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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Braking and Stopping

A variable frequency drive will slow down a motor by reducing the output frequency; the drive can also simply switch off and allow the motor to stop in its own time. DC injection can also be used to produce a braking and holding torque, but for real stopping power an electromagnetic brake is needed. These are usually built into the motor, and are powered by DC – often from the supply via a rectifier unit. They are energised to release the brake, so with a loss of power the brake will engage. If we are using a variable frequency drive with a brake, we need to ensure that the brake is released and engaged at the right time.

It is usual to increase the frequency to a level where we can expect to have torque on the motor before releasing the brake, and to apply the brake at about the same frequency when stopping. The brake can be controlled using the drive’s built in relay, but not by direct connection – use a suitable contactor (with separate coil supply) and suppress the contacts.

Invertek’s P2 drive has a hoist mode which allows the drive to hold at a frequency while the brake is released, and will even check that there is enough torque on the motor. The drive will also hold during the stopping sequence to allow the brake to engage.

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Supplies and Supply Problems

Variable frequency drives need power from the mains. They generally can’t produce an output voltage that is higher than the input voltage, so if you only have 230V supply, you’ll only get 230V out. Make sure your motor is connected correctly for the supply you have; many motors can be configured for different voltages (e.g. 230/400 or 400/690).

A 230V single phase input drive will supply a 230V three phase variable voltage, variable frequency supply to control the motor. A 400V three phase input drive will supply up to 400V three phase; some 400V drives will work on a single phase 400V supply with derating; consult the factory on this.

Drives generally operate over a wide voltage range (200-240V, or 380-480V) with an additional tolerance for supply variation of +/-10%. Outside of this tolerance the drive may trip or fail. Connect a 230V drive to 400V and it will certainly be destroyed.

Most countries work with supplies directly suited to 230V or 400V drives with these tolerances, but there are exceptions such as South Africa (525V) and Canada (575V). Check if you are shipping to an unfamiliar country; drives are usually available for these supplies. Drives must be grounded for safety and correct electromagnetic compatibility; check with the factory if you have an ‘IT’ supply, which has an isolated ground.

Generators present special problems; application note AN-ODP-2-060 gives good advice. Voltage surges can damage a drive if there is too much energy for the protection to absorb -Input chokes offer additional protection.

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Relays and Analogue Outputs

A variable frequency drive nearly always includes a relay and an analogue output for monitoring purposes. The E3 has one relay and one analogue output, the P2 and Eco has two of each. Relay are useful because they give a simple on or off function that is isolated from the drive electronics, so there are fewer problems with interference. However, if you are switching an inductive load like a contactor or brake, you must fit suppression components to prevent interference and damage to the relay contacts. Always check the relay rating as well; Invertek relays are mains rated, other manufacturers may not be.

The relays can be programmed with many different functions; simply change the relevant parameters. A useful feature is the ability to set a threshold so the relay will operate a particular load or frequency is exceeded. Some relays have special functions related to the drive functions; for example, a relay may control the brake on a crane in conjunction with the hoist settings on a P2 drive. Eco drives can control external pumps in cascade control via the built in relay.

The analogue outputs have two separate functions; analogue and digital. In digital mode, they duplicate many of the relay functions and simply switch between 0 and 24V. In analogue mode, they can be used to indicate the output frequency, current or power of the drive. The P2 and Eco drive will also indicate torque and PID output.

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