VFD vs Soft Starter - Inrush

[Jody Boehs]

Hello,
In my facility we have an emulsifier that is powered by a 125 HP Motor. This motor is operated by a soft starter.
This emulsifier is NOT on our backup power system. We are in the very early stages of figuring out if we have enough backup power capability to handle it.
We have 2 600KW Generators that are independent of each other, each with a UPS system that allows them to start up and transfer without any loss of power in the plant. Each of these units are running at about 50% capacity during the summer (this decreases during the winter).
My main question, however, is as follows:
This emulsifier has an inrush current of 830+ amps. We were told by our Generator vendor that we would need another 600KW Generator to handle that inrush (Keep in mind this 125 HP motor starts and stops approximately 10-15 times per 24 hrs).

Would a VFD instead of a soft starter on that motor reduce that inrush current?

Thank You!

 

[ptonsparky]

Is this a pump type load or constant torque?

 

[petersonra]

There is basically no inrush to a VFD.

 

[Jody Boehs]

Is this a pump type load or constant torque?

I believe it is constant torque. It run a grinder/cutter wheel at high speed. There is a gear pump that feeds product into it and then that grinder/cutter literally liquifies the product and pushes it out into a kettle.

 

[Jody Boehs]

There is basically no inrush to a VFD.

Well that's good to hear! Even with a high torque startup?

 

[petersonra]

Well that's good to hear! Even with a high torque startup?

Most of the high current in a motor is when it starts up, but it is at a very low power factor, and doesn't actually have a whole lot of power used. The input of the drive itself only takes in the actual power needed to replace what is going to the motor, and generally is close to a unity power factor.

 

[4-20mA]

There is basically no inrush to a VFD.

Especially when you set up a 60sec (or whatever you decide, longer if needed) ramp up. Seems like a VFD is a good choice for this scenario.

 

[Jraef]

To expand a bit on what Bob said, when you start a motor at a fixed frequency, such as with a motor starter or soft start, most of the current is not producing torque at first because until the motor reaches at least 80% speed it is reactive (low power factor) current, not active current. From the supply side, the difference is irrelevant because all of the current still has to be supplied. You can think of that starting current in terms of a curve, where with Across The Line starting, that curve is really high, as in 600% of FLA, but really short, as in 1-2 seconds. But at that initial 600% current, it is only producing 150% torque. A soft starter just basically trades the big peak height for a longer time, so maybe 300% but for 30 seconds. Yet at 300% current, it is still only producing 75% torque at first (again until it gets to about 80% speed), which is why it takes longer to accelerate. The total amount of energy in either curve remains the same no matter what though.

What’s different about a VFD is that by manipulating the voltage AND frequency together that gets to the motor, it can make the motor produce 100% of rated torque from the very start, but will only draw 100% of FLA when doing so, because the VFD is going to correct the power factor seen by the supply side the entire time. So in general, a VFD can often start a load FASTER that it can start Across The Line without exceeding the motor FLA. You CAN program a VFD to use higher current, often 150% for up to a minute on a heavy duty rated drive, but in a lot of cases that is unnecessary.

The only drawback is that above about 10HP, a VFD will cost more than a soft starter, and always more than an A-T-L starter, then the larger the motor, the wider that gap in cost difference. So although VFDs do a better job at soft starting, the added cost is often not practical if there is no need to change the motor speed.

 

[Jody Boehs]

To expand a bit on what Bob said, when you start a motor at a fixed frequency, such as with a motor starter or soft start, most of the current is not producing torque at first because until the motor reaches at least 80% speed it is reactive (low power factor) current, not active current. From the supply side, the difference is irrelevant because all of the current still has to be supplied. You can think of that starting current in terms of a curve, where with Across The Line starting, that curve is really high, as in 600% of FLA, but really short, as in 1-2 seconds. But at that initial 600% current, it is only producing 150% torque. A soft starter just basically trades the big peak height for a longer time, so maybe 300% but for 30 seconds. Yet at 300% current, it is still only producing 75% torque at first (again until it gets to about 80% speed), which is why it takes longer to accelerate. The total amount of energy in either curve remains the same no matter what though.

What’s different about a VFD is that by manipulating the voltage AND frequency together that gets to the motor, it can make the motor produce 100% of rated torque from the very start, but will only draw 100% of FLA when doing so, because the VFD is going to correct the power factor seen by the supply side the entire time. So in general, a VFD can often start a load FASTER that it can start Across The Line without exceeding the motor FLA. You CAN program a VFD to use higher current, often 150% for up to a minute on a heavy duty rated drive, but in a lot of cases that is unnecessary.

The only drawback is that above about 10HP, a VFD will cost more than a soft starter, and always more than an A-T-L starter, then the larger the motor, the wider that gap in cost difference. So although VFDs do a better job at soft starting, the added cost is often not practical if there is no need to change the motor speed.

This is a really good explanation. Just to reiterate how I am understanding this... Using a VFD, should not allow the current to go over the FLA of the motor which is 163.3 (142 x SF of 1.15 = 163.3).

The Accel time would need to be quicker than 60 sec. I was hoping to go with 10-15 sec. But, tell me how that would affect my current and possibly the life of the drive?

Currently, this motor does not have a backup power source. So, when power goes out (which is quite often where I am at), this unit being down prevents us from running product on all 4 production lines. A 6 hour power outage on 1 line costs us around $1,500 per hr., so that times 4 makes $6,000 per hour downtime for all 4 lines. The cost of a drive (around $10,000) is negligible in that sense.

Thank You!

 

[synchro]

Currently, this motor does not have a backup power source. So, when power goes out (which is quite often where I am at), this unit being down prevents us from running product on all 4 production lines. A 6 hour power outage on 1 line costs us around $1,500 per hr., so that times 4 makes $6,000 per hour downtime for all 4 lines. The cost of a drive (around $10,000) is negligible in that sense.

Since an outage is so costly, if you go with a VFD you might also consider having a spare one on hand, especially considering current supply chain issues. Perhaps others could weigh in on whether having a spare could be appropriate in this scenario.

 

[garbo]

Hello,
In my facility we have an emulsifier that is powered by a 125 HP Motor. This motor is operated by a soft starter.
This emulsifier is NOT on our backup power system. We are in the very early stages of figuring out if we have enough backup power capability to handle it.
We have 2 600KW Generators that are independent of each other, each with a UPS system that allows them to start up and transfer without any loss of power in the plant. Each of these units are running at about 50% capacity during the summer (this decreases during the winter).
My main question, however, is as follows:
This emulsifier has an inrush current of 830+ amps. We were told by our Generator vendor that we would need another 600KW Generator to handle that inrush (Keep in mind this 125 HP motor starts and stops approximately 10-15 times per 24 hrs).

Would a VFD instead of a soft starter on that motor reduce that inrush current?

Thank You!

I think you would be okay especially if you have at least a 10 second ramp up time to give it a very low start up current. At the large hospital/research center that I retired from we have over 500 drives from 2 to 1,750 HP. I often watched the ampere reading on drive screens from start-up to full speed and from 5 to an amazing 10 minute ramp up time. Current on say a 150 HP Chiller pump would only have a few amps first few seconds then gradually increase to motor nameplate amps at full speed. They used a 10 minute ramp up time on old Air handlers during they winter months to prevent free stats from shutting units down. Great things about guess all of the VFD'S made in last 20 years they have what one drive company called flying start. When we did our monthly testing of over 75 ATS'S when power is transferred from normal power to emergency power the drives calculate how fast the motors are turning compared to set speed and restarts a fast spinning motor on the fly.

 

[Jody Boehs]

I think you would be okay especially if you have at least a 10 second ramp up time to give it a very low start up current. At the large hospital/research center that I retired from we have over 500 drives from 2 to 1,750 HP. I often watched the ampere reading on drive screens from start-up to full speed and from 5 to an amazing 10 minute ramp up time. Current on say a 150 HP Chiller pump would only have a few amps first few seconds then gradually increase to motor nameplate amps at full speed. They used a 10 minute ramp up time on old Air handlers during they winter months to prevent free stats from shutting units down. Great things about guess all of the VFD'S made in last 20 years they have what one drive company called flying start. When we did our monthly testing of over 75 ATS'S when power is transferred from normal power to emergency power the drives calculate how fast the motors are turning compared to set speed and restarts a fast spinning motor on the fly.

That's good to hear! I appreciate all the input on this subject.

I won't be needing a flying start feature. I was actually thinking of installing a phase monitor and using it to operate a contactor. The said contactor would kill power and prevent a restart in the event of a power loss or loss of a phase.

 

[Jody Boehs]

Since an outage is so costly, if you go with a VFD you might also consider having a spare one on hand, especially considering current supply chain issues. Perhaps others could weigh in on whether having a spare could be appropriate in this scenario.

Absolutely! We will have a spare.

 

[ptonsparky]

That's good to hear! I appreciate all the input on this subject.

I won't be needing a flying start feature. I was actually thinking of installing a phase monitor and using it to operate a contactor. The said contactor would kill power and prevent a restart in the event of a power loss or loss of a phase.

That is built into the drive and most likely default parameters.

 

[Jraef]

A contactor opening and closing ahead of a VFD shortens it’s lifespan. It’s OK once in a while, but once you have a circuit, people have a tendency to over use it.

Side issue from earlier:

Using a VFD, should not allow the current to go over the FLA of the motor which is 163.3 (142 x SF of 1.15 = 163.3).

Just so you know, motor mfrs will tell you that when a motor is powered from a VFD, you cannot use the Service Factor. This is usually indicated by telling you that when powered by a VFD, the SF = 1.0.

 

[ptonsparky]

A contactor opening and closing ahead of a VFD shortens it’s lifespan. It’s OK once in a while, but once you have a circuit, people have a tendency to over use it.

Side issue from earlier:

Just so you know, motor mfrs will tell you that when a motor is powered from a VFD, you cannot use the Service Factor. This is usually indicated by telling you that when powered by a VFD, the SF = 1.0.

This forum is about the only place I hear that.

I just had a pump supply tell me that I needed to use the next size up drive so we could run the motor into its SFA. Should have taken it, cause we went back,it was gone.

 

[Jraef]

NEMA MG-1 Part 31, the section for "Inverter fed polyphase motors",
Paragraph 31.3.7 entitled "Service Factor" says
"A motor covered by this Part 31 shall have a Service Factor of 1.0"

 

[Jody Boehs]

Here is the nameplate of the motor I want to run with a VFD. We have 480V Wye supply. Any other issues that anyone can see that I may be missing?

I know there will be some programming issues that I will face while integrating a VFD into the program that operates the original soft starter, but I can get that figured out.

 

[ptonsparky]

NEMA MG-1 Part 31, the section for "Inverter fed polyphase motors",
Paragraph 31.3.7 entitled "Service Factor" says
"A motor covered by this Part 31 shall have a Service Factor of 1.0"

I believe you entirely. Others aren't getting the word.

 

[petersonra]

I believe you entirely. Others aren't getting the word.

I've seen motors running well above there service factor for years both on VFDs and contactors. They get a little warm but really all you have to do is turn a fan on them.