Induction Motors – Mechanical Design and Installation Considerations

Motor manufacturers go to great lengths to design, build and test their motors to comply with applicable NEMA and IEC guidelines. In nearly all cases, customers can be assured that a motor provided by a name-brand manufacturer will meet their needs IF properly specified, installed, and maintained. Because applications vary so widely, sometimes that “IF” can be a big one. There are many design and installation considerations, mechanical as well as electrical, which must be clearly understood and adhered to for a successful application. Let’s examine some of the more typical mechanical considerations and how they might affect your motor application. By the way, much of which follows is referenced against NEMA guidelines; however, the same general concepts apply whether US domestic or international standards apply.

  • Ambient considerations: NEMA MG 1, American National Standard for Motors and Generators, specifies usual service conditions under which motors are designed to operate. These include an upper ambient temperature rating of 40C, above which adequate cooling is difficult to provide and the motor internal components are at greater risk of exceeding operating temperature design limits. This is important because it has been estimated that for every 10C above design […]
By |2020-05-07T10:55:32-05:00October 26th, 2012|0 Comments

Mitigating Harmonics Induced by Adjustable Speed Drives

The prevalence of adjustable speed drives and other non-linear loads in industry has heightened awareness of the effects of harmonic current distortion on utility capacities and energy costs. It is not uncommon for utility companies to demand that harmonic-mitigating strategies be applied to larger drive installations. Let’s examine drive-induced harmonics in more detail, and look at ways that the problems they might cause can be reduced or eliminated.

Harmonics arise when current and/or voltage waveforms deviate from sinusoidal. The design of the front end (rectifier) section of an AC adjustable speed drive incorporates rectifying devices (typically diodes) to convert AC to DC, which then links by bus to the output section of the drive, where it is converted to a pulse-width modulated sine-wave for supply to the load. The DC bus incorporates a capacitor to smooth out the DC signal by damping the ripple in the line resulting from the rectification process. This capacitor does not draw current continuously, but only when discharging to the output section and when the rectifier section is forward biased; i.e when the instantaneous AC voltage is higher than the DC bus voltage. Because the current draw is not continuous, […]

By |2020-05-07T10:55:32-05:00October 11th, 2012|0 Comments

Variable Frequency Drives and Motor Cabling – Part II

Output cabling from a variable frequency drive (VFD) can act as a significant source of electromagnetic emission resulting from the cables’ characteristics in response to the high-frequency voltage pulses they send to the motor. In effect, the cables serve as antennae. For drives using IGBT’s to generate the voltage pulses – the most common method used in drives today – the emission spectrum coming from the output section of an AC drive may extend to 50 MHz; it is typically strongest in the 100 kHz – 10 MHz range. Below this range, electromagnetic interference (EMI) issues are rare, but in or above it the EMI emitted and its effects on cables and connected equipment can have a negative impact on drive system controls and any adjacent, sensitive equipment. Let’s examine more closely the role that cabling plays, and ways to mitigate EMI problems through cable specification and installation practices.

At high frequencies, current will flow primarily in the outer surfaces of the conductors due to skin effects, which raises the effective impedance of the cables and increases voltage and the potential for noise. Also, “stray” or “leakage” currents can arise via capacitive coupling and, if […]

By |2020-05-07T10:55:33-05:00September 21st, 2012|0 Comments

Variable Frequency Drives and Motor Cabling – Part I

Because of the high-frequency switching characteristics of the outputs of modern variable frequency drives (VFDs), additional attention should be paid to the cables connecting a VFD to its motor. Modern pulse-width modulated (PWM) VFDs use sets of controlled transistors turning on and off at frequencies from 2 – 20 kHz to generate voltage pulses which, taken together, approximate the sine wave an AC motor requires. These transistors, typically Insulated Gate Bipolar Transistors (IGBTs), switch very rapidly, and are capable of reaching 90% of rated output voltage in less than 0.1 microsecond. This results in very steep wave fronts on voltage pulses sent down the cables at very high frequency. In turn, this places additional design demands on cable capacitance, impedance, electro-magnetic (EM) shielding, and length in order to ensure a high-quality, EM-compliant installation that is safe for the equipment and dose not create interference with other connected loads.  In Part I of this series, we are going to discuss cable capacitance and impedance characteristics and their impact on the VFD and motor.

All cables have a characteristic capacitance determined by insulation type and thickness and shielding material, and influenced by conductor configuration. At fundamental frequencies […]

By |2020-05-07T10:55:33-05:00September 6th, 2012|1 Comment

Adjustable Speed Drives and Current De-rating

Adjustable speed drives are rated by their manufacturers to conduct a specified amount of current under specified conditions. While each manufacturer sets their own specifications, ratings generally share these common characteristics:

  • Drive PWM carrier (switching) frequency: There is variation here among manufacturers, but rated current output is based on a set carrier frequency. If that frequency is adjusted upward, de-rating is required.
  • Maximum ambient temperature: typically 40C, with a 1% drop in rated current for each degree from 41C to 50C. Most manufacturers will not rate their drives for operation above 50C. Ambient temperature is of course related to installation method, and so sometimes these two factors are taken together to generate a single de-rating factor. That is the case with Yaskawa drives in the examples below.
  • Installation spacing: although there is more variation among manufacturers here than with other factors, to obtain full-rated current the drive will typically need to be installed with a clearance of 100mm above and below, to allow proper ventilation airflow (usually from bottom to top), and from 0 – 30mm side clearance. If the drive is self-contained, the type of enclosure will also factor into this.
  • Altitude: due to the decreased cooling […]
By |2020-05-07T10:55:34-05:00August 15th, 2012|0 Comments

Variable Speed Drives and Motor Braking

Inertial loads, and/or those in processes requiring rapid deceleration, present special challenges for variable speed drives (VSDs) and the motors they control. As power is removed or frequency reduced at the VSD output, a load with high inertia will prevent its motor from slowing down as quickly as it would under light- or no-load conditions. In such cases, the motor will change from its normal operation as a current “consumer” and become a current generator, wherein the motor shaft’s mechanical movement produces a negative current flow before the motor’s flux degrades. During this process, called regeneration, the power flow is back-fed into the DC bus of the VSD. The rectifier bridge upstream of the DC bus blocks the flow of this current, and unless the drive is properly equipped the DC link risks over-heating and damage.

So how does a VSD compensate? There are several techniques used depending on process requirements, capital cost, and other factors. Of these, two of the more common are brake choppers and the use of rectifiers with regenerative diode bridges. Brake choppers are essentially switches which close when the DC bus voltage reaches a defined limit and divert the back-flowing […]

By |2020-05-07T10:55:34-05:00August 2nd, 2012|0 Comments

NEMA Motor Speed/Torque Characteristics

NEMA MG 1 assigns code letters to AC induction motor designs to indicate relationships between speed and torque. These relationships reflect the torque capabilities of various motor designs from zero (locked rotor) to synchronous speed. Why is knowing the torque characteristics of a motor important? Well, the most obvious reason is to ensure that the motor’s rated torque can supply the force needed to drive the load. This must be considered at all operating speeds – at start-up, the motor’s starting torque (also called “locked rotor” or “breakaway” torque) must be sufficient to move the load in order to avoid stalling the motor; and while running, the load torque requirements must not exceed the motor’s breakdown torque (also known as maximum or “pull-out” torque) or else the motor will see a steep drop in speed and rapid current and temperature rise.

For 3-phase motors up to 500 hp, there are basically three classes of speed/torque designs: A & B; C; and D. Let’s examine each in more detail; you can find representative curves and data in the figures below:

  • A & B: these are similar in characteristics, differing principally in terms of […]
By |2020-05-07T10:55:34-05:00July 19th, 2012|0 Comments

Defining Your Variable Speed Drive Requirements – A Checklist

Here’s an essential truism – knowing what to ask for is key to getting what you want. What follows is a checklist, with comments as appropriate, intended to help you define your variable speed drive (VSD) needs and ensure you get exactly the drive your application requires.

Motor Data:

  • Full load amperage (from the motor nameplate)

This will typically be labelled as “F.L.A.” on AC induction motors; “ARM Amps” and “FLD Amps” on DC motors. Rated amperage, NOT horsepower, is used to properly size VSD’s, because the power electronics are sized to conduct a certain amount of current for a certain amount of time, which can vary for a given motor horsepower depending on torque required.

  • Rated phase, frequency and voltage

This information can again be found on the motor nameplate. Often AC motors are rated for dual frequency (50/60 Hz); if so, this information should be included on the nameplate. If you intend to operate such a motor above rated speed, take care to note any changes to Service Factor; the dual frequency rating usually means a lower Service Factor when supplied […]

By |2020-05-07T10:55:35-05:00July 11th, 2012|0 Comments
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