The issue of mandating screened cables with VFDs is an interesting one and it is not strictly correct in that it is possible to minimise the spread of common mode switching currents without the use of a screened cable, but there must be an alternative low impedance return path for the capacitive currents flowing from the motor windings to the motor frame, back to the source of the PWM waveform, the DC Bus of the VFD.
The VFD converts incoming AC voltage to DC and charges a bank of DC Bus capacitors.
The DC is then chopped up by two series AC switches across the DC Bus with the junction of the AC switches connected to the phase output terminal.
A sinewave current is synthesised by modulating the action of the two switches using PWM techniques. There are essentially three instantanious states, output connected to DC Bus Positive, output connected to DC Bus negative and DC Bus disconnected.
Modern VFDs use IGBTs or MOS FETs as switching elements and these can switch in less than 100 nano Seconds. - Very fast opening and closing.
The Output voltage is essentially a PWM modulated waveform switching between the positive DC Bus and the Negative DC bus, so with a 400 volt motor, this waveform is rapidly switching from +300Volt to - 300 volts.
In the motor, there is capacitance between the motor windings and the motor frame.
The switched voltage waveform is applied to the motor windings and this results in a high charging and discharging current to flow from the motor windings to the motor frame through the parasitic capacitance between the motor windings and the motor frame.
The charging and discharging currents flow at the operation of each switch (there are six switches in a three phase inverter).
The current transients are very short and high in amplitude. - High frequency components when analized on the frequency domain.
If the frame of the motor is not bonded to earth at all, the motor would be subjected to voltage impulses in sympathy with the charging currents, but the frame voltage would be swtiching between the DC bus polarities minus the leakage current through the other phases. The frame voltage will be pulsing at hundreds of volts.
The capacitive current must find paths back to the DC bus and that can be via any equipment connected between earth and the incoming supply.
To minimise the switching voltage on the frame of the motor, it is imperative to provide a very low impedance path back to the DC bus. This impedance must be low at the frequencies of the noise spectrum caused by the switching waveform and this is typically in the order of 150KHz.
At 150KHz, skin effect is a very real problem with the current only flowing in the very outer surface of the conductor.
To achieve a good return path that minimises the switching voltage on the frame of the motor, the earth return path back to the VFD must exhibit a large surface area over the whole length of the return path.
A screened cable is a very effective means to provide that return path. At 150KHz, the impedance of the screen as a conductor is typically fifty times lower than the impedance of the internal earth conductor. This will reduce the voltage on the frame of the motor by a factor of fifty.
Suitable alternatives are a steel pipe of a simliar diameter as the screened cable, a flat braid equal in width to the screen opened out, A copper, or aluminium, or steel strip 1.5mm thick with a width equal to or greater than the screen of the screened cable opened out.
A screened cable is specified because is is an easy way to achieve the low impedance, high frequency return path, not because is actually blocks the conductors from radiating.
Unfortunately, many "experts" believe that the screen is used to form a faraday cage rather than a low impedance return path for the switching currents.
Many experts also claim that the screen must be terminated at one end only. This is true for control circuits, but definitely not VFD output circuits.
Horner have released the much publicised CScape 9.9 SP3
This service pack is primarily aimed at the Advanced Relay logic programming environment with enhancements such as UDFBs, Gmail support for emails, downloadable protocol enable/disable, DHCP and DNS support, MQTT Sparkplug IOT support.
Some of these additional functions will require a firmware update which is also available for all processors.
Should you upgrade?
If you have a program that works and does not reuire the additional enhancements, then leave it alone.
If you are starting a new project and the enhancements are of value to you, or if you are using one of the newer processors, then you should upgrade.
NOTE: while Horner endeavour to keep as much upwards compatbility as possible, there can be changes in some established functions in new versions of CScape that may require tweaks in the code. Avoid upgrading the CScape version of code that is running in the field without fully testing it first.
You can have multiple copies of CScape on a single machine provided that they are installed in their own directory.
Adding a VFD (Variable Frequency Drive) to a motor running at a fixed speed and continuing to run the motor at that speed, does not make the motor efficiency improve except where:
The motor is very underloaded and a reduction in voltage will reduce the flux in the iron and thereby reduce the iron losses in the motor, BUT
The reduction in voltage applied to the motor will :
reduce the magnetising current
reduce the flux in the iron and therebye reduce the inductive current flowing in the motor
Increase the slip in the motor, thereby increasing the slip losses
increase the work component of the current flowing in the motor
Reducing the flux in the stator can result in an element of energy saving BUT
Reduced Flux ==> reduced Iron loss
Reduced Flux ==> increase slip loss
Reduced Flux with an open shaft motor ==> reduced line current and reduced copper loss
Reduced Flux in the motor with load current equal to or greater than the magnetising current of the motor ==> increased motor current and increased copper loss.
At line speed, the potential to save energy by using a VFD is limited by the true losses due to the use of an induction motor. With many modern machines, the efficiency is is typically quite high and until the load is very low and the actual iron loss is commonly less than a few percent of the full load of the induction motor.
The iron loss is independent of the load, but there will only be a net gain when the load is very low, BUT the addition of a VFD to control the motor can reduce the iron loss by a small amount, but there is an immediate loss of two to three percent due to the the additional losses inherent in the additional losses in the from the use of a VFD.