Motor Acceleration and Motor Life

No... there will be benefits to making this change.

1) Reduced peak demand from your power supply.

2) Reduced winding heating as the starting current applied to the motor will be lower.

3) Reduced stress on the rotor and shaft of the motor.

4) Easier on the bearings of the motor.

5) Easier on the rotating portions of the compressor as the "shock of being accelerated" is lessened.

6) If needed, more frequent motor starts per hour could be permitted.

Have I missed any?

I agree, there are lots of benefits to a soft start..

The only caution is that if the compressor has a lateral or torsional critical speed between 0 and running speed, then you may see high vibration or shaft stress as you hang in the critical speed range for a longer period of time.

A rotordynamics study should be considered, but otherwise it is good for the motor...

BTW for a 350 HP train, there is not likely going to be a critical speed problem, that is a pretty small machine. What is the running speed rpm of the compressor? Is there a gear increaser in the machine train?

Corrections to the above:

1) Reduced peak demand from your power supply.

Not usually true. Peak Demand charges are largely based on demand windows of 15-20 minutes. You cannot stretch out the ramp time long enough to have an effect.

2) Reduced winding heating as the starting current applied to the motor will be lower.

Applies only to the VFD. The energy used in starting with a soft starter is exactly the same as Across-the-Line (DOL), just spread out over a longer time. There is debate over the issue of the extra time allowing more heat conduction away from the rotor and thus avoiding hot spots, but that is not proven. By the same token, there is an opposite school of thought that soft starters CAUSE more motor heating, and I don't believe that either.

6) If needed, more frequent motor starts per hour could be permitted.

Not at all true, in fact sometimes it is less because the soft starter itself may not be capable of as many starts as the motor is.

Don't get me wrong, I am a big fan of soft starters, and all the other points are very good. These are just common misconceptions.

Interesting... I'll get back to you... once I check some long held assumptions.

You said...

"Not usually true. Peak Demand charges are largely based on demand windows of 15-20 minutes. You cannot stretch out the ramp time long enough to have an effect."

There are lots of different schemes on how peak demand charges are applied. For example, here in the north, meters have "instantaneous peak" indicating dials on them. There is no demand window of any sort. They simply record the peak demand. So your energy charge from the supply authority includes both a KWh consumed component and a peak demand charge component.

And here is the real rub, once you hit a new peak, it stays as your demand charge for a year. Unless of course you hit a new peak and then the year starts over once more. The supply authority will only lower it once a year (or so I'm told) and then they lower it to the peak recorded in the first month after the year is up. So large users of power up here really watch their peak demand.

Given the scenario above, the AB 150 series of soft starts would possibly save lots of money. This device allows for many modes of operation. One of them, the "current limit" mode allows a user to program the device to supply from 50% to 600% of the motor FLA with a ramp of up to 30 seconds. This will definitely save on peak charges. It won't save on KWh consumed, it will only limit peak demand from the supply.

You also said...

"Applies only to the VFD. The energy used in starting with a soft starter is exactly the same as Across-the-Line (DOL), just spread out over a longer time. There is debate over the issue of the extra time allowing more heat conduction away from the rotor and thus avoiding hot spots, but that is not proven. By the same token, there is an opposite school of thought that soft starters CAUSE more motor heating, and I don't believe that either."

I agree that the energy consumed is equal to across the line starting. The largest component by far of motor winding heating is the level of current flow in the winding. I can not remember where I know this from, at the moment, but motor winding heating rises as a square of the current. I'll try and dig this up.

So I can not see a way in which reducing peak current flow in a winding, by the use of a properly configured soft start in "current limit" mode does not result in a cooler winding.

I agree that a soft start may introduce its own impact on start cycles per hour. Can you not agree that if we do see a reduction in overall winding temperature due to reduced current flow, we should be able to start that motor more often?

Do you have a URL for some of those debates you mentioned or were they found in a journal or magazine? If so, which ones may I ask?

For anyone interested in a little reading... here is an excellent technical document from AB on the device I mentioned.

http://literature.rockwellautomation.com/idc/groups/literature/documents/at/150-at002_-en-p.pdf

You said;
For example, here in the north, meters have "instantaneous peak" indicating dials on them. There is no demand window of any sort. They simply record the peak demand. So your energy charge from the supply authority includes both a KWh consumed component and a peak demand charge component.

I did not say ALL utilities use a demand window, I said they are "largely based...", meaning most. There are the odd exceptions, but remote areas with small generating capacities such as Nunavut Canada near the arctic circle are hardly the norm for most of North America and should not be consider the basis for making a sweeping statement like you originally did. While I believe it is true for you, this myth has been perpetrated and perpetuated by the soft starter industry and is largely (that term again) responsible for the disappointment that a lot of people have in soft starters. I was in that industry for 25 years with various manufacturers and have witnessed this marketing scam being used to falsely convince people that they will save money on their utility bills using soft starters. It just is not true in the greater sense, yet the perpetrators use rare situations such as yours to foster and promote the myth. I happen to feel that the true benefits of soft starters are great enough to stand on their own, not on exaggerated benefits. I accept that your statement was made in all earnestness, but I like for people to know the truth in general, not the exceptions.

You said;
I agree that the energy consumed is equal to across the line starting. The largest component by far of motor winding heating is the level of current flow in the winding. I can not remember where I know this from, at the moment, but motor winding heating rises as a square of the current. I'll try and dig this up. ...

Energy is not current. Energy is not even power. Energy is power over time. As I said, the energy needed to start a load from a dead stop to full speed is a fixed value, calculable based on mass, inertia, torque etc. If starting across the line, the energy curve is represented by a high peak and a short duration. With a soft starter, it is a lower peak, but a longer duration. THE AREA OF EITHER CURVE IS EXACTLY THE SAME. The heat put into a motor rotor is relative to the amount of energy used in acceleration vs the total amount of energy put in. In both cases, more energy is put in than can be used in acceleration because the slip is so high in the beginning. Nothing changes in that equation except time when you use a soft starter, so the amount of excess energy that the motor mass must absorb and dissipate is exactly the same in either case. Sure the heating effect is relative to the sqare of the current, hence the discussion regarding hot spot issues, but even if the heating effect is lower because of a reduced current, it is present for longer and the net energy put into heating the motor is still the same.

A VFD on the other hand, actually reduces the amount of ENERGY applied to the load. So you can set it up to produce only as much torque as is absolutely necessary to maintain acceleration without dumping excess energy into the motor. Hence, you can do that all day, as many times as you like.

By the way, one of the best arguments I have seen written on this subject of motor heating being equal in soft starters vs any other fixed frequency starting method came from an Allen Bradley engineer. But A-B have apparently stopped publishing it on the web; it probably embarrassed their marketing people by interfering with their efforts to find new ways to convince people to buy them. You can however still read it on the IEEE website if you belong.

Starting high inertia loads with AC motors article

Another resource:

http://www.lmphotonics.com/hi_inert.htm

In general, a properly applied soft starter will neither hurt or help the life of the motor. If misapplied, it can damage it.

Often times, extending the motor acceleration time will cause the motor to fail to accelerate altogether, so THAT can damage the motor. In general, you cannot "force" an acceleration time with a soft starter, you can only set the ramp time. There is a difference. Acceleration time is a function of torque and inertia, in other words it is load dependent. Ramp time is only a function of voltage and time. If the voltage and time values provide enough torque, the motor accelerates even if the ramp time is not yet complete. Most soft starters will come with an automatic detection circuit to override the ramp settings if it detects that the motor accelerated anyway.

A VFD does not have that issue as it directly controls the speed all the time. So you can take as long as you like to accelerate the motor. A VFD will technically extend the life of the motor IF you deal with a few ancillary issues that come along with VFD use.

• VFDs cause additional heating in the motor due to harmonics. This can be mitigated with filters / reactors, but that costs more $ and many people forgo them, which ultimately costs them more when the motor dies prematurely. If the motor is run at reduced speed most of the time, this may not be an issue because with reduced speed on a VFD, the power is reduced as well, so even though the harmonics heat up the winding more, proportionally the net effect can be minimal. But if the VFD runs at full speed / load most of the time, you have to deal with that issue.
• Motors run with VFDs can usually be slowed down to speeds at which the motor's cooling system will no longer function. Care must be taken when designing the system. External cooling is usually required for low speed operation.
• VFDs can cause ground currents to flow between the rotor and frame, so that means they pass across the bearings and races. Long term use eventually causes fluting of the races and damage to the bearings. Shaft grounding bushings are necessary in most cases, but again, cost $$ and thus are often ignored.
• VFDs can cause high voltage spikes to be formed in the conductors going from the drive to the motor. If long enough, these spikes can reach 2.5 x the line voltage, punching through the winding insulation of standard and older motors. Again, filtering can mitigate this, but cost $.
  

You may find it necessary to modify the compressor unloader valve strategy as frequently the unloader is disabled as soon as a start is initiated.

If you have a long run up time the pressure and thus the torque demand on the motor may exceed what the soft start or vfd provides and the drive will stall,because the pressure is building more rapidly than the motor torque.

I have had star-delta compressor drives stall if the star time is set too long.

You can however control the unloader from the soft start or vfd end of ramp signal,provided the unloader can stand the sudden operation with the compressor at full speed.