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How to Select the Right Motor for High Speed Applications | Parker Hannifin Electromechanical and Drives

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Ensuring you have selected the right motor for an application is never easy, especially when choosing for the demands of high speed applications.
In this presentation, and with the help of a simple case study, you will discover a step by step process for how to choose the best motor / drive combination from a technical and economical point of view.
Today, more and more applications utilise synchronous motors with permanent magnets and the choice of motors and drives are made with the same precautions as asynchronous motors so we will outline some additional recommendations. You will find typical use of high speed servo motors in machine tools (spindle) and test rigs (motor and component testing) applications where high dynamic is also required.

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How to Select the Right Motor for High Speed Applications | Parker Hannifin Electromechanical and Drives

  1. 1. April 26, 2016 How to Select the Right Motor & Drive for High Speed Applications Olivier Delor Product Manager, Motors
  2. 2. Agenda • Before Starting • A quick recap of motor technology • Application Requirements • How to calculate power • Motor Selection • Drive Selection • Field weakening mode • Permanent current at low speed • Frequency • Maximum BEMF capability • Protection module selection • Summary Chart • Market & Applications 2
  3. 3. Before Starting: Recap of Motor Technology Synchronous PMAC brushless motors offer many advantages over asynchronous motors (often used in electro-spindle applications): • 1.5 time more compact for the equivalent power (reduced footprint) • Higher efficiency => Energy saving • Full torque at low speed • Lower rotor temperature for : • better accuracy • lower vibration level • 3x longer bearings lifecycle • Inertia 2 times less (to increase acceleration / deceleration and decrease cycle time) 3 Parker synchronous motor
  4. 4. Application Requirements • The maximum speed, • The maximum continuous torque that will be used at low speeds for the roughing machining operation or for big diameter machining 4 In the first instance, we have to understand the application requirements in continuous mode: Torque Max continuous Torque Max Speed Speed Case study Maximum speed = 20000 tr/mn Maximum continuous torque = 22 Nm However, if the power (torque x speed) is calculated at this stage, the result will often be so huge that it will exceed what is needed – the drive will be oversized and far too expensive. Power (kW) = Torque (Nm) x Speed (tr/mn) / 9549 = 22 x 20000 / 9549 = 46 kW
  5. 5. Application Requirements How to Calculate the Power For this reason, the maximum continuous power required by the spindle must be defined, running with a field weakening mode to limit the drive size. 5 Speed Torque Max Power and constant Base speed Field Weakening mode Max Torque Max Speed Oversized Power At this stage we should choose at least a drive with power rating of 55 kW (> 46 kW). But the application requires 12 kW Case study
  6. 6. Motor Selection: Needs When these data are clearly defined, the motor choice is easy. It has to be able to: - Reach the maximum speed, because the centrifugal force generates heavy stress on the magnets and on the rotor up to a limit in rotation. The rotors of HKW and MGV motors have embedded magnets in the rotor that assure the best fitting. - Provide the maximum torque at low speeds - Have a winding configuration able to provide the power - Run in field weakening mode, which is not a common capability of synchronous motors with permanent magnets. 6 Case study (HKW Parker range as an example) Max. speed = 20000 tr/mn Max. torque = 22 Nm
  7. 7. Motor Selection A high speed motor is generally operated at constant torque up to its base speed and at constant power but reduced torque above the base speed. Indeed, a high torque output is required for starting and for acceleration, but above base speed, a reduced torque at rated power is enough to overcome friction effects (Figures 1 and 2). So the motor can run in field weakening mode at rated power up to its maximum speed. The ratio of the maximum speed to the base speed is called the field-weakening ratio. The higher that field-weakening ratio, the smaller and less expensive the drive required to run the motor to its maximum speed. 7
  8. 8. Drive Selection : Field Weakening Mode When it comes to drive, the first step is to check if it has a field weakening mode, it means it manages the phase advance between the current I and the back electromotive force (BEMF) of the motor in accordance with the power need: 8
  9. 9. Drive Selection: Permanent Current at Low Speed Next, we select the drive size to provide suitable rated current (Io) to the motor which will generate the maximum torque needed. To calculate this current, we will use the torque constant (Kt) of the motor expressed in Nm/Arms. Indeed, when we divide the desired torque by this constant, the result is the minimum current that the drive will need to provide. Io(Arms) = Torque(Nm) Kt(Nm/Arms) 9 Case study Io(Arms)= Torque(Nm) / Kt(Nm/Arms) = 22 / 0.772 = 28.5 A Parker AC890 18.5 kW ouput current = 29 A (instead of 55 kW with the primary rating data)
  10. 10. Drive Selection: Frequency 10 We have to be careful selecting the drive size because when as size increases, the output frequency has a nasty habit of decreasing. For this reason you should double-check the drive’s ability to provide sufficient frequency. Here, you should calculate the minimum frequency requested by the motor to reach maximum speed with the following formula: Frequency(Hz) ≥ MaxSpeed(tr/min) × Nomber of Pole 60 × 2 Case study Maximum speed = 20000 tr/mn HKW155 : 6 poles Parker AC890 max. frequency : 1000 Hz Frequency(Hz) ≥ 20000 ×6 60×2 = 1000 Hz The result must be less than or equal to the maximum output frequency of the drive. This is not to be confused with the PWM frequency of the drive that corresponds to the switching frequency.
  11. 11. Drive Selection: Maximum BEMF Capability The next step is specific to the synchronous motor with magnets, because this device is, as well as being a motor, and a very powerful generator able to reach high voltages. So, during the high speed operation, the back electromotive force of the motor has to be below the maximum DC voltage of the drive’s power bus. The Typical power bus values are 540VDC for a 400VAC power supply and do not exceed 850VDC in general. Check the value of the voltage of the back electromotive force of the motor at maximum speed. This limit is given by another motor constant: Ke expressed in Vrms / 1000rpm: BEMFPhase to phase(Vpeak) = Ke(Vrms / 1000rpm) × MaxSpeed(rpm) 1000 × 2 11 Case study BEMFPhase to phase(Vpeak) = Ke(Vrms / 1000rpm) × MaxSpeed(rpm) 1000 × 2 = 46.20 x 20000/1000 x 2 = 1307 Vpeak
  12. 12. The results are best plotted in vector diagrams showing axe mode, field weakening mode below the maximum voltage of the drive, and field weakening mode beyond the maximum voltage of the drive. 12 Drive Selection: Maximum BEMF Capability
  13. 13. Drive Selection: Protection Module As shown in the latter diagram, the voltage of the back electromotive force is over the drive limit due the high maximum speed. In this case and only in this case, a protection module has to be added. This is because, in the event of an incident or motor control loss (resolver fault), the motor can become a generator (driven by the load inertia). Here, the BEMF is on the terminals of the drive and, at extreme speeds, the motor and the drive will be damaged by excessive voltage. In general, the protection modules shall short-circuit the motor or the motor voltage are clipped. In all cases, the motor, the drive and the protection module have a maximum voltage limit (around 2000Vpeak or 1400Vrms phase to phase). Finally, the protection module and the drive must withstand the short-circuit of the motor current (Icc) to ensure the integrity of the components. 13 BEMFPhase to phase(Vpeak) = 1307 Vpeak > 850V (for AC890)  A protection module is needed Case study
  14. 14. Summary chart This chart will help you find the best solution to meet your technical and economic application requirements. 14
  15. 15. Markets & Applications Machine tools Parker HKW motors are high performance permanent magnet synchronous servomotors for spindle applications up to 276 kW (up to 50000 tr/mn). • High speed, • Compactness, • Low inertia, • Motion quality 15 www.parker.com/eme/hkw Test rigs Highly responsive with exceptional dynamic performance, MGV motors are ideally suited to the needs of simulation testing in many automotive or aeronautical applications up to 230 kW (up to 45000 tr/mn) • High acceleration, • High speed, • Low inertia
  16. 16. Questions? Olivier Delor - Product Manager, Motors Electromechanical & Drives Division Europe E-Mail: odelor@parker.com Parker Hannifin Manufacturing France SAS 4 Boulevard Eiffel - CS40090 21604 Longvic Cedex - France www.parker.com/eme 16

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