Daniel Lang, P.Eng. discusses various methods for supervised open transition bus transfer to maintain continuity of critical industrial loads during power source switching. There are two main categories of bus transfer - closed transition (parallel) and open transition. Open transition methods aim to minimize transient torque on induction motors during transfer by interrupting the original source before connecting the new source. Supervised open transition methods like fast, in-phase, and residual voltage transfers monitor conditions to safely reconnect motors before their terminal voltage and phase decay too much from the alternate source. Proper bus transfer supervision is needed to prevent catastrophic damage to motors from excessive transient torque during re-energization.

1. Daniel Lang, P.Eng.
GP Technologies Ltd.
Induction Motor Dynamics of Fast Bus Transfer
Daniel Lang, P.Eng.
GP Technologies Ltd.
200 9016-51 Ave Edmonton, AB
dlang@gptechnologies.com
Abstract
Process conditions in most industrial facilities
often cannot tolerate discontinuity of critical
loads, even momentarily. Considering plant
loads are only as reliable as their distribution
system, operational continuity is maintained by
utilizing fast bus transfer schemes.
There are two general categories of bus transfer:
closed transition (parallel) bus transfer and open
transition bus transfer. Closed transition is the
process of transferring load from one bus to
another source while briefly paralleling the two
sources. Open transition is just the opposite,
interrupting the original source prior to
connecting to the new source.
The disadvantages of closed transition bus
transfer generally outweigh the advantages.
Paralleling two main sources may exceed
interrupting ratings of the equipment as well as
result in unacceptable utility system transients.
This white paper focuses on the dynamics of
various supervised open transition bus transfer
methods on induction motor loads.
Introduction
There are various methods in which to achieve
open transition bus transfer while maintaining
full process continuity, which include:
 Supervised Transfer
o Fast
o In-Phase
o Residual
 Unsupervised Transfer
o Time Delay
To begin with, an obvious question arises: why
is there a need to supervise the transfer process?
The simple answer is, by closing in a new source
on a momentarily disconnected motor bus,
exceedingly large transient torque may be
imposed on an induction motor shaft, causing
catastrophic damage.
A more comprehensive explanation is this:
consider the case where the alternate source
voltage and motor terminal voltage are 180
degrees out of phase. Assuming that the motor
terminal voltage has not greatly reduced, closing
in on the alternate source effectively doubles the
applied stator voltage. This in turn may cause
extremely high sub-transient current to flow. As
electromagnetic torque is proportional to the
square of the current, a large opposing force on
the shaft is thus generated. This force can break
rotor bars and end rings, loosen stator windings,
wrench the motor from its baseplate, and in
some cases even snap the shaft.
Motor Dynamics During Bus Transfer
The transient response of a bus with multiple
motors is more involved than the case of a single
motor, and thus this paper will focus solely on
the single motor case.
During the process of a bus transfer, the
following occurs:
1. The normally closed breaker opens.
2. The phase A zero crossing occurs, and
phase A arc is quenched.
3. The motor operates asymmetrically for a
fraction of a cycle.
4. The remaining phase arcs are quenched.
5. The flux trapped within the motor air
gap decays, resulting in a decay of the
stator terminal voltage magnitude.
6. The rotor angular velocity decays,
resulting in an increase in slip frequency
between the motor terminal voltage and
the alternate source voltage.
7. The alternate source breaker closes,
restoring the service.

2. Daniel Lang, P.Eng.
GP Technologies Ltd.
The moment a motor is disconnected from its
power source, the stator voltage magnitude and
angle will decay as a function of the open circuit
time constant and the total rotor inertial (shaft
and load), respectively.
Fundamentally, if the motor terminal voltage has
not completely decayed when source voltage is
restored, the torque applied to the rotor is
proportional to the vectorial difference between
the source and motor volts per hertz phasors at
the instant the source voltage is applied.
Magnetic flux is the quotient of voltage and
frequency. As torque is proportional to flux
squared, the magnitude of transient torque
imposed on the rotor can be quantified by the
following vector summation:
(
𝑉̅
𝜔
)
𝑟𝑒𝑠𝑢𝑙𝑡𝑎𝑛𝑡
≔
𝑉̅𝑠𝑜𝑢𝑟𝑐𝑒
𝜔𝑠𝑜𝑢𝑟𝑐𝑒
−
𝑉̅ 𝑚𝑜𝑡𝑜𝑟
𝜔 𝑚𝑜𝑡𝑜𝑟
The image below shows the locus of the motor
terminal voltage phasor following disconnection.
The V/Hz at any point in time is simply the
vectorial difference between the motor and
source phasors, each divided by their respective
frequencies.
The V/Hz between the motor terminal and the
alternate source will increase due to a voltage
gradient or advancement in phase angle, or both.
ANSI Std. C50.41-2000 states that the
maximum magnitude of the volts per hertz
vector between the source and motor cannot
exceed 1.33 per unit during a transfer. However,
many studies have been published which
question the validity of the torque limiting the
1.33 per unit V/Hz constraint provides.
Decay in Voltage Magnitude
The open circuit time constant of an induction
motor is quantified by the following equation:
𝜏 =
𝑋2
′
+ 𝑋 𝑚
𝜔𝑠 𝑅2
′
Thus, the terminal voltage magnitude of an
induction motor disconnected from its supply
source will decay at the following rate:
𝑉𝑡𝑒𝑟𝑚𝑖𝑛𝑎𝑙 = 𝑉0 𝑒
−𝑡
𝜏⁄
Decay in Phase Angle
A bus disconnected from its source, through the
inertia in rotating motors, will supply current to
all loads connected to that bus. The energy
stored within a momentarily disconnected motor
will be lost to the load, back feeding any loads
on the same bus, and to friction.
The rate of change of rotor angular velocity is
defined by Newton’s second law:
𝑑𝜔
𝑑𝑡
= 𝛼 =
𝑇𝑡𝑜𝑡𝑎𝑙
𝐼
I is the combined rotor and load moment of inertia
T is the combined rotor and load torque

3. Daniel Lang, P.Eng.
GP Technologies Ltd.
The combined machine inertia dictates rotor
acceleration. High inertia loads (fans) will yield
slow phase angle decay, whereas the opposite
occurs with low inertia loads (centrifugal
pumps).
More specifically however, a parameter of
rotating machinery defined as the inertia
constant is used to approximate rotor
acceleration.
𝑑𝜔
𝑑𝑡
= 𝛼 =
1
2𝐻
(𝑇𝑒 − 𝑇𝑙)
H is the inertia constant
Te is the electromagnetic torque
Tl is the load torque
It is apparent that rotor acceleration is the
difference between electromagnetic torque
(dependent on air gap flux), and the load torque
(dependent on dynamics of the load).
For this reason, it is somewhat difficult to
predict the rate of change of the phase angle,
especially with non-linear (quadratic) torque
loads.
Fast Bus Transfer
Fast Bus Transfer is the process of connecting
the dead bus to the alternate source with no
intentional time delay. This method minimizes
the bus dead time, and attempts to restore bus
voltage before the motor terminal voltage and
phase angle has greatly deviated from the
alternate source.
As the motor slows down, the slip frequency
between the motor terminals and the alternate
source begins to increase. Fast bus transfer
attempts to reclose on the alternate source before
the change in slip frequency is too rapid.
The alternate source breaker is supervised by
upper and lower voltage and phase angle limits
between the dead bus and alternate source.
In-Phase Bus Transfer
In-Phase Bus Transfer is the process of
connecting the dead bus to the alternate source
by predicting a zero phase coincidence between
them. This is the same process as auto
synchronization, but with the added difficulty of
a high slip frequency. Due to this high slip
frequency, the calculation of predicted phase
coincidence and the breaker close advance angle
requires solving a second order partial
differential equation every processing cycle:
0 = 𝑘𝑇𝑐
𝜕2
∅
𝜕𝑡2
+ 𝑇𝑐
𝜕∅
𝜕𝑡
+ 𝐶∅
k is a system constant
Tc is the circuit breaker closing time
C defines system initial conditions
Ø is the circuit breaker closing advance angle
In-Phase Bus transfer is supervised by an upper
and lower voltage limit as well as a slip
frequency limit.
Residual Voltage Bus Transfer
Residual Voltage Bus Transfer requires that the
motor terminal voltage decay to approximately
33% of nominal. This ensures that even in the
worst case (closing 180° out of phase), the
maximum voltage across the stator windings
will be 1.33 per unit.
Residual voltage bus transfer can in some cases
result in loss of process continuity. If the load
inertia is low or the voltage decay is slow
enough, process conditions need to be
considered to produce a maximum time delay to
ensure continuity.
Residual Voltage Bus Transfer also requires
coordination with electrically (AC) held
contactors which may drop out prior to closing.
Sequential Transfer Mode
With a sequential transfer mode, confirmation of
normal source breaker opening via breaker
status contact (52a/b) is required as a permissive
in the transfer logic. Bus transfer only occurs
following confirmation from the breaker status

4. Daniel Lang, P.Eng.
GP Technologies Ltd.
contact and once adequate conditions from
transfer supervision is achieved.
Simultaneous Transfer Mode
With a simultaneous transfer mode, all three
transfer methods are enabled to supervise and
permit transfer without confirmation that the
normal source breaker has opened via the
breaker status contact. This provides a time
saving of the breaker trip coil signal delay and
breaker opening time. In most cases, this is a
minimum three cycle delay.
Brief Note on Practical Considerations
In practice, relays will incorporate all transfer
methods simultaneously and close as soon as
adequate conditions are realized. The speed at
which the fastest transfer sequence can occur
depends on the modes of transfer (sequential or
simultaneous). Modern microprocessor relays
have sample rates in the tens of samples per
cycle and can achieve protection element
processing as fast as 4 times per cycle.
The below images show the voltage and phase angle waveforms of high and low inertia loads, as a
function of time, illustrating the regions of each transfer methods. It is apparent that the slip speed of the
stator voltage and alternate source increases at a slower rate for higher inertia loads.

5. Daniel Lang, P.Eng.
GP Technologies Ltd.
References
1. Collum et. al., Enhanced Load Transfer
Schemes for Very Reliable Service, 62nd
Annual Georgia Tech Protective Relaying
Conference, May 2008.
2. Raje et. al., Fast Bus Transfer Systems – A
System Solution Approach, NPSC, IIT
Bombay, Dec. 2008.
3. Hunswadkar et. al., Considerations and
Methods for an Effective Fast Bus Transfer
System, Power System Protection and
Automation Conference, New Delhi, India,
Dec. 2010.
4. Kolodziej, J., Induction Motor Torque
During Fast Bus Transfers, M.Sc. Thesis,
University of Illinois, 2012.
About the Author
Daniel Lang holds a
Bachelor of Science in
Electrical Engineering
from the University of
Alberta (2010). He has
worked as an engineering
consultant in all areas of
power systems design and
analysis, from small
commercial buildings to
large scale heavy
industrial installations.
His work has included power system analyses and
design with an emphasis on protective relaying,
unbalanced fault analysis, steady-state / loadflow and
transient analysis, arc flash analysis, and protection
logic.
Daniel is a Professional Engineer registered in the
Province of Alberta, and is a member of the IEEE.