Selection and Installation of Variable Frequency Drives


Variable Speed Drive Selection and Installation

Let’s look at how to choose and install a drive. You can choose a drive based on the power and voltage of the motor you are using. Make sure the motor is correctly configured (star or delta) to match the supply. Check the full load current of the motor and the variable frequency drive; sometimes you can choose a lower power drive; it’s the current that matters.

If the drive is mounted in an exposed area, a protection level of IP55 or IP66 should be used. If the drive is in a cubicle, IP20 will usually be enough, but make sure there is no chance of dirt, dust or liquid getting into the drive.

EMC regulations should be followed; that usually means choosing a drive with a built-in EMC filter to limit interference into and out of the drive.

If you need high torque at low speed, or high performance at high power, choose an ‘industrial’ or constant torque drive such as the P2. For pump and fan applications which don’t need a high starting torque, a variable torque drive such as the Eco includes many useful features for these applications. A general purpose drive such as the E3 will generally do both jobs up to 22kW.

If you install the drive in a cubicle or machine, check the manual or application note AN-ODE-2-070, available on the Invertek website (invertekdrives.com) for information concerning cooling and EMC recommendations. There must be enough air, free from dust, dirt or liquids around the drive to cool it. Make sure the drive is properly earthed, and that the motor cables are screened or armoured.

Connect the wiring as described in the manual. Double check the motor and supply connections, and that you have the correct voltage unit. On units without built in controls, you’ll probably need a run stop switch (control terminals 1 and 2) and a potentiometer as a minimum to get started.

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Braking and Regeneration

Normally the power in a variable frequency drive system flows from the drive to the motor and then to the load. However, sometimes the energy can flow in reverse. This is called regeneration. If a crane is lowering a load, or if a conveyer is going downhill, then regeneration can occur. If you try to decelerate a high inertia load such as a fan, then regeneration can also happen. When the energy goes back to the drive, it cannot go back to the supply – the rectifier diodes block this, so the DC link capacitor absorbs the energy as a rising voltage. If the voltage gets too high the drive will detect this and trip on ‘overvoltage’. When this happens the drive stops supplying magnetising current to the motor, so the motor no longer regenerates, but coasts to a stop in its own time.

To prevent tripping when decelerating high inertia loads, we can increase the ramp down time, so the drive is more able to absorb the energy. Alternatively, we can stop the drive immediately by changing the Stop Mode parameter P-05 (P1-05 on P2 and Eco) from 0 to 1. Now there is no ramp down, the drive shuts off and the motor coasts.

For controlled braking in applications like cranes and downhill conveyers, we can fit a braking resistor to drives that have a built in braking IGBT (braking chopper). Now the drive will detect the increasing voltage on the DC link and will switch on the IGBT to burn the energy in the external resistor.

For very high power applications fully regenerative drives are available that have a second inverter instead of a rectifier, and can feed the energy back to the supply.

Most Invertek industrial drives have braking IGBTs built in as standard.

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Cranes, Hoists and Elevators

When Cranes lift a load, they are doing work; when they lower a load, the energy is coming from the load to the drive, that is, the load is regenerating. An elevator usually has a counterweight, so it is not always clear if the system is motoring or generating. These applications nearly always work with a drive fitted with a braking resistor to absorb the regenerative energy.

Stopping and starting a crane or elevator requires care. Before the brake is released, the drive can’t tell if the load is heavy or light, or if it will have a tendency to fall or (in the case of an empty elevator) rise on release. So it is usual to run the motor to a low frequency to build up torque, and then release the brake. Similarly, when the motor is slowed down the brake is applied before reaching zero frequency. The output relay on many drives can be programmed to switch at these times, and can be connected to a contactor to control the brake.

An encoder is often fitted to a motor and used to feedback to the drive the shaft speed. This can be used to detect any movement when the brake is released, simplifying control under these conditions. However, with modern Vector Control systems encoders are often not needed.

Elevators need many extra features such as smooth acceleration and deceleration and motor contactor control. Drives with special software will offer this. Emergency operation - when the power fails – is also important, and switching the drives supply to a small, low voltage uninterruptible power supply (UPS) will provide enough energy to get to the next floor.

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Safety Inputs and Safety

Safety Regulations are now an important part of engineering design and installation. Rules and standards must be met; ignorance is not an acceptable defence.

In the past, safety could be seen, with interlocks, disconnections and lock off systems. These days systems are more complex with remote control and automated systems that seem to stop and start at random. Regulations have been updated to meet these systems. New regulations such as IEC 61508 are relatively easy to make sense of, and offer guidance in risk assessment and reduction. Many variable frequency drives now incorporate safety certified inputs. These inputs are externally tested and guarantee that the drive will stop (or run at a certain speed for example). The external testing is comprehensive and takes some time, because the test house must understand the function of the equipment to be confident in approving the control. Some Invertek drives offer a pair of Safe Torque Off (STO) inputs, which, if either is open, guarantee that the drive will not turn the motor. The inputs will control the software and inhibit the drive immediately, but they also supply the power to the IGBTs (the power devices) so there is no possibility that the motor will be driven. Of course, an Emergency Stop system that disconnects the power will always be needed.

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