SERVOSTAR S- and CD-Series Regeneration Requirements

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Jocelin Chambers
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www.danaherotion.com SEROSTAR S- and CD-Series Regeneration Requirements One of the key (and often overlooked) considerations for servo system sizing is regeneration requirements.

1 SEROSTAR S- and CD-Series Regeneration Requirements One of the key (and often overlooked) considerations for servo system sizing is regeneration requirements. An undersized regeneration resistor can lead to blown fuses or overload relay trips in the regen protection circuitry. If proper protection is not applied, damage to the BUS module capacitors and drive transistors can occur. Therefore, an evaluation of regeneration requirements is essential to ensuring that the customer purchases a system that will perform the desired task. Energy Calculations A rotating motor and load has kinetic energy. When this motor and load stops rotating, the energy must either be stored or dissipated. The BUS module capacitors are capable of storing a certain amount of this energy. Any energy beyond this must be dissipated by the regen resistor. If the BUS module capacitors are capable of storing all of the kinetic energy, the system needs no regen package. To determine if a system needs a regeneration resistor, use the following procedure: Define the term E which is the kinetic energy of the motor and load minus the system losses. EQUATIO 1 E (1.356/)(J + J L )ω -3I (R /)t d -(1.356/)T F ω t d Joules Where: J rotor inertia (lb ft sec ) J L load inertia (lb ft sec ) ω motor speed before decel (rad/sec) RP I motor current during deceleration (A RS /phase) R motor resistance (Ω L-L) t d time to decel (sec) T F friction torque (lb-ft) If this energy is less than that which the BUS module can store, no regen resistor is needed. 9.55

2 For a single-axis system, the condition for which no regen resistor is required is: EQUATIO E < ½C( O) Where: C BUS module capacitance (Farads) max BUS voltage () O nominal BUS voltage () (L-L) For an n-axis system, the condition for no regeneration resistor is: EQUATIO 3 E < ½C( O) Where: all negative E are set equal to zero before summation (sum all non-negative E ). This represents a worst case in which only the motors that are regenerating (E J > 0) decelerate while those whose system losses exceed their regenerative energy (E J < 0) remain idle. If Equation or Equation 3 is not satisfied, a regeneration resistor is required. Regeneration Calculations The procedure for calculating regeneration requirements is twofold. Both the regen resistance value and the resistor wattage rating must be determined. Determining Resistance alue The maximum allowable resistance of the regen resistor is that value that holds the BUS under its maximum value when the regen circuit is initially switched on. For a single-axis AC servo system, the maximum allowable regen resistance is: EQUATIO 4 R AX B 3 I Where: maximum BUS voltage B motor back EF less motor losses EQUATIO 5 B KB - 3 I (R / ) Where: K B back EF constant ( (L-L) /K RP ) motor speed prior to decel (K RP ) I deceleration current in motor (A RS /phase) R motor resistance (Ω L-L) I deceleration current in motor (A RS/phase) Revised 11/0/003 Page

3 Equation 4 is easily expanded for an n-axis system with all axes decelerating simultaneously. R AX EQUATIO 6 I 3 + I I 3 B1 1 B B R AX I 3 BJ J Determining Dissipated Power The average wattage rating of the regen resistor is a function of energy to be dissipated and the time between decelerations. This average wattage rating for a single axis system is given by: EQUATIO 7 P A E Where: cycle HYS -(1/)C( - t ) t cycle time between decels + time to decel (sec) HYS hysteresis point of regen circuit The hysteresis point of the regen circuit is the voltage level at which the resistor switch opens following an "on" cycle. For example, consider a BUS module with nominal DC bus 35 and maximum DC bus of 390. When the motor decelerates, it "pumps" the bus up to 390. When the voltage reaches 390, the regen circuit gates on and bleeds the bus down. When the bus bleeds down to a certain level, the regen circuit opens the resistor path thus allowing the bus to "pump" up again. The voltage at which this occurs is the hysteresis point. For an n-axis system, again set all negative E equal to zero and sum E as: EQUATIO 8 P A Σ J HYS E - ( 1/ ) C( ) t cycle Where : t cycle longest t cycle of the n-axis This is the average power dissipated over one time period which all axes simultaneously decelerate to a stop. This represents the worst case for a multi-axis system. When the time between decelerations becomes very large, Equations 7 and 8 become very small. In cases such as these, the average wattage is not a meaningful number. Peak wattage and the timethe resistor sees peak wattage become the main concerns. Revised 11/0/003 Page 3

4 The peak wattage of the regen resistor is: P PK R REGE Revised 11/0/003 Where: R REGE REGE Resistance In summary, Equations 4 and 5 are used to determine if regeneration is needed for a system. If a resistor is required, then Equations 4 and 7 define those requirements for a single-axis system. Equations 6 and 8 define the requirements for a multi-axis system. Regeneration Calculation Example A customer's machine is configured so two B-404-B motors with SR10000 amplifiers share the same PA800 BUS module. The load parameters are: otor 1: J LOAD lb-ft-sec T F 1.5 lb-ft AX 500 RP otor : J LOAD lb-ft-sec T F 1.0 lb-ft AX 500 RP inimum time between decelerations 5 sec Assume that both motors decelerate at peak current simultaneously from maximum speed to zero. What, if any, regeneration capacity must the BUS module provide? otor 1 Calculation for E Where: E 1 (1.356/)(J +J L )ω - 3I (R /)t d - (1.356/)T F ω t d Joules J lb-ft-sec (From motor specification) J L lb-ft-sec ω 500/ rad/sec I 0 amps (aximum drive peak current of the SR R Do not exceed I PEAK from the motor. 1.3 ohms (From motor specifications) t d (J +J L )(ω )/(T +T F ) sec T K I lb-ft T K T 0.99 lb-ft/a (From motor specifications ) T D 0.99 (0) 19.8 lb-ft T F 0.5 lb-ft t d ( )(51)/( ) sec sec E 1 (1.356/)( )(51 ) - 3(0 )(1.3/)(0.0175) - (1.356/)(1.5)(51)(0.0175) 45.1 Joules Page 4

5 otor Calculations for E t d (J +J L )(ω )/(T +T F ) sec ( )(51)/( ) sec E (1.356/)( )(51 ) - 3(0 )(1.3/)(0.01) -(1.356/)(1.0)(51)(0.01) 30.5 Joules ow use Equation 3 to determine if a regeneration resistor is required. Condition for no regeneration resistor required: Σ E < ½C( O E J E 30.5 J C Farads (From BUS module data on Table 1) 390 (ibid.) O 35 (ibid.) 45.1 J J < 1/( )( ) 75.6 J < 46 J This is obviously false, therefore, a regeneration resistor is required. Determine aximum Resistance alue The maximum resistance is given by Equation 6: R AX Z Σ I 3 BJ J Knowns: B1 K B - 3 I (R /) K B1 81.(/KRP)(From motor specifications).5 KRP I 0 A R 1.3 ohms B1 81.(.5) - 3 (0)(1.3/) 180 B 180 R AX B1 1 I 3 + I 3 B 390 (From BUS module data) R AX 390 (180)(0) 3 + (180)(0) 3 1. ohms Revised 11/0/003 Page 5

6 Determine Dissipated Power The average power is given by Equation 8: P A E -1/ C ( J t cycle HYS ) Knowns: E J E 30.5 J C F 390 HYS 370 t cycle (1 / )( )(390 P A ) 1.1 Watts The PA800 BUS module by this calculation must have a regen resistor with a maximum resistance of 1. ohms and a minimum average wattage rating of 1.1 watts. Table shows the standard internal regeneration resistor for the PA800 has 1.5 ohms and is capable of 40 watts continuous power dissipation. Comparing these values indicates adequate wattage, but marginally high resistance for the internal resistor. Prudence would dictate the use of the external ER-30 resistor. On a high volume OE application, testing of the BUS module s internal resistor might prove it adequate when considering additional friction or perhaps the IR drop in the motor cable. BUS module Specifications odel Capacitance (Farads) O (DC) HYS (DC) AX (DC) PA PA PA PA PA PA CR,CE03* CR,CE06* Table 1: BUS odule Data Revised 11/0/003 Page 6

7 odel Standard Internal Resistor Optional External Resistor Kit PA08 OE /A PA14 40W, 1.5 Ω ER W, 8.8Ω ERH W, 8.8Ω PA8 40W, 1.5 Ω ER W, 8.8Ω ERH W, 8.8Ω PA50 OE ER-0 500W, 4.5Ω ER W, 4.4Ω PA75 OE ER- 1000W,.Ω ER-3 000W,.Ω PA85 OE ER- 1000W,.Ω ER-3 000W,.Ω CR, CExx * OE ERH-6 00W, 0Ω * Integrated Amplifier and Bus module Table : Available Regeneration Resistors Typical Servo System Architecture PA BUS odule with Regeneration Servo Amplifier Position Feedback Regen Transistor Brushless Servomotor 1-phase or 3- phase power Diode Bridge 3-phase PW Amplifier Filter Capacitors Regen Resistor Inertia Load Revised 11/0/003 Page 7

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