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Diagnostics Optimization CT MODBUS RTU Technical Data
106 Installation and System Design Guide
Issue Number: 1
4.12 Output circuit and motor protection
The output circuit has fast-acting electronic short-circuit protection which limits the fault current to typically no more than five times the rated output
current, and interrupts the current in approximately 20 µs. No additional short-circuit protection devices are required. The drive provides overload
protection for the motor and its cable. For this to be effective, Motor Rated Current (B02) must be set to suit the motor.
There is also provision for the use of a motor thermistor to prevent over-heating of the motor, e.g. due to loss of cooling.
4.12.1 Motor winding voltage
The PWM output voltage can adversely affect the inter-turn insulation in the motor. This is because of the high rate of change of voltage, in
conjunction with the impedance of the motor cable and the distributed nature of the motor winding.
For normal operation with AC supplies up to 500 Vac and a standard motor with a good quality insulation system, there is no need for any special
precautions. In case of doubt the motor supplier should be consulted. Special precautions are recommended under the following conditions, but only
if the motor cable length exceeds 10 m:
• AC supply voltage exceeds 500 V
• DC supply voltage exceeds 670 V
• Operation of 400 V drive with continuous or very frequent sustained braking
For the other cases listed, it is recommended that an inverter-rated motor be used taking into account the voltage rating of the inverter. This has a
reinforced insulation system intended by the manufacturer for repetitive fast-rising pulsed voltage operation.
Users of 575 V NEMA rated motors should note that the specification for inverter-rated motors given in NEMA MG1 section 31 is sufficient for
motoring operation but not where the motor spends significant periods braking. In that case an insulation peak voltage rating of 2.2 kV is
recommended.
If it is not practical to use an inverter-rated motor, an output inductor should be used. The recommended type is a simple iron-cored component with
a reactance of about 2 %. The exact value is not critical. This operates in conjunction with the capacitance of the motor cable to increase the rise-time
of the motor terminal voltage and prevent excessive electrical stress.
4.12.2 Star / Delta motor operation
The voltage rating for Star and Delta connections of the motor should always be checked before attempting to run the motor. The default setting of the
motor rated voltage parameter is the same as the drive rated voltage, i.e:
400 V drive 400 V rated voltage
230 V drive 230 V rated voltage
A typical 3 phase motor would be connected in Star
for 400 V operation or Dela for 230 V operation, however, variations on this are common e.g.
Star 690 V Delta 400 V.
Incorrect connection of the windings will cause severe under or over fluxing of the motor, leading to a very poor output torque or motor saturation and
overheating respectively.
4.12.3 Output contactor
A contactor is sometimes required to be installed between the drive and motor for safety purposes. The recommended motor contactor is the AC3
type. Switching of an output contactor should only occur when the output of the drive is disabled. Opening or closing of the contactor with the drive
enabled will lead to:
1. OI ac trips (which cannot be reset for 10 seconds)
2. High levels of radio frequency noise emission
3. Increased contactor wear and tear
The Elevator drive has TUV Nord approval to EN 81 which allows the use of the Elevator drive secure Safe Torque Off (STO) control input to achieve
a zero output motor contactor solution.
4.13 Braking
Braking occurs when the drive is decelerating the motor, or is preventing the motor from gaining speed due to mechanical influences. During braking,
energy is returned to the drive from the motor. When motor braking is applied by the drive, the maximum regenerated power that the drive can absorb
is equal to the power dissipation (losses) of the drive.
When the regenerated power is likely to exceed these losses, the DC bus voltage of the drive increases. Under default conditions, the drive brakes
the motor under PI control, which extends the deceleration time as necessary in order to prevent the DC bus voltage from rising above a user defined
set-point. If the drive is expected to rapidly decelerate a load, or to hold back an overhauling load, a braking resistor must be installed.
Table 4-19 shows the default DC voltage level at which the drive turns on the braking transistor. However the braking resistor turn on and the turn Off
voltages are programmable with Braking IGBT Lower Threshold (D19) and Braking IGBT (D20) upper threshold.
Motor Rated Current (B02) must be set correctly to avoid a risk of fire in the event of motor overload.
If the cable between the drive and the motor is to be interrupted by a contactor ensure that the drive is disabled before the contactor is
opened or closed. Severe arcing may occur if this circuit is interrupted with the motor running at high current and low speed.
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