# Resistance of DC Motor Experiment

• bluey107
In summary, to test the resistance of a DC motor, one can connect a power supply, an ammeter and a DC motor in series, and measure the potential difference across the motor with a voltmeter. Alternatively, one can measure the back emf and use it to calculate the effective resistance of the motor. However, it is important to use a low voltage to avoid burning out the motor coil.
bluey107
Hey all, I have a small question that's been bugging me and I can't find an answer anywhere. How do you test the resistance of a DC motor?

My main "practical" idea is to test it as you'd test any other piece of metal: connect a power supply, an ammeter and a dc motor in series, and connect a voltmeter to measure the potential difference across the motor (in parallel), then take all the readings and calculate using the R=V/I formula.

However, I thought maybe it has something to do with back emf/induced current? In this case, would I connect a galvanometer instead of an ammeter (same setup as above), start up the motor, record the reading, then hold down the motor to see the supply current, and then measure the induced current? But then, would I use the induced current, the supply current or the net current in the R=V/I calculations?

My third thought is very vague, and I have almost no backing for it, but would there be any way to measure the back emf? Because isn't that kind of the resistance of a motor?

Please help, we were just given the aim of this experiment in class and told to figure out the rest if we wanted a head start for a practical exam coming up.

bluey107 said:
Hey all, I have a small question that's been bugging me and I can't find an answer anywhere. How do you test the resistance of a DC motor?

My main "practical" idea is to test it as you'd test any other piece of metal: connect a power supply, an ammeter and a dc motor in series, and connect a voltmeter to measure the potential difference across the motor (in parallel), then take all the readings and calculate using the R=V/I formula.

However, I thought maybe it has something to do with back emf/induced current? In this case, would I connect a galvanometer instead of an ammeter (same setup as above), start up the motor, record the reading, then hold down the motor to see the supply current, and then measure the induced current? But then, would I use the induced current, the supply current or the net current in the R=V/I calculations?

My third thought is very vague, and I have almost no backing for it, but would there be any way to measure the back emf? Because isn't that kind of the resistance of a motor?

Please help, we were just given the aim of this experiment in class and told to figure out the rest if we wanted a head start for a practical exam coming up.

Presumably you are after the "effective resistance" of the motor.

If you connected the motor and a fixed resistor in series with a given power supply [battery?] you could measure the current through the circuit.

If you then replaced the the motor with a rheostat [variable resistor], you could adjust that rheostat until the same current was flowing.

Then use what ever is necessary to measure the resistance the rheostat was set to to achieve that - should/must be the effective resistance of the motor.

If, by resistance, you mean the resistance of the wires of the motor you may be able to measre this with an ohm meter.
If you connect a battery and measure a current it must be the current when the motor is prevented from turning (stalled) you must take care to use low voltage so that you do not burn out the motor coil.
When the motor is running the current will be much less than the stalled current because of the back emf you have identified.
Once the motor is running the volts and amps tell you the power being supplied and not much about 'resistance'

## 1. What is the purpose of the Resistance of DC Motor Experiment?

The purpose of this experiment is to determine the relationship between the resistance applied to a DC motor and its speed of rotation. This allows us to understand the effect of resistance on the performance of a DC motor and how it can be controlled.

## 2. How is the resistance of a DC motor varied in this experiment?

The resistance of the DC motor is varied by changing the load on the motor. This can be done by adding or removing weights from the motor's shaft or by adjusting the resistance of an external circuit connected to the motor.

## 3. What equipment is needed for the Resistance of DC Motor Experiment?

The equipment needed for this experiment includes a DC motor, a power supply, a voltmeter, an ammeter, a variable resistor, and a tachometer. Optional equipment may include weights for varying the load on the motor and a data acquisition system for recording data.

## 4. How is the speed of the DC motor measured in this experiment?

The speed of the DC motor is measured using a tachometer, which is a device that measures the rotational speed of a shaft. The tachometer is connected to the motor's shaft and provides a reading of the motor's speed in revolutions per minute (RPM).

## 5. What is the expected outcome of the Resistance of DC Motor Experiment?

The expected outcome of this experiment is a direct relationship between the resistance applied to the DC motor and its speed of rotation. As the resistance increases, the speed of the motor decreases. This relationship can be represented by a graph known as the speed-torque curve.

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