Your questions answered: AC Drives: Efficient Motor Controller for Optimal Energy Consumption

The Sept. 28, 2017, “AC Drives: Efficient Motor Controller for Optimal Energy Consumption” webcast presenter addressed questions not covered during the live event.

By Boima Morray, Danfoss Drives October 10, 2017

Motor-driven systems accounts for about half of the global electricity consumption, so having an efficient ac drive motor controller as part of the system will result in huge energy savings because the ac drive matches true energy demand to energy supply for optimal energy consumption. Viewers will learn the value of ac drives in a motor-driven system (MDS) with respect to energy savings and consumption, guidelines for selecting the right ac drive based on customer application needs, methods for mitigating harmonics in an MDS with ac drives to meet IEEE 519-2014: IEEE Recommended Practice and Requirements for Harmonic Control in Electric Power Systems, and utility company requirements. Electrical energy demand will continue to rise with population growth, and the best way for meeting the demand is to bridge the gap between energy supply and energy consumption by eliminating inefficiencies in MDS with the use of ac drives.

Presenter Boima Morray, vice president of Marketing, Danfoss Drives, Americas responded to questions not answered during the live AC Drives: Efficient Motor Controller for Optimal Energy Consumption on Sept. 28, 2017.

Question: Please describe the three motor controller types you mentioned: variable speed ac drive,

soft starter, and contactor.

Boima Morray: A variable speed drive is a type of motor controller that drives an electric motor by varying the frequency and voltage supplied to the electric motor. It also has the capacity to control the ramp-up and ramp-down of the motor during start and stop, respectively.

A soft starter is a solid-state device and type of motor controller that only provides a gentle ramp-up to full speed during startup of an electric motor.

An across-the-line contactor is a type of motor controller that applies the full line voltage to an electric motor. 

Q: How does a variable speed ac drive actually save energy or optimize energy consumption?

Morray: Power is the rate of doing work, which is the amount of energy transferred in a given timeframe. Because the energy requirement for the driven equipment (pump, fan, compressor, or any other mechanical movement) in a motor-driven system (MDS) typically varies over the operating period, supplying more than the power needed will result in wasted energy or inefficient use of energy. A variable speed ac drive controller has the ability to vary the power supplied to match the energy requirement of the driven equipment, and this is how it saves energy or optimizes energy consumption.

Q: Besides energy savings, why should a variable speed ac drive be the preferred controller of

choice in an MDS?

Morray: It’s a very good question. The ability of a variable speed ac drive to gradually ramp the motor up to speed will reduce peak current drawn during startup, which delivers two main benefits to users:

1.      Protection from peak-demand electricity prices that the utility charges to companies for exceeding preset limits.

2.      Protection of the motor equipment against premature damage due to exceedingly high peak currents exposure during startup.

In addition, a variable speed ac drive controller today integrates networking and diagnostic capabilities to better control performance and increase productivity. So, energy savings, intelligent motor control, and reduction of peak-current drawn are three great reasons to choose a variable speed ac drive as the controller in every MDS.

Q: What’s the typical payback period for having variable speed ac drive controller in an MDS?

Morray: Installing an ac drive as a motor controller in an MDS can increase the system efficiency by up to 30%. The average payback period varies but rarely exceeds 3 to 4 years, making such an investment a highly cost-effective measure with good ROI.

Q: How does pulse width modulation (PWM) vary voltage and frequency?

Morray: The PWM technique requires switching the ac drive’s inverter power devices—transistors or insulated-gate bipolar transistors (IGBTs)—on and off many times in order to generate the proper root-mean-square (RMS) voltage levels. Controlling and varying the width of the pulses is how PWM varies the output frequency and voltage.

Q: Do you have a certain line of drives targeted to the low speed high torque motors for applications like cranes or movable draw bridges?

Morray: Danfoss Drives VACON NXP product line is best suited for low-speed high-torque motors for applications such as cranes or movable draw bridges. You can learn more about our NXP drive at our website or download our free product portfolio app ”MyDrive Portofolio.” 

Q: What are the pros and cons of specifying 5% line reactors instead of 3% line reactors? How common is this, and what is the relative costs difference?

Morray: Typical ac line reactors are either 3% or 5%, with the latter offering better harmonic control and surge resistance but at the slightly higher cost. The incremental cost to go from 3% line reactor to 5% line reactor is in the range of 5% to 25%. A 3% line reactor is most commonly used.

Q: Is there a typical payback or ROI range for variable speed ac drives when used on HVAC equipment, such as chillers, pumps, or fans?

Morray: Based on reports and studies I have seen and my own experience, the payback period for variable speed ac drives used on HVAC equipment averages 18 to 24 months, but can be less depending on the type and size of the system and how much time the motor is operating, as well as how much flow is actually required to heat or cool the building space.

Q: Are techniques other than PWM used?

Morray: The three major variable frequency designs used today are PWM, current source inverter, and voltage source inverter. The PWM technique is the most common.

Q: For an ac drive to control the speed of an ac induction motor, what harmonic correction method is perfect for industrial facilities?

Morray: Generally, for industrial facilities, the measured value of total harmonic distortion at any point of common coupling (PCC) should not exceed 5%. Based on this, my recommended correction methods are active front end drive, low harmonic drive with active filter, and 18-pulse drive.

Q: Adjustable frequency drives (AFDs) are offered with or without bypass. What percentage of drives are provided with bypass?

Morray: The percentage of drives provided with bypass varies by vertical market segment. For HVAC and water/wastewater segments, the percentage is about 40% to 50%.

Q: Must ac drives reside in air-conditioned temperature-controlled environments or can they reside in ambient temperatures above 104°F?

Morray: Yes, but derating may/will be required where the total output of the drive will be limited at such high operating temperature environment.

Q: What are the upper and lower horsepower limits for variable frequency drives (VFDs)?

Morray: From fractional (0.5 hp) to about 1,000 hp for low-voltage VFDs.

Q: Can you have energy savings using a VFD/motor controller in a constant flow/constant speed application?

Morray: In such an application, there’s no need to vary the speed because the driven equipment (load) is required to run at a constant speed all the time, so the VFD may not be the most cost-effective controller to use.

Q: Does Danfoss have a guide for applying drives in retrofit HVAC applications?

Morray: Yes. Please contact Danfoss at 1-888-DANFOSS for support.

Q: Can you speak to the benefits of Danfoss’s drive offerings versus other manufacturers?

Morray: Yes.

·        Broad product portfolio: Two brands (VACON and VLT) with power ratings from 0.5 hp to 3,000 hp.

·        Application-customized drives: HVAC drives, water drives, lift drives, refrigeration drives, etc.

·        Drive modification flexibility: Ability to modify drives to meet customer-specific needs.

·        Industry expertise: many years of experience in the drives industry.

·        Price competitive drives.

Q: Is there a software/calculation sheet available from Danfoss to quickly calculate the energy/cost saving by using VFD? Link?

Morray: Yes. It is called Danfoss HCS, 

Q: Can you talk about the torque? What should we be looking for when specifying one? When to spec. variable versus constant torques?

Morray: The characteristics of the driven equipment (fan, pump, compressor, etc.) will determine whether to select a variable torque (110% overload) or constant torque (150% overload) drive. For example, a centrifugal pump is variable torque driven equipment and a compressor is constant torque driven equipment.

Q: Not all motors are listed as “inverter duty.” What is the specific insulation rating required for an existing motor to be used with a drive?

Morray: I will recommend checking out NEMA and IEC standards for motor insulation ratings and what are better suited for drives. Inverter-duty motors are relatively expensive, but can withstand higher voltage spikes and run at low speed without overheating.

Q: What is the life span of a AC Drive vs a Soft starter, or contactor?

Morray: A lot depends on the environment (ambient temperature variations, humidity, quality of the incoming power, operating utilization, etc.). In a perfect environment:

·        A VFD should have a lifespan of about 7 to 10 years.

·        A soft starter should have a lifespan of more than 10 years.

·        A contactor should have a lifespan of more than 10,000 operations.

Q: Can any ac induction motor be controlled by a VSD or should the motor be designed for that?

Morray: Any Induction motor (squirrel cage or wound rotor) can be controlled by VSD.

Q: Do you have to use drive-rated cable from the drive to the motor in all applications?

Morray: No, not always. However, VFD cables can experience high voltage spikes as a result of the high-frequency switching technology used in drives—the high voltage spikes can generate high levels of radiated/conducted noise as well as premature insulation failure for longer cable lengths. So, my recommendation is to use drive-rated cables for cases where the distance between the drive and motor requires long cables and when the drive is operating in an environment with sensitive electronic devices.

Q: What is open loop versus closed loop operation?

Morray: A closed-loop control system has a feedback loop (sensor and transmitter) between the controller and process. A good example of this is a thermostat controlling a furnace to maintain the temperature in a room.

Open-loop control systems do not use feedback. A control effort is applied with the assumption that the desired result will be achieved. The drawback is potential loss of accuracy. Without feedback, there is no guarantee that the control efforts applied to the process will actually have the desired effect.

Q: What is the frequency range required to operate the VFD in safe operation of the motor?

Morray: For a 60 Hz motor, as long as the power source to the VSD is 60 Hz, then the drive can safely operate the motor at frequency range of 0 to 60 Hz.

Q: What does THiD stand for in your Harmonics Mitigation Solutions chart? (THD is total harmonic distortion.)

Morray: THiD means “total harmonic current distortion.” Also, THvD means “total harmonic voltage distortion.”

Q: How does the variable speed drive monitor the frequency of the motor?

Morray: Variable speed ac drives can estimate motor rotational speed without an external measurement sensor on the motor shaft, and this is how it monitors the frequency of the motor.

Q: What distinguishes Danfoss as a premier league AC drive product?

Morray: Product leadership (breadth and depth of low-voltage and medium-voltage ac drives), application know-how (Danfoss has been in the ac drives business since 1968), and customer-centric operating model (Danfoss makes it easy to do business with customers). Our product portfolio consists of the VACON and VLT brands.

Q: Is solar VFD drive with MPPT is different from the normal VFD controller?

Morray: Normal VFDs have three major components: converter, dc link, and inverter sections. The converter section converts ac power to dc power.

Q: Does the 30% energy savings represented by using an Ac drive include the energy consumption of the ac drive itself?

Morray: No. The 30% energy savings is due to the fact that the motor is not always running at full (or 100%) load, the variable speed ac drive actually runs the motor at the load level required by the driven equipment at any given time.

Q: Are all VFDs for 3-phase motors or can they be used on single-phase motors?

Morray: Based on my experience, variable speed ac drives are for controlling 3-phase motors. I am not familiar with single-phase motor controllers.