Motor Starting Temp Rise- FVNR vs. RVSS

[SteamDonkey]

Has anyone here conducted tests, done research, or heard anecdotal evidence that suggests a motor's start-up temp rise can be reduced by using a soft starter instead of a full voltage starter? Related to this is the question of allowable starts per hour. Have soft starters been shown to allow more frequent starting in some applications?

 

[Jraef]

There is a lot of debate on this actually, but the consensus is that at best, a soft starter does not significantly add to the motor heating during start-up, as long as it is a successful start. The added acceleration time is somewhat offset by the lower current and vice versa. That of course is the opposite of what you wanted to hear, sorry to burst your bubble. Adding a soft start will not increase the starts-per-hour capability of a motor, but if properly applied will not likely decrease it either (at least in my opinion, but there are many people who disagree). You should also note that each Decel ramp must be counted the same as a start cycle in the starts-per-hour capability of the motor, such as in the case of a pump application.

The type of soft starter also makes a difference by the way. Many newer-smaller-cheaper versions are being made now with what is called "2-phase control" which means they have SCRs on only 2 of the 3 phases, the middle phase is a piece of bus bar. This style, while not unsuccessful, does increase the motor heating during starting because you will have severely unbalanced current in the earliest stages. Unbalanced current creates negative sequence currents in the motor which disproportionately heats the windings above what current flow alone would do. My rule-of-thumb on these is to cut the starts-per-hour capability of the motor in half when using this kind of soft starter. I have no mathematical basis for it, it's just my conservative nature.

 

[weressl]

There is a lot of debate on this actually, but the consensus is that at best, a soft starter does not significantly add to the motor heating during start-up, as long as it is a successful start. The added acceleration time is somewhat offset by the lower current and vice versa. That of course is the opposite of what you wanted to hear, sorry to burst your bubble. Adding a soft start will not increase the starts-per-hour capability of a motor, but if properly applied will not likely decrease it either (at least in my opinion, but there are many people who disagree). You should also note that each Decel ramp must be counted the same as a start cycle in the starts-per-hour capability of the motor, such as in the case of a pump application.

The type of soft starter also makes a difference by the way. Many newer-smaller-cheaper versions are being made now with what is called "2-phase control" which means they have SCRs on only 2 of the 3 phases, the middle phase is a piece of bus bar. This style, while not unsuccessful, does increase the motor heating during starting because you will have severely unbalanced current in the earliest stages. Unbalanced current creates negative sequence currents in the motor which disproportionately heats the windings above what current flow alone would do. My rule-of-thumb on these is to cut the starts-per-hour capability of the motor in half when using this kind of soft starter. I have no mathematical basis for it, it's just my conservative nature.

The same I^2*t that produces heat produces the necessary flux to accelerate the same mass, The only component that will be reduced is the initial magnetizing current. Dependent on your load this can be significant enough to change the number of starts that can be thermally sustained. If you have an RTD integrated thermal management relay that actually models the motor, you can choose to ignore the manufacturers recommended maximum number of starts regardless of what your starting method is.

 

[SteamDonkey]

Jraef,

That's pretty much the understanding I have and is the answer I expected to get, but I'd like to know what this consensus is based on. When people (especially clients) ask me if a soft starter will allow more frequent starts, it'd be nice to reference a published study or something.

 

[Besoeker]

Jraef,

That's pretty much the understanding I have and is the answer I expected to get, but I'd like to know what this consensus is based on. When people (especially clients) ask me if a soft starter will allow more frequent starts, it'd be nice to reference a published study or something.

Steam, I think Weressl has it about right.
We have sold quite a number of soft starts and other reduced voltage starters, usually as part of a system. They have ranged from a few tens of kW up to several MW.
As far as I can recollect, the justification was either supply capacity limitations or the desired requirement to avoid mechanical shock.

 

[Jraef]

Jraef,

That's pretty much the understanding I have and is the answer I expected to get, but I'd like to know what this consensus is based on. When people (especially clients) ask me if a soft starter will allow more frequent starts, it'd be nice to reference a published study or something.

Most of what is available comes from soft starter mfrs, which may or may not help your case if you are selling a competing brand. But there is an IEEE published paper (albeit written by Eaton engineers) that discusses this very subject and is the basis for my opinion on it. You would have to buy it though, it will cost $30 unless you are already a subscriber.

 

[SteamDonkey]

Most of what is available comes from soft starter mfrs, which may or may not help your case if you are selling a competing brand. But there is an IEEE published paper (albeit written by Eaton engineers) that discusses this very subject and is the basis for my opinion on it. You would have to buy it though, it will cost $30 unless you are already a subscriber.

Perfect, thanks.

 

[jdsmith]

Most of what is available comes from soft starter mfrs, which may or may not help your case if you are selling a competing brand. But there is an IEEE published paper (albeit written by Eaton engineers) that discusses this very subject and is the basis for my opinion on it. You would have to buy it though, it will cost $30 unless you are already a subscriber.

This is incidentally one of the benefits of IEEE membership - many engineers don't feel the yearly fees are justified, but as a member of the IEEE Industry Applications Society I can view this paper and anything else that came out of the IAS for free. The majority of papers relevant to industrial system come from the IAS transactions and conferences, however some protective relaying topics are covered by the Power and Energy Society (formerly the Power Engineering Society).

 

[weressl]

This is incidentally one of the benefits of IEEE membership - many engineers don't feel the yearly fees are justified, but as a member of the IEEE Industry Applications Society I can view this paper and anything else that came out of the IAS for free. The majority of papers relevant to industrial system come from the IAS transactions and conferences, however some protective relaying topics are covered by the Power and Energy Society (formerly the Power Engineering Society).

True, true. However, on this forum engineers are a micronity(smaller than a minority), so we can help out by refering - or even directly copying - to relevant sections of published papers, without violating the copyright regulations. The majority of that article is scientific and only the conclusions are relevant. The basic conclusion was that the highest torque kick on the startup ramp provided the lowest thermal buildup fro THAT application and modeling, so it can be concluded that SSRV aplications indeed can result in lower thermal duty, prolonged insulation life and the potential to have higher number of restarts within one hour than the NEMA limit of 2 per hour. As I said if you have a protective realy with actual temperature feedback, in addition to current, this can be safely accomplished.