VFDs in Cow Sheds (Dairy Parlors)

VFDs are used in Cow Sheds (Dairy Parlors)

VTdrive ® VFDs are commonly used in cow sheds for a number of applications:

1. Vacuum Pump. VFDs commmonly are used to control vacuum pumps to give a constant vacuum in the milk line. The VFD is installed with a vaccum transducer and the speed of the pump is regulated to keep the vacuum level at a preset level. This minimises the damage to the teat end that can be caused by excess vacuum. The use of a VFD makes the vacuum level independant of the number of clusters being used. Slowing the pump down reduces wasted energy, so the VFD saves energy as well as improving the animal health.


2. Milk Pump. VFDs are becoming commonly used to control the milk pump with the speed adjusted to match the milk flow.


3. Platform Drive. VFDs are used in Rotary Cow Sheds to control the rotational speed of the platform. Adjusting the speed of the platform enables all cows onn the platform to be milked within one rotation. If the rotation is too slow, the total milking time is extended. If the platform speed is too high, some/many of the cows will have to go round twice which also results in extended milking times.


4. Water Pumps. VFDs are used to control water pumps, especially for washdown pumps tht are used on demand with variable flow. The sleep function of the VFD allows the pump to be stopped until a hose is turned ON, and by running in constant pressure mode with a pressure transducer, the speed of the pump is controlled to suit the required flow.

Stray Voltage in Cow Sheds (Dairy Parlors)

Stray voltages in cow sheds have been recongnised as a problem since initial research work done in New Zealand about 40 years ago. The research carried out was focused on electrical transients resulting from the switching of circuits and electric fences and did not involve VFDs as this technology was not a practical consideration in those days.
Little research has been conducted into the effects of the stray voltage caused by VFDs, but there are many actual installations where the VFDs have been proven to create a number of problems with the herd unless the VTdrive ® VFDs are correctly installed. Typical problems are :

1. Poor Milk Let Down.
2. Increased Somatic Cell Count.
3. Increased Mastitis incidence.
4. Increased defacation while in the bales.
5. Reluctance to enter the milking bales.

VFD Installation in Cow Sheds (Dairy Parlors)

Cows are very sensitive to stray voltages.
As a result, it is very important that VFDs installed in a cow shed must be installed to minimise the common mode conducted noise that can circulate through the shed and onto the platform.

1. Use VFDs rated for "Domestic Levels" of conducted emissions.
2. Install any EMC filters on the line side recommended for Domestic level emissions.
3. Install any output ferrites (common mode filters) recommended for Domestic level emissions.
4. Used high grade EMC cable between the VTdrive ® VFD and the motor.
5. Terminate the cable into the VFD using a suitable clamp around the screen to bond the screen to the VFD chassis. If there is no clamp, (small plastic VFDs), mount the VFD on a metal gear tray and clamp the cable screen to the gear tray as close to the VFD as possible. - DO NOT USE Pigtails!!
6. Terminate the cable into the motor using a proper EMC screened cable gland. - DO NOT USE Pigtails!!

EMC Testing in the Cow Shed.

Use a wide bandwidth Oscilloscope with a 470 ohm terminating resistor across the input.
Measure the pk-pk voltage between the Rump Rail and the cow bail (on the platform).

1. Measure the voltage with no VFDs running to get a base line measurement. If the background voltage is higher than 100mV, find out the source by turning things OFF.
2. Repeat the test for each VTdrive ® VFD by running that VFD on it's own and recording the pk-pk voltage.
3. Measure the voltage with all VFDs running.
4. Repeat this at multiple points around the platform, a minimum of three points, first, close to the entry point to the platform, second mid way around the platform and third, close to the exit point from the platform.
5. If the voltage at any point around the platform exceeds 300mV, then alter the installation of the offending VFD to reduce this level. This may entail improving the screen bonding, and/or adding additional EMC filtering.

Submersible Pump VFD installation for minimum EMC problems

Submersible Pump Installation

1. Determine the environmental requirements for the EMC levels.
1. High sensitivity area will require installation to domestic levels.
2. Standard environment in remote area will require industrial levels.


2. The variable speed controller shall be connected to the supply in the manner described in the manufacturers installation guide for the domestic or industrial levels.
1. Where an input filter is specified for compliance with EMC requirements, that filter shall be installed as close to the input of the VTdrive ® VFD as possible with cabling and earthing as specified by the manufacturer.
2. Where an output filter or ferrite ring is specified by the manufacturer for compliance with EMC requirements, such filter shall be installed.
   
   Note : Some Variable Frequency Drives require both input and output filters fitted in order to comply with  the industrial EMC levels. Other Variable Frequency Drives only require additional filtering to be added for compliance with domestic EMC levels.
3. The length of cable on the output or the drive shall be less than the maximum length specified by the manufacturer.
   Note : The maximum length of screened cable may depend on the presence of input and/or output filters.

3. The Variable Frequency Drive shall be connected to the well head terminal box via one or more screened cables.

1. The well head terminal box shall be metal and securely bonded to the well head structure.
2. The screen is to be terminated to the VTdrive ® VFD output earth or chassis via a screened gland or clamped by a copper saddle or clamping device as supplied/recommended by the VFD manufacturer.
3. The other end of the screened cable shall be connected to the well head terminal box by a screened gland or similar clamping device.
4. Where there is an output filter, output reactors, or output isolator in series with the output, such devices shall be mounted in the drive enclosure with the screen terminated after these devices, or they shal be mounted in a separate metal enclosure with the screen glanded or clamped to the enclosure at the input and also at the output of the enclosure.
5. Connections from the well head terminal box can be made by single core or multicore cables with minimum length outside of the terminal box and well casing.

4. The well head shall be earthed to the output of the VTdrive ® VFD and shall not be additionally earthed back to the supply. – There shall only be one source of earth connection to the motor.

1. Where the screened cable is an EMC cable or a four core cable, the earth conductor shall be connected to the earth point in the VFD and to the earth point in the motor.
2. Where a three core neutral screened cable is used, an additional flexible earth conductor of not less than half the cable core rating shall be run with the main cable to further reduce the earth impedance between the VFD and the motor.

5. The motor earth shall be bonded to the well head at the well head terminal box.


6. Control cables connected to the VTdrive ® VFD shall be screened with the screen bonded  to the VFD chassis.

1. If the controls are mounted in a separate enclosure which does not have an independent earth connection, then the screen shall be bonded to the controls enclosure.
2. If the controls are mounted in a separate enclosure which has an independent earth connection, then the screen shall not be bonded to the controls enclosure.

Standard VFD installation to minimise EMC

Standard Induction Motor

1. Determine the environmental requirements for the EMC levels.
1. High sensitivity area will require installation to domestic levels.
2. Standard environment in remote area will require industrial levels.


2. The variable speed controller shall be connected to the supply in the manner described in the manufacturers installation guide for the domestic or industrial levels.
1. Where an input filter is specified for compliance with EMC requirements, that filter shall be installed as close to the input of the VTdrive ® VFD as possible with cabling and earthing as specified by the manufacturer.
2. Where an output filter or ferrite ring is specified by the manufacturer for compliance with EMC requirements, such filter shall be installed.
   
   Note : Some Variable Frequency Drives require both input and output filters fitted in order to comply with  the industrial EMC levels. Other Variable Frequency Drives only require additional filtering to be added for compliance with domestic EMC levels.
3. The length of cable on the output or the drive shall be less than the maximum length specified by the manufacturer.
   Note : The maximum length of screened cable may depend on the presence of input and/or output filters.

3. The Variable Frequency Drive shall be connected to the motor via one or more screened cables.
1. The screen is to be terminated to the VTdrive ® VFD output earth or chassis via a screened gland or clamped by a copper saddle or clamping device as supplied/recommended by the VFD manufacturer.
2. The other end of the screened cable shall be connected to the motor terminal box by a screened gland or similar clamping device.
3. Where there is an output filter, output reactors, or output isolator in series with the output, such devices shall be mounted in the drive enclosure with the screen terminated after these devices, or they shal be mounted in a separate metal enclosure with the screen glanded or clamped to the enclosure at the input and also at the output of the enclosure.


4. The motor shall be earthed to the output of the VTdrive ® VFD and shall not be additionally earthed back to the supply. – There shall only be one source of earth connection to the motor.
1. Where the screened cable is an EMC cable or a four core cable, the earth conductor shal be connected to the earth point in the VFD and to the earth point in the motor.
2. Where a three core neutral screened cable is used, an additional flexible earth conductor of not less than half the cable core rating shall be run with the main cable to further reduce the earth impedance between the VFD and the motor.


5. Control cables connected to the VTdrive ® VFD shall be screened with the screen bonded  to the VFD chassis.
1. If the controls are mounted in a separate enclosure which does not have an independent earth connection, then the screen shall be bonded to the controls enclosure.
2. If the controls are mounted in a separate enclosure which has an independent earth connection, then the screen shall not be bonded to the controls enclosure.

Screened Cable from the VFD to the Motor

Why use screened cable?

A screened cable is used betwen the output of the VTdrive® VFD and the motor because, provided that it is corectly terminated, it will reduce the return impedance for the noise coupled into the motor frame by a factor of 20 - greater than 100 and this in turn will reduce the motor frame noise voltage proportionaly. Recent tests done with a VFD connected to a motor showed that where a screened cable, glanded into the VTdrive® VFD and the motor with EMC glands, was used, the motor frame voltage was 0.82V pk-pk and where the same motor was used with a four core cable unscreened, the motor frame voltage was 96V pk-pk.

The importance of termination

There are a number of ways of terminating the screen of the screened cable, but only some are useful in minimising the noise produced in the frame of the motor.
It is important that the screen is bonded around the full 360 degrees of it's surface either by the use of an EMC screened gland, or by a clamp mechanism such as a saddle clamp.

The use of pigtails is one of the best ways to defeat the use of the screened cable.
Typical test results showing the ffects of the use of pigtails:

Pigtail Length	EMC voltage
0	0.82V
50mm	3V
100mm	7V
250mm	15V
500mm	30V

The pigtail length refers to the total length of pigtails between the VTdrive® VFD and the motor. Two pigtails, one at the motor and one at the VFD, each with a length of 50mm give a total length of 100mm and an increase in EMC noise voltage in this installation of more that eight fold. 

500mm of pigtail length, and the screen is only halving the EMC noise!!

What is a pigtail?

There are two major techniques of terminating the screen on a screened cable.
One way is to clamp around the exposed screen and cable providing a 360 degree connection to the screen using a metalic clamp or an EMC screened gland. This provides a large surface area connection to the earth or chassis of the equipment.
The alternative is to terminate the screen to a standard circular cable and connect this to a ground or chassis connection. This can also be achieved by peeling the screen off the cable and twisting it together and sleeving it to make it into a standard conductor.
The smaller conductor is known as a pigtail connection and has a much lower surface area than the screen and so exhibits a much higher impedance at the high frequency than the screen.

VFDs and EMC

Understanding Conducted EMC reduction

The PWM waveform at the output of the VTdrive® VFD exists on virtually all VFDs sold in New Zealand. Induced noise voltage on the frame of the motor is a direct result of the switched waveform and the capacitance within the motor.
The best means of minimising the stray currents to flow in other circuits and earth paths, is to minimise the noise voltage on the motor frame.
At high frequencies, the current flows on the surface of the conductor. Providing a very low impedance earth return from the motor frame to the VFD frame will reduce the voltage on the isolated motor frame by the effect of the voltage divider.
Using a cable screen as an earth return path provides a surface area that is much larger than the surface area of a standard circular conductor. 
Laboratory tests indicate that the voltage on the frame of the motor can be reduced by a factor of 20 to 50. One test carried out by UL laboratories showed a noise voltage drop from 69 volts to 1.4 volts with a 5 meter length of cable when an unscreened cable was replaced by an equivalent correctly terminated screened cable.

Many field problems, where stray voltages in the order of 50 to 100 volts have been measured, have been corrected by the fitting of a screened cable with the screen correctly terminated at each end with stray voltages commonly less than 2 volts after correction at the same point of measurement.

The effectiveness of the screen as a low impedance earth return is inversely proportional to the frequency of the noise current. Lower frequencies are not attenuated to the same degree as higher frequencies when a screened cable is employed.

Another solution, is to completely isolate the motor and the cables from all other conductive paths in the environment. This is not a practical option in most installations and is of academic interest only.

The only other way to overcome this problem, is to eliminate all high frequency components from the output voltage of the VTdrive® VFD. This can be done, but is expensive and has a major impact on operating efficiency. Filtering on the output of the VFD can result in the bandwidth of the noise being compressed into the lower frequencies where the screen is less effective, but the existing standards are met. In this situation, because the screen is less effective, the stray voltages can be higher and harder to eliminate than the unfiltered VFDs where the potential divider using the screen is much more effective.

Sine wave filters are available for use with VFDs, but the commonly used filters are designed to produce a sinewave voltage between phases, but not relative to earth. The voltage waveform can have a very high common mode PWM component that still causes high stray voltages and currents.  The dv/dt of the common mode voltage is greatly reduced by the filter, reducing the effectiveness of the decoupling within the drive and forcing much of the return path current to flow via the incoming phases. Noise voltages in excess of 100V have been superimposed on the phase to earth voltages causing lamp failures, switchmode supply failures and many other issues. To be effective, a sinewave filter would need to filter relative to earth as well as between phases.

VFDs and conducted emissions

VFD Operation

A VTdrive® VFD comprises three major power sections, i) a rectifier, ii) a DC filter and iii) a three phase inverter. The noise is primarily generated by the inverter section. The inverter section operates by chopping up the DC bus voltage (typically around 570Volts DC) and creating a PWM output voltage that causes an average sinusoidal current of variable frequency to flow through the motor. In modern inverters, the PWM waveform is generated by IGBTs operating as switches, so they are either turned fully ON or fully OFF. On each phase, there are two switches in series and across each switch, there is a flyback diode.

There is a transition period between the ON state and the OFF state, and the OFF state and the ON state. This is referred to as the switching state and has inherent switching losses.

There are three phases out of the inverter, each phase is generated by two switching devices, one from the output to the negative DC bus rail and the other to the positive DC Bus rail. This gives a total of six switching devices that are being controlled to produce the PWM output waveform. Switching or carrier frequencies are typically in the range of 1kHz to as high as 16 kHz.

For each cycle of the switching frequency, there are three positive transitions and three negative transitions resulting in transients occurring at six times the carrier frequency. These switching transitions are effectively phase modulated by the PWM waveform so the noise is aperiodic or semi random in nature. During the ON state, a switching element carries the output current for that phase and has an ON state voltage of between one and three volts depending on the device and current. Power dissipated in the switching element during the ON state is known as a static loss. When the device begins to turn OFF, because it is carrying an inductive current, the voltage across the device is driven to the opposite rail voltage and the current not passing through the switching device, is driven through the flyback diode on the opposite switching device. The voltage across the switching device during the switching OFF period is in the order of 600V with the current through the device reducing from 100% to 0% over the period of the switching. At switch OFF, there is a very high switching power dissipated (600V x 0.5 x Io x Ts). The slower the switching time, the higher the switching losses.

Modern IGBTs are able to switch from ON to OFF in periods as low as 100nS. There is a tremendous thermal advantage in “fast” switching resulting in smaller silicon, smaller heatsinks and lower costs. The disadvantage of high speed switching, is that the noise bandwidth is much greater. The operation of switching current ON or OFF results in the generation of electromagnetic noise. The bandwidth of that noise is a function of the switching time (Time of transition from ON to OFF). This is well accepted as a cause of Electromagnetic Interference.

When the IGBTs are switched ON and OFF, there are leakage currents to earth caused by stray capacitance within the VFD and external to the VFD.

In older VFD designs, there was no decoupling of the DC bus to the chassis. In these cases, the high current noise transients are conducted via the earth connection to the outside world and then via capacitance in the wiring and other equipment, to the phase wiring and back to the DC bus via the bridge rectifier. Modern designs that comply with the European requirements include decoupling between the DC bus and the chassis to provide a return path back to the DC bus. (completing the circuit.)

The traditional EMC conducted emissions tests do show the effects of the noise generated by the stray capacitance within the VFD and the rapid switching voltages. The amplitude of the noise transient current is a function of the value of the stray capacitance and the rate of rise of voltage (dv/dt). The length of the current pulse is a function of the DC bus voltage and the dv/dt. It is imperative that the design of the decoupling offers a very low impedance path across the full spectrum of the noise generated in this circuit. Poor designs will fail the conducted emissions test. This mechanism is exactly the same mechanism that occurs in switchmode power supplies, but in small switchmode power supplies, it is possible to isolate the heatsink from earth and reduce the transient noise currents flowing in the earth circuits.

Conducted Emissions.

In a practical VTdrive® VFD installation, the output of the VFD is connected to the windings of the motor via a length of cable. 

The voltage applied to the motor is a PWM waveform generated by the inverter section of the VFD with fast rising and falling edges.
There is considerable capacitance between the stator windings of the motor and the motor frame and just as the stray capacitance in the VFD causes noise currents in the chassis of the VFD, the capacitance within the motor causes  high noise currents to flow between the motor windings and the frame of the motor.
The noise current spectrum covers the frequency range from KHz to tens of MHz.
The circuit is closed by conductive paths from the motor frame back to the DC Bus of the VFD.
One path, is the return earth path provided by the wiring between the motor and the VFD. Current will flow on the earth conductor between the motor frame and the VFD frame and via the DC decoupling back to the DC Bus.

Another path is back to the DC bus via capacitance between the incoming three phases and earth to the incoming three phases and then through the rectifier to the DC bus.

The conductors between the motor and the VFD have an impedance that is made up of resistance, inductance and capacitance. At high frequencies, due to skin effect, the current flows on the surface of the conductor. At 100KHz, most of the current is concentrated in the outer 0.2mm of the conductor. With a circular conductor, this can increase the effective impedance of that conductor dramatically relative to the DC impedance of the cable.
If we look at the circuit between the output of the VFD and the motor, we have a phase impedance Zp and an earth return impedance Ze. In series with this we have the impedance of the motor capacitance Zm and this is very low at high frequencies.

The voltage that appears on the frame of the motor where it is totally isolated, is determined by the potential divider comprising Zp plus Zm plus Ze. The voltage being the ratio of (Zp +Zm)/(Zp + Zm + Ze). In some cases, the size of the earth conductor is half the size of the phase conductor so the impedance of Ze is up to twice Zp. The impedance of the capacitance can be very low relative to Zp, so it is possible for the noise voltage to be hundreds of volts on a 400 volt motor frame.

When the motor is physically installed, there are many other paths between the motor and the DC bus of the VFD. Some of these paths are direct through the installation from the motor frame to the VFD frame, and others will be through the installation via other capacitance to the phase conductors and then through the rectifier back to the DC bus.
Current will flow in all paths, the magnitude of the currents is a function of the “stray voltage” on the motor frame and the individual impedances of the other paths.

“Stray” or “Leakage” currents, flowing through the installation cause “stray” noise voltage to be developed and induced in other circuits and this can result in problems with the operation and integrity of other equipment. These voltages have been measured in some installations at levels in excess of 100 volts peak and have caused failure of equipment such as switchmode power supplies and flow meters. There can be significant energy in these stray voltages with MOVs being damaged by the voltages.
The noise can cause interference with the operation of equipment, blocking serial communications circuits, inducing resets in microprocessors and false input operations on electronic equipment.
Motors can be started and stopped intermittently by extraneous noise voltages which are well in excess of the EMC immunity requirements.
Stray voltages in sensitive areas such as cow sheds can have a severe effect on the health and behaviour of the cows with documented evidence of issues caused by stray voltages.
Recent cases have come to light of severe herd mastitis, loss of production and elevated cell counts  caused by VTdrive® VFD induces stray voltages.

There are additional stray capacitance paths between the cable and the environment through which the cable passes. This includes capacitance to other conductors and to earth and structures.
The cable capacitances also cause stray noise currents to flow in other circuits.

VFDs and EMC

What is EMC?

EMC is an acronym for ElectroMagnetic Compatibility and is a regime of standards designed to ensure that interference between items of equipment is minimized. It is some times referred to as EMI or RFI, but with most electronic equipment, there is a) noise generated by the equipment (emissions) and b) a maximum threshold of noise that the equipment can withstand without causing problems. (Immunity)
There are two types of emissions, radiated emissions and conducted emissions. Radiated emissions are generally high frequency emissions above several megahertz and the conducted emissions are the lower frequency emissions. (below ten megahertz)

What is EMI?

EMC is an acronym for ElectroMagnetic Interference and is the effect that a strong electromagnetic field can have on the correct operation of equipment.

What does EMI do?

Electromagnetic Interference causes other equipment to alter the way in which it performs and this can range from very minor interference to catastrophic interference.
An example of EMI, is the buzzing noise that can be heard on an AM radio station. If this is minor, it will be a background noise, but if it is major, it will totally block reception of the radio program. EMI can cause lines to appear on top of a TV program, it can cause measuring equipment to read incorrectly and it can even cause equipment to fail.
In cow shed (dairy parlor) installations, incorrectly installed or designed VFDs can cause the cows to be subjected to electric shocks and this in turn can increase the SCC and indirectly cause an increase in mastitis.

What causes EMI with VFDs?

The output stage of a VTdrive VFD comprises six solid state switching devices which are switched in a manner to provide a PWM sinusoidal output current to the motor. Modern switching elements are usually MOS FETs or IGBTs. These devices switch from fully OFF to fully ON very quickly, typically 150 - 250 nS.
The very rapid transition of the output voltage from low to high results in switching noise being produced and this can be coupled to the frame of the motor and from there conducted by the ground circuits, eventually returning to the DC Bus in the VFD.

What are conducted emissions and conducted noise?

When an electrical or electronic switch operates, there is a noise impulse that can cause high frequency radiated emissions, and low frequency conducted emissions. The conducted emissions or noise, are carried by the phase and earth conductors to other equipment. noise generated by the equipment that is below 1 Mz is usually conducted between equipment rather than radiated.
The current carried in the earth conductor between the VFD and the Motor can be very high in amplitude although short in duration. The source voltage is high, (hundreds of volts) and a poor connection on the earth circuit can result in major arcing which is very dangerous in a hazadous environment.

How do you minimize EMI and conducted emissions from VTdrive VFDs?

The best way to minimize the emissions from VFDs, is by firstly selecting a VFD that has been correctly designed to minimize the noise produced, and then to install the VFD correctly. The most important part of the installation, is to provide a very low impedance path from the frame of the motor to the chassis of the VFD.
This would usually entail the use of a screened cable with proper EMC glands or clamps at each end of the cable. The screen must be correctly terminated at the VFD end and the motor end. Pigtails severly reduce the effect of the low impedance screen path.

What is the impact of dv/dt on conducted emissions?

The rate of rise of voltage determines the bandwidth of the noise produced. Steeper wave fronts produce noise over a wider spectrum (to a higher frequency) than less steep wave fronts. Reducing the dv/dt (rate of rise of voltage) concentrates the noise produced to a lower frequency range. This can actually make it harder to eliminate interference in a practical installation.

What is the impact of pigtails on conducted emissions?

The use of pigtails to terminate the screen on a screened cable increases the return earth impedance for the noise voltage on the frame of the motor and increases the conducted emmissions considerably.

NO B.S. Guide to installing VTdrive VFDs for minimum conducted noise problems.

A simple step by step guide for installing VFDs (VSDs) to minimise interference problems. This applies to all VFDs.