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Selecting a Drive

3 Selecting a Drive

Often drive selection is straight forward, as a motor is already installed and the speed range requirement is not excessive. However, when a drive system is selected from first principles, careful consideration may avoid problems in installation and operation, and may also save significant cost.

3.1Overall Considerations

Check the Current rating of the inverter and the motor. Power rating is only a rough guide.

Check that you have selected the correct operating voltage. 230 V three-phase input MICROMASTERs will operate with single or three-phase inputs. 400 V MICROMASTERs are for three-phase application only. Single-phase input units can be more cost effective in some cases, but note that 230 V units will be damaged if operated at 400 V. See section 3.2.1.

Check the speed range you require. Operation above normal supply frequency (50 or 60 Hz) is usually only possible at reduced power. Operation at low frequency and high torque can cause the motor to overheat due to lack of cooling.

Synchronous motors require de-rating, typically by 2-3 times. This is because the power factor, and hence the current, can be very high at low frequency.

Check overload performance. The inverter will limit current to 150 or 200% of full current very quickly. A standard, fixed speed motor will tolerate these overloads.

Do you need to stop quickly? If so, consider using a braking resistor to absorb the energy. A separate braking unit may be required for some VSDs. See section 0,

Do you need to operate with cables longer than 50m, or screened or armored cables longer than 25m? If so, it may be necessary to de-rate, or fit a choke to compensate for the cable capacitance.

3.2Supply Side Requirements

In order to achieve reliable operation, the main power supply to the inverter system must be suited to the inverter and the anticipated power supplied. The following points should be considered:

3.2.1Supply Tolerance

Inverters are usually designed to operate on a wide range of supply voltages. For example:

Low Voltage units

200 - 240 V ±10% i.e.

180 - 264 V

High Voltage units

380 - 480 V

±10% i.e.

342 - 528 V

Very High Voltage units

500 - 600 V

±10% i.e.

450 - 660 V

Inverters will operate over a wide supply frequency range, typically 47 - 63 Hz

However, many supplies vary outside these voltage levels. For example:

MICROMASTER Applications Handbook

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