Lecture 7 BEHAVIOR OF THE MOTOR UNDER LOAD

Lecture # 7 BEHAVIOR OF THE MOTOR UNDER LOAD AND LOAD 1

BEHAVIOR OF THE MOTOR UNDER LOAD AND LOAD o For constant magnetic field, when a load is placed on the motor, it will slow down causing Eg to decrease. ⇒ Eg ↓ α ϕ ω ↓ o If back EMF decreases, Ia increases which causes the torque T to increase. Vt − Eg ↓ Ia ↑= Ra ⇒ T ↑ α ϕ Ia ↑ o This process continues until the speed is restored to a rated speed (i. e. an increase in torque is equal to the increase in load placed on the motor’s shaft). o If you remove the load, Ia decreases which will reduce the torque, and the motor will accelerate until it reaches the original (no-load) speed. Pin Pout 9

TORQUE-SPEED CURVE TL o At no-load, the motor runs at no-load speed NL. Tstall o If load increases to torque value of TL 1, speed decreases to 1. T TL stall Tstall NL T 1 o As load further increases, decreases until motor finally stalls. This load torque is called Stall torque Tstall. NL Typical motor torque-speed curve o The behavior of the Load Torque vs. Motor Speed can be predicted by using the line equation: y = m x + yb ; where m: slope and yb: y @ x=0. Slope: m T Tstall 0 T stall 0 NL NL T Line Eq: TL stall Tstall NL 10

TORQUE-SPEED CURVE Example 5 -2: A permanent-magnet dc motor has a no-load speed of 120 rad/s. Its stall torque is 160 N. m. (a) At what speed will it run if a 60 N. m load is applied? (b) What is its output power under this load condition? Sol. (a) T TL stall Tstall NL Tstall TL Tstall (b) 160 N. m 120 160 75 rad/s 160 P T 103 60 75 103 4. 5 k. W 120 rad/s P P 746 4. 5 103 746 6. 0 hp 11

SPEED REGULATION o Speed regulation represents how the speed changed from no-load to a full-load conditions. Or % Speed Regulation where FL N NL N FL NFL 2 rad N rpm 60 100 rad / s NL FL Rated Current % Speed Regulation NL FL Speed (rad/s) o The smaller the speed regulation, the better the motor’s characteristics. Ia (A) Typical motor load characteristics 12

SPEED REGULATION Example 5 -3: For the motor whose characteristic is shown in the figure, find the percent speed regulation. Sol. % Speed Regulation NL FL FL NL= 100 FL= 160 120 100 33. 3 % 120 13

MOTOR EFFICIENCY Pi Po PLosses Efficiency (%) P o x 100 Pi o Pi is an electrical input power and easy to calculate, Pi = Vt IL o Po is the mechanical output power and is equal to: Po = Pi – PLosses o Therefore, the efficiency could be also calculated by: Efficiency (%) P i Plosses Pi x 100 14

POWER FLOW DIAGRAM o The developed power is the electrical power which is converted to a mechanical power by the armature’s and can be calculated by: Pdev = Eg Ia = Pi – Pcu= Po + Pst o Note that this developed power is not the actual output power from the motor; to get the actual output power, the stray power losses must be taken into account since there is an internal friction and core losses: Pdev = Eg Ia Electrical Input Power Pi = V t I L Mechanical Output Power Po = Pin – PCu – Pst Copper losses Pcu Stray Power Losses (mechanical and core losses) Pst 15

POWER FLOW DIAGRAM Example 5 -4: Find the nameplate efficiency of a dc motor rated 3 hp, 220 V, 12 A. Sol. Pin = Vt. IL = (220) (12) = 2640 W Po = 3 x 746 = 2238 W P % o x 100 Pi % 2238 x 100 84. 8 % 2640 16

POWER FLOW DIAGRAM Example 5 -5: A 230 V dc motor draws 3 A at no-load. From the measurements and calculations the copper losses are found to be 370 W. What are the stray power losses? Sol. Pout no-load = 0 W Pi Pdev Pcu 370 W Pst Po 0 W Pin no-load = INL x Vt = 3 x 230 = 690 W Pin = Pcu + Pst = Pin – Pcu Pst = 690 – 370 = 320 W 17