Low Inrush Motor Starting met.

The current level of the motor never exceeds

the full load amp rating of the motor at any time
Motor manufacturers face difficult design choices. during its start or operation. In addition to the benefit
Designs optimized for low starting current often of low starting current, motor designs can now be
sacrifice efficiency, power factor, size, and cost. optimized for high efficiency.
With these considerations in mind, it is common for
AC induction motors to draw 6 to 8 times their full Table B
load amps when they are started across the line. Comparison of Starter Types Based on Inrush
When large amounts of current are drawn on the Starter Type Starting Current (% of FLA)
transformers, a voltage drop can occur2, adversely VFD 100%
affecting other equipment on the same electrical Wye-Delta Starter 200-275%
system. Some voltage sensitive applications may Solid State Soft Starter 200%
even trip off line. For this reason, many engineers Autotransformer Starter 400-500%
specify a means of reducing the starting current of Part Winding Starter 400-500%
large AC induction motors. Across the Line Starter 600-800%

Soft Starters
Easy Installation
Wye-delta, part winding, autotransformer, and solid-
state starters are often used to reduce inrush during Many pieces of equipment are factory shipped with
motor starting. All of these starters deliver power to unit mounted VFDs that arrive pre-programmed and
the motor at a constant frequency and therefore must factory wired. Motor leads, control power for
limit the current by controlling the voltage supplied auxiliaries, and communication lines are all factory
to the motor. Wye delta, part winding, and auto- wired. The VFD cooling lines on unit-mounted
transformer starters use special electrical chiller VFDs are also factory installed. The
connections to reduce the voltage. Solid-state installing contractor needs only to connect the line
starters use SCRs to reduce the voltage. The amount power supply to the VFD.
of voltage reduction possible is limited because the
High Power Factor
motor needs enough voltage to generate torque to
accelerate. With maximum allowable voltage Power converted to motion, heat, sound, etc. is
reduction, the motor will still draw two to four times called real power and is measured in kilowatts (kW).
the full load amps (FLA) during starting. Power that charges capacitors or builds magnetic
Additionally, rapid acceleration associated with fields is called reactive power and is measured in
wye-delta starters can wear belts and other power Kilovolts Amps Reactive (kVAR). The vector sum
transmission components. of the kW and the kVAR is the Total Power (energy)
and is measured in Kilovolt Amperes (KVA)
VFDs as Starters (Figure 5). Power factor is the ratio of kW/KVA.
A VFD is the ideal soft starter since it provides the
lowest inrush of any starter type as shown in Table Motors draw reactive current to support their
B. Unlike all other types of starters, the VFD can magnetic fields in order to cause rotation.
use frequency to limit the power and current Excessive reactive current is undesirable because it
delivered to the motor. The VFD will start the motor creates additional resistance losses and can require
by delivering power at a low frequency. At this low the use of larger transformers and wires. In addition,
frequency, the motor does not require a high level of utilities often penalize owners for low power
current. The VFD incrementally increases the factor. Decreasing reactive current will increase
frequency and motor speed until the desired speed is power factor.

2
This is a significant consideration for "soft" systems
such as backup generators.

5
 Total
volts x
Energy
amps used to
trans build /
mitte decay
d. 
magnetic
field s in
motors,
transfor
Power mers etc.
consu med
as heat,
sound,
work
Fig. 5. Measuring power
etc.
Typical AC motors may have a full load power factor Table C
ranging from 0.84 to 0.88. As the motor load is Power Factors and Energy Usage
reduced, the power factor becomes lower. Utilities Input Power Amps Volts KVA
may require site power factor values ranging from kW Factor
0.85 to 0.95 and impose penalties to enforce this 350.4 .84 502 Nominal 480 417
requirement. Power factor correction capacitors can 350.4 .99 426 Nominal 480 354
be added to reduce the reactive current measured NOTE: KVA = Volts x Amps x 1.732
upstream of the capacitors and increase the
measured power factor. To prevent damage to the Backup generators are typically sized to closely
motor, power factor correction capacitors should not match the load. Lowering KVA can reduce the size
exceed the motor manufacturer’s recommendations. of the generator required. When VFDs with active
In most cases, this results in maximum corrected front ends are used, the generator size can approach
values of 0.90 to 0.95. an ideal 1:1 ratio of kW/KVA because the power
factor is near unity (1.0) and the harmonics
The VFDs include capacitors in the DC Bus that per- produced by the VFD are extremely low.
form the same function and maintain high power
factor on the line side of the VFD. This eliminates Lower KVA also benefits utilities. When the power
the need to add power factor correction equipment to factor is higher, more power (kW) can be delivered
the motor or use expensive capacitor banks. In through the same transmission equipment.
addition, VFDs often result in higher line side power
factor values than constant speed motors equipped HARMONIC DISTORTION AND INDUSTRY
with correction capacitors. STANDARDS

Low Full Load KVA A discussion of the benefits of VFDs often leads to
a question regarding harmonics. When evaluating
Total Power (KVA) is often the limiting factor in the VFDs, it is important to understand how harmonics
amount of energy that can be transmitted through an are provided and the circumstances under which
electrical device or system. If the KVA required by harmonics are harmful.
equipment can be reduced during periods of peak
demand, it will help alleviate voltage sags, brown Harmonic Definition
outs, and power outages. The unit efficiency and
power factor are equally weighted when calculating In the United States, three-phase AC power typically
KVA. Therefore, equipment that may be equal or operates at 60 hertz (60 cycles in one second). This
worse in efficiency, but higher in power factor has is called the fundamental frequency.
significantly lower KVA (Table C).

In this example, equipment with a higher power

factor uses 15% less KVA while performing the
same job. This can lower electrical system cost on
new projects and free up KVA capacity on existing
systems. 6
A harmonic is any current form at an integral When you calculate harmonics you are calculating
multiple of the fundamental frequency. For example, the effect of the harmonics on the fundamental cur-
for 60-hertz power supplies, harmonics would be at rent wave form in a particular distribution system.
120 hertz (2 x fundamental), 180 hertz, 240 hertz, There are several programs that can perform
300 hertz, etc. estimated calculations. All of them take into account
the amount of linear loads (loads drawing power
What Causes Harmonics? throughout the entire sine wave) relative to non-
linear loads (loads drawing power during only a
VFDs draw current from the line only when the line fraction of the sine wave). The higher the ratio of
voltage is greater than the DC Bus voltage inside the linear loads to non-linear loads, the less effect the
drive. This occurs only near the peaks of the sine non-linear loads will have on the current wave form.
wave. As a result, all of the current is drawn in short
intervals (i.e., at higher frequencies). Variation in Are Harmonics Harmful?
VFD design affects the harmonics produced. For Harmonics that are multiples of 2 are not harmful
example, VFDs equipped with DC link inductors because they cancel out. The same is true for 3rd
produce different levels of harmonics than similar order harmonics (3rd, 6th, 9th etc.). Because the power
VFDs without DC link inductors. The VFDs with supply is 3 phase, the third order harmonics cancel
active front ends utilizing transistors in the rectifier each other out in each phase 3. This leaves only the
section have much lower harmonic levels than 5th, 7th, 11th, 13th etc. to discuss. The magnitude of the
VFDs using diodes or silicon controlled rectifiers harmonics produced by a VFD is greatest for the
(SCRs). lower order harmonics (5th, 7th and 11th) and drops
quickly as you move into the higher order
Electronic lighting ballasts, uninterruptible power harmonics (13th and greater).
supplies, computers, office equipment, ozone
generators, and other high intensity lighting are also Harmonics can cause some disturbances in electrical
sources of harmonics. systems. Higher order harmonics can interfere with
sensitive electronics and communications systems,
Rocks and Ponds
while lower order harmonics can cause overheating
Obviously, the magnitude of the contributing wave of motors, transformers, and conductors. The
forms has an effect on the shape of the resultant opportunity for harmonics to be harmful, however,
wave form. If the fundamental wave form (60 Hz) is dependent upon the electrical system in which
has a very large magnitude (5,000 amps) and the they are present and whether or not any harmonic
harmonic wave forms are very low (10 amps), then sensitive equipment is located on that same
the resultant wave form will not be very distorted electrical system.
and total harmonic distortion will be low. If the
Understanding IEEE 519
harmonic wave form current value is high relative
to the fundamental, the effect will be more dramatic. IEEE (Institute of Electrical and Electronics
Engineers) created a recommendation for evaluating
In nature, we see this effect with waves in water. If harmonics. The IEEE-519 standard provides
you continually throw baseball size rocks into the recommended limits for harmonic distortion
ocean, you would not expect to change the shape of measured at the point of common coupling. The
the waves crashing onto the beach. However, if you point of common coupling is the point at which the
threw those same size rocks into a bathtub, you customer’s electrical system is connected to the
would definitely observe the effects. It is similar utility.
with electrical waves and harmonics.

3
The neutral wire sizing should account for 3rd order
harmonic current.
7