Download Tech Note 4/16 Meg-alert VFD and Solid State Soft Starter

Tech Note
4/16
Meg-alert VFD and Solid State Soft Starter Applications
Most Variable Frequency Drives have internal fault protection circuits used to prevent
the VFD from operating on a grounded motor. However, those circuits are designed
only to protect the VFD and not the motor. Most often by the time the VFD fault
protection circuit detects a ground in the motor, the damage is already done to the motor
windings no matter how fast the protection circuit is. Catching motor ground problems
ahead of time not only ensures that the VFD won't be damaged, but will also save the
motor as well. This is one of the important reasons why Meg-alert systems are needed to
protect motors being operated by a VFD.
Another crucial reason for using Meg-alert systems on PWM (Pulse Width Modulated)
VFD applications is that ever since the conception of the VFD, there have always been
inherent problems with them damaging the electrical insulation in the motor and power
cables. VFD-induced motor insulation break-down is the result of an interaction
between the voltage pulses that the VFD applies to the motor and the stator coils
of the motor. The motor’s coils have an electrical property called “inductive
reactance”. This causes these coils to react to a change in the current through the
coils by producing a voltage that opposes this change in current flow. This is
sometimes called a “back-voltage”.
If the change in current flow is gradual, such as when a sine-wave AC line voltage
is applied to the motor, the motor’s reaction is also gradual. The inductive
reactance of the motor simply delays the change of current through the motor,
making it lag behind the applied voltage. When a fast-rising voltage pulse is
applied to the stator coils of an AC induction motor, the back voltage generated by
the motor’s coils can cause the voltage pulse to overshoot the voltage that was
applied by the VFD. This overshoot interacts with the inductance of motor,
the inductance of the motor cables, and the capacitance of the motor cables and the
motor to cause the voltage to oscillate. The peak voltage caused by this oscillation
is the major point of concern. If this voltage gets to be too high, it can break
through the motor’s insulation and cause the motor’s windings to fail.
1.
This AC transient spiking in the VFD output waveform breaks down the electrical
insulation during normal operation, which then reduces the operating life of the motors
and the power cables. There have been many improvements over the years to the
original VFD designs. However even with today's latest technology and extensive
filtering, special VFD duty motors and power cables are still required for a VFD
application due to the damage being done by the drive output voltage. Accordingly, it's
become a well-known fact that motors and power cables will have shortened operating
lives and will incur pre-mature insulation breakdown problems when operating on a
VFD. With Meg-alert automatic permanently installed I/R testing and monitoring
systems installed on the motor circuits, customers are able to detect the insulation
breakdown in time to prevent an insulation failure.
When a motor is operated from a VFD or SS Soft Starter, it is necessary to have an
output Isolation contactor installed between the VFD or SS Soft Starter and the motor to
isolate the motor circuit when the Motorguard system is testing. Because most VFD or
SS Soft Starters do not isolate the motor circuit electrically from the power source, the
Motorguard test voltage is allowed to go back through the VFD or SS Soft Starter and
find the ground in the power source. This causes a low insulation reading and a false
alarm condition when testing. Note: The Meg-alert test voltage does not damage the
VFD or SS Soft Starter in anyway because the Meg-alert DC test voltage level is on the
same order of the motor operating voltage and far below the breakdown voltage of the
system. Also, the test voltage is only applied from the windings to ground and not
across the phases, so there is no voltage potential placed on the VFD or SS Soft Starter
circuitry that could cause any internal damage.
Some VFD and SS Soft Starters systems are already equipped with an output isolation
contactor in the motor circuit and many systems have Bypass contactors, to allow for a
full voltage start should the VFD or SS Soft Starter ever fail. In most cases the Bypass
contactors can provide the isolation needed for the Meg-alert system to operate
correctly, as long as the controller is removed from the motor circuit while in the
Bypass mode. If the VFD or SS Soft Starter remains in parallel with the motor circuit,
then an output isolation contactor must be installed between the motor and the
controller. This will allow for automatic and continuous insulation resistance testing by
the Meg-alert system whenever the motor is offline. The insulation condition is then
displayed outside the control cabinet on a Megohm meter, so technicians are no longer
required to access the control cabinets during testing. Note: Meg-alert can supply the
output isolation contactor loose with the Motorguard system, to be installed in the
existing control cabinet or included in a packaged wall mount system that can be
installed near the VFD or SS Soft Starter.
2.
Typical GP500-M Motorguard VFD wiring diagram (Figure 1)
3.