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Electric Motor Soft Starter
Some questions on electric motor soft starters


I have an application where an electric motor is connected to a hydraulic pump. Consider that the max motor rpm is well within the pump specifications. The input torque while running is also within specifications. We however keep braking pump shafts. I think we can solve this problem by replacing the motor start contactor by a soft starter.

However, in our internal discussions, the requirement of having a torque limiting soft starter (more complex and more expensive) instead of a common soft starter has created some confusion.

Will a non-torque limiting soft starter gradually increase motor speed and avoid the sudden start that I believe is damaging the pump, or is this type of unit only protecting the motor from current peaks and effectively starting the pump with the same sudden force as before?

FYI the conventional soft starter we are considering is the Siemens 3rw4036-1bb04.

The motor is here:

I hope this is clear.

Many thanks in advance,

moderator's note: I could not access the web page.

By Curt Wuollet on 10 January, 2013 - 2:52 pm

Before you do that, check the alignment of the shafts. Many types of shafting fatigue under any significant bending stress. This is if inline. If belt driven the same applies to overhang or excessive belt tension. These are more likely than inertial stress.



Many thanks for your answer!

Yes, we checked the shafts' alignment. I'm pretty confident alignment is ok, but we will take a closer look at that again.

Now, about the soft starter, if not only for exercise: what is the difference between conventional soft starter and torque-limiting soft starter? is it electrical protection only on the first case and electrical and mechanical protection on the second case? does a conventional soft starter provide some level of mechanical protection?

Just curious and confused ;-)

Many thanks,

By Curt Wuollet on 11 January, 2013 - 10:53 am

Soft starters are commonly used to reduce the peak starting current by spreading it out over time. Because current generally equals torque, this does also reduce the starting torque which can be good or bad depending on the nature of the load. A load like a blower will simply come up to speed more slowly. When spreading the starting current over time there is usually some sort of integration calculation taking time and current into account to prevent overheating the windings with the (still elevated) starting current. Some loads like compressors and pumps without unloaders need the high starting torque to overcome residual pressure, etc. These are prone to stall and cause trips if soft started. That's why the caution. If your start loading _is_ high enough to fatigue the shaft, it may not start at all with a soft start. Soft starts generally provide all the protection normal starters do, which isn't much, simply ensuring that starting current goes away before the thermal device has become hot. They do it electronically. This could be yet another instance of trying to solve a mechanical problem with an electrical solution. I think it unlikely that a pump that is reasonably matched to a motor would have enough inertia to break the shaft. And the pump people point to shaft alignment whenever you are breaking shafts. after all, even a little bending thousands of time a minute adds up to fatigue faster than the number of starts.


Many thanks Curt! Pretty clear comments.

Now I need to go align that shaft ;-)