Variable speed drives bring great advantages in controlling motors but care needs to be taken to match the characteristics of the drive to the motor to ensure the combination is a winning one. Geoff Brown, drive applications consultant for ABB investigates
Of the approximately 10m motors installed in UK industry, only some 3% are controlled by variable speed drives. Despite the huge energy savings to be gained, often in excess of 50%, many companies are still not making use of variable speed drives to run their motors.
Yet, process operators cannot simply connect a drive to any old motor and expect huge energy savings overnight or even a successful motor and drive match.
To minimize the risk of selected motor failing, users need to understand the required operating and environmental characteristics of the application. Motors have to cope with all sorts of environments, from high ambient temperatures, to being immersed in sewage, to operating in dust or gas hazards.
Special designs exist for all of these cases and the user must ensure he follows the motor manufacturer’s instructions. Getting all the help you can from motor and drive manufacturers is also a good idea in general; their experience with motors and drives will help find the most compatible motor and drive combination. Many will have local service representatives who can assist with setting up the drive. Users installing their own drives need to read up about the issues that exist when connecting AC motors and drives.
Drives and their effects on motors
Variable speed drives come in standard voltage ratings, which must be chosen to match your line voltage. In general, the lower the voltage, the easier it is on the motor.
The high switching rates of inverter power devices can place a rain of high switching voltage pulses at the motor terminals, which will cause an electrical stress on the windings, which is partly dependent on the length of the cable connecting the inverter to the motor. The drive manufacturer will usually advise on the maximum practical cable lengths between 15m and 300m depending on the power rating. In some cases long cable runs may also require additional drive components such as du/dt filters. Long cable runs can also lead to EMC issues.
Because a higher carrier frequency means more frequent pulses, a useful feature of the drive is an adjustable carrier frequency. Lower carrier frequencies place lower stress on the motor insulation system and reduce the incidence of damage due to bearing currents. However, higher carrier frequencies have a positive effect on reducing motor noise levels. Some switching strategies such as direct torque control have no fixed carrier frequency, which can also help, while ensuring a low noise spectrum.
Frequency converters with non-sinusoidal current can also cause additional losses in the motor and an increase in motor losses of up to 15% was not uncommon in early PWM inverter designs, which translates into an overall reduction in motor efficiency of up to 1%.
Modern inverter designs still increase motor losses, beyond those of a true sinusoidal supply, but in practice the effect is less than that caused by connecting to the supply network.
Major factors causing an apparent reduction in output with modern drives is the fact that the output voltage is lower than the input voltage, due mainly to the presence of chokes and other components used to limit harmonics, and the improved switching patterns in the inverter. The reduction in voltage can often be compensated by using a low harmonic “active rectifier” drive solution.
Choosing a motor for
Given these points, how do you go about choosing a suitable drive for a motor? Firstly, always choose a good quality motor. High quality materials will extend the life of a motor, as well as improve efficiency. Look for thinner core plates giving lower iron losses, good slot fill giving improved stator performance, good bearings reducing rolling resistance. Reduced losses make for smaller fans, cutting noise and windage losses.
Another important quality factor is the level of insulation of the windings. Voltage stress acting on microscopic air bubbles in the winding varnish can cause ionization flash-over, known as a partial discharge, breaking down the insulation. Different insulation materials can withstand different levels known as the partial discharge inception voltage (PDIV), so you need to make sure the insulation level is adequate. Standard motors commonly have a PDIV in the region of 1350 to 1600V. A higher withstand voltage is better in variable speed drive applications. Unfortunately as yet there is no common visible classification on a motor nameplate, the use of Class B, or F or H materials does not in itself confer a specific PDIV withstand level.
Inverters also have common mode voltages in their outputs, which can give rise to induced voltages in the rotor, and if the path is not blocked can give rise to circulating currents, which can destroy bearings. This problem is solved by breaking the circuit by using insulated bearings.
Choose the right combination for the environment
A particular concern is the use of variable speed drives to power motors in hazardous areas. The main sources of risk are high surface temperature and sparks in either the winding or the bearings. This can result in increased temperature rises and higher voltage stresses on the motor insulation. These increase when self-cooled motors are used, as the speed of the cooling fan is reduced along with the motor speed.
These factors can combine to create a source powerful enough to ignite an explosion. The best way to reduce this risk is to choose a combined Atex package, which gives end users the assurance that the motor and drive combination is optimised for their application.
Note that the application of a drive with an existing, pre Atex motor is at the owners risk, and possible only in a Zone 2 area. In any case the product certification is the responsibility of the motor manufacturer.
This practice of supplying matched drive and motor pairs is a growing trend and one that progressive vendors have adopted to help cut users’ workload to a minimum.
Choose high efficiency
The efficiency of the motor is always a major factor in the choice. Although a VSD will bring system efficiency gains, it will not compensate for a poor or inefficient motor. Always use the highest efficiency motor possible. Ideally, the motor should have a good efficiency across the load range.
Motor power plays a major part because AC motors work at their peak efficiency over a limited range of their power output. Modern EFF1 electric motors usually produce peak efficiency at around 75 per cent of rated load. By contrast, older designs often have peak efficiency in a very narrow band around full load.
This is important in energy saving installations because the object of a drive is to vary the speed of the load, especially with centrifugal fans or pumps. The time spent running at full load will therefore normally be limited to emergency situations, such as extracting smoke in the event of a fire.
A new high efficiency EFF1 motor rated at 90kW with 95.2% efficiency, will cost around £5,900 and will use electricity costing around £37,250 per year, but will save nearly £9,000 compared to a standard efficiency EFF3 motor with 93% efficiency, over a 10-year service life. For companies operating large industrial complexes with many motor driven machines, such savings can mean tens of thousands of pounds, and tonnes of CO2 emissions annually.
Although an existing motor already in place can usually be used with a drive, it may not be known how well the motor has been treated and higher efficiency may be gained by using a new motor.
Choose the right speed profile
It is important when designing a system to consider the motor as a source of torque. Torque equates both power and speed, and with variable speed it is the torque profile which is of importance.
The two most common profiles are variable torque and constant torque. The first is used for centrifugal fans and pumps while the second is used for conveyors, extruders, positive displacement pumps, and similar loads.
Variable torque loads are the easiest applications for motors and drives because load power is governed by the cube of the shaft rpm for centrifugal loads acting with little static head.
It is also worth considering most load machinery is designed for sale in both 50 Hz locations such as Europe, and 60 Hz locations such as the US. Due to this the best efficiency is often between 50 Hz and 60 Hz nominal speeds, i.e. between 1500 and 1800 r/min. A variable speed drive allows this to be exploited. The freedom to select the output shaft speed can also be used to advantage to eliminate inefficiencies in belt drives.
Constant torque can pose problems because in order to maintain a constant torque at low speeds, the motor needs to be supplied with a relatively constant current throughout its speed range. This mode of operation will continually produce more heat, which will need to be dissipated at low speeds.
The current ratings of the inverter must also match the motor’s current requirements both at full load and during acceleration. The drive’s current rating and its suitability for the motor needs to be checked with the motor manufacturer, especially on motors operating below 30Hz and whenever acceleration torque is critical.