Step motors
Posted by
Mariss Freimanis
on 2000-06-17 20:30:45 UTC
Hi,
I am updating the white paper on our web-site because of numerous
similar questions I have been getting recently . I feel the update is
of a generic nature and may be helpful to others as well so I am
posting it here. If anyone feels this is cluttering this news group
please let me know and I will desist in the future.
Mariss
----------------------------------------------------------------------
UNIPOLAR, BIPOLAR, SERIES, PARALLEL, FULL-WINDING, HALF-WINDING
These terms can be confusing but they don't have to be. In the
end, 4
wires from the motor must be connected to the drive; it's all a
matter of which ones go where. The following definitions will clear
things up.
UNIPOLAR: An outdated motor connection requiring only 4 power
transistors. It also refers to 6-wire motors, (not outdated). Motor
current is applied to the center tap of a winding. One end of the
winding or the other is switched to ground by the power transistors
but never at the same time, though both transistors may be
simultaneously off.
ADVANTAGES: Simple
DISAVANTAGES: Many. The connection acts a 2:1 step-up transformer, so
the transistors "see" twice the power supply voltage.
Inductive
energy is not inherently clamped, so complex and inefficient clamping
circuits must be used to prevent damage to the transistors.
BIPOLAR: Almost universally used in modern, high-performance drives.
Bipolar refers to how the winding is driven; when one end of the
winding is grounded, the other end is connected to the power supply
voltage and visa versa. This method requires a full-bridge driver per
winding for a total of 8 transistors.
ADVANTAGES: Many. The transistor voltages never exceed the power
supply voltage. Inductive energy is clamped to the power supply
voltage rails and is efficiently recirculated back to the power
supply. No circuit voltage is above or below the power supply voltage
rails at any time.
DISADVANTAGES: More complex than unipolar.
SERIES: A method of connecting a split winding (8-wire motor) end to
end so that the current flows through one winding and then through
the other one. Mind the phasing of the windings; they must be
connected so that they "add" (boost). If they are connected
to "subtract" (buck), the result will be no torque and
possible
damage to a switching drive since the inductance will be zero as well.
ADVANTAGES: Smaller power supply because only half of the rated
current is necessary for the same holding torque. Less motor heating
at a given voltage.
DISADVANTAGES: The motor will develop only half the power at higher
speeds because this connection has 4 times more inductance.
PARALLEL: A method of connecting a split winding (8-wire motor) side
by side so that current flows through both windings simultaneously.
Mind the phasing again (see SERIES: above).
ADVANTAGES: Develops maximum power at higher speeds.
DISAVANTAGES: Greater motor heating at a given power supply voltage
when compared to the series connection.
FULL-WINDING: A method of connecting the end wires of a 6-wire motor
leaving the center tap unused. It is exactly identical to a series
connected 8-wire motor since the winding is internally connected in
series at the center tap. The advantages and disadvantages are
identical to the series connection as well (see SERIES: above).
HALF-WINDING: A method of connecting the center tap and one end wire
of a 6-wire motor leaving the other end wire unused. It makes no
difference which end wire is used. It is almost identical to a
parallel-connected 8-wire motor.
The single difference is only half the copper (hence half-winding) is
used, so the winding resistance is twice what it would be for an
identical 8-wire motor. At higher speeds this higher resistance
develops a slightly greater voltage drop, leaving a little less for
the motor to work with, and so the motor develops a little less power
compared to an 8-wire motor.
The difference is almost insignificant; dynamometer tests show only a
3% difference in power output between a half-winding 6-wire motor and
a parallel-connected 8-wire motor. It only stands to reason; at
higher speeds phase currents decrease and therefore the IR squared
loss differences decreases as well.
The advantages and disadvantages are the same as a parallel
connection (see PARALLEL: above).
CURRENT SETTINGS FOR WINDING CONNECTIONS:
The proper phase current is very important for best low speed
performance of a microstep drive. It relates to how smoothly and low
speed resonance free a good microstepping motor will be.
There is a common misperception some users make regarding current set
with microstep drives. It has to do with the fact microstepping
drives the motor with sine and cosine weighed currents. When the
current in one winding reaches its maximum the other winding current
is at zero. From this some surmise that it should be possible to
increase the set current to 141% of rated since the motor power
dissipation would then be identical to a full step drive. That is
correct as far as power dissipation goes but it neglects a very
important point most are not aware of.
The magnetic flux path is not shared by the windings. This means the
current in one winding does not contribute by adding or subtracting
to the to the magnetic flux of the other winding. Though the
dissipation may equal a full step drive by running the current at
141% of rated, the iron magnetic saturation is dependent on the
current of one winding entirely.
The result is the iron in the motor saturates at the higher current
and degrades the motor's linearity. Consequently the microstep
placement is not optimal anymore and the motor will exhibit greater
low speed resonance than it otherwise would have.
There is in fact a free lunch. A microstepping drive may not have as
much holding torque as a full step drive, but this is meaningless in
practical terms. A full step drive invests about 35 to 40 % of its
available torque in vibrating the motor and mechanism once it begins
to turn. A microstepping drive wastes less than 1% of available
torque this way.
Though it may have less torque to work with, a microstepper applies
nearly all of it to the load, so in the end it is at no disadvantage
compared to a full step drive. There is no point to trying to bring
up a microstepper drive torque to the same level as a full step drive.
The conclusion is: set the current to the motor's rated current.
If
it is a unipolar (6-wire motor) use the nameplate rating. If it is a
parallel wired 8-wire motor, use the parallel rating.
UNIPOLAR: Use the motor's name plate rating
SERIES: Use one-half the parallel rating
PARALLEL: Use the motor's name plate rating (usually the parallel
rating)
FULL-WINDING: Use one-half the name plate rating
HALF-WINDING: Use the motor's name plate rating
I am updating the white paper on our web-site because of numerous
similar questions I have been getting recently . I feel the update is
of a generic nature and may be helpful to others as well so I am
posting it here. If anyone feels this is cluttering this news group
please let me know and I will desist in the future.
Mariss
----------------------------------------------------------------------
UNIPOLAR, BIPOLAR, SERIES, PARALLEL, FULL-WINDING, HALF-WINDING
These terms can be confusing but they don't have to be. In the
end, 4
wires from the motor must be connected to the drive; it's all a
matter of which ones go where. The following definitions will clear
things up.
UNIPOLAR: An outdated motor connection requiring only 4 power
transistors. It also refers to 6-wire motors, (not outdated). Motor
current is applied to the center tap of a winding. One end of the
winding or the other is switched to ground by the power transistors
but never at the same time, though both transistors may be
simultaneously off.
ADVANTAGES: Simple
DISAVANTAGES: Many. The connection acts a 2:1 step-up transformer, so
the transistors "see" twice the power supply voltage.
Inductive
energy is not inherently clamped, so complex and inefficient clamping
circuits must be used to prevent damage to the transistors.
BIPOLAR: Almost universally used in modern, high-performance drives.
Bipolar refers to how the winding is driven; when one end of the
winding is grounded, the other end is connected to the power supply
voltage and visa versa. This method requires a full-bridge driver per
winding for a total of 8 transistors.
ADVANTAGES: Many. The transistor voltages never exceed the power
supply voltage. Inductive energy is clamped to the power supply
voltage rails and is efficiently recirculated back to the power
supply. No circuit voltage is above or below the power supply voltage
rails at any time.
DISADVANTAGES: More complex than unipolar.
SERIES: A method of connecting a split winding (8-wire motor) end to
end so that the current flows through one winding and then through
the other one. Mind the phasing of the windings; they must be
connected so that they "add" (boost). If they are connected
to "subtract" (buck), the result will be no torque and
possible
damage to a switching drive since the inductance will be zero as well.
ADVANTAGES: Smaller power supply because only half of the rated
current is necessary for the same holding torque. Less motor heating
at a given voltage.
DISADVANTAGES: The motor will develop only half the power at higher
speeds because this connection has 4 times more inductance.
PARALLEL: A method of connecting a split winding (8-wire motor) side
by side so that current flows through both windings simultaneously.
Mind the phasing again (see SERIES: above).
ADVANTAGES: Develops maximum power at higher speeds.
DISAVANTAGES: Greater motor heating at a given power supply voltage
when compared to the series connection.
FULL-WINDING: A method of connecting the end wires of a 6-wire motor
leaving the center tap unused. It is exactly identical to a series
connected 8-wire motor since the winding is internally connected in
series at the center tap. The advantages and disadvantages are
identical to the series connection as well (see SERIES: above).
HALF-WINDING: A method of connecting the center tap and one end wire
of a 6-wire motor leaving the other end wire unused. It makes no
difference which end wire is used. It is almost identical to a
parallel-connected 8-wire motor.
The single difference is only half the copper (hence half-winding) is
used, so the winding resistance is twice what it would be for an
identical 8-wire motor. At higher speeds this higher resistance
develops a slightly greater voltage drop, leaving a little less for
the motor to work with, and so the motor develops a little less power
compared to an 8-wire motor.
The difference is almost insignificant; dynamometer tests show only a
3% difference in power output between a half-winding 6-wire motor and
a parallel-connected 8-wire motor. It only stands to reason; at
higher speeds phase currents decrease and therefore the IR squared
loss differences decreases as well.
The advantages and disadvantages are the same as a parallel
connection (see PARALLEL: above).
CURRENT SETTINGS FOR WINDING CONNECTIONS:
The proper phase current is very important for best low speed
performance of a microstep drive. It relates to how smoothly and low
speed resonance free a good microstepping motor will be.
There is a common misperception some users make regarding current set
with microstep drives. It has to do with the fact microstepping
drives the motor with sine and cosine weighed currents. When the
current in one winding reaches its maximum the other winding current
is at zero. From this some surmise that it should be possible to
increase the set current to 141% of rated since the motor power
dissipation would then be identical to a full step drive. That is
correct as far as power dissipation goes but it neglects a very
important point most are not aware of.
The magnetic flux path is not shared by the windings. This means the
current in one winding does not contribute by adding or subtracting
to the to the magnetic flux of the other winding. Though the
dissipation may equal a full step drive by running the current at
141% of rated, the iron magnetic saturation is dependent on the
current of one winding entirely.
The result is the iron in the motor saturates at the higher current
and degrades the motor's linearity. Consequently the microstep
placement is not optimal anymore and the motor will exhibit greater
low speed resonance than it otherwise would have.
There is in fact a free lunch. A microstepping drive may not have as
much holding torque as a full step drive, but this is meaningless in
practical terms. A full step drive invests about 35 to 40 % of its
available torque in vibrating the motor and mechanism once it begins
to turn. A microstepping drive wastes less than 1% of available
torque this way.
Though it may have less torque to work with, a microstepper applies
nearly all of it to the load, so in the end it is at no disadvantage
compared to a full step drive. There is no point to trying to bring
up a microstepper drive torque to the same level as a full step drive.
The conclusion is: set the current to the motor's rated current.
If
it is a unipolar (6-wire motor) use the nameplate rating. If it is a
parallel wired 8-wire motor, use the parallel rating.
UNIPOLAR: Use the motor's name plate rating
SERIES: Use one-half the parallel rating
PARALLEL: Use the motor's name plate rating (usually the parallel
rating)
FULL-WINDING: Use one-half the name plate rating
HALF-WINDING: Use the motor's name plate rating