Induction motor losses and Power flow diagram

Losses in Induction Motors: Constant and Variable Losses Explained

Like transformers, induction motors also suffer from losses, which reduce efficiency. While transformers have only iron (core) losses and copper losses, induction motors additionally have mechanical losses because of their rotating parts.

Thus, induction motor losses are broadly classified into two categories:

1. Constant (Fixed) Losses

2. Variable Losses

Let’s study them in detail.

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1. Constant (Fixed) Losses

These are independent of motor load and remain nearly constant whether the motor runs at no-load or full load.

(a) Core (Iron) Losses

• Hysteresis Loss

  • Caused by magnetization and demagnetization of the core during each AC cycle.

  • Reduced by using high-grade silicon steel laminations.

  • Depends on supply frequency and maximum flux density.

• Eddy Current Loss

  • Induced circulating currents in the core cause unwanted heating.

  • Reduced by laminating the stator core, which increases resistance and decreases circulating currents.

⚡ Note:

• Stator core losses are significant because the frequency is constant (= supply frequency).

• Rotor core losses are negligible since rotor frequency = slip × supply frequency, and slip is small at normal load.

(b) Mechanical (Friction & Windage) Losses

• Friction in bearings.

• Windage losses due to air resistance around the rotating parts (fan, rotor).

• Brush friction loss (only in wound rotor induction motors).

• Slightly dependent on speed, but generally taken as fixed losses.

👉 Formula for Fixed Losses:

$$\text{Fixed Losses} = \text{No-load Power Input} - \text{Stator I²R loss at no-load}$$

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2. Variable Losses (Load Dependent)

These depend on the load because current in stator and rotor windings varies with load.

(a) Stator Copper Losses (I²R losses)

• Losses due to resistance in stator windings.

• Directly proportional to square of stator current.

(b) Rotor Copper Losses (I²R losses)

• Losses due to current in rotor conductors.

• Also proportional to rotor current (which increases with load).

(c) Stray Load Losses

• Occur due to leakage flux, harmonics, and non-uniform current distribution.

• Appear in both iron and copper parts.

• Usually taken as ~0.5% of motor input power in calculations.

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Power Flow in a 3-Phase Induction Motor

The energy conversion process can be shown in the Power Flow Diagram:

1. Input Power (P_in) → Applied to stator.

2. Subtract Stator Copper Losses (I²R losses).

3. Remaining = Air-Gap Power (P_g).

4. Subtract Rotor Copper Losses → gives Mechanical Power Developed (P_m).

5. Subtract Friction & Windage Losses → gives Net Shaft Power (P_out).

$$P_\text{in} \; \rightarrow \; P_g \; \rightarrow \; P_m \; \rightarrow \; P_\text{out}$$

⚠️ Key Difference from Transformers:

• Transformers: Only Iron + Copper Losses (no rotation).

• Induction Motors: Iron + Copper + Mechanical Losses.

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In Case of transformers there were only two types of losses i.e. constant losses and fixed losses same is true for Induction motors but as transformer don’t have any rotating part so there were not any mechanical associated with transformers. But since in motors there is rotating parts so they have mechanical losses which are also covered in constant losses. Lets discuss about these losses one by one.

To know about Transformer losses visit link below:-

http://electricalsystembasics.com/2014/04/transformer-losses-efficiency-transformer.html

Induction motor losses are divided as below:-

1.    Constant losses also known as fixed losses
These are known as constant losses as they will remains constant even if induction motor kept running without load. This means that these losses remains fixed irrespective of load on motor. These are further divided as below:-

(a)  Core losses known as iron losses

(b)  Mechanical losses or Friction losses

     

      2.     Variable losses
These losses are also called copper losses as These losses occur due to current flowing in stator and rotor windings. As the load changes, the current flowing in rotor and stator winding also changes and hence these losses also changes. Therefore these losses are called variable losses. These losses are occurred in both stator and rotor as current flows in both.

(a)  Stator copper losses or Stator ohmic losses

(b)  Rotor copper losses or rotor ohmic losses

(c)  Stray Load losses

Let’s discuss in details about these losses:-

     

Constant or Fixed Losses

1.    Core Losses Or Iron losses

As in case of transformers core losses or iron losses are further divided into two parts :-

(a)   Hysteresis losses

(b)  Eddy current losses

Eddy current losses can be minimized by using lamination of core. By laminating the core area decreases and hence resistance increases, which results in decrease in eddy currents.

Hysteresis losses are minimized by using high grade silicon steel.

The core losses depend upon frequency.

Stator frequency is always equal to supply frequency and rotor frequency is equal to slip multiplied by frequency so usually rotor core losses are very small and can be neglected.

2.    Mechanical losses or Friction losses

Mechanical losses occur at the bearing and brush friction loss occurs in wound rotor induction motor. These losses vary slightly with the change in speed.

Fixed losses = Power I/P at no load- Stator I² R loss at no load

Variable Losses:-

The copper losses or Ohmic losses are obtained by performing blocked rotor test on three phase induction motor. The stator and rotor ohmic losses or copper losses can be calculated directly if stator and rotor winding losses are known.

Stray load losses occur in iron as well as in conductors. They are usually taken as 0.5% of motor efficiency.

Power flow diagram for 3-phase induction motor is given below:-

Power flow diagram has shown how these losses takes place during conversion of electrical power to mechanical power. Under normal running conditions rotor core losses are neglected.  Power developed at shaft differs from mechanical power developed by friction and windage losses.