Download When should inverter-duty motors be specified?

When should inverter-duty motors
be specified?
The Growing Use of Inverters
The premature failures of motor insulation systems began to occur as fastswitching, pulse-width-modulated (PWM) VFDs were introduced. The
switching rates of modern power semiconductors can lead to voltage
overshoots. These voltage spikes can rapidly damage a motor’s insulation
system, resulting in premature motor failure. These high voltages can break
down the motor’s insulation system.
Additionally, the harmonics in a PWM (pulse-width modulation) wave form can
lower the motor’s efficiency when compared to operation on a sine wave.
Additional heating occurs in the motor, which may also lower the amount of
torque provided at low speed.
How Inverters Affect Motors
Early inverters or VFDs caused significantly higher temperature rise in the
motor, and mismatched could easily burn out the motor. As new transistor
devices and software attempted to minimize this effect, they introduced other
stresses on the motor’s insulation system. It is time to design motors
specifically to operate on these new power sources. New IGBT, PWM inverters
can output very high switching frequencies, very rapid changes in voltage, and
transient voltage spikes that can burn pin holes in the motors insulation
causing short circuits and premature motor failure .Insulation systems must be
improved to prevent this cause of unscheduled and costly downtime. Other
issues are increased motor noise and bearing failure.
A VFD driven motor can overheat if it is run too slowly as motors can get hot if
they are run slower than their rated speed. Since most motors cool themselves
with shaft- mounted fans, slow speeds means less cooling. If the motor overheats bearing and insulation life will be reduced. However, providing an
external fan with motor will be a great idea to reduce the issues related to
over-heating of motor.
High switching rates of modern power semiconductors in VFD lead to rapid
changes in voltage in relatively short periods of time, (dV/dt, quantified in units
of volts per microsecond). Steep-fronted waves with large dV/dt or very fast
rise times lead to voltage overshoots and other power supply problems. Also,
when the motor impedance is greater than the impedance of the conductor
cable between the motor and the drive, the voltage waveform will reflect at
the motor terminals.
This creates a standing wave as shown in figure.
Standing waves
created by
reflected
voltage.
Figure: PWM pulse with reflected voltage
Preventive measures can be taken to avoid motor failures caused by voltage
spikes. These include using power conditioning equipment (such as dV/dt
filters or load reactors) and restricting the distance or cable length between
the drive and the motor.
Inverter-Duty Motor
Most motor manufacturers offer general-purpose, three-phase premium
efficiency motors that feature “inverter-friendly” insulation systems. These
“inverter-ready” motors are suitable for use with variable torque loads over a
wide speed range.
An inverter duty motor can withstand the higher voltage spikes produced by all
VFDs (amplified at longer cable lengths) and can run at very slow speeds
without overheating. This performance comes at a cost: inverter-duty motors
can be much more expensive as compared to general purpose motors.
Medium-voltage inverter-duty motors with a base rating that exceeds 600 V
must be able to withstand a peak voltage equal to 2.04 times the motor’s rated
line-to-line voltage. Rise times must equal or exceed 1 microsecond. However,
larger inverter-duty motors often have a constant-speed auxiliary blower to
provide adequate cooling for motors operating at low speeds
For extreme constant-torque speed ranges, inverter-duty motors should be
specified. Such inverter-duty motors may have special windings that do not
allow operation without an inverter, or they may have separately powered
auxiliary cooling fans. Use of inverter-duty motors for variable torque loads is
overkill as inverter-ready general-purpose motors are well suited for VFD
control with variable torque loads.
Conclusion
In summary, general purpose motors can be run with drives in many
applications ; however inverter-duty motors are designed to handle much
lower speeds without overheating and they are capable of withstanding higher
voltage spikes without their insulation failing. With the increased performance
comes an increase in cost. This additional cost can be worth it if you need
greater performance.