AdministrativeFAQ
1. Can I buy samples for immediate delivery?
2. Can I buy ex-works (I have my own courier/agent)?
3. Is there a minimum order quantity?
4. I would like to promote/market/distribute this technology in my area. What do I do?
5. Are Cognito Quam soft starters certified for maritime use?

Technical FAQ

1. Can I control motor speed with a soft starter?

No. In operation, the AC induction motor constantly tries to reach, but never reaches, its synchronous speed which is proportional to the applied AC voltage frequency. To control the speed of an AC induction motor both the voltage and the frequency must be varied as VSDs (variable speed drives) do. A soft starter only reduces the applied voltage to the load while accelerating or decelerating the load and the motor is always driven at the constant line frequency. In effect the motor is starved for current while accelerating towards its synchronous speed. Depending on the load, it will settle within its normal slip range which is between its nameplate speed (full load speed) and the synchronous (practically at no load) speed.

2. Why is there a limit in the number of times a motor can be started in an hour?

An AC induction motor is designed to operate within its normal slip range which typically is within 3% of the synchronous speed. When started, the motor dissipates extra heat due to the large starting current (typically six times the full load current) until it reaches its normal operating area. This heat is generated in the stator windings and the rotor cage and is eventually dissipated in the environment after the motor reaches its normal operating speed and becomes properly ventilated by its rotor fan. For this reason motor manufacturers specify the maximum number of times a motor can be safely started in an hour. Exceeding this limit will overheat the motor, shorten its service life and eventually lead to its destruction.

Both the NSFTxx soft starters and the iSFT controller feature a limiting mechanism so that this limit is not exceeded.

3. What methods are there to start an AC induction motor?

There are a number of ways to start an AC motor depending on the supply, motor and load characteristics. These are:

• Direct starting: the motor is connected to the line and draws all the current it can take. The load is accelerated uncontrollably at the maximum possible acceleration. This method is the simplest (and cheapest) and is suitable for small motors which are connected to a powerful supply and drive a stiff and robust load.
• Star/Delta starting: the motor is started with its windings in star (or wye) connection. The reduced voltage at the windings (equal to the supply phase voltage) limits the starting current while the load is accelerated. After a set time, the winding connection is switched to delta and the full line voltage is applied to each winding. Effectively the load is accelerated twice, first at phase voltage and then at line voltage. At the switch-over to delta extra current is drawn and extra torque is generated. This method is suitable for most loads, assumes a healthy line (to provide the current spike at the switch-over) and a stiff mechanical load. This was the most popular method before soft starters became available and, as it uses electromechanical contactors to switch between the two connections, it requires frequent maintainance.
• Reactor start: a reactor is connected in series with the motor to limit starting current and after a set time is shorted out of the circuit. While the current is limited by the reactor, load torque is also substantially limited (the torque being proportional to the square of the current). This method is simple but limited by its starting torque characteristic.
• Autotransformer start: an autotransformer is used to limit the starting current by applying a reduced voltage to the motor. This is similar to the reactor start but more complex in wiring.
• Rheostat starting: the rotor resistance is increased to control starting current and torque. This method assumes a wound rotor with slip-rings to externally connect additional resistors (typically a rheostat) and is employed in special cases where high starting torque is required from a limited line such as in the case of very large loads.
• Reduced voltage soft starting: thyristors are used to partly conduct during each line cycle and as a result drive the motor with reduced voltage. The applied voltage is progressively increased to the full line voltage providing smooth torque and no current spikes. Soft stopping is also possible giving smooth load retardation with the most notable application being hydraulic shock (or "hammer") elimination. Torque can also be controlled by monitoring the load current and any acceleration/deceleration profile can be realized by using standard digital techniques.
• Variable Speed Drive start: the VSD ramp-up/down facility is used to accelerate/decelerate the load to/from the rated speed. As the VSD is designed to drive the motor at variable frequency and voltage this method is an overkill and suitable for small motors where the costs of soft starters and VSDs are comparable.

4. Why use a by-pass contactor?

While motoring the soft starter thyristors conduct throughout the line cycle and dissipate heat as defined by their forward conduction voltage (typically <1 Vrms) and the current flowing to the load. By shorting ("by-passing") the thyristors while in full conduction, this energy is saved. Also note that as the contactor shorts about 1 Vrms with the thyristors fully conducting there is no contact arcing and the contactor duty rating can safely be AC1.

The NSFTxx soft starter "By-pass" control output provides the necessary signalling for this while any iSFT control output can be programmed for this task.

5. How can I activate a braking/immobilizing mechanism while the load is not driven?

Depending on the employed logic type, by using one of the NSFTxx soft starter "Idle" or "Run" control outputs. At the iSFT controller any control output can be set to the "Idle" state or its complement.

6. How can I use the temperature sensor in the motor windings?

By connecting it directly (or via a suitable interface) to the NSFTxx soft starter "Enable" control input. In such a case the soft starter is enabled by the low temperature sensor state and disabled when overheated.

For such a task a number of options are available at the iSFT controller:

• One of the control inputs can be programmed to enable/disable operation depending on the sensor state.
• One of the external fault/event inputs can be used to control operation.
• The Pt100 measurement facility can be used if the motor sensor is of this type. In such a case the winding temperature is monitored continuously and an alarm or fault can be flagged when greater than a preset limit.

7. Can I control the NSFTxx overcurrent limit by an external potentiometer or from a PLC analog output?

By removing the relevant strap and feeding the controlling voltage to the appropriate connector as described in the manual.

8. How are faults handled?

A fault at the NSFTxx soft starter, if enabled, turns off the soft starter immediately and turns on the "Fault" control output. Following this, a valid turn-on command clears all fault conditions other than the invalid phase sequence and normal operation (including fault monitoring) will be entered. A phase sequence fault can only be cleared by repowering the unit at the correct sequence.

Fault handling at the iSFT controller is completely configurable/programmable and any condition can trigger a fault. The controller can also be set to try a preset number of times to recover from a specific fault before giving up.

9. How can I read the load current for PLC use?

By using the analog output available at both, the NSFTxx soft starter and iSFT controller.

10. Why have separate control and power circuits?

When the control and power circuits are powered separately, faults in the power line/load do not affect the soft starter control and monitoring functions. Typically in such an installation the power circuit is enabled/activated after the soft starter (powered separately from the control line) reports "Ready/Idle" and the absence of power line voltage (if enabled). In the case of a fault, the soft starter will halt operation, turn the "Fault" output on and wait in "Ready/Idle". (See also how faults are handled).

11. Why are your soft starters particularly suited for pump and compressor applications?

Our soft starters package under one enclosure all the features and facilities required for the comprehensive implementation of fully protected, robust and versatile soft starting applications. They are ideally suited for constant speed fluid transport applications especially when upgrading, retrofitting or modernizing existing installations.

When compared to the alternatives, the cost and operational advantages are numerous:

• Very high efficiency and practically lossless motoring operation when combined with a bypass contactor.
• Smooth, noise-free, reduced load starting current ensures minimal plant wear and a quieter operating environment. Hydraulic shock (or "hammer") is eliminated when accelerating or decelerating a pump load.
• Their simple and non-demanding installation requirements save time, money and effort. When upgrading an existing installation, typically a star/delta circuit, no additional rewiring from the line or to the load is needed.
• The bookcase style enclosure small footprint further saves cabinet space and real-estate.
• Protection is complete and user-programmable against all contingencies: overloads, underloads (typically caused by a broken transmission belt or a dry pump), disruptive phase sequence/line faults and internal overheating (typically caused by blockage in the cooling flow inlet or outlet). Faults from any phase to the ground are detected as load current is monitored in each phase line. As a result robust and dependable operation is ensured.
• The versatile and galvanically isolated control input and output interface allows for integration within any PLC-based system or simple two command start/stop operation.
• The provided 24 VDC supply can power local sensors or other small automation devices.
• Power to the soft starter control circuits can be separate from the three-phase power line enabling continued normal monitoring/control operation while the power circuit is down.
• Optional network connectivity is available for integration in distributed multi pump/compressor control systems.

12. How can I soft start a small single phase motor?

By using our Solid State Relay Analog Controller and a suitably sized solid state relay.

The ramp-up profile can be generated by a simple RC network at the controller input or any 0-10 VDC analog or PWM source such as a PLC output.

13. What are the considerations when using the inside-delta connection?

The inside-delta connection reduces the power handled by the soft starter by controlling the current of each motor winding within each delta connection branch individually. To do this, six conductors to the motor windings are required as opposed to the three needed for the simple delta connected load.

Thus the main consideration is balancing the cost saving of the smaller sized soft starter with the extra cost of the cabling, the more complex installation and additional protection required for the current flowing outside the soft starter. Note that when upgrading a star/delta starting installation the six conductors to the motor windings are already in place and can either be paralleled to minimize cable losses in a simple delta connection or used as they stand for the inside-delta connection.

Administrative FAQ

1. How can I buy samples for immediate delivery?

By contacting us with your special instructions. We make every effort to have sampling quantities of the most popular models ex-stock for quick worldwide delivery.

2. Can I buy ex-works (I have my own courier/agent)?

Yes, by contacting us with your requirements and instructions.

3. Is there a minimum order quantity?

No, but note that shipping/handling charges become proportionally higher with smaller orders.

4. I would like to promote/market/distribute this technology in my area. What do I do?

Contact us!

5. Are Cognito Quam soft starters certified for maritime use?

Yes, but as an optional service. Please contact us with your certification requirements.