Your questions answered: Motors and drives: Advantages of using adjustable speed drives

Advantages of using adjustable speed drives vary, and different applications may require different options.

By John Malinowski and Tim Albers March 2, 2023
Courtesy: Plant Engineering

VFD insights

  • This Q&A helps readers learn the energy savings and process control available with adjustable speed drives
  • After watching the webcast and reviewing these questions, readers will learn when to specify inverter ready or inverter duty motors.
  • It is important to understand what advantages a synchronous motor provides over an induction motor.

What are the advantages of using adjustable speed drives? The combination of a motor and adjustable speed drive (ASD) is called a power drive system (PDS). Specifying adjustable speed drives can help a manufacturing process become more energy efficient and productive. On variable torque applications (pumps and fans), one can expect energy savings of 20 to 40% with a PDS when compared to an application using a valve or damper for flow control.

During this live webcast entitled “Motors and drives: Advantages of using adjustable speed drives,” on Feb. 16, 2023, several questions were left unanswered. The presenters take on several audience questions here.


  • John Malinowski, IEEE Fellow, NEMA MG1 Past Chairman, JMAL Consulting, Fort Smith, Arkansas
  • Tim Albers, IEEE Senior Member, NEMA MG1 Technical Committee Chairman, Chair of the Associate Member Council, Hydraulic Institute, St. Louis

What hp range of synchronous motors are available in the North American market?

John Malinowski: 0.5 to 500+.

Name two factors that are important to optimize peak efficiency on a motor with a variable frequency drive (VFD).

Tim Albers: Two factors:

  • Finding the best efficiency point (BEP) of the pump or fan.

  • Providing system feedback (temperature, pressure, flow, level) to the VFD to maintain.

Is a microdrive and three-phase motor the same as an electronically commutated motor (ECM)?

John Malinowski: A Microdrive is usually a pulse width modulation inverter paired with a three-phase induction motor where an ECM is a synchronous motor.

Does VFD come with integral or external fused disconnect for short circuit/fault current protection? Is it required by code?

Tim Albers: Most locations code will require a fused disconnect for a VFD. Some VFDs (smaller and then very large panel built) do have an internal fused disconnect, most do not.

Is there any established practice of running the VFD at a frequency higher than 60 Hz to gain some additional process control and/or more horsepower out of a motor?

Tim Albers: I would say that there is not an established practice. It is pretty common to run a motor over base speed to increase output and increase the available horsepower. The tricky or most important item is that to increase the hp above 60 hz, the voltage must be set up to peak at the higher frequency and not at 60 hz. So, if, for example, a pump is going to be run at 75 hz instead of at 60, then the 460 volts would have to occur at 75 hz. So, the voltage at 60 hz is going to be the ratio below 75 hz. The 60 hz volts will be 368 and the 75 hz volts is then 460. The motor must be designed for this or it will not work. If a motor is a 230/460-volt motor, then (for example) a 4-pole motor could be connected at 230 volt and then run up to 120 hz at 460 volts. The motor then would be a 3,600-rpm motor that would double in horsepower.

Power drive systems (PDS), electronically commutated motors (ECM), adjustable speed drives (ASD) and variable frequency drives (VFD) were all highlighted in this webcast. Courtesy: Plant Engineering

Power drive systems (PDS), electronically commutated motors (ECM), adjustable speed drives (ASD) and variable frequency drives (VFD) were all highlighted in this webcast. Courtesy: Plant Engineering

What is a “KPI”?

Tim Albers: Key performance indicator (what do you want to measure?).

What about soft start?

John Malinowski: A soft start is a type of control that ramps up the voltage to a motor when starting to reduce the inrush current and starting torque. On many applications it is bypassed once the motor reaches speed so there are no additional losses.

How does the cost of cooling, HVAC equipment, maintenance and power consumption of cooling equipment play into this?

Tim Albers: In many cases, no extra cooling is required. A VFD will have about 3% total losses, so that heat does need to be taken into account, but the VFD does not have to be in a cool location. Most VFDs are rated for 40C so they can be in nonconditioned air. If they are placed in the conditioned air, then those 3% of losses in watts must be taken into account

How is energy efficiency related to motor size?

John Malinowski: Lower horsepower NEMA Premium motors have nominal efficiencies starting around 82% and the larger ones are 96.2%.

What are the largest variable speed drive? I have a scope in an RFP that indicates it includes a 20,000 hp synchronous compressor drive motor with variable speed control.

John Malinowski: Medium voltage drives are available to 100 MW from several manufacturers. Larger sizes may be available on a custom basis.

Could manufacturer supply harmonics spectrum, as many applications request harmonics mitigation?

Tim Albers: This is difficult as many items come into play on harmonics. How large is the power supply system? What is the cabling like? Are there any magnetics added such as a line reactor? For the cost of evaluating the harmonics, in most cases, a line reactor will fix most issues and not cost that much. I strongly recommend talking to a VFD rep, distributor or manufacturer about your specifics.

Grounding rings need to be installed to protect bearing?

John Malinowski: Shaft grounding systems are helpful in installations where there is poor grounding or where common mode voltage is present.

What are some considerations that need to be incorporated in an electrical power design if the distance from the VFD to the motor exceeds 100 feet or more?

John Malinowski: Where long cable runs are present between the drive and motor, line reactors are often used in the system. Check with the drive manufacturer for recommendations.

How come a system, such as a fan, cannot have both damper and VFD? The damper changes the duct system curve, while the VFD varies the fan curve, until they both meet at a “mutual” operating point? What is wrong with that to make a system work?

Tim Albers: You absolutely can do this, but the VFD on its own can do 99% of the work and save a significant amount of energy. Having both in the system will require a fairly complex control system to make sure they two work together to save the most energy. Again, it is possible but a VFD alone is much easier to program and control.

Ground currents through the bearings?

John Malinowski: Shaft grounding systems are helpful in installations where there is poor grounding or where common mode voltage is present.

In my experience, a drive working under a 60% to 50% nominal horsepower, efficiency and harmonics become much worse than nominal values? Could such values could be supplied by manufacturers?

Tim Albers: Harmonics are really an individual installation discussion. I agree that lightly loaded systems, without any mitigation, can have harmonic issues. I think this should be a discussion with the suppliers of the VFD. The installation is a part of this so the VFD alone will not have a harmonic that you can just apply.

Are there guidelines/estimates for the increase in motor losses for large low-voltage motors (i.e., 300 hp) driven by standard VFDs?

Tim Albers: IEC Standard 60034-31 provides calculations that are used for estimating the incremental losses of a motor while running on a VFD. For larger motors it is an incremental 25% of losses over the base motor losses. For 1-125 hp it is 15%. So, if a 300 hp is 95% efficient, then the remaining losses are 5%*0.25% is 0.0125%. So, the new motor efficiency is 93.75% instead of 95%. On smaller VFDs it is 15%. Those values are now incorporated into a number of Department of Energy efficiency rules and the European efficiency rules as well, so they are well accepted.

When do we need to spec line and load side reactors? What do they do for us?

Tim Albers: Line reactors protect the plant/location electrical from harmonics the VFD sends to the system, load reactors (I strongly prefer load filters over reactors) lower the spikes going down the line to the motor from the VFD. A filter versus a reactor on the load side of the VFD also will have less losses in efficiency which is another reason I prefer them.

When do we specify shaft grounding rings?

John Malinowski: Shaft grounding systems are helpful in installations where there is poor grounding or where common mode voltage is present.

When do we specify ceramic bearings in the motors?

John Malinowski: Ceramic bearings are an alternative to shaft grounding systems. They are more expensive than standard stainless bearings and may be difficult to get in larger sizes.

When do specify both shaft grounding rings and ceramic bearings?

John Malinowski: Such a system would be like having a belt and suspenders, a backup system.

Tim Albers: On larger hp motors such as 100 hp and up, circulating currents become an issue. An insulated bearing of some type (could be ceramic or a coating on the shaft or in the end shield) will stop circulating currents which may not have anything to do with the VFD. The grounding ring then still serves the purpose of taking away the shaft voltage.

Examples of when to use a vector flux drive?

John Malinowski: A flux vector drive controls current to the motor better than a simple volt/hertz inverter. Vector drives are most often used for constant torque applications and accurate speed operation down to zero speed. Adding feedback with an encoder will allow the vector drive to do positioning like a servo.

Can a capacitor-start, single-phase motor be run on a drive?

John Malinowski: No, but a permanent split capacitor motor is sometimes used with a triac control for fan applications.

What class of motor insulation will result in an acceptable operation with a drive?

John Malinowski: Most industrial motors are manufactured with a Class F insulation system and operate at Class B rive online power. This gives them the margin for additional losses when operated with a drive.

Inverter duty rated versus normal motor?

John Malinowski: General purpose motors are usually capable of operation with an inverter and often termed as “inverter-capable.” CSA Group makes manufacturers mark these as inverter duty motors and show the constant and variable torque speed ranges on the nameplate. These motors are identified in NEMA Part 30.

Inverter only motors have different commercial names from each manufacturer. These motors are designed for use with inverters or vector drives only and not designed for line start. Such motors are meant for wide constant torque speed use.

Synchronous motors have low starting torque. I’ve witnessed problems starting a 600 hp pump with a synchronous motor. How is this problem addressed?

Tim Albers: Many synchronous do have lower staring torque. A few ways to address are designing the motor and system for the torque. Sound simple, but important. Some synchronous are designed with more robust start systems. Also, for larger hp synchronous, an induction pony motor is also a good idea. It engages at start and during accel and then drops off closer to full load speed. It is a fairly inexpensive way to fix a big problem (used it before and it still working over 20 years later).

Do permanent magnet (PM) motors rely on rare earth magnets? Will they become difficult to obtain?

John Malinowski: PM synchronous motors with high power densities like servo motors use rare earth magnets, most other PM synchronous motors use ferrite magnets like PM DC brush type motors.

The government has identified rare earths are a critical material and are working to develop domestic sources.

Does soft start qualify as a variable torque application?

John Malinowski: A soft start only gives reduced torque and inrush current and not variable speed.

What are the biggest challenges you see in VFD installs? Too long cables? Are shunt trips required if a remote disconnect is installed on the load side?

Tim Albers: Shunt trips should not be required. Proper fusing is. Correct grounding from the motor back to the VFD and the cabling required for a good installation are a couple of challenges.

We’ve seen some instances of instability in VFD speed control due to high electromagnetic interference/radio frequency on the encoder signals. What are your recommendations to mitigate encoder noise issues?

Tim Albers: There are encoders with more isolation. Also, be very sure the encoder feedback signals and cabling are well isolated from the motor cables. The motor cables will overwhelm the control cables for sure. Cannot get commercial, but I do know of some encoders with high isolation for higher electromagnetic frequency locations.

Is there a 120 V 1 hp catalogue I’m looking at fractional horsepower range under 1 hp for a closed loop heating system instead of closing valves to allow heat gain in the boiler.

John Malinowski: Some manufacturers have single phase ECMs in their product lines.

What is the recommended high frequency grounding method for the motors?

Tim Albers: This comes down to the connection. A cable with an external shielding that is designed to capture and transmit the high frequency currents. From motor to ground a strap instead of a cable is better. This is a potential long answer so I can also say to recommend talking to a VFD manufacturer.

How to determine if a blower fan can run direct-online using contactor-overload combo or a motor soft-starter is required?

John Malinowski: Drive bypass systems are sometimes used to eliminate drive losses if the motor will be operated only at full speed.

What method should be used to ground the motor for high-frequency grounding? Assuming that the standard electrical grounding conductor is already connected to the motor electrical junction box.

Tim Albers: Standard grounds will not capture and transmit high frequency currents. Specific cables that are designed with an external braided jacket are the best. That is what would be used to ground the motor back to the VFD. From Motor to earth ground, a strap is good as it captures the high frequency as well.

Do you have to do something different if motor is a distance for drive?

John Malinowski: Where long cable runs are present between the drive and motor, line reactors are often used in the system. Check with the drive manufacturer for recommendations.

Author Bio: John Malinowski, IEEE Fellow, NEMA MG1 Past Chairman, JMAL Consulting; and Tim Albers, IEEE Senior Member, NEMA MG1 Technical Committee Chairman, Chair of the Associate Member Council, Hydraulic Institute