Soft Starts and bypasses..... 1

[dannychristie]

I have been involved in insalling soft starts on large motors. I have an understand of what they are used for. But i am not sure why a by pass contactor is used. Is there any advantage to using one. I have done a little research and only found that it would reduce the heat. Which makes sense because the soft start is bypassed. I thought that soft starts can be used continuously

 

[jack6238]

This is quick.
The purpose of the bypass switch is to be able to start the motor should the electronic package fail.The larger the motor and the higher the voltage generally results in an important motor. Also the electronics in systems above 480 volts are not as reliable and you may need to start the motor acorss the line. I can send more on this later. It's now time to take the family to church

 

[Concepts]

A bypass uses a reduced voltage transfomer like one leg of 480 two reduced to get the motor turning then a set of timers time out and induce the voltage to 3 legs of 480 it is a hardware softstart. It is used only as a back up to the softstart. Or in most cases a VFD which is in reality a softstart so to speak

 

[DrivesGuy]

It really depends on the design of the soft starter as to whether it can be used continuously or not. As to the purpose of the bypass, it can be for various reasons. The two most common are those mentioned above: to take the electronics out of the circuit (and their associated losses) once the motor is up to speed; to start the motor across the line when the electronics fail.

I have seen one softstarter that was actually a VFD with the cooling system removed. It was limited to about a 1 in 20 duty cycle with a maximum of about 5 minutes at full current. After bringing the motor to full speed, it performed a line synchronization and closed the bypass for a bumpless transfer.

I, too, do quite a bit of large motor work, and for most large motor applications, the cost and foot print of the bypass are well worth the investment. I've seen start ups last over 6 months for older systems. Were I a salesman (I'm not a good enough liar), I'd try to get the bypass added to every large motor control scheme.

In your case, since you are running at a fixed maximum speed once you are up to speed, your process controls (vanes, valves, etc) must already be in place. A VFD for you would be redundant to these controls (and probably worth the money in the long run, regardless).

 

[jbartos]

Comment: It appears that the by-pass contactor paralleling a soft start arrangement is used to:
1. Eliminate the soft start arrangement to:
1a. Reduce heat
1b. Save the soft start arrangement run time
1c. Increase reliability and availability of more expensive processes
2. Reduce voltage drop
3. Improve power quality including the mitigation of harmonics
4. Reduce or eliminate common mode currents
5. Suppress electromagnetic interference
6. Other aspects mentioned in above postings

 

[ICEMAN]

The bypass contactor in a soft starter is normally used to switch the unit out circuit after start-up, thus reducing heat and perhaps the use of large cooling fans.

Most microprocessor based soft starters have software which allows energy optimisation of the motor. This entails phasing back the voltage (since the torque is proportional to the voltage squared) to match the instantaneous torque requirment of the load to reduce magnetising current and improve the motor power factor. This phase control produces large levels of harmonics and line notching due to the SCRs. Since there is no additional ac line inductance in circuit (as usally with dc drives) these harmonic current levels can be quite high.

Using a bypass contactor simply eliminates all these problems.