34 n MOTORS May 2023 www.drivesncontrols.com because the excessive heat can degrade the insulation. If you examine the voltage, current and torque levels when a motor starts, you can see the levels of stress it is experiencing. As a result, motors often fail during start-ups. When loads and power conditions are fluctuating – typically at start-up – rotors can experience tremendous stresses. Monitoring these conditions is vital to determining the motor’s ability to continue operating. Cracked or broken rotor bars can cause dramatic increases in thermal stress, affecting efficiency. Parameters that can help to deduce a motor’s health include its load, % load, horsepower demand, kilowatt usage, and power factor. If motors are forced to meet load demands above their capacity, their windings can run at unsafe temperatures, even if the current is at nameplate values. A motor running above its nameplate power rating can also suffer from higher torque demands, putting stress on the rotor. Motors are often oversized and sometimes undersized: both of these have cost implications. Oversized motors can result in higher initial costs, energy consumption and repair costs. Undersized motors perform poorly, suffer from higher losses, and can fail prematurely. Some of these problems can be detected and corrected quickly by using digital motor load monitors (see right). n Measure power, not current Digital load monitors measure the load on the motor electronically. They can replace mechanical forms of protection such as friction clutches, ball detent clutches, shear pins and tacho controllers. The motor acts as a sensor to detect the state of the equipment being driven. The main aim is to protect the machine from damage, but it can also protect the motor. In abnormal running conditions, load monitors can measure the load being drawn by a motor and compare it to normal values. The most common use for such monitors is to detect when a pump is running dry. Many pumps cannot be allowed to dry-run, especially if they are pumping hazardous chemicals. In the water industry, dry running and dead heading due to a shut discharge can also result in abnormal motor loads. Many monitors operate by measuring currents. But this can be a problem if the motor is oversized. In such cases, the motor will never reach its rated load because it is too big for the job and will operate where current is a flat line. Current cannot therefore be measured to assess variations in the work done by the motor. Phase angle detection used to be one way to get around this, but with advances in technology and the higher efficiency of newer motors, this technique is no longer a reliable way of measuring the work done by the motor. The only electrical measurement of work that is linear with and proportional to load is power. Power consumption to be used to measure torque indirectly. This measurement needs to be fast and accurate, reaction times must be short, and measurements must be valid for non-sineshaped currents – for example from frequency inverters generating high, short current peaks (with crest factors of up to 10). One manufacturer offering digital load monitors capable of calculating power in this way is the Danish company Unipower, whose monitors have been used in motordriven applications for more than 20 years. They are available in the UK from Unipower UK, trading as Charter Controls. A Swedish manufacturer of vinyl flooring tiles has boosted the efficiency of ten motors at its factory by more than 15% by replacing them with synchronous reluctance machines with 95% efficiency ratings – equivalent to IE5. Before making the upgrade, the company – called Tarkett – gathered data from its ten existing machines using ABB’s Ability Digital Powertrain Energy Appraisal service. This revealed that upgrading the motors to an IE5 synchronous reluctance technology would raise the efficiency of the installation from 80% to 95%, saving the flooring manufacturer 800MWh per year, or 1% of total annual energy consumption for the site in Ronneby. Based on these findings, Tarkett upgraded the motors to ABB SynRM machines. With current energy prices, the payback period is predicted to be 18 months or less. “The biggest lesson for us is that we’ve been maintaining motors that we should have exchanged years ago,” admits Ted Evaldsson, Tarkett’s supervisor of electrical maintenance. “We are really impressed by the savings obtained by just upgrading 10 of our motors to more efficient ones.” Tarkett is now planning to link further motors to the digital energy audit service with the aim of achieving even larger savings greater energy savings. By extending the coverage of its assets being monitored, the company will be better positioned to identify and prioritise energy-saving opportunities at a larger scale, while also cutting its CO2 emissions considerably. Tarkett, founded in 1880, has 34 industrial sites and 12,000 employees around the world, and sells 1.3 million m2 of flooring every day in more than 100 countries. n IE5 upgrade boosts motor efficiencies by 15% for flooring manufacturer Five of the ten IE5 synchronous reluctance motors that are saving Tarkett 800MWh a year
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