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In a
permanent magnet or hybrid stepping motor, the
magnetic field of the motor rotor changes with
changes in shaft angle. The result of this is that
turning the motor rotor induces an AC voltage in
each motor winding. This is referred to as the
counter EMF because the voltage induced in each
motor winding is always in phase with and counter to
the ideal waveform required to turn the motor in the
same direction. Both the frequency and amplitude of
the counter EMF increase with rotor speed, and
therefore, counter EMF contributes to the decline in
torque with increased stepping rate.
Variable
reluctance stepping motors also induce counter EMF!
This is because, as the stator winding pulls a tooth
of the rotor towards its equilibrium position, the
reluctance of the magnetic circuit declines. This
decline increases the inductance of the stator
winding, and this change in inductance demands a
decrease in the current through the winding in order
to conserve energy. This decrease is evidenced as a
counter EMF.
The
reactance (inductance and resistance) of the motor
windings limits the current flowing through them.
Thus, by ohms law, increasing the voltage will
increase the current, and therefore increase the
available torque. The increased voltage also serves
to overcome the counter EMF induced in the motor
windings, but the voltage cannot be increased
arbitrarily! Thermal, magnetic and electronic
considerations all serve to limit the useful torque
that a motor can produce.
The heat
given off by the motor windings is due to both
simple resistive losses, eddy current losses, and
hysteresis losses. If this heat is not conducted
away from the motor adequately, the motor windings
will overheat. The simplest failure this can cause
is insulation breakdown, but it can also heat a
permanent magnet rotor to above its curie
temperature, the temperature at which permanent
magnets lose their magnetization. This is a
particular risk with many modern high strength
magnetic alloys.
Even if
the motor is attached to an adequate heat sink,
increased drive voltage will not necessarily lead to
increased torque. Most motors are designed so that,
with the rated current flowing through the windings,
the magnetic circuits of the motor are near
saturation. Increased current will not lead to an
appreciably increased magnetic field in such a
motor!
Given a
drive system that limits the current through each
motor winding to the rated maximum for that winding,
but uses high voltages to achieve a higher cutoff
torque and higher torques above cutoff, there are
other limits that come into play. At high speeds,
the motor windings must, of necessity, carry high
frequency AC signals. This leads to eddy current
losses in the magnetic circuits of the motor, and it
leads to skin effect losses in the motor windings.
Motors
designed for very high speed running should,
therefore, have magnetic structures using very thin
laminations or even nonconductive ferrite materials,
and they should have small gauge wire in their
windings to minimize skin effect losses. Common high
torque motors have large-gauge motor windings and
coarse core laminations, and at high speeds, such
motors can easily overheat and should therefore be
derated accordingly for high speed running!
It is
also worth noting that the best way to demagnetize
something is to expose it to a high frequency-high
amplitude magnetic field. Running the control system
to spin the rotor at high speed when the rotor is
actually stalled, or spinning the rotor at high
speed against a control system trying to hold the
rotor in a fixed position will both expose the rotor
to a high amplitude high-frequency field. If such
operating conditions are common, particularly if the
motor is run near the curie temperature of the
permanent magnets, demagnetization is a serious risk
and the field strengths (and expected torques)
should be reduced accordingly! |
