When a motor’s rotor speed is greater than its synchronous, or designated, speed, it can act as a generator, creating electrical energy from mechanical energy. This electrical energy needs somewhere to go, and there are two ways to deal with it — dissipate it as heat, or reuse it. Dynamic braking takes the energy generated […]
AC Motors
What are bearing currents and what causes them?
Pulse-width modulation is a fundamental operating principle of variable frequency drives (VFDs), but the high-frequency switching that delivers pulses of voltage from the drive to the motor can cause bearing currents — high-frequency currents that flow through the motor bearings, often leading to damage and premature failure. To achieve pulse-width modulation, insulated gate bipolar transistors (IGBTs) […]
What is cable ringing between AC drives and motors and how can it be avoided?
Variable frequency drives use a technique referred to as pulse-width modulation (PWM) to provide voltage of varying frequency to an AC induction motor. The inverter, or output, stage of the VFD uses insulated gate bipolar transistors (IGBTs) that switch on-and-off rapidly, creating trapezoidal pulses of varying width to simulate AC sinusoidal voltage. But the windings in […]
What are drive isolation transformers and how do they differ from line reactors?
A variable frequency drive alters the speed of an AC induction motor by rectifying incoming AC power to DC, storing the DC power in a bus, and then converting the DC power back to AC at the voltage and frequency required to meet the load demands. Although this matching of the motor speed to the […]
Nippon Pulse introduces new high-resolution, high-torque miniature AC servomotors
Nippon Pulse America, Inc. is pleased to introduce its new series of high-performance micro direct-drive (MDD) AC servomotors. The new MDD motors will join Nippon Pulse’s existing line of Linear Shaft Motor servomotors. Each compact MDD motor comes with a built-in high-performance encoder (either absolute or incremental) that enables direct fine positioning. The motors are […]
What are power factor correction capacitors?
The measure of how efficiently a piece of equipment or facility uses power to produce work is known as its power factor. Although resistive loads, such as heating and lighting, have a high power factor, inductive loads, including AC induction motors, don’t use all of their supplied power to produce useful work, so their power […]
What is flux braking and how does it differ from regenerative braking?
AC induction motors are used to drive loads such as fans, pumps, lifts, and conveyors. But in some applications, the motor is also used as a brake to stop the load, reverse its direction, or hold the load and prevent it from moving. And while there are several methods of braking for AC motors, if […]
ABB’s new AC motors offer power-dense, water cooled design
ABB’s AXW 5000 and 5800 motors offer more horsepower per pound than conventional totally enclosed fan cooled (TEFC) motors but with a smaller frame size. This helps save space, up to 8 in. in overall length in some cases, which enables more compact installations. Advantages of AXW motors include higher power densities, smaller overall footprint […]
What is direct torque control (DTC) for AC induction motors?
Variable frequency drives control AC induction motors through one of several control schemes. Scalar control (also referred to as V/Hz or V/f control) varies both the voltage and frequency of power supplied to the motor to maintain a fixed ratio between the two. This keeps the strength of the magnetic field at a constant level […]
What is a DC bus choke and why is it used?
In a recent post, we looked at motor chokes — inductive devices installed between a drive and motor — and why they’re used in servo systems and variable frequency drives. But in variable frequency drives (VFDs), inductors can also be placed after the drive’s input diodes, between the input rectifier and the DC bus link. […]
