Soft starter ramp-up time vs. motor RPM

[schow]

Dear all,

 

I have bought an European "D" brand 2-phase SCR controlled ("L2" is directly connected to "V" of 3-phase AC motor) soft starter recently for one of my conveyor (I suppose conveyor should be constant torque application) driven by a 4-pole, 7.5HP, 1450rpm motor.

 

The soft starter c/w adjustable torque of 0-85% of full voltage and kick-start of 100% full voltage for 200msec; ramp-up & ramp-down time of 0.5 to 10 sec.

 

Here are some of my testings done with 3 different setting on "Torque" only, with ramp-up set to max 10 sec and no setting on ramp-down (0 Sec) on 3 cases:-

 

1) With "Torque" set to 25%:-

Motor can be started, ramp-up indination LED showed about 10 sec to stay still (the LED will blink during ramp up) but motor full rpm, 1450 achieved in about 5 sec

 

2) With "Torque" set to 85%:-

Motor can be started, ramp-up indination LED showed about 10 sec to stay still but full rpm achieved in time shorter than case "1", which approx 3 sec

 

3) With "Torque" set to 0%:-

Motor started to generate noise when soft starter was enabled, ramp-up indination LED showed about 10 sec to stay still but motor still reached full speed in less than 10 sec, approx 8 sec.

 

My question is why the soft starter & the motor did not behave linearly? I suppose when the soft starter's ramp-up time is set to 10 sec, no matter what the "Torque" is, it shall reach full rpm in 10 sec, right?

 

Another question about this type of soft starter; if I set the ramp-up time to minimum (0.5 sec), is my motor going to behave like DOL if I enabled the soft starter? If this is true, is this means that the soft starter will experience almost 7 to 8 times of the motor rated current of 11A? Can the 7.5HP soft starter withstand this huge current of approx 80A for 3 to 4 sec? I dare not try this since I am afraid that it will damage the SCR of the soft starter.

 

Are all soft starters makers out there designed their soft starters to cater for 3-4 times motor rated current only (just to withstand the motor soft start current)? How do they derive the rated current of soft starters? Normally, they just mentioned the kW-rating only!

 

All comments on these issues are most welcome!

 

Cheers! :cool:

[marke]

Hello Schow

 

Soft starters reduce the voltage applied to the motor during start. The reduction in voltage results in a reduction in torque. The torque is reduced by the voltage reduction squared.

Two phase controllers are reasonably common and are satisfactory for low inertia loads with a relatively short start time. The two phase controller introduces negative sequency torques to the rotor and this increases the heat developed in the rotor during start. Some adjust the current magnitude by skewing the phase angle. These do not eliminate the negative sequence currents.

If the initial start voltage is too low, the motor will not rotate until the voltage has ramped up to the point where the torque developed is equal to or greater than the breakaway torque of the driven load.

The motor is presented with a rotating torque field which is spinning at the line frequency. Being essentially a pseudo synchronous machine, the motor will spin the load up to full speed as fast as the load will allow. The rate of acceleration is a function of the acceleration torque and the load inertia. The acceleration torque is the difference between the load torque and the torque developed by the motor.

The ramp rate will not determine the acceleration time of the motor and load. It will influence the start time, but do not expect the acceleration time to match the ramp time, it just wont happen. If you want this level of control over the acceleration time, buy a VSD at a much higher price.

 

Some manufacturers rate their soft starters at 300% start current, other rate their starters at 500% start current and some do not rate the start current at all.

If you need a high start torque, make sure that you select a starter that is capable of suplying a high start current.

 

With a voltage ramp soft starter, if you set the ramp time too fast, you will reach full voltage before the motor reaches full speed and the motor will draw a very high current. If you set the start ramp too long, you will have the motor up to full speed at a very reduced voltage and you will increase the slip losses in the motor until the voltage approaches full voltage.

For a low inertia load, you want a short ramp, for a high inertia load, you want a long ramp.

 

Best regards,

[schow]

Hi Mark,

 

Thanks for your detailed reply. So in other words, can I say that the output voltage of a soft starter is not linear proportianal to the speed of an AC induction motor? Thus even the output voltage has been increased linearly, the rpm of the motor will not be increased linearly?

 

Lastly, can you advice whether I am going to blow the SCR off if I set the ramp-up time to 0.5 sec? If this will happen, why the manufacturer still allows users to set to a minimum ramp-up time of 0.5 sec?

 

Pls advice.

 

Many thanks!

[marke]

Hello Schow

 

There is no relationship between the sped of the motor and the voltage applied to it. The voltage reduction causes a torque reduction by the squre of the voltage reduction. The motor will accelerate up to the speed where the torque output from the motor is equal to the torque required by the load.

If the motor is open shaft, it will accelerate to full speed quickly at 20% voltage.

 

The botom line with a soft starter is that it does not necessarily create a slow start, but it does create a gentle start.

 

If you overload the starter, you will probably not blow the SCR immediately, but you will weaken the SCR and shorten it's life. You may find that the starter fails after a few months rather than lasting for 20 + years.

Some manufacturers sell light duty units to keep the price down. They expect you to select the appropriate model for your application.

 

Best regards

[schow]

Hi Mark,

 

I noticed that most of the soft starters in the market with 2-phase are below 22kW and from 30kW onwards are normally 3-phase controlled, may I know why? Can we applied the 2-phase SCR controlled technology for higher kW rating says up to 110kW?

 

Pls advice.

 

Cheers! ;c;

[jraef]

I'll chime in now to give Marke's "e" key time to heal, it seems to be intermittant. ;-)

 

There are some manufacturers who still offer a 2 phase control soft starter in large power ratings, but as Marke explained this technique is not good for the motor, regardless of the size. The additional heating is essentially lowering the duty rating capability of the motor. In smaller motors, the duty rating is higher than most people use anyway, and most loads are not heavy duty, hence the lack of concern on the soft starter mfgrs part.

 

As the motors get larger however, the number of starts-per-hour that the motor can safely handle goes down rapidly and the likelyhood of the load being heavy goes up, so that additional heating becomes more critical. For example, if a 7.5kW motor can handle 15 starts-per-hour, sacrificing 2 starts for getting a cheap soft starter may be a "devil's bargain" you are willing to try. If your 110kW motor is rated for 6 starts-per-hour, sacrificing 2 of them just cut your duty cycle capability by 1/3. Soft starter manufacturers know this, and most will excercise some restraint in allowing users to destroy their own equipment. As you already noticed with the issue of allowing a .5 sec. ramp, that concept is not universal.

 

Many Soft Starter manufacturers will not offer the 2 phase option at all because the concept requires the user knowing what your load needs, and most people don't understand the nuances. For example, you have apparently selected one for a conveyor application, which is probably not a good choice. I am not saying anything about you, I am pointing out the problem. How were you supposed to know? They do not do a good job of educating people about the right and wrong way to apply them, because they do not want to expose their lack of concern.

[schow]

Hi jraef,

 

Thanks for your advice, I am more than happy to learn from you guys as an expert in this field with open heart since I am just a beginner in this, so feel free to comment on any mistake I have made.

 

Just to explore more on the 2-phase controlled technique, besides the additional heat generated at the motor end, would it be any other disadvantage? For example, if I used a 2-phase soft starter to start a 90kW motor, would the starting current drawn be same as if I used a 3-phase controlled soft starter?

 

Would it be any different in heat generation if I used a by-pass magnetic contactor to by-pass the 2-phase soft starter after the motor is ramped to full voltage (with the by-pass signal output from the soft starter)?

 

What is the reasonable ramp-up time do you suggest if I used a 2-phase and 3-phase controlled soft starter to start a 90kW motor?

 

Cheers!

[jraef]

The only other disadvantage is regarding the failure mode. In a 3 phase soft starter should one SCR short it will cause problems in ramping, but will not damage the motor. You need to have at least 2 SCRs in opposing phases short in order for there to be a current path through the windings. In a 2 phase soft starter, 1/2 of the circuit is always hot (L2) so if any 1 SCR shorts, you have an unrestricted current path through the motor windings and you may lose the motor. This means that you MUST use a shunt trip circuit breaker or an In-Line isolation contactor in front of them. Adding that contactor is more expensive than buying a 3 phase soft starter. A shunt trip added to a circuit breaker is less expensive, IF you are using a circuit breaker. If you use fuses, you must add the contactor to be safe.

 

Starting current should be the same between the 2 styles, but Marke would know that answer better than me, I'm not sure if there is less torque per amp generated in a 2 phase scheme.

 

Once any soft starter is bypassed, nothing about it's design has any more effect on the motor, so no, the additional heating issue would be gone.

 

Because of the additional heating efect, I would not ramp any longer than 15 seconds with a 2 phase, most are limited to that anyway. 3 phase soft starters can be adjusted out to 2 minutes, some even longer, although I have yet to see an application where that would be appropriate. I personally rarely go beyoind 30 seconds. How long you need for your application is totally dependant on the driven load.

[marke]

Hi guys

 

With the two phase control system, there are negative sequence currents generated. This will reduce the net torque output for a given start current, so in many cases, the start current will be higher.

 

Best regards,

[jraef]

Good to know, thanks Mark.

[marke]

The higher the start current, the lower the influence of two phase control. A little bit like th old SCR diode days, but not nearly as bad.

 

A local manufacturer here makes up to 110KW units. We supply a Danish unit up to 25 Amps only. I am hesitant to go beyond that size, but these small ones are great for applications subjected to frequent broken chains. - They have their place, but they are not a subsitute for three phase control.

You can also find similar two phase control in small autotransformer starters with the emphasis in small. I have not seen large autotransformer starters with two phase control.

 

Best regards,

[GGOSS]

Good thread!

 

Larger (higher kW rated) 2-phase auto-transformer starters are less common in Australia now than they were say 10 or 15 years ago. Back then you could expect up to 10% of auto-transformer starters sold were of the 2-phase type, at a guess it's now less than 0.5%. 2-phase auto-transformer starters are very rarely used these days, even for small motors.

 

Regards,

GGOSS