Almost all industrial applications require using electric motors in some form. You can see them being used in factory robotics, compressors, blowers, cooling and recirculating pumps, lifts, hoists, mixers, cranes, paper mills, printing presses, conveyor belts, fans and in many other applications. Worldwide, over a 300 million electric motors are in use, and their numbers are growing steadily every year.
When dealing with adjustable-speed electric power-drive systems, it is necessary to isolate the low-voltage control system from the actual motor as it most often runs on a higher voltage. Such smart motor-control systems have other names also, such as AC motor drives or variable-frequency drives. Instead of running the motor at a fixed speed or using mechanical elements to control it, smart control systems employ sophisticated power electronics to control the speed, torque, and position of a motor. Adjustable speed drives improve the efficiency and controlling of motor drive systems substantially, and therefore, are widely used in motor drive applications.
The IEC 61800-5-1 is a safety standard specified by the International Electrotechnical Commission for adjustable-speed electrical power drive systems. It covers the safety aspects related to electrical, thermal and energy. In the part covering electrical safety, the standard defines requirements for ensuring proper insulation between circuits carrying voltages higher than 50 V and any drive system connectors or parts that humans may be able to access. What this means is any part of the system that a person can touch – motor, panel, switch, connector, cable, etc. – must be adequately insulated, if it is carrying a voltage higher than 50 V.
Most adjustable-speed electrical power drives use isolators as one of their key electronic components. For instance, by employing isolated gate drives, isolators control the turning On/Off the power transistors such as MOSFETs or metal-oxide semiconductor field-effect transistors used in the power stages. In addition, there are isolated ADCs or analog-to-digital converters and isolated amplifiers to convey voltage and current feedback from the high-voltage inverter output to the low-voltage control system. Moreover, power drives also need general-purpose communication links, where isolators transfer information from high-voltage circuits to earthed circuits. In this capacity, isolators also act as insulators.
An adjustable speed motor drive system has a grid input, which is typically a three-phase AC power supply typically at 400 V, 690 V, or 830 V at frequencies of 50 or 60 Hz. This is followed by a rectifier stage that converts the AC voltage into DC, filtered by high-voltage DC capacitors. A three-phase inverter usually follows, made up of IGBT, insulated gate bipolar transistor modules. IGBTs have isolated gates through which gate drivers provide the necessary drive voltages to turn the IGBT on and off. The control system uses a closed-loop and receives feedbacks through isolated voltage and current sense elements.
To conform to the IEC 61800-5-1 safety standard, the designer of a motor drive system needs to understand a few definitions such as creepage, clearance, system voltage, working voltage, and overvoltage category. Most industrial motor drives fall under Category-III, as equipment is connected permanently to supply mains, downstream of the distribution board.
