HVAC rating of soft starters?

[schow]

Dear all,

 

I believe most of you have ever come accross the term "HVAC Rating" for AC drives, where the drives manufacturers claimed that the same VSD could be applied for one size higher HVAC application (eg. a 22kW constant torque VSD could be used for 30kW HVAC application).

 

I trust this is mainly due to the typical "square torque" characteristics of HVAC application (on the other hand, perhaps this is one of the marketing strategics of drives manufacturers so that their VSDs are more competitive, AGAIN in terms of pricings!)

 

Do you think that the same concept is applicable for soft starters, where a 22kW soft starters used for constant torque application could be applied for 30kW HVAC application?

 

danke!

[mariomaggi]

Dear schow,

the extra-sizing of inverter is not related to square or linear torque/speed curve, but to overload possibilities.

A fan don't ask more current because there are no overloads, therefore the nominal current level could be increased. The same for certain types of pumps.

Something similar happens with soft starters, but in this case you have to consider the inertia of rotor+load: at a very low inertia level you could start a bigger motor. I2t is important on soft starters.

 

Regards

Mario

 

 

[marke]

Hello Schow

 

As mariomaggi has replied, the ratings are basically thermally rated.

 

When you operate a pump or a fan under variable speed control, you alsways operate at rated load or less. There is no transient overload requirement, whereas for many other applications, there are overloads as part of the operation.

This enables the drive to have one rating for pumps and fans where the maximum loading is 110% or 120% and an industrial rating of 150%. I have also been in situations were 150% is not enough and have installed drives for higher ratings.

 

With a soft starter, the overload is during start and this determines the starter rating.

With submersible pumps, it is common to have a start current of 300% or less, but for surface pumps, the start current can be as high as 450% depending on the motor characteristics.

Fans tend to have quite a high inertia and commonly require 400 - 450% current to accelerate to full speed in an acceptable period of time, so a fan is definitely not a light duty application on a soft starter.

 

The start current is dependent on the torque required to spin the load up to full speed, and the ability of the motor to develop that torque. There are very large variations between motors in the high slip portion of the curve with some motors having a very poor start torque characteristic. As an indication of the starting efficiency of motors, take the LRT (Locked Rotor Torque) and divide that by the LRC (Locked Rotor Current) with both in percentage. This show an indication of the range that can occur.

 

One local manufacturer did actually produce a range of HVAC soft starters based on their drives experience. - they learned the hard way!!

 

Best regards,

[schow]

Dear Mario & Mark,

 

Thanks for both of your info and detailed explainations.

 

Actually, I am getting a bit confused now, since HVAC rating has nothing to do with their square torque characteristic but only their over load requirement, then can I say that a 30kW AC drive could be used for all 45kW applications but only without over load capability for constant torque applications?

 

Initially, I thought that it could be used for bigger size HVAC applications mainly due to the full torque is only required at full speed but it seems that I was wrong all this while.

 

But like Mark said, fan application would not be considered as light duty application, then why a smaller drive could be used for a bigger size fan?

 

As for soft starters concern, what would you guys categorize as "light duty applications", which a smaller size sst could be used and what would be "heavy applications" that a bigger size sst should be applied.

 

danke!

[marke]

Hello schow

 

The ratings of both the soft starter and the VSD are thermal.

 

There is a big difference however between the two technologies and how they operate on a given load.

 

A VSD always operates in the low slip sector of the induction motor curves. Over current is directly related to over toques. Provided that the torque is never above 100%, the output current of a VSD will never go above 100%. The inertia of the load is only relevant if the load is required to accelerate very fast, at which time it may be necessary to supply more than 100% torque to the load.

 

A soft starter operates in the high slip sector of the induction motor curves during start. The start current is very high per unit of torque developed relative to the VSD because the motor is very inefficient in this sector. The start current is dependent on the load torque requirements and the efficiency of the motor during start.

 

With a VSD, the only difference in controlling a fan and a pump, is the inertia of the load which requires that the ramp rates are slower for a fan than for a pump.

 

With a soft starter, the pump has a very low inertia and so the starter sees the high start current for a very short time. For the fan, the high inertia results in the high start current being required for a much longer period of time and there fore there is more heat dissipated in the SCRs, the junction temperature will be higher and so the rating must be reduced.

 

If you try to use the same applications logic for soft starters and VSDs, you will end up with total confusion. They are totally different technologies and operate on different sectors of the motor curves.

A VSD ALWAYS operates at low slip. A Soft Starter Starts the motor at high slip.

 

Applications such as pumps and fans, do not require an overload characteristic from the VSD, hence the thermal requirements are less and the VSD can be connected to a larger motor.

Applications such as conveyors, crushers etc do require an over load torque requirement and are typically supplied by VSDs with a maximum torque rating of 150%.

 

Pumps have a low inertia and therefore start very quickly, whereas fans have a higher inertia and take longer to accelerate to full speed. On a soft starter, the heat dissipated in the SCR junction is a function of the starting current and time. Longer time, more heat, smaller motor connected.

 

Best regards,

[jraef]

Well stated.

 

To more directly answer one of your your questions though schow, theoretically you can "say" that a 30kW drive can run a 45kW motor if you don't want any overload capabilities, but it's not a good practice to do so unilaterally. Other than these "variable torque" loads such as pumps and fans, there are a multitude of conditions that can cause a motor to experience brief overloads, i.e. large step changes in load, restrictions in feeder systems, worn power transmission components etc. etc. If a VFD had severely restricted overload capabilities, its only option to protect itself would be to shut down every time something slightly out of the ordinary happened. In other words, you can SAY what ever you like, but the field conditions dictate what happens.

 

In the case of purely centrifugal load such as pumps and fans, what they are saying is that the nature of the load itself is that it cannot experience those kinds of conditions, so it physically cannot make those demands on the motor or drive. Therefore, it is more acceptable to re-rate a drive for those specific conditions.

 

The 'square law" issue you are referring to is a side issue that just happens to go along side-by-side with re-rated drives. If you are sure that your load is centrifugal and CAN use the re-rated VFD, then you can also take advantage of the VFDs ability to alter its V/Hz pattern to be V/Hz squared, which can maximize energy savings in light load conditions. This feature helps to justify the expense through additional energy savings, but is not the reason why the VFD can be re-rated. They just tend to go hand in hand.

[mariomaggi]

Dear jraef,

theoretically you can "say" that a 30kW drive can run a 45kW motor if you don't want any overload capabilities

 

to avoid confusion, I would recommend to qualify inverters in "A" or "kVA at stated voltage" (never in kW).

 

For example, we can "say" that a "60 A" drive can run a 45 kW motor if you don't want any overload capabilities. .... etc.

 

One reason of re-rating is this one: for fans, sometimes the motor size is selected "by experience" and not using good engineering rules.

For example, in a certain plant someone installed a fan with a 30 kW motor, star/delta starting. The fan had problems to start. Simple solution: to install a bigger motor, i.e.: 45 kW. Starting is now O.K.

By experience in that company they could tell that for that fan it is necessary to have a 45 kW motor.

In fact a 30 kW motor is enough, specially if started by an inverter. Therefore in this case a 60 A inverter is the most economic choice, and it could be acceptable if total output inductance is not so low.

Regards

Mario

 

Note for moderators: my English is bad, you are allowed to modify my wrong phrases.

 

[schow]

Dear all,

 

Thanks for all your replies and valued experiences in this topic, I have learned another lesson today!!!

 

Well, jraef, I like the word you said: "In other words, you can SAY what ever you like, but the field conditions dictate what happens" since I trust most of the great human inventions come from assumptions initially (although there is another saying "assumption is the mother of all screwed up!!!")

 

I would say that in nowadays VFD and sst market, stuffs are getting more and more competitive, especially after the giant sleeping dragon, China woke up! Almost everybody starts to compare the prices with Chinese pricings and in our domestic market, sorry to say that it has become a trend that users would say price comes first, quality later (I know you will say good things never come cheap but sorry this is not appliable here, not to all clients but unfortunately most of them).

 

I totally agreed with what GGOSS mentioned in the other topic "2 leg control in soft starters" that "however I still firmly believe that it is up to us to advise potential customers of the issues prior to taking an order. This might be an old school attitude but it allows me to sleep well at nights, and it keeps my customers coming back" but I am afraid I won't sleep well at night not because of customers called for help BUT my boss is calling chasing for figures!

 

Sometime, we wanted to be honest to the customers but we are "forced" by them to hide something up since we need to meet their budget (he! he! I throw the ball back to the customers)

 

Lastly, can I say ("say" again) that there are not such thing as HVAC rating for soft starters BUT only Light or Heavy duty sst? So from all replies, if I understand everthing correctly, it seems that I may quote a 15kW sst to drive a 18.5kW or 22kW for 30kW water pumps (but not fan applications) in order to win the bid, please correct me if I was wrong AGAIN!!!???

 

I am a bit curious when Mark said he has a local sst maker has a range of HVAC ssts, this seems a bit strange since if the same sst used for pumps application, it will be over-sized and when used for fan, it would be under size, or they are having 2 different ranges of ssts, one for fans and the other one for pumps?

 

danke!

[marke]

Hello schow

 

You must be very careful in up and down rating the manufacturers ratings.

With VFDs, there are two ratings that can be applied as a rule. They are usually rated at 110% torque, or 150% torque. You need to ensure that the rating is based on 150% torque before you change the modle size for pumps and fans. - sone VFDs are already rated for pumps and fans and you need to use a larger VFD for industrial applications.

 

With soft starters, there is no defacto standard that has evolved in the manner that there is in the drves market.

Some soft starters come standard with a light duty rating (suitable for submersible pumps) and a heavy duty rating, others are rated standard as heavy duty and others are part way between the two.

There is a standard that covers how soft starters shal be rated, it is part of IEC 947-4-2 and they are given an AC53 category with AC53A for non bypassed application and AC53B for bypassed applications. You can find a breakdown of these ratings at http://www.lmphotonics.com/sstart.htm.

 

Unfortunately, there is no rule of thumb that you can aply to uprating/downrating soft starters. You need to determine what the start current and time will be and then select the model accordingly.

Beware of what is meant by heavy duty, severe duty etc. Some manufacturers call a start current of 400% for 30 seconds severe duty. This is about normal start current with modern "high efficiency" motors these days. for a heavy duty start, you need to allow at least 500% current, and with some motors, even more.

 

Best regards,

[GGOSS]

This is where I have a real problem with the ratings given to soft starters.

 

In my opinion soft starter ratings need to take into account overload conditions that can and do occur during run mode. I firmly believe that this should apply to soft starters that are AC53a and AC53b rated. To the best of my knowledge, there isn't a single soft start manufacturer out there that has given this due consideration. All appear to have made compromises!

 

To extend on the above, I believe soft start ratings should be linked directly to 'universally recognised' motor overload trip classes. This would bring about uniformity in product ratings and allow customers to make informed decisions about the products they select & use.

 

I do not see any sense in offering a soft start product rated to 3 x FLC, 30 seconds particularly when it includes motor protection that enables adjustability to Motor Overload Trip Class 30. Whilst this philosophy remains in place, soft starters will never be as robust as electro-mechanical RVS alternatives and therefore they will never replace them entirely.

 

Regards,

GGOSS