What is Eddy Current Dynamometer : Construction & Its Working

An eddy current dynamometer is a special device with fewer losses, high efficiency, and more versatile as compared to the conventional mechanical dynamometer. In the eddy current dynamometer, the losses are less due to the absence of any physical contact between windings and excitation. Due to its small size and combability, it has numerous applications, and even for some cases like testing the performance of an internal combustion engine, it is used as a load. This article discusses an overview of an eddy current dynamometer.

What is the Eddy Current Dynamometer?

An eddy current dynamometer is an electromechanical energy conversion device, which converts mechanical energy to electrical energy. It fundamentally uses Faraday’s Law of electromagnetic induction as its working principle. A schematic of the dynamometer is shown below.

The constructional aspects of the eddy current dynamometer are shown in the above figure. It consists of the outer frame as the stator, which is also called a stationary member of the machine. The stator consists of windings, which are placed in stator slots. When the stator windings are excited, a stator magnetic field is produced in the stator coils. In the case of high rated machines, 3 phase windings are placed in the stator slots.

The stator windings are made of copper. The outer frame, i.e. the stator is made of a magnetic material like cast iron or silicon steel in case of delicate applications. The rotating member is called a rotor, which is kept below the stator coils. The rotor is placed on a shaft so that it can rotate. Rotor windings are placed on the rotor slots. In the case of heavy machines, three-phase rotor windings are used to be kept on rotor slots.

The rotor must be connected to the prime mover, such that when the prime mover rotates, it provides the mechanical input to the device. A D.C supply is used to excite the stator windings. In the case of large machines, rectifier units are used to achieve this DC supply. For large machines, oil is used for cooling and insulation of the stator windings. This is important to dissipate the heat generated.

Once current meter as shown in the diagram is used to measure the current produced and torque induced. A pointer is connected by an arm to the stator, which can measure the torque generated in the rotor. And with the knowledge of speed, by using this torque value, we can calculate the power generated in the machine.

Dynamometer Working

An eddy current dynamometer works on the principle of Faradays’ Law of electromagnetic induction. As per the law, whenever there is a relative displacement between a set of conductors and a magnetic field, an emf is induced on the set of the conductor. This emf is called is dynamically induced emf. In the case of the dynamometer, when the stator poles are excited with a DC supply which is connected to the stator.

When the DC supply is connected, the stator coils are excited and a magnetic field is produced in the stator coils. In the case of a three-phase machine, we obtain a 3 phase rotating magnetic field, in the stator coils when the coils are excited with the three-phase supply. When the prime mover rotates, the rotor, the rotor coils rotate and interact with the stator magnetic field.

It must be noted that in this the stator magnetic field is static in nature. Since the excitation is DC, we get a static magnetic field. When the rotor coils cut the stator magnetic field, an emf is induced since in this case the magnetic field is static and the conductors are rotating. So there is a relative displacement between the magnetic field and the conductors.

Features of Eddy Current Dynamometer

It must be observed that the eddy current dynamometer is different from than conventional mechanical dynamometer. In this case, when the rotor of the dynamometer cuts the stator magnetic field, an emf is induced on the rotor conductors. It causes eddy currents to flow in the rotor conductors. The direction of the eddy currents is opposite to the change in the magnetic flux and is generated in the rotor.

The rotor opposes the force exerted due to the magnetic flux, but due to the prime mover input, it keeps rotating. And since there is no physical contact between the magnetic field and the conductors, the losses produced are very less as compare to a conventional generator.

Unlike in a conventional mechanical dynamometer, in an eddy current dynamometer, an arm is connected to the body of the stator. At the end of the arm, a pointer is connected, which can measure the torque produced in the rotor winding. By knowing the speed of the rotor, the amount of power can be known, as the power is equal to the product of torque and speed.

Dynamometer Advantages

The advantages of eddy current dynamometer are

1. It is more efficient as compared to conventional mechanical dynamometer due to low frictional losses.
2. Its structure is simple
3. It can be operated more conveniently as compared to conventional dynamometers
4. It has a fast dynamic response because of low rotational inertia.
5. Due to the absence of huge windings, the number of copper losses is less.
6. It can be connected to an external control unit easily to monitor the flow of currents and even control it.
7. The braking torque is very high
8. It is highly precise and stable

Applications

The major applications are

• Performance testing of the internal combustion engine
• Used in small power motor
• Automobile transmission parts
• Gas turbines
• Water turbines

Hence we have seen the working principles of dynamometers that are compact and versatile in nature. It must be thought, how one can bring the operating characteristics of an eddy current dynamometer up to the level of conventional mechanical dynamometers?