Guide to dimensioning of a drive system

Drive system

Dimensioning of a drive system is a task where all factors have to be considered carefully. Dimensioning requires knowledge of the whole system including electric supply, driven machine, environmental conditions, motors and drives etc.

Time spent at the dimensioning phase can mean considerable cost savings. A single AC drive system consists typically of:

1. An input transformer or an electric supply,
2. Frequency converter,
3. An AC motor and
4. Load

Inside the single frequency converter there is a rectifier, DC-link and inverter unit. In multi-drive systems a separate rectifier unit is commonly used. Inverter units are connected directly to a common DClink.

General description of a dimensioning procedure

This chapter gives the general steps for dimensioning the motor and the frequency converter.

1. First check the initial conditions

In order to select the correct frequency converter and motor, check the mains supply voltage level (380 V …690 V) and frequency (50 Hz … 60 Hz). The mains supply network’s frequency doesn’t limit the speed range of the application.

2. Check the process requirements

Is there a need for starting torque? What is the speed range used? What type of load will there be? Some of the typical load types are described later.

3. Select the motor

An electrical motor should be seen as a source of torque. The motor must withstand process overloads and be able to produce a specified amount of torque. The motor’s thermal overloadability should not be exceeded.

4. Select the frequency converter.

The frequency converter is selected according to the initial conditions and the selected motor. The frequency converter’s capability of producing the required current and power should be checked.

Advantage should be taken of the frequency converter’s potential overloadability in case of a short term cyclical load.

An induction (AC) motor

Induction motors are widely used in industry. In this chapter some of the basic features are described. An induction motor converts electrical energy into mechanical energy.

The slip is often defined at the motor’s nominal point (frequency ( f_n ), speed ( n_n ), torque ( T_n ), voltage ( U_n ), current ( I_n ) and power ( P_n )).