Affect of Soft Starters on Motor/Rotor Heating 1

[gordonl]

I'm wondering if a start of a load using a soft starter is easier on a motor from a thermal capacity point of view compared to a DOL start. I realize mechanically it is easier on the motor, but will the use of soft starters improve the number of starts a motor is capable of?

 

[CB2]

No and No. The power required to start the motor is what it is, the variable here is how you present the power to the motor (across the line vs. various reduced voltage starters including a soft start).

 

[electricpete]

From the equivalent circuit, with some reasonable assumptions, it can be shown that:
#1 - For inertia-only loads, the total rotor and stator I^2*R energy dissipated in the motor over the course of the start is constant, regardless of applied voltage (soft start).

#2 - For inertia-plus-torque loads, the total rotor and stator I^2*R energy dissipated in the motor over the course of the start INCREASES as we decrease voltage (soft start). To convince yourself, look at the limiting case where we decrease voltage to the point that the motor stalls (Tmotor<=Tload), then the total heating grows without bound (until motor trips).

Here is the analysis upon which these conclusions were based:

http://www.geocities.com/pschimpf/RotorHeatingSymbols.htm

 

[GusD]

Hi gordonl

The best example that probably addresses your question would be starting a HI ENERTIA load such as a centrifuge,
Induced draft fan or similar load.All of these loads are usually started with very long accelerating times rather than a quick short DOL start.
If the accelerating period is long, the motor has enough time to dissipate the heat,if you start this motor DOL, it will develop a lot of heat very rapidly without enough time for dissipation.If you had to restart motor in short order,the windings might not like it.
As for the # of starts on SoftStarts,it is pretty well regulated by the Ramaining Thermal Capacity available after successive starts.Most newer Motor Management relays measure your available Thermal Capacity.
Standards are also available for # of starts on a particular motor.Cold stars,Hot starts and any combination.





GusD

 

[electricpete]

Good points Gus that the heat is spread out over a longer time for reduced voltage start. For the specific case of high-inertia, low-torque load this might make soft start less severe as you say.

gordon - A standard motor purchased per NEMA MG-1 is assured to safely accelerate a pump/fan load of the standard inertia listed in the tables ONLY IF the terminal voltage is at least 90% during starting (many users specify 80%). Any lower voltage (soft start) must be spec'd/reviewed by the motor manufacturer (along with load inertia and torque characteristics) because the increased total I^2*R heat energy from reduced voltage start can have significant effect, especially for rotors of large motors.

 

[jraef]

gordonl,
In concert with the above posts, my experience is that there is no gain or loss of starting capacity in the motor, provided that the RVSS is properly applied and set up.

Obviously if the torque is reduced to the point of stalling the motor, it will increase heating. This is not really a valid issue however because if the motor is stalled, it is not a successful application to begin with!

As to the issue of increased heat dissipation time, I have had a motor design engineer argue the point that although you have more time to dissipate the heat, you have less air movement as well, so again the net effect is negligible in both directions.

He was older than me and had less hair, so I believed him.


Quando Omni Flunkus Moritati

 

[Marke]

Yes, I agree with the above. The power dissipated in the rotor is dependent on the load inertia for a purely inertial system. There is additional power dissipated in the rotor due to the work torque required during acceleration. With a purely inertial load, the total power dissipated in the rotor shold be the same for full voltage or reduced voltage starting, just the rate at which it is dissipated changes with start voltage.
With a load requiring work torque during run up, provided that the acceleration torque is significantly higher than the work torque during start, the increase in heating will not be significant. If the acceleration torque is small relative to the work torque, then there will be a major increase in the rotor dissipation and corresponding reduction in number of starts per hour.
In Laymans terms, if the motor accelerates freely and easily under the reduced voltage condition, you will not get a significant increase in heating. If the motor is struggling to accelerate, then the heating will be increased. As was stated above, it is dependant on the setup of the starter relative to the motor and load.
The setings must be such that the motor imediately begins to spin. Some ramp systems I have seen, have a period of time before the voltage ramps up high enough to break the machine away, and that is unecessary heat.
There is an argument that due to the lower rate of dissipation with reduced voltage starting, the heat has time to dissipate through the iron of the rotor, allowing a greater overall dissipation. I am not convinced either way, but would epect less hot spoting in the bars and that could be significant.
Best regards,


Mark Empson

http://www.lmphotonics.com

 

[electricpete]

I am respectful of all the opinions offered in this post.

In the interest of clarity I have to say that I believe jraef has mischaracterized the issue of increased heating when he stated there is in general no loss of starting capability. Reduced voltage creates increased total heating for any application that has load torque, regardless of stall or not. I mentioned stall situation because it is a limiting case consistent with my statement, not because it is the only thermal concern. Mark has provided some good comments on determining whether that increase in heating is significant for a given application.

I agree with jraef's statement that reduced voltage is acceptable &quot;...provided that the RVSS is properly applied and set up.&quot; In my opinion, &quot;properly applied&quot; includes an evaluation of thermal performance or consulation with the manufacturer. That should be self-evident since the NEMA standard does not require/assure any reduced voltage starting capability below 90% voltage.

Good discussion!

 

[Farkel]

Break down torque is usually thought of with rotor heating, while locked rotor torque is thought of with stator heating. While I concur with most of the statements made to date; one can get more starts usually by reduced voltage starting over longer periods of time, because torque or amperage is reduced, creating less heating of the motor in primarly the stator.

 

[Modula2]

I agree with electricpete and I have seen a stall at an intermediate speed, such as 50%. For those who want experimental proof, try this:
1. Set the soft starter as current limit and adjust it down until stall at 50% speed is achieved.
2. Set the acceleration timer,overload settings to max. or off, and other thermal trips to off, to avoid interference with test outcomes.
3. Press &quot;Start motor&quot;, take the day off, then revisit next day.
4. Send us the motor before and after pictures.