Hysteresis Motors
• Stator
– same as for induction
motor
• Rotor
Smooth cylinder
Principle of Operation
Stator Flux
establishes these
magnetic poles
Rotor poles
“induced” by Stator
Flux
Spin the stator poles
with the rotor blocked
Rotor poles follow
the rotating flux,
but lag behind by
angle δh
Stator poles
moving CCW
Spin the stator poles
with the rotor blocked
If the rotor is released, it will accelerate to synchronous speed
Hysteresis Power Loss, Ph
Ph  kh f r B
n
max
where
fr = frequency of flux reversal in the rotor (Hz)
Bmax = maximum value of flux density in the air
gap (T)
Ph = heat-power loss due to hysteresis (W)
kh = constant
Mechanical Power developed
 1 s 
Pmech  Ph 

 s 
n
Ph  kh  f  Bmax
Th nr
n  1 s 
 kh  f  Bmax 

5252
 s 
nr  ns (1  s )
f r  sf s
Mechanical Power Developed (cont)
n
 5252kh f s Bmax

Th  

ns


120  f s
ns 
P
n
5252kh Bmax Independent of frequency
Th 
and speed!
120
P
Hysteresis Motor at Synchronous Speed
No load and
negligible
rotational losses
Induced rotor
magnets remain
locked with the
rotating poles
produced by the
stator
Hysteresis Motor at Synchronous Speed
Apply a step
increase in shaft
load.
The rotor slows
down and the
induced rotor
magnets lag the
rotating poles of the
stator by an angle
δmag .
The rotor returns to
synchronous speed
at the new torque
angle.
Hysteresis Motor at Synchronous Speed
Tmag  sin( mag )
Tmagmax occurs @  mag  90
If shaft load causes δmag>90°,
the rotor pulls out if
synchronism, the magnet torque
drops to zero, and the machine
develops hysteresis torque. This
torque is not sufficient to carry
the load.
Torque-Speed Characteristic
Constant Hysteresis Torque allows the motor to
synchronize any load it can accelerate
“Normal”
Operating
Range