Induction Machines-Performance

• In induction motor, the magnetizing current is much larger than the active current, so that the no-load power factor is low,0.1 or 0.25
• If a load torque applied to the shaft, the active component of stator current increases and magnetizing current remains constant, the power factor is raised
• The maximum effieciency ocurs roughly where the I²R losses in stator and rotor sum to ana amount equal to the no-load constant losses.
• Usual reltio to maximum to full load torque is 2:1
Cage motors:
• Motors with normal starting torque and current , having cages with low resistance and reactance, low full load slip, good efficiency and power factor
• Motors with high starting torque and low starting current, using deep-bar rotor or double cage rotors.
• Motors with high full load slip, using a comparatively high resistance cage , with large starting torque, low starting current and low efficiency. Used for fluctuating torque drives(punching machines)
Multispeed Motors:
• variable-torque, power output proportional to square of speed-fans,centrifugal pumps
• Constant torque, power output proportional to speed-conveyors,stokers, reciprocating compressors,printing presses
• Inverse torque, power output rating constant-machine tools,laths,boring mills,drills,planners
• Gear motors- high speed motors-conveyors,agitators,blowers and screens
• A startor may either increase rotor resistance or decrease starting current in induction machine
• The inclusion of rotor resistance will reduce the stator current at standstill and simultaneously raise its power factor.
• The starting current of a cage motor can only be reduced by lowering the voltage applied to the stator
• The output and torque for a given slip change as the square of the allied voltage and current reduces in proportionally
Direct online startor
• For direct switching case, if T_SC be the torque developed on short circuit at normal voltage with currents I_SC then starting torque is T_S = T_SC and the starting current per phase is I_S = I_SC the ratio of starting to full load torque is T_S/T_n=(I_SC/I_n)²s_n
Stator resistor startor:
• The normal voltage per phase to the fraction x is applied to the termianls. The starting current is I_s = xI_sc and starting torque T_s = x²T_sc
• The ratio of statig torque to full load torque is T_s/T_n = (I_s/I_n)²s_n = x²(I_sc/I_n)²s_n
Auto-trnasformer startor:
• The auto-transformer starter is used to reduce the phase voltage to the fraction x of normal value. The starting current is I_s = xI_sc and starting torque T_s = x²T_sc
• The ratio of statig torque to full load torque is T_s/T_n = (I_s/I_n)²s_n = x²(I_sc/I_n)²s_n
Star- Delta startor
• A motor must be built to run normally with a mesh connected stator winding. At starting the winding is connected temporarily in star. The phase voltage is thus reduced to 1/√3 = 0.58 of normal.
• The starting current per pahse is I_s = 0.58I_sc, the line current is (0.58)²I_sc = 0.33I_sc, the starting torque is one third of short circuit value.
• the ratio to starting torque to full load torque is T_s/T_n = (1/3)(I_sc/I_n)²s_n
Cogging and Crawling:
• If the number of stator slots S1 equal to the rotor slots S2, the machine may efuse to start at all, this is called cogging or teeth locking.
• With other ratios S2/S1, the motor may exhibit a tendency to run stabnly at low speed (one-seventh of normal): this is called crawling.
• The 5th harmonic r.m.f rotates agaonst the fundamental at one-fifth synchronous speed.
• The 7th harmonic r.m.f rotates with the fundamental at one-seventh synchronous speed.
• If the harmonic torques are sufficiently pronounced, the seventh harmonic may prevent the motor speed exceeding abiut one-seventh of normal, since the downward slope of the resultant torque at this speed is a stable running condition over the torque range between the maximum and minimum points. The motor therefore crawls.
• If n₁ is the synchronous speed of fundamental, and n the speed of the rotor, then the synchronous speed of the thirteenth harmonic is +n₁/13, or relative to the rotor -(n₁-n)/13. The rotor runs at a speed n, and so rotates its own thirteenth harmonic at a speed -(n₁-n)/13+n relative to the stator. The sttor and rotor thirteenth harmonic fall into step when +n₁/13 = -(n₁-n)/13+n i.e. n= (1/7)n₁. Thus the torque discontinuity at one seventh synchronous speed is here produced not by the seventh harmonic, but by the thirteen.