Variable speed drive

A variable speed drive (VSD), also called a variable frequency drive (VFD), supplies an induction motor at an adjustable frequency, so the motor speed can be varied steplessly. It also starts the motor, and can stop it, on a ramp: the frequency rises or falls evenly over a set time. The acceleration time is the time it takes the frequency to rise from zero to a frequency set by a parameter, which on the ACS580 is 50 Hz by default.

Power circuit

The mains supply is connected to terminals L1, L2 and L3, and the motor to the output terminals U, V and W. The markings vary a little: on the ABB ACS580 the output terminals are T1/U, T2/V and T3/W. On the Schneider Altivar 320 the supply is R/L1, S/L2 and T/L3 and the output U/T1, V/T2 and W/T3. The motor cable is a symmetrical screened cable, with the screen earthed at both ends.

Control connections

• Digital inputs DI. Run, direction, preset speeds and reset. In the ACS580 factory settings, DI1 is run (0 stop, 1 run), DI2 is direction (0 forward, 1 reverse), DI3 and DI4 select a preset speed and DI5 selects the ramp. The common terminal of the inputs is DCOM.
• Speed reference AI. An analogue input takes the speed reference as a voltage, 0–10 V, or as a current, for example 4–20 mA. On the ACS580, AI1 is changed from voltage to current with a switch or a parameter. On the Altivar 320, AI1 is a voltage input and AI3 a current input. The input common is AGND or COM.
• Relay outputs RO. Changeover contacts for status signals. In the ACS580 factory settings, RO1 signals that the drive is ready to run, RO2 that it is running and RO3 a fault. The ACS580 contacts are rated 250 V AC or 30 V DC, 2 A.
• Reset. A fault can be reset with a digital input. The reset happens on the rising edge of the input, and it only works once the cause of the fault has gone.

Preset speeds are selected in binary with two inputs. On the ACS580, DI3 alone selects preset speed 1, DI4 alone preset speed 2, and both together preset speed 3. When neither is on, the speed comes from the analogue input.

The functions of the inputs and outputs are set with parameters or a ready-made macro, so the same function can be on a different terminal on different drives. In the Johdota game tasks, DI3 is reset, and the preset speeds are set on inputs DI4 and DI5.

Connecting a PLC

The contact of a PLC relay output brings +24 V to a digital input of the drive. The voltage can come from the drive's own +24 V output (at most 250 mA on the ACS580) or from the control circuit's own 24 V supply. With a PLC that has transistor outputs, the input type is chosen to match the outputs: on the Altivar 320 the switch is in the Source position for PNP outputs (factory setting), and an input is then on at a voltage above 11 V.

The speed reference comes from a PLC analogue output over a screened cable. The scale is set with parameters, for example 0 V is 0 Hz and 10 V is 50 Hz. The ready signal comes through a relay output: +24 V to the common terminal of the contact, and the NO terminal to a PLC input. The contact is closed only when the drive is ready, so a broken wire also shows up as a fault to the PLC.

Stopping

The stop mode is chosen with a parameter. With a ramp stop the motor slows down over the set time. With a coast stop the drive switches off its output and the motor coasts to a stop on its own. On the ACS580, coast stop is the factory setting. In the Johdota game the drive stops on a ramp, but on a fault it switches off its output and the motor coasts to a stop.

Remember

• On the ACS580, the Safe torque off inputs are closed with jumpers at the factory. The drive starts only when both circuits are closed.
• After the supply has been switched off, wait before you touch the drive, the motor cable or the motor: 5 minutes for the ACS580, 15 minutes for the Altivar 320.