Current limit

[theDOG]

Dear Forum members;

 

I would like to know what other members think is the best form of "Current limit" starting used on modern Soft Starters ie. closed loop current feedback, fixed voltage etc. Also the opinions on benefits/drawbacks of each of the different types of current limit would be good.

 

Look forward to the responses...

[GGOSS]

Hello theDOG,

 

Some people appear to have a knack for asking 'short/simple' questions to which the response must be 'long and detailed'.

 

It should be needless to say that if the main focus is to reduce starting current, and more importantly limit it to a specified level, you can't go past closed loop controlled current technology.

 

Open loop 'current limiting soft starters' achieve their current limit functionality by maintaining a reduced (fixed) voltage on the motor terminals during the start period. Imbedded in the micro-controller are certain 'typical motor characteristics' for example Locked Rotor Current equals 6 times motor rated current. That is, in order to achieve a current limit start of say 3 times FLC, the micro-controller will perform a calculation and fire the SCRs' such that half supply voltage appears at the motor terminals.

 

More often than not however, the actual LRC for the connected motor is not equal to 6 times rated current, hence the current that is flowing is not the same as the user specified start current. In other words, using the above example with a motor exhibiting an LRC equal to 7 x FLC, the user has requested (through DIP switches, poetetiometers etc) 3 x FLC during start, however the actual or measured start current will be 3.5 x FLC.

 

Another disadvantage of open loop current limiting starters is that the starter does not measure supply voltage and can therefore not 'adjust itself' to compensate for voltage drop on the supply during motor starting. Therefore the current delivered to the motor always 'tracks' what is happening on the supply. Using the above example again however this time assuming an LRC of 6 x FLC, if the supply voltage drops by 10% during start, the current delivered to the motor will be 2.7 x FLC (or 3 x FLC -10%) and this may result in the motor developing insufficient torque to accelerate the motor/load combination to rated speed.

 

With closed loop controlled current soft starter technology, the starter can dynamically adjust its output to ensure the user specified current limit level is maintained during start. It is not dependant upon motor LRC or what is happening to the supply.

 

Hope this helps.

 

Regards,

GGOSS

[marke]

It really depends on what you want to do, what results you want to see.

 

Strictly speaking, to get a current limit, you need to have a feedback regime. Constant voltage, voltage profiles etc and pseudo constant current devices and do not respond well to changing conditions.

 

There are essentially three levels,

 

• pseudo current limit

• current limit

• controled current

 

If we look at each of these:

Pseudo Current Limit. Essentially an open loop controller with a voltage ramp profile designed to emulate the approximate shape of the start voltage under a current limit start. The profile has a voltage plateau at part voltage for a programmable period of time and then the voltage increases to full voltage. Being an open loop contoller, the starter is less responsive to variations in starting conditions. It could hold the voltage low for longer than required, or elevate the voltage before the motor has reached full speed, causing a high start current.

Current Limit Essentially a Timed Voltage Ramp soft starter with a means of monitoring the current and freezing the ramp. If the current goes too high, the starter does nothing about it. Some implementations have a limit on how long they can freeze the ramp with a step to full voltage at the end of the period. Typically, these starters only monitor one phase current so can be fooled by supply problems etc.

Controlled Curent These starters monitor the current on all three phases (can be achieved by using two CTs in a three wire installation) and adjust the output voltage to achieve the required output current. Totally responsive to changes in loading, the ouptut voltage can go up or down. A constant current or a current ramp or a pre programmed current profile can be implemented.

 

The controlled current starter is able to accelerate the motor to full speed in the minimum time without exceeding a predetermined current. It is also able to have different profiles to accomodate different load types or variable start torque requirements. It is generally the most expensive system.

 

The open loop systems will eventually get to full voltage (just like a star / delta) and so are more impervious to incorrect setups, but the start current can be much higher than with the controlled current starter.

In a nutshell, I would say that the controlled current starter can provide shorter start times and lower start currents than the open loop types.

 

Best regards

[theDOG]

I am about to play the devil's advocate..

What about in a pumping system? If you do the comparison between the two start methods, as the breakdown torque speed approaches, the closed loop current controller allows the motor to draw significantly more current than a fixed voltage controller. The result? A massive torque transient. Surges (pressure transients) occur as a result of a sudden change in flow, which equals fluid hammer.

Any comments???

[GGOSS]

Hey DOG,

 

What did you take for breakfast this morning?

 

With a closed loop controller, the starting current is maintained at the user specified level throughout the speed range. It does not increase as breakdown torque speed is approached!

 

As we approach full speed, the amount of torque produced by a motor controlled by a closed loop current controller can be much higher than that produced by the same motor under the control of a voltage/current limiting starter.

 

The higher magnitude of motor torque may or may not be an issue dependant upon pump type. For example, in many cases the pump must be accelerated to rated speed within a period of say 2 to 3 seconds. This would compromise hydraulic performace moreso than anything else.

 

Question:

 

Is one method better than the other?

 

Answer:

 

I would have to agree with Marke and say that it all depends on the driven machine and/or the expected outcome.

 

Regards,

GGOSS

[GGOSS]

Hey DOG,

 

Just me again!

 

If you were do do a comparison of acceleration times for 2 identical motor/load combinations started at the same current limit level, #1 via a closed loop controlled current soft starter, #2 via a voltage/current limiting controller, the acceleration time for #1 would be much shorter. This of-course reflects the amount of acceleration torque that the motor is able to produce as it approaches rated speed!

 

Shorter starting times can be extremely beneficial particularly for highly inertial loads, as they minimise thermal stresses on the motor.

 

Regards,

GGOSS:

[marke]

Hello theDog

 

A massive torque transient. Surges (pressure transients) occur as a result of a sudden change in flow, which equals fluid hammer.

If you use a true current limit with a very fast reaction time, then depending on the shape of the current speed curve, and the torque speed curve, you can certainly have a "boost" in torque towards the end of the acceleration time due to the voltage reaching full voltageat say 80 - 90% full speed. This will apply full voltage torque which can be upto 250% FLT. This is definitely not a torque transient. It is certainly a rise in torque which can cause a rise in acceleration rate. In some circumstances, this can cause quite a pressure surge, or can cause belts to slip momentarily as the motor reaches full speed etc, but the torque is definitely not a torque transien, nor is it any higher than it would be if DOL started, and it is less than it would be if star delta started.

 

As GGOSS says, this mode of soft start will get the driven load up to full speed in the shortest possible time without exceeding a current set point.

 

The magnitude of the maximum torque, is very much a function of the motor curves. If you have a motor with a very high LRC and a very square curve, the problem will be exagerated. Additionally, if your motor has a very low LRT with a very peaky torque curve, the problem will be exagerated. If both of these conditions exist together, then the characteristic is most pronounced.

Typically, submersible pumps have a low LRC, high LRT and a very non peaky curve so the characteristic is less of a problem than in the case of surace mount pumps employing shallow bar two pole motors.

There are some situations where a different profile can be an advantage. There is no one ideal start method!!

Best regards,

[theDOG]

GGOSS;

 

I think you have mis-interpreted my last post. I stated that in COMPARISON to a fixed voltage controller, the closed loop current controller allows the motor to draw more current. I agree that this will be limited to the level set by the user, but it will still be at a level that is higher than a fixed voltage controller. Therefore, the acceleration torque is greater, leading to a decreased acceleration time.

Just because the motor gets to speed quicker, doesn't mean it is a better form of starting ie DOL Vs solid state. If you are concerned about the motor thermal stresses, DOL is the best form of starting! As for submersible pumps, do you agree that a Timed Voltage Ramp would be a better starting method?

 

marke;

Most soft starters are marketed as being "better" than other forms of starting. It is important to note that there are many different starting methods, and no single method is the ultimate. It will be dependent on the application as you stated.

 

I'm glad I stirred up the pot!!! I agree with most of the comments posted by other forum members, but I thought it was important to point out the difference between the two methods.

 

Thanks for your replies.

[GGOSS]

theDOG,

 

My appology for the late response to your post:D

 

No, I don't think I mis-interpreted your post at all. In fact you have repeated the same confusing information in your most recent one. Therefore I would ask that you please clarify what you mean by:

 

"in COMPARISON to a fixed voltage controller, the closed loop current controller allows the motor to draw more current".

 

How is this possible???

 

I believe it is this portion of your post that is causing the confusion;p;

 

Regards,

GGOSS

[theDOG]

GGOSS;

 

The closed loop current controller will have a "flat" current curve, by increasing the voltage to maintain the current setpoint. On the other hand, the fixed voltage controller will reduce the (DOL) current by the same factor of the voltage reduction ie. it will have the same "curve", at a different magnitude.

Conclusion:

The closed loop current controller will increase the voltage (in comparison to a fixed voltage) as the motor speed approaches rated speed. This means greater current, greater torque and reduced acceleration time (as you point out in your second post on the 20/9).

Does this clarify?

[marke]

Hey guys,

you are both correct. If you measure the maximum current drawn by a constant current soft starter, it will invariably be less than a voltage ramp starter. (assuming both are appropriately setup)

 

If we measure the current as the motor approaches full speed, say from 85% speed upwards, then the voltage ramp starter will often have a lower current. The constant current starter is able to lift the voltage quicker as the motor impedance changes rapidly. This leads to faster acelleration. Good for high inertia loads, but not so good with very peaky torque curves on surface pumps with long lines.

 

Best regards,

Mark.