Integrated drive systems webcast: More answers on energy savings

Siemens discusses more issues around VFDs, motor efficiency and over-sizing motors

By CFE Media April 23, 2015

Plant Engineering and Control Engineering recently presented the Webcast, “Integrated Drive Systems: A Holistic Approach to Energy Savings,” presented by Rich Mintz, senior market development manager for the gear motor business segment, Siemens Industry, Inc. A link to the Webcast is here:

There were many audience questions that could not be answered during the Webcast, but Mintz provided additional answers to those questions here:

Q: What is meant by inverter compatible motor?
A: Typically this means that the insulation system can handle high voltage spikes seen with high switching frequencies of the transistors inside the drive. They can be over 1000V peak to peak. Motors that are not somehow marked as inverter duty may not be able to handle that, and will fail pretty quickly.

Q: If I have a heavy constant load (say a chipper or a refiner) needing say 1000 HP and use an 1800 RPM motor but need to reduce it to say 900 RPM. Is the power output now equivalent to a 500 HP motor? Would it not be more efficient to use a belt to reduce the speed and keep the power less 10% belt loss?
A: I will have to make some assumptions. The torque would remain the same at a reduce speed for therefore the motor losses and temperature rise would remain the roughly same.

Q: For one of our previous projects, client decided to use one VFD to start three compressor motors (one at a time) and after start-up, the motors were on DOL. How common is this strategy and what other alternatives are there to handle poor upstream system?
A: This is very common method of starting motors on a weak power system. The type of compressor will determine starting torque demand. Applications such as screw compressor will demand 40-50% of the FLT. While a specially designed motor can meet the demand, however the efficiency of the motor can drop a full 2-3%.

Q: What is the difference between soft start and VSD driven inverter compatible motors?
A: Soft starters do not typically create the high switching frequencies associated with drives; therefore the motors may not need to be inverter duty. You would need to confirm with the manufacturer of the soft starter and the motor. Best bet is to use one supplier for both to ensure compatibility.

Q: How do VSD systems compare with hydraulic couplings (VOITH Type) in overall efficiency?
A: It all depends on the application. The fluid coupling is mostly for over torque protection. It does also act as a soft starter somewhat. But it has losses associated all the time. A drive would give you more flexibility on the process control which would likely give better energy reduction.

Q: What is limiting size of integrated drives?
A: At Siemens, basically none. We can do fractional HP to tens of thousands of HP.

Q: What is the difference between improving efficiency and reducing losses?
A: Efficiency is defined as energy used for work divided by total energy consumed. It is a ratio often expressed as a percentage. Losses are wasted energy. Losses are energy you pay for and can’t use. So having a very efficient machine is great, but you can still run it in a way that produces too many losses. One of the best ways to minimize losses is by maximizing efficiency. But you can reduce losses without changing efficiency by implementing process controls like slowing down or switching off when not in use.

Q: What is the max power available under SIMOGEAR system?
A: Currently 40HP. We have other integrated drive systems that go to thousands of HP. See

Q: What advantage does the integral gearmotor provide vs. a direct coupled gearmotor?
A: Coupled systems have more parts (bearings, seals) than integral systems. Also, integral units have perfect alignment. Misalignment results in losses (proven by premature failures in misaligned systems). C-face gear units with C-face motors are the next best thing, because they maintain the alignment pretty well. Footed motors that are aligned in the field are not as good for efficiency.

Q: How does over-sizing a motor affect efficiency?
A: Motor efficiency is a rating value that relates to a particular place on the motor torque curve. It is typically at full load and rated voltage and speed. The efficiency of the motor drops significantly when you move away from that point. So, under loading drops the efficiency a lot. You have to look at the particular application and motor for full details. For the Siemens SIMOGEAR gear motors, our efficiency rating is pretty consistent even down to 75% load. Great question that I forgot to address in the presentation.

Q: What are the main advantages of using VFDs instead of fluid coupling?
A: It all depends on the application. The fluid coupling is mostly for over torque protection. It does also act as a soft starter somewhat. But it has losses associated all the time. A drive would give you more flexibility on the process control—speed, acceleration, deceleration—which would likely give better energy reduction.

Q: Is the extra cost of a 6-pole motor offset by energy savings because of reduced input shaft speed?
A: There are a lot of variables at work there. You have to look at the lifetime of the application and the torque/speed requirements. We can help you do that.

Q: If you use a 6 poles motor there will be a lower speed on the gear unit input shaft and most likely one last stage equals higher efficiency.
A: There are a lot of variables at work there. You have to look at the lifetime of the application and the torque/speed requirements. We can help you do that.

Q: You can only squeeze so much juice out of the fruit.  With the forthcoming efficiency legislation of 2016 on the horizon, do you feel motor manufactures going to be able to get another 1 or 2% efficiency out of a motor before we top out?
A: Yes. There is still room for improved motor efficiency as this is attainable through utilization of higher grade metals and/or added active material.  Of course, this practice always comes with added manufacturing costs.

Q: Up which power range, for example until 20 hp, could we have better efficiency with such IDS systems, or the gain would always remain in the same level?
A: The integrated approach allows you to take efficiency into consideration always and lets the supplier, preferably Siemens, help you do it.

Q: Any thoughts about keeping managers trained in energy management?
A: More CFE Media webinars! Q: What is the largest VSD Siemens has offered for a motor driven largest CO2 integral gear compressor.

A: Up to date largest drive we built for CO2 compressor is 21.1MW

Q: Do you know of a stable algorithm to calculate energy savings for VFDs on pumps with static pressure head? For example VFDs on city well pumps.
A: The use of VFD’s to save energy in a pump application with mostly static head is not very effective because there is no way to avoid consuming the energy required to raise the water over the head distance (as in a well pump). Only if there is pressure caused by the friction of the fluid flow through the pipes can you save energy at lower flow rates. If the head is a mixture, you have to separate the static and dynamic parts. Only the dynamic head represents the opportunity to save energy at lower flow rates.

Q: Is it true that most VSD’s impose torsional sub frequency on the strings. How to avoid it?
A: It is true that VFD’s cause the motors they are feeding to generate harmonic torques. The current distortion in the waveforms causes small torque pulsations at multiples of the fundamental output frequency. There are generally no torque pulsations at lower than fundamental frequency. However, in modern VFD’s these torque pulsations are small no more than a few percent of motor rated torque.

Q: Is it not true that the VSD Technology is still in its prime? They require diode replacements frequently.
A: It depends from expertise and experience of the VFD designers as well as maturity of the VFD technology applied. Siemens perfect Harmony Medium voltage drives utilize robust and conservative design practices, state of the art process and quality controls, as well as mature and therefore most popular technology on the market which going back to 1994. Frequent Diodes replacements are not required for this topology.                     

Q: Can you discuss larger (100+ hp) VFD efficiencies at lower loads (5 to 15 hz)?
A: It depends on product line and application. For perfect Harmony drive estimated overall VFD efficiency (all VFD components included) at 20% speed and 100% rated motor torque is approximately 93%. We will be happy to discuss further if are any additional questions or concerns.

Q: What types of VFDs are provided in IDSs? Are they strictly 6-pulse or are 12 and 18-pulse versions available? How is compliance with IEEE 519 obtained?
A: VFD topology within Siemens portfolio which best fits application will be offered. Typically we provide Perfect Harmony product which offers at least 18 pulse configurations and IEEE519 compliant without use of input filters. This is fully integrated general purpose drive which fits most typical VFD applications.

Q: What are some of the drive system comparables between DC and AC motors?  Replacing some older (large) DC motors creates access issues within a building.
A: Modern SIEMENS MV AC drives offer DC drives/motor train performance characteristics while taking advantage of lower cost and maintenance requirements as compares to DC drive/motors system.

Q: What are Bearing Currents due to induction type motors controlled by VFDs?
A: Siemens induction motors are fully compatible with Siemens MV drives offered in IDS configuration. As standard in Siemens IDS package motor is provided with insulated bearing on Non Drive End to eliminate motor bearing currents.

Q: What HP do the motors go up to with the integrated VFD’s?
A: 5HP. See

Q: We use a VFD with a brake resistor to stop a high inertia load. You mentioned eliminating the resistor and rather use the system to generate energy, how does this work? Are other components needed?
A: Drives with regenerating capability can be used instead of using braking resistor. Active front end converter is used on this drives which allows energy flow in both directions: from power source to the load (motoring) as well as from load to the power source (regenerating). 

Q: Is there any more information available on the AFE and Regenerative Energy solutions you mentioned earlier, like application notes, integration strategies, user guides, selection tools, product catalogs etc?
A: Yes. We will be happy to assist on case by case basis to address specifics of your application. 

Also see

Author Bio: Since its founding in 2010, CFE Media and Technology has provided engineers in manufacturing, commercial and industrial buildings, and manufacturing control systems with the knowledge they need to improve their operational efficiency. CFE delivers the right information at the right time around the world through a variety of platforms.