AC drives: Your questions answered

An AC drive is a type of motor controller that drives an electric motor by varying the frequency and voltage supplied to the electric motor. It also has the capacity to control the ramp-up and ramp-down of the motor during start and stop, respectively.

A soft starter is a solid-state device and type of motor controller that only provides a gentle ramp-up to full speed during the startup of an electric motor.

An across-the-line contactor is a type of motor controller that applies the full line voltage to an electric motor.

Power is the rate of doing work, which is the amount of energy transferred in a given timeframe. Because the energy requirement for the driven equipment (pump, fan, compressor or any other mechanical movement) in an MDS typically varies over the operating period, supplying more than the power needed will result in wasted energy or inefficient use of energy. An AC drive has the ability to vary the power supplied to match the energy requirement of the driven equipment, and this is how it saves energy or optimizes energy consumption.

The ability of an AC drive to gradually ramp the motor up to speed will reduce peak current drawn during startup, which delivers two main benefits to users:

1. Protection from peak-demand electricity prices that the utility charges to companies for exceeding preset limits.
2. Protection of the motor equipment against premature damage due to exceedingly high peak currents exposure during startup.

In addition, today’s AC drives integrate networking and diagnostic capabilities to better control performance and increase productivity. So, energy savings, intelligent motor control and reduction of peak-current drawn are three great reasons to choose an AC drive as the controller in every MDS.

Installing an AC drive as a motor controller in an MDS can increase the system efficiency by up to 30%. The average payback period varies but rarely exceeds 3 to 4 years, making such an investment a highly cost-effective measure with a good ROI.

The PWM technique requires switching the AC drive’s inverter power devices – transistors or insulated-gate bipolar transistors (IGBTs) – on and off many times in order to generate the proper root-mean-square (RMS) voltage levels. Controlling and varying the width of the pulses is how PWM varies the output frequency and voltage.

The VACON® NXP product line is best suited for low-speed high-torque motors for applications such as cranes or movable draw bridges.

Typical AC line reactors are either 3% or 5%, with the latter offering better harmonic control and surge resistance but at a slightly higher cost. The incremental cost to go from a 3% line reactor to a 5% line reactor is in the range of 5% to 25%. A 3% line reactor is most commonly used.

Based on reports and studies I have seen and my own experience, the payback period for AC drives used on HVAC equipment is 18 to 24 months on average but can be less depending on the type and size of the system and how much time the motor is operating, as well as how much flow is actually required to heat or cool the building space.

The three major variable frequency designs used today are PWM, current source inverter and voltage source inverter. The PWM technique is the most common.

Generally, for industrial facilities, the measured value of total harmonic distortion at any point of common coupling (PCC) should not exceed 5%. Based on this, my recommended correction methods are an active front-end drive, a low harmonic drive with an active filter and an 18-pulse drive.