This document discusses hysteresis motors. It describes the construction of hysteresis motors, which have a smooth cylinder rotor like induction motors but without windings. It explains that hysteresis motors operate based on the principle of a rotating stator flux inducing poles in the rotor that lag behind, causing it to accelerate to synchronous speed. The document also outlines the torque-speed characteristic of hysteresis motors, noting they can develop constant hysteresis torque independent of speed and frequency in their normal operating range.

1. Special Machines:
Hysteresis Motors
• Construction
• Principle of Operation
• Power Flow
• Torque-Speed Characteristic
• Edited by: Engineer.Ahmed Amin Hassan
• Date : 1.10.2015

2. Hysteresis Motors
• Stator
– same as for induction
motor
• Rotor
Smooth cylinder

3. Principle of Operation
Stator Flux
establishes these
magnetic poles
Rotor poles
“induced” by Stator
Flux

4. Spin the stator poles
with the rotor blocked
Stator poles
moving CCW
Rotor poles follow
the rotating flux,
but lag behind by
angle δh

5. Spin the stator poles
with the rotor blocked
If the rotor is released, it will accelerate to synchronous speed

6. Hysteresis Power Loss, Ph
max
n
h h rP k f B
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

7. Mechanical Power developed
max
max
1
1
5252
(1 )
mech h
n
h h
nh r
h
r s
r s
s
P P
s
P k f B
T n s
k f B
s
n n s
f sf
 
  
 
  
 
    
 
 


8. Mechanical Power Developed (cont)
max
max
5252
120
5252
120
n
h s
h
s
s
s
n
h
h
k f B
T
n
f
n
P
k B
T
P
 
  
 



Independent of frequency
and speed!

9. Hysteresis Motor at Synchronous Speed
No load and
negligible
rotational losses
Induced rotor
magnets remain
locked with the
rotating poles
produced by the
stator

10. 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.

11. Hysteresis Motor at Synchronous Speed
max
sin( )
@ 90
mag mag
mag mag
T
T occurs



 
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.

12. Torque-Speed Characteristic
“Normal”
Operating
Range
Constant Hysteresis Torque allows the motor to
synchronize any load it can accelerate