What to consider for high speed motor applications
21 December 2016
Need to drive a motor of 0.75KW to 1MW well in excess of 10,000rpm? As well as compiling to EU regulation 428/2009, the decision on which inverter drive to use for your high speed application needs to be considered and taken.
KEB has been providing solutions for these applications since 1988. The applications have seen motor speeds up to 100,000rpm and the ability to drive not just induction (asynchronous) but PM (synchronous) and IPM motors too. KEB’s applications in this field range from compressors, blowers, vacuum pumps, turbines to spindles (normally high speed cutting tools) and rotating flywheels to name the most common. The drives are often unseen in control cabinets but used by leading companies within these fields.
Some of the main factors to consider for these applications are as follows and how KEB addresses some of them is explained:
• A high power factor,
• Low current harmonics (it helps to reduce the rotor temperature),
• low dV/dt (so the motor windings are protected),
• high frequency PWM up to 16KHz,
• High speed voltage and current calculations,
• And experienced and knowledgeable application technicians
KEB uses its Sensorless Closed Loop control which not only means that closed loop control can be achieved on SMPM and IPM motors without a feedback device but some of the benefits of using it is an integral way of driving the motors at the high speeds by:
• determining the rotor position in locked condition,
• torque and speed control
• as well as achieving 50 percent rated torque at zero speed
The control algorithm used with the sensorless control calculates the optimum momentary value of the voltage and current for the motor which provides a stable and dynamic operation of the system in all load conditions. The fast processor updates provides fast model updates leading to torque limitations which protects both drive and motor. KEB developed a simulation program to simulate motor and drive system using SCL software that allows possible resonance to be identified and the system to be optimised prior to initial testing.
There is no complex parameter adjustment necessary, just simple step-by-step-start-up via its CombiVIS-Software. Automatic measurement of all the relevant motor data for the regulation is done after entering the name plate data and if using sine filter, KEB have also developed a program to create a download list of relevant motor/filter data.
The other elements to KEB’s success are the filters that KEB build and supply which help to reduce the dv/dt and current harmonics normally seen.
By smoothing the motor current, the rotor heat can be reduced so KEB recommend a filter/choke. For higher powers, it is advisable to reduce the harmonics in order to reduce the transformer and cable overload. For that reason KEB has designed special input filters to reduce the current harmonics to values down to maximum 8 percent Total Harmonic Distortion (THDi):
• THDi with a standard 3 phase inverter: 60…80 percent
• THDi with a standard 3 phase inverter with mains choke: 30…40 percent
• THDi with a standard 3 phase inverter with KEB THD Filter: <8 percent
KEB guarantees that the THD Filter will perform as advertised to reduce harmonic distortion caused by AC Variable Speed Drives and other non-linear loads equipped with 3-phase, 6-pulse, diode bridge rectifiers. A properly selected and installed KEB THD Filter Type Z1 will:
1. Reduce current Total Harmonic Distortion (THDi), measured at the THD Filter
2. NOT become overloaded by other upstream harmonic sources.
3. NOT resonate with other power system components.
4. NOT have compatibility problems with engine generator sets properly sized for the load.
By selecting and using the correct sinus filter, again KEB build and offer a suitable range, the heating loses can be reduced by over 90 percent and the rotor temperature halved. This will result in improved reliability and motor life – especially important in these applications when the motor is often more expensive than the drive.
Physical heat dissipation within the drive and control cabinet also needs to be considered when driving at these high speeds and KEB has a couple of options designed to alleviate this issues: –
Through mounted, air cooled unit extends the heat sink outside of the electrical cabinet so 70 percent of heat is dissipated outside of the enclosure leading to a lower cost for overall cooling and a reduction in the enclosure depth.
The second option KEB offer is the liquid cooled heat sink which provides even more benefits - 80 percent of heat dissipated outside enclosures, additionally a physically smaller housing for the same rated power. All liquid cooled heat sinks are all rated for water (open and closed), glycol & R134a, and with built in flow control scheme it solves condensation worries. The coolant connections are outside of the enclosure and all of KEB heat sinks are tested at 20bar and UR labeled at 10 bar. If there is a scale build up the coolant channels are straight through so end caps can be removed and the channels cleared.
To help integrate into most systems the KEB inverters have various bus communication systems, including the most popular ones. When used with the software package mentioned, fault analysis and the scope functionality helps monitor what’s happening at all times.
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