ECE 3502 :

POWER ELECTRONICS, MACHINES AND SYSTEMS

DC MOTORS AND DRIVES

BY

MELANIE OOI

13094637

CONTENTS

OBJECTIVE

The laboratory experiment was carried out to aid in the understanding of the circuit parameters of a DC machine. From the understanding of circuit parameters, an equivalent circuit is drawn up and load predictions of the equivalent circuit are compared to actual experimental measurements. The variable speed of DC drive systems is also examined.

INTRODUCTION

Direct current motors essentially convert electrical energy into mechanical output. The type of load it is used to drive will determine type of motor to be used. There are 3 types of DC motors, which are the shunt motor, series motor and compound motor. The speed of which the motor runs can be controlled by either varying the armature voltage or varying the field flux.

Armature speed control is achieved by setting the field current (which in turn is set by setting the field current) at its rated value and varying the armature voltage. The speed of the motor will rise or fall proportionally with an increase or decrease in armature voltage respectively. The maximum achievable speed is limited by the machine’s rated armature voltage.

Field speed control is achieved by setting the armature voltage to its rated value and varying the field flux (which is achieved by varying the field current). When flux/current is increased, the speed of the motor will decrease and vice versa. This means that the speed of the motor is inversely proportional to the flux. This method of speed control is used frequency when the motor is required to run higher than its rated speed. Above the rated speed, the motor speed would increase non-linearly to a decrease in field current.

The equations of interest here are:

  where:              w  =  motor speed (rad/sec)

ff  =  field flux (linearly proportional to If when magnetic saturation is

          ignored)

                        t    =  electrical torque developed by the motor

                        Ke = motor electrical constant

                        Va = Armature supply voltage

                        Ra  = Armature resistance

                        Ia   = Armature current

                         e_a = Counter-emf developed in the motor

By rearranging the equations, the speed of the motor in terms of the circuit parameters can be written as:

For the purpose of this experiment, the electrical and flux constants Ke and Kf are calculated as a single mechanical constant, Km since we cannot measure the flux within a DC motor experimentally. Also, saturation effects at higher field currents (whereby the motor speed changes non-linearly with field current) must be neglected. Assuming this is true, the equations can be rewritten as: 

                                                            e_a           = Km . If .w

                                                            Va          = e_a  + Ra .Ia

                                                            t          = Km . If . Ia

EXPERIMENTAL AND EQUIPMENT DETAILS

Measurement of DC motor parameters

The equipment is connected as shown in the figure below:

1. The motor, M2 is used to drive the generator M1 at the speeds of 500rpm, 750rpm, 1000rpm, 1200rpm and 1400rpm. The armature voltage of the DC generator, M1 is measured at each speed and the same process is repeated for decreasing motor speed to determine if the direction of speed change would affect the motor backemf.

2.      The motor M2 is used to maintain the motor speed at its rated value (1400rpm) and the field current of the generator M1 is varied gradually from 0 to 0.58A. The backemf at each increase is taken and the result is plotted to determine the saturation of the machine.

3.      Now the machine is set at a constant speed of 1400rpm using the motor M2. M1 is loaded with an armature current from 0-5A in 4 steps. The speed is adjusted according for each current increment to ensure that the speed is constantly at 1400rpm and the armature voltage of M1 is noted. The armature resistance R_a is determined.

4.      M2 is used to run the machine at 1000rpm and 1400rpm, while M1 is left unconnected to the supply. The electrical power into M2 is measured. Then the field winding of M1 is energized and the electrical power into M2 is measured. The change in electrical power is the rotational magnetic losses for the DC motor at the speed it is taken.

5.      Finally M1 is run at as A DC motor and M2 is left disconnected from the supply. The same procedure is carried as in Step 4. The DC power into the armature is noted at each speed and the mechanical rotational losses can be calculated by subtracting the armature resistive losses and rotational losses from this measurement.

Determination of DC machine performance

M1 is run as a DC motor and M2 as a DC generator. The speed is adjusted to 1400rpm and the load bank is adjusted to its maximum. The DC armature voltage and current, armature and field input power, speed and toque produced by the DC motor are measured. The load is reduced so that armature voltage is increased and all measurements are again noted. This is done in 5 steps for armature current up to 5A.

The same as above is repeated for M1 running as a DC generator and M2 as a DC motor.

RESULTS

Measurement of DC Motor Parameters

1. The mechanical constant is calculated from the equation: V_a = K_m * I_f * w

Table 1: Increasing speed

Table 2: Decreasing speed

Km(average) = -0.2939

2. With M2 set to maintain the speed at 1400rpm, The measurements below are taken.

Table 3: Decreasing field current

Table 4: Increasing field current

Note that at I_f  = 0.55 A, the backemf = -227V

3. M2 is used to set the speed at 1400rpm and current is incremented. Note that the field current is 0.55A here. The calculations for armature resistance is done using these two equations: e_a = Km . If .w  and Va= e_a + Ra .Ia

4. The machine is running at 1000 and 1400 rpm. Electrical power with and without field winding is calculated.

 At the speed 1000rpm

Without field current,

Ia = 0.78 A, Va = 141.5 V,

Pa  = Ia*Va

      = 0.78*141.5

      = 110.37 W

With field current,

Ia = 0.92 A, Va = 140.8 V,

Pa  = Ia*Va

      = 0.92*140.8

      = 129.5 W

Rotational losses     = 129.5 – 110.37

                              = 19.13 W

At the speed of 1400rpm

Without field current,

Ia = 0.83 A, Va = 197.7 V,

Pa  = Ia*Va

      = 0.83*197.7

      = 164.1 W

With field current,

Ia = 1 A, Va = 197.3 V,

Pa  = Ia*Va

      = 0.92*140.8

      = 197.3 W

Rotational losses     = 197.3 – 164.1

                              = 33.2 W

5. M1 is run as a DC motor and M2 is disconnected.

At the speed of 1000rpm

I = 0.64 A, V = 171.3 V,

P    = I*V

      = 0.64*171.3

      = 109.6 W

Mechanical rotational losses

= 109.6 – (0.64²)(6.579) – 19.13

= 87.78 W

At the speed of 1400rpm

I = 0.68 A, V = 273.0 V,

P    = I*V

      = 0.68*273.0

      = 185.64 W

Mechanical rotational losses

= 185.64 – (0.68²)(6.579) – 33.2

= 149.4 W

Using Microsoft Excel, the linear relationship between mechanical and magnetic rotational losses with speed can be established.

Determination of DC Machine Performance

1. With M1 acting as a DC motor and M2 as a generator:

At maximum load

V_a        = 233V   

I_a          = 0.80A

V_f         = 240V

I_f          = 0.53A

P_f         = 129.6W

Speed = 1419rpm

Torque = 8.39

Decreasing load

Experimental results

Speed is kept constant at around 1400 rpm and V_f = 240V

Theoretical Calculations

Developed torque = Km If  Ia

Load torque   = Developed torque – Mechanical Rotational Losses/Speed

Mechanical losses = 0.0352*w – 16.045 (taken from the earlier section)

2. With M1 acting as a DC generator and M2 as a DC motor:

Speed is kept constant at 1414rpm.

Experimental results

Theoretical Calculations

Developed torque = Km If  Ia

Load torque   = Developed torque – Mechanical Rotational Losses/Speed

Mechanical losses = 0.0352*w – 16.045 (taken from the earlier section)

DISCUSSION

By comparing Table 1 and Tabl2 2, it can be seen that the mechanical constant is independent of the direction of change in speed. This constant is calculated to be an average of –0.2939. From Graph 1, it can be seen that the saturation of the motor is independent of the direction of change in field current. It should also be noted that the when the field current is varied above 0.3A, the back emf no longer behaves in a linear fashion to the field current, indicating that saturation of the motor has occurred.

To increase the speed of the motor above the rated speed, the field current is decreased. However, when the motor speed is too high, the losses through heat in the motor windings would increase dramatically and can cause damage to internal parts of the motor. In other words, one can note that as speed increases, the mechanical rotational losses increases as well. This is because the machine is loaded with a higher armature current, causing I²R losses to increase rather significantly. These losses are dissipated as heat in the motor. Moreover, due to the high flux, magnetic saturation effects would also set in.

Using all the motor parameters calculated, the equivalent circuit of the motor is drawn up as below:

In the experiment, the speed was kept constant instead of varied to find load torque. Therefore the load-speed characteristic curve could not be obtained. However, the expected discrepancy between the calculated and actual results can still be discussed.

A significant difference between the predicted results and the experimental results are well expected. Since the predicted results are purely theoretical expectations based on the DC motor parameters whereby the saturation of the armature reaction was ignored in all the calculations, this would lead to variation from the actual experimental readings. Armature reaction would reduce flux and therefore the assumption of V_a = K_m * I_f * w would not hold true.

It can be seen from theoretical calculations that the efficiency of the motor increases as the armature voltage is decreased. From the expected load-speed curve, it can be seen that after a certain load amount of load introduced, the speed increases instead of continually decreasing, which is quite unexpected.

In the laboratory experiment whereby the speed was kept constant, the results obtained were somewhat peculiar and not at all in the behavior which was expected, since as input power increase, the torque seems to be decreasing. This is actually impossible, since if electrical power into the motor increases, the developed mechanical output (torque) should obviously increase. This peculiar problem is most likely caused by instrumentation problems and cannot be avoided unless proper inspection and testing of the motor and generator was done before the laboratory experiment was carried out. It could also be caused by misinterpretation of experimental data.

CONCLUSION

The speed of a DC motor can be controlled by either varying the field current at a rated armature voltage, or varying the armature voltage at the rated field current. Although saturation effects are ignored in the theoretical calculations, it should not be overlooked when performing actual running of motors.

It is very desirable to have a high output mechanical power relative to the electrical input power (high efficiency). This high efficiency conversion can be obtained for a pre-determined speed by decreasing the input armature voltage slightly. This is possible since mechanical losses at a steady speed are pretty much constant and therefore the output to input power ratio would be at a higher value.

Although high motor speeds may be somewhat more desirable in industrial use, it is not advisable to continually increase the motor speed beyond its rated speed, which is done by decreasing the field current well below its rated value. When motor speed is too high, the corresponding mechanical, electrical and magnetic rotational losses increase quite significantly. These power losses are dissipated as heat and can caused much damage to the internal components of the motor. The magnetic saturation of the motor would also occur, causing unpredictable change in motor behavior. Speed of the motor decreases when the load applied is increased. This is obvious, since the load would introduce a torque in the opposite direction to the developed torque of the motor.

To summarize, the droop characteristic of the motor speed due to increasing motor load, and the risk of damaging motor components due to over-rated field currents makes the system studied in this lab highly inappropriate for industrial use. A more practical approach would be to introduce a closed loop system whereby the speed control loop can adjust the machine rotational speed to match the specified target, by demanding more or less torque as required.

REFERENCES

1.      Wildi, Theodore; Electrical Machines, Drives and Power Systems, 4^th edition; Prentice Hall