DRIVING SYNCHRO LOADS With
Synchro Amplifiers and Converters
Synchro Torque (Producing) Receivers
When driving Synchro Torque Recievers, power is pulled
from and against the 2 following sources:
1) The Torque Reciever itself: Via 26 or 115VAC on
it's rotor coil, R1 & R2 rotor coil.
----------- AND -----------
2) The Power Amplifiers 3 wire synchro outputs, S1,
S2 and S3 stator coils.
Torque recievers provide torque as a result of the
interaction of the two magnetic fields introduced
through these coils within the torque reciever itself.
The torque reciever is considered an active load in
that it works against the opposing stator coil inputs,
thereby loading them to produce the torque required of
Torque is produced whenever the torque recievers
shaft angle differs from the angle dictated by it's 3
wire stator input.
The angular difference is reflected as a voltage
gradient that develops circulating currents in the
stators, working against the rotors magnetic field.
These opposing stator currents provide the
magnetomotive force against the rotors magnetic field,
to move the rotor shaft.
Theoretically, when the shaft angle is positioned
exactly to the angle dictated by it's 3 wire synchro
input (respective of the phasing of it's rotor input);
the load impedence is infinite, the shaft is nulled and
the load is null.
In practice however, the amplifiers outputs must
still accomodate the load incurred by virtue of both the
voltage and phase differentials existing between the
amplifier outputs and the actual characteristics of the
torque recievers imperfect stator coils. These
differential effects are significant, and must be
considered when specifiing appropriate amplifiers for a
Driving Synchro Torque (Producing) Receivers
When driving torque recievers: the amplifier must be able to
handle both: the peak transient power required to be able to
drive the torque reciever to a null (close the loop), in
addition to being able to supply enough steady state, continuous
power to maintain the torque reciever at null, accomodating the
at the null resulting from phase shift and voltage
differentials in the driven synchro, the amplifier, and the D-S
converter or other synchro driving the amp.
When driving a Torque Receiver, like driving a servo, we are
constantly attempting to null the circuit to achieve any desired
position, to null a 3 wire synchro consider the voltage required
at null in a 3 wire synchro format as:
sine(120O)(Vm) Vm = voltage magnitude
Vm for a 115V/90V L-L synchro: (.866)(90V.L-L) = 78 V. L-L
Vm for a 26V/11.8V L-L synchro: (.866)(11.8) = 10.2 V. L-L
This Vm (Voltage magnitude) is the voltage that will be
measured with two stator legs shorted accross the remaining
winding. See the following illustration "Zss":
power amplifier's Zss (output impedence) must be low enough to
drive the combined Zss of all the Torque receivers being driven, (plusthe
Zso of all the CT's and CDX's being driven off the same load
(less if these are tuned)), to accomodate the peak transient
power required to be able to drive the torque reciever/'s shaft
to a null (close the loop).
Additionally, the Synchro
Amplifier must be
able to provide enough continuous power to accomodate the
circulating currents respective of the phase shift and voltage
missmatch (Vmm), required just to maintain a null.
Fig. Zss, Driving Torque Receiver Loads
Calculating Load Impedances Required to Maintain a Null
(To maintain desired position, minimum continuous current flow)
The criteria used to determine the effective load impedence
at null, or effectively how much power will be required just to
maintain a constant position, we must consider the voltage
mismatch and phase shift respective of the components used in
1) Line to
line voltage difference
between the amplifiers outputs and the torque recievers stator,
(differential voltage, mismatch or
magnitude error, = Vmm).
---------------- AND ----------------------
Shift: Line to line phase-shift
the amplifiers outputs and the torque recievers stator.
The active load calculations derived by these two variable
differentials may be referred to as the "null wattage" or the
"VA @ null required", the power exhibited as circulating
currents flowing just to maintain a null (any constant
position), which is lost wattage above and in addition to the VA
required of the amplifier to produce any torque.
Calculating line to line "Voltage Missmatch" @ null
When driven from a digital to synchro converter, that part's
"Transformation Ratio Accuracy" is the criteria required, it is
not usually specified on D-S converters which normally only
specify accuracy with respect to the ratio accuracy.
standard CCC D-S converters this is typically +/-2%, and may be
trimmed to +/-1% on request. It is benificial to source both the
Amp and the converters from the same source and request they be
matched interchangeably, this will minimize the voltage
B) The Scale factor accuracy specified of a good
Reference Powered Synchro Amplifier is +/-1%.
On conventional DC powered (non-pulsating) amplifiers this
absolute scale factor accuracy may be as much as +/- 8%.
The Transformation Ratio (Input to Output) tolerance of most
synchro's is typically +/-2%:
Scale Factor accuracy:
Synchro Transformation Ratio:
Total Voltage missmatch:
Reference Powered type with D-S converters trimmed to match
where any combined represents +/-1%.
voltage differential (or missmatch) can be used for nominal
calculations, with 1 synchro TR and any of CCC's Reference
Powered Synchro Amplifiers.
When driving a Torque Receiver,
like driving a servo, we are constantly attempting to null the
circuit to achieve any desired position, to null a 3 wire
synchro consider the voltage required at null in a 3 wire
synchro format as:
for a 115V/90V L-L synchro:
78 V. L-L
for a 26V/11.8V L-L synchro:
(.866)(11.8) = 10.2
When driving a synchro Torque Reciever using 115VAC reference
and 90 V.L-L stators, anticipate this 3.% in synchro system
tolerances, will yield a Voltage
78V.L-L (.03 System Tolerance) =
2.34V.L-L = Vmm,
The following figure further illustrates the power required
at null (actually to satisfy the circulating currents that will
be flowing) attributed to the voltage differential (magnitude
error, or component missmatch) in the synchro system driving a
Fig. Vmm, voltage mismatch currents @ null
|Take the magnitude of
the voltage missmatcth (V1 -V2) over the total impedence of the
circuit (output impedence of the transmitter or amplifier (ZSS =
ZT ) - the
impedence of the Receiver (ZSS = ZR).
times the nominal voltage (78V. for 115/90V systems) represents
the power flowing at null in VA or watts, just to satisfy the
multiple synchro's are being driven, only the impedence of the
receivers side needs to be changed, calculate like adding
resistors in parallel:
1/R1 + 1/R2 + 1/R3 = 1/Total =
ZSS = ZR
Voltage Mismatch and Amplifier Headroom:
mismatch must also be considered with respect to the negative
potential of the missmatch verses the amplifiers voltage
envelope, to insure there is sufficient headroom such that the
negative flowing currents do not try to backfeed or buck the
amplifier outputs, possibly causing damage to the amplifier.
This is explained in greater detail in the following section
regarding large phase shifts, and includes both the tolerance of
different synchro amplifiers, and the means to increase the
headroom and ZSS in the system, and at the synchro amplifier
Calculating line to line phase-shift differential @ null:
(between the amplifiers outputs and the torque recievers
When reference powered amplifiers are used, the 3
wire synchro outputs are in phase with the reference input, the
phase shift specified of the Torque Reciever being driven
provides the line to line phase-shift differential.
Theoretically, when driving only one synchro this effect can
be minimized by adding a phase shift compensation RC (Resistor
Capacitor Network) in series with the rotor input of and at the
source of each synchro being driven, on many preinstalled
synchro applications this luxury is usually not a practical
Alternatively, adding a large capacitor in series with the
Reference input of the Synchro Amplifier can be considered, but
this also requires a RC phase lead/lag network be added to the
synchro or D-S converter driving the amplifier itself.
Large Phase Shift Effects:
Most installations specify
power sufficient to accomodate the increased load required to
maintain a null, respective of both the phase shift and voltage
mismatch differentials required of the synchro's employed, but
the voltage tolerance especially with respect to the phase shift
should be calculated to insure there is enough voltage mismatch
headroom, that the negative flowing currents do not try to
backfeed or buck the amplifier outputs, possibly causing damage
to the amplifier.
Phase shift differentials can be significant, example: a
typical 15TRX6a has a 20 degree phase shift, a 23TR6 is 9.1
degrees, also consider the phase shift tolerence between like
manufactured synchro's is approx. +/-20% of that nominally
specified. This is further complicated when driving multiples of
differing synchro's off of one common amplifier.
When Reference Powered Synchro Amplifiers are used to drive
synchro Torque Receivers having large phase shifts, the phase
shift limits the peak voltages available from the pulsating
power supplies, this is because the pulsating power supplies'
peak voltages are full-wave rectified, and in phase with, the
reference (power) input. The peak magnitude of the voltage seen
as phase shifted away from the reference is less. This makes the
amplifiers effective output voltage envelope smaller, limiting
or reducing the peak amplitude available on the outputs with
respect to the synchro's desired phasing.
The more undesireable effect (from phase shift) is when the
synchro stator signals being driven by the amp., exhibit a
higher voltage (by virtue of the induced rotor voltage coupling
working against the stators) than the peak voltages being
produced by the amplifier. This results in a negative
voltage mismatch which,
if significant enough will
try to backfeed or buck the synchro amplifiers output stages.
for the effects of Phase Shift on Synchro Amp.
calculate the circulating currents that will be flowing at null
attributed to phase-shift, first calculate the Voltage magnitude
error, (or the voltage offset) incurred by the phaseshift, over
the total impedence of the circuit (output impedence of the
transmitter or amplifier ZSS = ZT) + the impedence of
the Receiver (ZSS = Z R ):
shift in degrees) (Vm) = Vme
Fig. PS-(Vme), Phase Shift Voltage
calculate the power that will be lost to phase shift to simply
maintain a null: I (Vm) = VA
Where Vm = Voltage magnitude used
to Drive Synchro,
Vm = for 115/90 Systems use 78V,
Vm = for 26/11.8V Systems use 10.2V
Vme = Voltage Magnitude Error, this voltage will be present
on the driven synchro's leads, fighting against the power
amplifiers outputs, at the zero-crossings of the reference input
sine wave, when the reference input is providing no
instantaneous power, and likewise, the dynamic pulsating supply
has no instantaneous power to transfer; at the instant of these
(reference/power input) zero crossings; the amplifier is
essentially driving 0V, 0 current (less mismatch), while the
driven synchro inductively applies to the same signal lines its
phase shifted voltage potentials.
The difference between the voltage seen from the driven
at the zero-crossings of the pulsating supply; must fit into the
headroom tolerated by the amp.
If the phase shift line to line voltage plus the mismatch
exceeds the voltage missmatch headroom tolerated by the amp.:
the phase shift must be compensated for, or external resistors
must be used, on the stator lines to increase the headroom, or
Synchro Amplifier Headroom:
The negative voltage
missmatch that CCC's Reference Powered Synchro Amp's. are
designed to tolerate for 115V/90V. units are as follows, (this
is your headroom tolerance):
25 VA unit: 6.7 Volts/leg, (2) = 13.4 Volts across winding
50 VA unit: 2.45 Volts/leg, (2) = 4.9 Volts across winding
100 VA unit: 1.73 Volts/leg, (2) = 3.46 Volts across winding
Techniques used to increase the Synchro Amplifier
Load Balancing Resistors (illustrated
in figure LBR)
To increase the amount of negative voltage mismatch tolerated by
the amp. without shutting down the outputs, the user can add
large (10 - 20) watt resistors in series with the amplifiers
synchro outputs representing upto 2/3rd's of the synchro load
being driven (represented as the ZSS "Stator impedence rotor
shorted", specified for the synchro (or synchro's) being driven
in each leg.
Load Balancing Resistors &
Phase Shift added to Synchro Amp.
resistors will effect the current flow through the synchro, the
synchro signals however will still read 90 V.L-L. The higher the
total line impedences, the lower the current flow at null.
Though this will certainly help minimize the voltage at null,
and lower the current flow. There may be a slight reduction in
peak torque available on the synchro's output shaft (when
driving very large shaft loads). When
driving multiple synchro loads, load sharing effects' can and
will minimize loss of torque.
Occasionally, when driving several different Torque Receivers
with large shaft loads, compromise may be required, and the user
may have to try a couple of different load balancing resistors,
or phase shift capacitors to the amp's. reference input to
optimize driving the loads.
When driving multiple synchro Torque Recievers, the phase
shift should be apportioned respective of the ZSS rating of each
synchro verses its phase shift, the larger that synchros load
(the lower its ZSS impedence), the more its effect of phase
shift will burden the system.
Adding Lead/Lag RC networks for phase shift compensation:
If the phase shift is large, the user may add (or order with
internal) a phase shift lead/lag RC (resistor/capacitor) network
on the D-S converters reference input lines, and, use a large
capacitor on the reference input of the synchro amplifier to
compensate for the average phase shift of the load driven.
If a 115V, 60 Hz. synchro system is being used, start with a
10 Uf. 400V cap. in series with RH on the synchro amp., simply
measure phase shift between S1-S3 out verses the RH-RL in , when
loaded, on a dual-trace scope.
The formula used to calculate the phase shift for the D-S are
||tan ø = XC
Where ø = the phase shift in degrees.
f = frequency, c = capacitance
impedence of the reference input specified for the D-S converter
is required as part of "R" resistor component (see Data Sheets). CCC
converters are available with internal phase shift.
Control Transformer CT's and CDX type Synchro's
are relatively high impedence rotary transformers that provide a
single phase AC rotor output representing the sine of the
difference between the absolute shaft angle of it's rotor and
it's 3 wire stator (command) inputs. CT's are typically coupled
directly on the apparatus being controlled, providing
instantaneous position feedback and control, it's output is
typically amplified to drive a servo motor direct, thus the
motor automatically nulls it's shaft to the command angle
dictated by the CT's 3 wire input.
CT's' are typically driven from a CX (control transmitter) or
CDX (control differential transmitters).
CDX's (control differential transformers) have a 3 wire
primary input, a physical rotor shaft angle input, and a 3 wire
secondary output used to drive CT's other CDX's or even TR
(Torque Receiver) inputs.
The CDX output is a 3 wire synchro format representing the
angular difference between the absolute shaft angle of it's
rotor input and the shaft angle command determined by it's 3
wire synchro input. Because the CDX is used to drive other
synchro's, it's load must be added to the loads required of all
the synchro's connected to it's outputs, to determine the full
magnitude of the load burden that will be required of it's
inputs. CDX's are typically driven from a CX (control
transmitter) or another CDX, the are used as active offsets in a
synchro chain to bias their synchro inputs by the shaft angle of
When driving CT's or CDX's the amplifier
must be able to supply enough steady state, continuous power to
drive the Zso of the load.
Fig. Zss, Driving Control Transformer (CT)
and CDX Loads
Correction by Tuning CT's and CDX's
Unlike the more
resistive load requirements of Torque Receivers; CT's and CDX's
are primarily inductive loads, whereby power factor correction
can be achieved to reduce the reactive component by simply
tuning the loads.
CT's and CDX's may be tuned by adding good grade,
unpolarized, poly-type, high voltage tuning capacitors in a
delta configuration, in parallel with the stator inputs (see
illustration, use 400V. min. capacitors for 90 V. L-L signals).
The use tuning capacitors can reduce the load burden to the
synchro, or even a whole chain of synchro's by as much as 50%.
Fig. TC, Adding Tuning Capacitors for CT
Download CCC app. note#G-SA1 "Driving