Study of DC Motor

Virtual Labs
IIT Kharagpur

Study of DC Motor

Control and Instrumentation Laboratory -->

Study of DC Motor

Instructions

Motor and Generator Characteristics

First click on "Tests" button and click on 'No Load' to perform the experiment for finding the dc motor and generator characteristics.

- Click on 'ON' button to switch on the motor unit. Connect (blue dots for wire connection) 1-3.
- Note: Example: connection point 1 - connection point 2 (drag the wire from connection point 1 by pressing left mouse button and release on connection point 2).
- Click on 'Check Connection' button.
- Note: Any wire connection can be deleted by clicking on the connected wire if required.
- Set 'MOTOR' switch to 'ON' by clicking on it.
- Set the 'RESET' switch to 'RESET'.
- Set load to 0 value.

- Apply armature voltage E_a through rotating knob1 (start with 3 volts).
- Note: To rotate any knob put the mouse cursor on the knob handle (blue line on the knob), a hand symbol will be showing. Press left mouse button, rotate clockwise to increase or anticlockwise to decrease values.
- Note: If the desired value does not appear while rotating the knob in one attempt, try rotating it back and forth to reach the correct value.
- Click on 'Simulate' button. Observe motor armature current (mA), speed (rpm).
- Now click on the connected wire (1-3) to delete that and connect 2-3.
- Click on 'Check Connection' button.
- Click on 'Simulate' button. Observe generator voltage E_g.
- Click on 'Table' button to get the observation table.

Delete connection 2-3 and again connect 1-3, click on 'Check Connection' button then on 'Simulate' button. Increase the armature voltage to 4 volts and follow step 3.

- Vary E_a from 5 - 12 volts and follow step 3.
- Now Click on 'Plot' button, then on 'Speed (N) Vs. Motor armature voltage (E_a)'.
- From a slope of the plot calculate motor constant K_M (rpm/volts) (calculation explained in theory).
- Again click on 'Generator voltage (E_g) Vs. Speed (N)' under plot button.
- From a slope of this plot calculate generator constant K_G (volts/rpm) (calculation explained in theory).
- Plots can be downloaded by clicking on 'Download Plot' button.
- Now enter the values of K_M and K_G in the corresponding box under 'Result Block'.

Bring back all the knobs to zero value. Set 'MOTOR' switch to 'OFF' by clicking on it again and switch off the motor unit.

Torque-Speed Characteristics

First click on "Tests" button and click on 'Load' to perform the experiment for finding the dc motor torque-speed characteristics.

- Click on 'ON' button to switch on the motor unit.
- Connect 1-3, click on 'Check Connection' button. set 'MOTOR' switch to 'ON' by clicking on it.
- Set the 'RESET' switch to 'RESET'. Set load to 0 value.

- Apply armature voltage E_a through knob1 and set E_a to 6 volts.
- Click on 'Simulate' button. Observe motor armature current (mA), speed (rpm).
- Now click on the connected wire (1-3) to delete that and connect 2-3 , click on 'Check Connection' button then on 'Simulate' button. Observe generator voltage E_g.
- Click on 'Table' button to get the observation data.

- Set load to 1 through knob3. Delete connection 2-3 and again connect 1-3. Click on 'Check Connection' button.
- Click on 'Simulate' button and follow step 3. Now vary the load from 2 to 5 through knob3 and take observations following step 3.
- Click on 'Check Connection' button then on 'Simulate' button each time after a wire connection is made.
- Click on 'Plot' then click on 'Torque (T_M) Vs. Speed (N)'.
- Now from a slope of the plot calculate viscous friction coefficient B (newton-meter/rad/sec) of motor and calculate average value of motor back emf constant K_b (volts/rad/sec) from the observation table.
- Now enter the values of B and K_b in the corresponding box under 'Result Block'.

- Bring back all the knobs to zero value.
- Follow steps 1-4 for E_a = 8, 10 and 12 volts.
- Set 'MOTOR' switch to 'OFF' by clicking on it again and switch off the motor unit.

Step Response of Motor

First click on "Tests" button and click on 'Step Response' to perform the experiment for finding the dc motor transfer function.

- Click on 'ON' button to switch on the motor unit.
- Connect 1-3, click on 'Check Connection' button, set 'MOTOR' switch to 'ON' by clicking on it.
- Set the 'RESET' switch to 'RESET'. Set load to 0 value.

- Apply armature voltage E_a through knob1 and set E_a to 8 volts. Click on 'Simulate' button.
- Observe motor armature current (mA), speed (rpm).
- Now click on the connected wire (1-3) to delete that and connect 2-3 , click on 'Check Connection' button then on 'Simulate'.
- Observe generator voltage E_g

- Now delete connection 2-3 and connect 4-3 , click on 'Check Connection' button then on 'Simulate' to observe E_s value in DVM.
- Set E_s through knob2 to 63.2 % of E_g (measured above).

- Set 'MOTOR' switch to 'OFF'. Set 'RESET' switch to 'READY'.
- Now set the 'MOTOR' switch to 'ON', click on 'Simulate' and observe the 'Time Constant (msec)'.

- Click on 'Table' to get the observation data. Observe the step response.
- Now find the motor inertia J using the values of B, K_b (calculated before) and τ_m from observation table following the formula provided in 'Result Block'.
- Now enter the value of J in corresponding box under 'Result Block'.
- Click on 'Show Transfer Function' button to get the transfer function of the motor.

Bring back all the knobs to zero value. Follow steps (1-6) for E_a= 10 and 12 volts. Set 'MOTOR' switch to 'OFF' and switch off the motor unit..

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Sl No.	E_a (volts)	I_a (mA)	N (rpm)	E_g (volts)

Sl No.	Load Step	I_a (mA)	N (rpm)	ω (rad/s)	E_b (volts)	K_b (volts/rad/s)	T_M (newton-meter)

Sl No.	E_a (volts)	E_g (volts)	N (rpm)	E_s (volts)	Time Constant (τ_m (msec))	Gain Constant (K_M (π N/30E_a))

Result Block

K_M (Motor Constant) = rpm/volts

K_G (Generator Constant) = volts/rpm

B (Viscous friction coefficient of Motor) = newton-meter/rad/sec

K_b (Back emf constant of Motor) = volts/rad/sec

$$J = \tau_m (B + \frac{{K_b}^2}{R_a}) \ take \ R_a = 4 \Omega$$ J (Motor inertia) = newton-meter/rad/sec²