Commissioning and Troubleshooting Variable Frequency Drives


Commissioning a Variable Frequency Drive

In the last article we described how to connect up the drive and how to install it. Now we’ll see if we can get it going.

Final Checks again:

  • Check the supply is correct for the variable speed drive.
  • Check the motor is also suited for the supply.
  • Check the control wiring: A switch between terminals 1 and 2 to stop and start the drive. A pot connected between terminals 5 (10V reference) 6 (analogue input) and 7 (0V) to control the output frequency. On P2 and Eco drives you’ll need to link the Safe Torque Off inputs as well by connecting terminals 1 and 12 and also terminals 9 and 13.

Now when you switch on, the display should light up and show STOP. Close the switch. The drive should start, and display the output frequency. Adjust the pot or the power supply connected to terminal 6 and the output frequency should change. The motor should be turning as well.

If the drive doesn’t work, check through everything logically (beginning with the mains supply) All the drives are fully tested before they leave the factory so are unlikely to be faulty at this stage.

Once everything is working we can look at setting up the drive to match the application. To do this, we’ll be using parameter settings. Most parameters can be adjusted by the user to suit their needs; parameter settings are retained at power down, and some parameters can be adjusted while the drive is running. In the next article we’ll go through some of the key parameters and explain why you may want to change them – or leave them alone.

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Setting up a Variable Frequency Drive

Now we need to look at the basic parameters that match the drive to the motor and the application. In Invertek variable frequency drive, the first thirteen parameters can be adjusted to suit most applications; more parameters are available for specialist applications – we’ll look at those later.

Changing parameters is easy. Long press on the navigate button to see the parameter number, use the up and down arrows to select the parameter of interest, short press on navigate and you can change the setting. Long press and you’re back to the display.

The motor parameters P-07, P-08, P-09 and P-10. (On Eco and P2 drives these basic parameters are numbered P1-07, P1-08 etc.) are important as they tell the drive the size and type of the motor. Set these parameters based on the motor rating plate information. P-07 and P-09 are probably already the correct Voltage and frequency; P-10 changes the display to RPM, otherwise it doesn’t need setting. P-08 is the important one as by setting the motor current the drive can then protect the motor.

The other basic parameters are about matching the drive to the load and application. P-01 and P-02 set the maximum and minimum frequency. Running the motor faster than its nominal frequency can cause problems.

P-03 and P-04 set the acceleration and deceleration rates. If the load has a high inertia you may get problems if you set fast rates here.

P-05 allows you to switch between a stop with a deceleration, or just simply switching off the drive and allowing the motor to coast to a stop.

P-06 reduces the magnetising current, so it saves energy on light loads.

P-11 adds some boost voltage to give more torque at low speed; don’t put too much in or you may overheat the motor.

P-12 Sets where the control signals come from. We’ll look at these options later; we can stick with control from the terminals for now.

P-13 Selects your application type; Industrial gives best torque at low speed, Pump and Fan gives better performance in those applications. It’s different on Eco and P2.

In the next article look at more advanced applications.

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Using the Inputs on a Variable Frequency Drive

There are many ways to control a variable frequency drive, but often the analogue and digital inputs are used. These can be programmed in many different ways. On Invertek’s E3 drive parameters P-15 and P-12 can be used to select favourite settings for the two digital and two analogue or digital inputs. On P2 and Eco drives P1-13 and P1-12 do this, and there is more flexibility with these drives. Note that the functions of controls built into some IP66 drives will change if these parameters are adjusted.

On E3, changing P-15 will change the individual functions of the inputs. With the default setting of P15=0 we have the functions shown in Figure 1. Changing P15 to 2 allows the use of preset frequencies as shown in Figure 2.

If we change P-12, which selects the primary command source, then the settings of P-15 adapt to suit. For example with keypad control selected (P-12=2) then with P-15=6, the terminals can be used for additional functions as shown in Figure 3. If P12=5 or 6, then the closed loop control system is enabled, and the digital and analogue inputs change again, as shown in Figure 4. Many possible configurations are therefore available; these are explained in detail in the handbook.

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Closed Loop Control with Variable Frequency Drives

A closed loop control system is one where the actual value that is desired (temperature, pressure, speed etc.) is measured, and the resulting signal connected back to the controlling device, in this case a variable frequency drive.

The drive compares the actual value with the required value (the Setpoint) and continually adjusts the speed of the motor to try to minimise the difference – the error - between the two. Different systems respond in different ways; for instance, there can often be a time lag between changing the motor speed and the system responding; sometimes using too much correction will make the system unstable. Therefore a general purpose closed loop controller allows tuning of the system by adjusting the Proportional, Integral and Differential factors which modify the error signal. Hence the alternate name - PID controller. The P term simply multiplies the error by a constant; the I term integrates the error value, that is, adds a filtering or damping effect. The D term isn’t used that much. Not enough P and you’ll always have an error – too much and you’ll be unstable. Use the I term to slow things down, especially when things are happening slowly – but too much and you’ll never catch up. Sounds a bit tricky? Usually you don’t need to adjust these terms too much, or it’s a case of trial and error. More important when you’re trying to set up a PID controller is making sure the feedback is giving the signal you expect and that it is properly scaled. Invertek drives allow all these factors to be adjusted using parameters (although there’s no D term on E3); P2 and Eco drives have additional features and functions which take account of practical problems like start up. On Eco drives even more parameters add extra functions like cascade control, making a highly versatile controller.

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Warnings, Trips, Faults and Failures

Variable speed drives are pretty reliable these days. Most failures are caused by contamination (dust, dirt, water etc.) or misuse, such as connecting the supply to the output. To help protect itself and the motor, the drive will give a warning (on Invertek drives the decimal points flash to show this) when there is a problem. If the problem persists, the drive will trip – that is, switch off its output to the motor and indicate a fault in the display.

Many warnings and faults are caused by overload. If the correct motor current has been set in the drive (P-08 or P1-08) then if the motor is in danger of overheating a warning will be indicated and the drive will eventually trip. So it is important to set the correct motor parameters for best motor protection. If the drive itself is overloaded it will protect itself as well. A short circuit or sudden overload will cause a rapid trip.

The drive monitors its temperature, and will warn and then trip if the heatsink or internal temperature is too high (or even too low).

Under voltage trips and warnings may be caused by poor supplies, but are more usually recorded as a result of switching off the drive while it is running. Overvoltage trips may also be caused by supply problems, but regeneration from the motor will result in these trips as well. Regeneration occurs when energy comes back from the load, either due to decelerating a high inertia load (such as a fan) or when a crane or conveyer is lowering a heavy load. External braking resistors will absorb this energy and prevent trips. Other warnings and trips are usually the result of settings, such as enabling an external trip or PTC input.

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How to Break a Variable frequency Drive

Most variable frequency drives are installed correctly and give reliable operation. However, if they are incorrectly wired up, or poorly protected, they can fail.

Connect a 230V drive to a 400V supply and you can be sure the protective Varistors will burn out, usually destroying the drive. Connect the supply to the motor terminals and the drive may survive – until you start it. Finally, connect the supply to the control terminals (except the relay connections), and you’ll certainly destroy the control board. If you’re not sure, read the manual or the help card, and double check connections.

Once the drive is correctly connected, set up and running, it’s pretty difficult to break it. The output is protected from overload and short circuit, and the temperature is monitored inside the drive to prevent overheating. However, poor grounding and interference from other equipment can still cause damage. Serial interface components can be damaged by overvoltage (fit an isolating module or improve grounding) and input components can be damaged by power surges (fit an input choke if supplies are prone to this).

Dirt, dust and liquid contamination are a major cause of drive failure. Dust – even non conductive dust – can build up inside the drive, causing local overheating or voltage breakdown. Liquids, corrosive gases, smuts from fires will all lead to failure eventually. Make sure your cooling air is clean and dry – unless you want to break the drive of course. An IP66 drive will give added protection to the drive internals.

There are a few other ways to break a drive, such as switching on and starting the drive after a long period of storage. Capacitors should be reformed after a year or more without power – any manufacturer will tell you how. So to avoid a warranty claim that may be refused (the service guys can tell if you’ve not followed the instructions) just remember:

  • Connect up correctly.
  • Follow basic rules to prevent Electromagnetic Interference (EMI) problems.
  • Set the parameters to suit your motor and application.
  • Make sure the drive is kept cool and free from dust, dirt and liquids.
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Electromagnetic Compatibility

Electronic systems and controls are everywhere, and as they have spread the problem of interference between equipment has grown. Unwanted signals were being transmitted from wires and printed circuit tracks, and were being received in other equipment. Signals were also finding their way along control cables and power connections. In some cases these signals would interfere with the operation of a machine, causing minor – or occasionally major problems and faults. Engineers began to understand the problem of Electromagnetic Compatibility (EMC) and modified their designs to reduce transmission of interference and improve the product’s immunity to incoming electrical noise. Regulations were introduced to ensure all equipment met certain basic standards. Designers of power supplies and variable frequency drives added input filters and made many other changes in order to meet regulations such as EMC Directive 2004/108/EC.

But poor installation can undo much of this. It is important to follow some basic rules when installing all electronic equipment, including variable frequency drives. Good grounding, with short, thick connections to a star point is important. Screened cables should be used, especially between the drive and motor. Power and signal cables should be separated, and contactor coils should be suppressed with varistors or resistor capacitor networks. There’s plenty of information on the Web; remember, prevention is better – and cheaper – than cure.

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Problems with motors and cables

Induction motors are designed to work from the mains. However, a variable frequency drive produces a series of high voltage pulses that build up an approximate sinusoidal current in the motor. Most motors run from a drive without problems, and the high switching frequencies used by modern drives create a pretty good sine wave current. The voltage waveform however, can cause problems. The waveform contains many frequencies higher than the switching frequency, and these can cause interference and leakage currents if there is any stray capacitance. Cables between the motor and the variable frequency drive will always have some capacitance to ground, and screened cables are generally worse than loose or unscreened cables – which aren’t recommended as they can cause interference. The longer the cable the greater the capacitance, so drives are usually specified with a maximum cable length. If the capacitance is too great, then the drive may trip on over current or overheating as the leakage current becomes excessive.

The same voltage waveform can damage insulation in older motors. Modern motors are designed to handle the voltages, but when drives were first introduced there were many instances of old ‘reliable’ motors failing.

Bearing failure caused by leakage currents is usually confined to higher voltage systems, and many motors have insulated bearings to prevent this damage. Good grounding systems will prevent leakage currents, and in extreme cases shaft grounding brushes can be used.

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Cooling Matters

All electronic equipment will fail if it gets too hot – usually if the semiconductors get above 125oC , so some thought needs to be given to cooling them, especially with variable frequency drives, which generate significant heat themselves.

Drives are carefully designed so that components such as the capacitors and power components are adequately cooled; the IGBTs and Diodes are usually mounted on a heatsink and are monitored with temperature sensors. On many drives fans are used to assist cooling; on Invertek drives the fans only switch on when needed, so the fans don’t wear unnecessarily. If the drive gets too hot it will display a warning, and eventually trip to protect itself.

When drives are installed in cubicles or similar enclosures the temperature rise can often be calculated. Bear in mind if it’s hot outside it will be even hotter inside! Make sure air can flow through the drive; don’t block the top or bottom of the drive; some drives need space at the side as well. Dirt or dust can impede cooling and stop the fans; make sure the cooling air is clean. Derating for altitude and higher switching frequency may also be necessary – check in the handbook or with the factory.

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Optitools Studio

Serial communications connections are available on most variable frequency drives and as well as simplifying wiring and control allow drives to be commissioned and monitored using a PC program such as Invertek’s Optitools Studio. This is free to download from the website and runs with most Windows operating systems.

Connecting between a PC and a serial communication connection – which is usually RS485 can be tricky. Use the recommended USB to RS485 adaptor, or simply plug the Optistick into a drive (or drive network) and connect by Bluetooth.

Once connected, you can scan the network to find the drive and read out the parameter settings. Optitools Studio then offers four possibilities:

  • You can use the Parameter Editor to upload and download the drive’s parameter settings. You can edit them offline and then upload them, or you can edit them in real time.
  • You can use the Drive Monitor to monitor any drives on the network, choosing which User variables (analogue input, output current, Torque Etc.) you wish to appear in the table
  • You can use the Function Block Editor to prepare a programme that can be uploaded to a P2 or Eco drive. There a many different function blocks available (Logic blocks, Arithmetic functions, Timers, Counters, Parameters, Display settings) that allow the drive to do the work of a PLC. You’ll need to buy a licence if you want to save or upload a programme.
  • You can use the ‘Scope function to observe up to four channels (such as current, speed, feedback etc.) either in real time, or as a ‘capture’ before and after a trigger event such as a trip.
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