Your questions answered: What is inverter duty anyway?
Reece Robinson, senior technical training specialist at Grundfos Pumps Corp. in Olathe, Kan., tackled unanswered questions from the May 24, 2016, webcast on inverters and inverter-duty motors.
Q: Please clarify the difference between 6-, 12-, and 18-pulse inverters. Also, please describe difference between inverter duty and NEMA Premium motors.
Reece Robinson: A 6-pulse inverter has 6 rectifiers and an 18-pulse inverter has 18 rectifiers. The main reason the 12- and 18-pulse inverters were developed was to mitigate harmonics. The 12- and 18-pulse inverters are larger and more expensive than standard 6-pulse inverters and usually are not a stocked/mass-produced drive. It is generally more cost-effective to use harmonic filters in conjunction with 6-pulse inverters, which are readily available.
Q: For a secondary pump with a differential pressure (DP) sensor at the pump header and using software to adjust speed, what other sensors are needed versus a more traditional DP sensor located at the farthest coil?
Robinson: Some pump controls have pump curves loaded into the controls, therefore no additional sensors are required to achieve the effect of a remote-mounted sensor with a header-mounted sensor. If pump curves are not loaded into the controls, a flow sensor can be used and the DP setpoint can be automatically adjusted based on the measured flow.
Q: What happens if we use an inverter duty motor without an inverter? Does it provide better service/performance?
Robinson: Definite-purpose, inverter-fed poly-phase motors are not designed for across-the-line starting. Therefore, they must be used with inverters. Successive across-the-line starts can damage the motor.
Q: What happens when motors are used above 3,000 ft (elevation)?
Robinson: If motors and drives are to be operated above 3,000 ft, the effect of proper cooling should be evaluated. Because the air is thinner at elevations above 3,000 ft, sufficient cooling may present a challenge on motors and drives that are operating near peak performance. Most motor and drive manufacturers have some kind of derating chart or table for operation above 3,000 ft.
Q: Why is 480 V used instead of 460 V for the rating of the motor in your voltage stress slide? Motors are rated at 460 V, not 480 V.
Robinson: It is more conservative to look at 480 V as opposed to 460 V. Most utilities supplying power in that range are supplying closer to 480 V than 460 V.
Q: If one specifies a motor to the energy code, will Part 31 meet that code?
Robinson: NEMA MG-1, Part 31 covers definite purpose motors, which are outside the scope of the energy code.
Q: Is there a difference in the winding gauge between a general and an inverter duty motor?
Robinson: From a practical standpoint, most motor manufacturers use only one size wire. Most, if not all, use a type of wire that’s spike-resistant enough to meet the peak voltage requirements of NEMA MG-1: 18.104.22.168.
Q: For inverter-rated motors, is inverter-rated defined on the motor nameplate?
Robinson: Most drives have an electronic thermal relay, the operation of which is based on a calculation. Certain situations will result in overhead detection. The drive essentially is the motor’s overload protection. Most drives have positive temperature coefficient (PTC) thermistor inputs so you can have thermal protection beyond what the drive supplies. The most common reason motors hit excessive temperatures is overloading, which most drives will detect.
Q: What will be the consequences if we operate a 50 Hz motor at 60 Hz when using a variable frequency drive (VFD)?
Robinson: If a motor’s maximum output (in horsepower or kW) is based on the rotational speed corresponding to 50-Hz operation, running that motor at 60 Hz can overload the motor. Power goes up by the cube of the speed, so a motor rated for 15 hp at 50 Hz would be at 20 hp at 60 Hz.
Q: True or false: A general-purpose motor driven by a VFD tends to overheat, compared to inverter-duty motors.
Robinson: For the most part this is false. A properly sized motor will not overheat when being driven by a VFD.
Q: What is considered a long cable run?
Robinson: Generally speaking, a "long" cable run would be in the hundreds of feet. When working with an application with long cable runs, consult the drive manufacturer for guidance.
Q: I have a consultant that runs 1,800 rpm, 60-Hz motors on 50-Hz power overseas. What are the pros and cons of this?
Robinson: Many motors are dual rated (50/60 Hz). If not on the nameplate, many motors designed for 60 Hz successfully can operate at 50 Hz if the motor was sized for 50-Hz operation. The output horsepower (or kW) of the motor will be less at 50 Hz, so if this is taken into consideration there should be no issues.
Q: We typically call for high winding-temperature sensors with totally-enclosed fan-cooled (TEFC) motors running on VFDs. As the speed goes down, the fan will slow down too. What is your opinion on the temperature sensor requirement?
Robinson: For centrifugal pumps with variable-torque loads, the cooling needed at slow speeds is substantially less than at full speed (as seen from the torque comparison earlier in the presentation). The electronic thermal protection provided in most drives normally will protect motors from problems associated with overheating.
Q: In general, for an application that could use an inverter-duty versus a general-duty motor, what is an approximate cost difference of the motors?
Robinson: A definite-purpose inverter-duty poly-phase motor will cost an additional 75% to 100% more than a general-purpose motor.