Difference From AC and DC Electric Motors

Difference from AC and DC Electric MotorsAC electric motors operate by applying alternating current (AC) power to the electric motor. The main parts of AC electric motors are the stator and rotor. AC electric motors stator consist of coils that are supplied with alternating current power and produce a rotating magnetic field. AC electric motors rotor will rotate inside the electric motor coils and the output shaft produces torque via the rotating magnetic field.

There are two different types of AC electric motors and each of them uses a different type of rotor. The first type of AC motor is called an induction motor. Induction motors use a magnetic field on the rotor of an induction motor that’s created by an induced current. The other type of AC motor is called a synchronous motor and rotates precisely at the supply frequency or on a sub-multiple of the supply frequency. Synchronous motors are able to operate with precision supply frequency because it doesn’t reply on an induced current. The magnetic field on a synchronous motor is generated by current delivered through slip rings or a permanent magnet. Synchronous motors run faster than induction motors because the speed is reduced by the slip of asynchronous motors.

DC Electric Motors

Difference from AC and DC Electric MotorsDC electric motors are powered from direct current (DC) power and are mechanically commutated. DC electric motors have a voltage induced rotating armature windings and non-rotating armature field frame coils that are a static field or permanent magnet. DC electric motors use different motor connections of the field and armature winding to produce variable speed and torque outputs.

DC electric motor speed can be controlled within the winding by changing the voltage sent to the motor armature or by adjusting field frame current. Most DC electric motors today are manufactured to be controlled with industrial electronic DC drives. DC electric motors are used in many applications across the globe such as paper producing machines, steel mill rolling machines and other applications that require variable speed and torque output.


Difference from AC and DC Electric MotorsDirect Current DC motors are usually seen in applications where the motor speed needs to be precision controlled. AC motors work best in applications when performance is needed for extended periods of time. DC motors are single phase and AC motors can be single phase or three phase. AC and DC motors use the same principle of using an armature winding and magnetic field except with DC motors, the armature rotates while the magnetic field doesn’t rotate. In AC motors the armature does not rotate and the magnetic field continuously rotates.

DC electric motors are commonly replaced by pairing an AC electric motor with an adjustable frequency speed drive (also known as variable frequency drives, VFD’s, ASD’s, etc). DC electric motors are replaced with an AC electric motor and an adjustable frequency drive as a more economical and less expensive solution to DC motors. DC electric motors have parts that are either obsolete or expensive, so usually it’s less expensive and more efficient to replace a DC motor with an AC motor paired with an AC speed drive.




Eaton SPI9000 VFD System Drives

A variable frequency drive (also known as a VFD) adjusts an electric motors speed to closely match output application requirements while also reduces energy savings of 10 to 50 percent. Soft start motor controls lower the demands on a motor during start-up, conserving energy and extending the life of the mechanical system. Whether the application calls for an ultra-compact solution, clean power, or future configurability, Eaton’s Cutler-Hammer drives answer Industrial, HVAC, Water/Wastewater Treatment and other application demands. Eaton offers VFD system drives for nearly every application.

SPI9000, SPA9000, SPN9000 Drives

Eaton SPI9000 VFD System DrivesEaton SPI9000 VFD system drives offer a comprehensive range of common direct current (DC) bus drive products. The front-end SPI9000 vfd units convert a mains alternating current (AC) voltage and current into a DC voltage and current. The power is transferred from the mains to a common DC bus (and, in certain cases, vice versa). The SPA9000 (active front-end) unit is a bi-directional (regenerative) power converter for the front end of a common DC bus driveline up. An external LCL filter is used at the input. This unit is suitable in applications where low mains harmonics are required. The SPN9000 (non-regenerative front-end) unit is a unidirectional (motoring) power converter for the front-end of a common DC bus drive line-up. The device operates as a diode bridge using diode/thyristor components. A dedicated external choke is used at the input. The unit has the capacity to charge a common DC bus. This unit is suitable as a rectifying device when a “normal” level of harmonics is accepted and no regeneration to the mains is required.

Features and Benefits

  • Comprehensive range available for almost every application
  • UL and cUL listed
  • Meets NEC, NEMA and IEEE standards
  • Alphanumeric keypad available for most drives
  • Air cooling
  • 460 V and 690 V, 1-1/2 to 2000 Hp

Eaton’s common DC bus product portfolio fulfills solution demands with a flexible architecture. Eaton SPI9000 VFD System Drives are built on Eaton SPX9000 technology which product family covers a number of front-end units and VFD units in the many power ranges.

To learn more about the Eaton SPI9000 System Drives or to download technical information, visit the Eaton Website.

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ABB DCS800-EP Drive Updates

ABB DCS800-EP Drive UpdatesABB is pleased to announce that the DCS800-EP now includes the easy-to-useQuickStart Assistant and the introduction of the ABB DCS800-EP User Manual andwall chart.The ABB DCS800-EP Panel Drive is a DCS800 power module and associated systemcomponents mounted and wired on a sub-panel, ready to be installed into anindustrial enclosure. Standard components include AC input fuses, DC outputfuses (regen), control transformer, AC contactor, plus a variety of optionalcomponents.

QUICKSTART ASSISTANT: The QuickStart Assistant has been added to the control panelsoftware as a new way to commission the drive. It mimics the QuickStart Assistant used inthe Reliance FlexPak 3000 drive and makes commissioning easier by shortening theprocess for the 80% of users who have simple, straightforward applications.

ABB DCS800-EP USER MANUAL: The brand new 120-page User Manual targets these sameusers who have simple, straightforward applications.

The DCS800-EP User Manual:

  • combines the best of the DCS800 Hardware and Firmware Manuals into one shorter
  • document,supports the QuickStart Assistant and the commissioning process in general
  • defines the twelve application macros,defines the most commonly used parameters and signals in the compact set, andclearly explains faults and alarms and the related parameters and signals

In light of the lead time reduction, DCS800-EP drives will be built-to-order only.Stock drives will be discontinued.

Due to more stringent requirements in the EU for the power usage offans, some of the DCS800 drives are transitioning to a design with fansthat use less energy with no reduction in airflow. The new designs will be
rolled out gradually over the next few quarters.

The ABB DCS800 D5 Frame, 500 Volt drives: DCS800-S01-1200-05 throughDCS800-S02-2000-05 become DCS800-S01-1200-05B throughDCS800-S02-2000-05B. These are the first to be transitioned as ofthis release.

Frame D6 & D7, 600 & 700 V drives: To be transitioned at a laterdate. The ABBDCS800-EP Panel Drives are NOT affected. Frames D1 D4 drives arealso NOT affected.

CHANGES TO THE D5 DRIVE: The fan that was used on the D5 drivewas a squirrel-cage blower that extended 2.5 inches below the DC busbars. The fan on the new drive is a box fan that is much thinner. Thisreduces the overall height of the D5 drive from 41.4 inches to 37.4 inches.Both the new fan and old fan are electrically equivalent, requiring 230 Vsingle-phase power.

To learn more about product changes or to download ABB DCS800 literature, please visit the ABB Website.


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ABB Frequency Converters

ABB Frequency Converters are used to change the frequency and magnitude of the constant grid voltage to a variable load voltage. Frequency converters are especially used in variable frequency AC motor drives.

Figure 1 shows the behavior of an induction motor with several motor input voltages. The bold blue curve represents the electrical torque as a function of rotor speed when the motor is connected directly to a constant supply network. The blue portion of the torque curve shows the nominal load region (-1+1 [T/TN]), which is very steep, resulting in low slip and power losses. Similar motor torque behavior with other motor input frequencies can be achieved by feeding the induction motor with a frequency converter and keeping the ratio of the magnitude and frequency of the motor voltage constant. As a result, the shape of the torque curve remains unchanged below the nominal speed (constant-flux region -1+1 [n/nN]). In the field weakening region the motor voltage is at its maximum and kept constant, resulting in the torque curves being flattened.

ABB Frequency Converters Figure 1Fig. 1. Operation principle of the frequency converter fed induction motor.

ABB Frequency converters can be classified according to their DC circuit structure to voltage-source (Fig. 4), current-source (Fig. 3) and direct converters (Fig. 2). With a voltage-source converter the variable frequency and magnitude output voltage is produced by pulse-width modulating (PWM) the fixed DC voltage, whereas with a current-source converter the output voltage is produced by modulating the fixed DC current. With a direct frequency converter the variable output voltage is formed directly by modulating the constant input voltage. At low voltage applications (<1000 V) the voltage-source topology is mainly used.

ABB Frequency Converters Figure 2Fig. 2. Main circuit of the direct converter

ABB Frequency Converters Figure 3Fig. 3 Main circuit of the current-source converter

Fig. 4 shows a typical voltage source frequency converter structure where a constant DC voltage is formed by using an input diode rectifier. The output voltage with variable frequency and magnitude is produced by pulse-width modulating the inverter bridge. In more sophisticated frequency converters the input diode bridge can be replaced with a PWM bridge enabling a higher DC voltage, reactive power control, nearly sinusoidal supply network currents and regenerative operation of the inverter.

ABB Frequency Converters Figure 4Fig. 4. Main circuit of the commonly used voltage-source frequency converter.

Fig. 5a shows the operation of the inverter bridge with two different output voltages. The desired output voltage is achieved by changing the width and polarity of the output voltage pulses. The higher the instantaneous value of the output voltage, the wider the output voltage pulse needed. Although the output voltage contains a lot of high order harmonic voltages due to the PWM, the motor current is nearly
sinusoidal since the motor inductances filter out the high order current harmonics.

Fig. 5b shows the construction of negative voltage pulses, circled in Fig.5, during one PWM period. The graphic on the right shows the switching states of the phases and the resulting U-V voltage. The graphic on the left shows the space vector presentation of the eight switching states of the voltage-source converter. These switching vectors are commonly used to achieve PWM (vector modulation). The + and - signs mean that the phase is connected to the positive or negative rail of the DC link respectively.

ABB Frequency Converters Figure 5Fig. 5a and 5b Operation of the voltage-source inverter with two different output voltage references and the principle of pulse width modulation.

To learn more about ABB Frequency Converters or to download technical information, visit the ABB Website.


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