Squirrel Cage Induction Motor
A squirrel cage induction motor is an alternating-current motor whose rotor carries no windings, brushes or slip rings — only conducting bars shorted together by end rings — and in which the rotor current is induced by the stator field rather than supplied to it.
The name describes the rotor. Bars running the length of the core, joined at each end by a ring, look like the exercise wheel the name comes from. Nothing connects that assembly to the outside world.
Because the rotor current has to be induced, the rotor must turn slower than the stator field under load. That difference is called slip, and without it there would be no induced current and therefore no torque. It is also why the machine is called asynchronous — the two words mean the same thing.
Also written: squirrel cage motor · cage induction motor · squirrel cage rotor · asynchronous motor
What the term fixes, and what it leaves open
| Rotor construction | Conducting bars shorted by end rings, cast or fabricated into the rotor core |
|---|---|
| How the rotor gets current | Induced by the stator field — nothing is fed to the rotor |
| Brushes or slip rings | None. No sliding contact anywhere in the machine |
| Slip | Always present under load. Zero slip means zero torque |
| Starting | Starts direct on line with no additional device |
| Also called | Asynchronous motor — the same machine, named after the slip rather than the rotor |
| Efficiency classification | GB 18613 / IEC 60034-30-1 for low voltage; IEC 60034-30-3 for high voltage, inside a defined window |
| What the cage shape decides | Starting torque and locked-rotor current — which is what a NEMA design letter describes |
Why it became the default machine
There is no sliding contact to wear, no rotor connection to fail, and no external device needed to start it. The rotor is a single cast component. For most industrial duties that combination has never been beaten on cost per kilowatt or on mean time between failures.
What it gives up is control. Speed is set by the supply frequency and the pole count, and the only way to vary it is to vary the supply — which is what a converter does, and why variable-speed duty is a separate specification rather than a property of the machine.
Where the cage shape shows up on a datasheet
The bars are not just conductors; their cross-section and depth shape the starting behaviour. A deep, narrow bar has more effective resistance at standstill than at speed, which raises starting torque and limits inrush.
That single design choice is what the NEMA design letters A, B, C and D encode, and it is why a Design D machine for a beam pump is not interchangeable with a Design B machine of the same rating.
What Squirrel Cage Induction Motor does not tell you
Cage does not mean the rotor is a serviceable part
A cast cage is one piece with the rotor. A broken bar is usually a rotor replacement, not a repair — which is the opposite of the intuition that a simple part must be easy to fix.
Asynchronous is not a lesser kind of synchronous
They are different machines. A synchronous machine turns exactly with the supply frequency and needs its own excitation; an induction machine cannot produce torque without slip. Neither is a degraded version of the other.
Squirrel cage does not mean fixed speed
The machine has one speed on a fixed supply, but on a converter it is a variable-speed machine. What changes is the supply, not the rotor.
It does not imply an efficiency class
Almost every efficiency class in the low-voltage standards is written for exactly this machine, which is precisely why the class has to be stated separately — saying "squirrel cage" narrows nothing.
Which LEADGO series are squirrel cage machines
Generated from the product data as everything in the range that is not a permanent magnet machine. There are no wound-rotor or slip-ring series here, so induction and squirrel cage mean the same thing across this range — and the two lists below therefore cover every motor series on the site.
DYG Series
8 – 60 kW
NEMA D
3 – 150 HP (2.2 – 110 kW)
NEMA JM / JP PE
1 – 75 HP
NEMA PE
Up to 350 HP (260 kW)
YBBP
0.18 – 315 kW
YBBPX3
0.18 – 315 kW
YBX3
0.18 – 315 kW
YBX4
0.18 – 315 kW
YBX5
15 – 90 kW
YD Series
0.35 – 90 kW
YDGJ Series
18.5 – 75 kW (dual rating per model)
YDT Series
0.17 – 160 kW
YE3 (IE3)
0.55 – 375 kW
YE3 Compact HV
185 – 3150 kW
YE3 Large-Power
200 – 1000 kW
YE4 (IE4)
0.55 – 375 kW
YE4 Large-Power
200 – 1000 kW
YE5 (IE5)
0.55 – 315 kW
YEJ Series
0.55 – 90 kW
YFBBPX3
0.18 – 315 kW
YFBX3
0.18 – 315 kW
YFBX4
0.18 – 315 kW
YKK / YXKK
6 kV: 185 – 3150 kW · 10 kV: 220 – 2500 kW
YVP Series
0.55 – 560 kW
And which are not
These are permanent magnet synchronous machines. They have no cage current at all, they turn exactly with the supply frequency, and most of them need a converter to run.
CYJTYP Series
15 – 75 kW
DZTYP Series
30 – 160 kW
GTYP Series
1 – 185 kW
GTYPS Series
1.1 – 200 kW
TYC Series
5.5 – 55 kW
TYP Series
0.37 – 400 kW
TYPL Series
8.4 – 55 kW
Frequently asked
What is a squirrel cage induction motor?
An AC motor whose rotor carries no windings, brushes or slip rings — only conducting bars shorted together by end rings. The stator field induces current into those bars, and the interaction between that current and the field produces torque. Because the current has to be induced, the rotor always turns slightly slower than the field under load.
Why is it called a squirrel cage?
Because of the shape of the rotor conductor assembly: bars running the length of the rotor, joined at both ends by rings, look like a cylindrical exercise wheel. The name describes the rotor, not the machine’s behaviour.
What is the difference between an induction motor and an asynchronous motor?
Nothing — they are two names for the same machine. "Induction" names the way the rotor gets its current; "asynchronous" names the consequence, which is that the rotor cannot turn at exactly the same speed as the stator field and still produce torque.
Does a squirrel cage motor need a starter?
Not to start. It will start direct on line with nothing between it and the supply. Starters and soft starters are fitted to limit the inrush current or the mechanical shock, not because the machine cannot start without them.
Can a broken rotor bar be repaired?
Usually not in the way the simplicity of the part suggests. On a cast rotor the cage is integral with the core, so a cracked bar generally means replacing the rotor. The symptom to watch for is a periodic variation in current and torque rather than a sudden failure.
Where this is worked through properly
Why NEMA Design D exists →
what the cage shape is doing on a beam pump
Where synchronous machines are used →
the other half of the AC machine family
GB 18613 and IE classes →
the classification written for this machine
NEMA MG 1 explained →
where the design letters come from