motor voltage increases with speed.
2. Closed transition starting.
3. Less expensive than auto.transformer starter in lower HP ratings.
4. Available with as many as 7 accelerating points.
|1. Low torque
2. Resistor gives off heat.
3. Starting time in excess of 5 seconds requires expensive resistor.
4. Starting voltage difficult to adjust to meet varying conditions.
5. High power factor during start.
||1. Least expensive reduced voltage starter.
2. Closed transition starting.
3. Most dual voltage motors can be started part winding on lower of two
4. Small size.
|1. Unsuited for high inertia, long starting loads.
2. Requires special motor design for voltages higher than 230 V.
||1. Moderate cost--less than primary resistor or auto-transformer.
2. Suitable for high inertia, long acceleration loads.
3. High torque efficiency.
|1. Requires special motor design.
2. Starting torque is low.
3. Inherently open transition--closed transition available at added cost.
Table 3: Reduced Voltage Starters Application Data
Because there will still be some voltage dip when the motor is started on a generator set, and that dip
further reduces available torque, it usually is not practical to use the 50%
tap with a generator set. Closed transition type autotransformer reduced voltage
starters can be helpful when starting large motors on generator sets.
We have had experience with
a variety of other special applications and are glad to provide assistance with
whatever motor starting problems you may encounter.