SSD 605C Product Manual - Pplication

Application Notes  

13-1

605C Frequency Inverter

13 

A

PPLICATION 

N

OTES

Application advice is available through our Technical Support Department, who can also

arrange for on-site assistance if required. Refer to Chapter 8: “Routine Maintenance and Repair”

for  the address of your local SSD Drives company.

•  Always use gold flash relays, or others designed for low current operation (5mA), on all

control wiring.

•  Remove all power factor correction equipment from the motor side of the Inverter before

use.

•  Avoid using motors with low efficiency and small cos ø (power factor) as they require a

larger kVA rated Inverter to produce the correct shaft kW.

Synchronous Motor Control

Although intended primarily for use with induction (asynchronous) motors, Inverters can also be

used for speed control of synchronous motors.  Synchronous motors can offer economic

solutions in applications where tight control of speed is required together with the low

maintenance characteristics of an ac motor.

The two most common types of synchronous ac motor are permanent magnet and wound rotor.

In contrast to induction motors, synchronous motors run at synchronous speed whether on full

load or no load. Synchronous speed is set by the frequency of the supply applied to the stator.

The stator flux can be kept constant by keeping the stator volts/frequency ratio constant, as with

an induction motor.

Torque is produced in the motor by an increase in load angle between the stator and rotor fluxes.

Maximum torque occurs when the load angle approaches 90

°. If the load angle exceeds this

value then torque drops and the motor will stall. Systems involving synchronous motors need

careful design to ensure that the motor can accelerate the load and handle transient load changes

without stalling.

Brake Motors

Brake motors are used in applications requiring a mechanical brake for safety or other

operational reasons. The motor can be a standard induction motor fitted with an electro-

mechanical brake, or it could be a special conical rotor machine. In the case of a conical rotor

machine the spring-loaded brake is controlled by the motor terminal voltage as follows:

•  At rest the motor is braked.

•  When the motor is energised an axial component of the magnetic field due to the conical

air-gap overcomes the force of the brake spring and draws the rotor into the stator. This

axial displacement releases the brake and allows the motor to accelerate like a normal

induction motor.

•  When the motor is de-energised the magnetic field collapses and the brake spring displaces

the rotor, pushing the brake disc against the braking surface.

Inverters can be used to control the speed of conical rotor brake motors since the linear V/F

characteristic maintains the motor magnetic field constant over the speed range.  It will be

necessary to set the  FIXED BOOST parameter to overcome motor losses at low speed (see

FLUXING menu at level 3).

Using Line Chokes

Line chokes are not required to limit input current to SSD Drives Inverters.   Controllers from

5.5kW ( 400v) or 2.2kW ( 230v) upwards are fitted with DC link chokes to limit the ripple

current seen by the DC link capacitors and thus prolong their life.