MG5017 Electrical Machines

Introduction

In this lab we will understand the characteristics of a D.C shunt connected motor. We shall look at Inrush current behavior, motor voltage-speed relationship, motor torque response and motor locked rotor test.

Objective.

The objective of our experiment was to understand the characteristics of a D.C machine configured in shunt motor mode.

Equipment Used. 

The equipment we used in conducting our experiment were:

1. 200V DC Machine – 400V 3Ph Induction machine coupled rig
2. 3Ph Variac (variable transformer) rated to 400V/10A
3. One DC Voltmeter rated to 200V
4. One AC Voltmeter rated to 400V
5. Two DC Ammeters, rated to 5Amperes
6. One AC Ammeter, rated to 5Amperes
7. Leads
8. Tachometer

Theory.

Inrush current is the is the instantaneous maximum input current that an electrical device draws when it is first turned on. In motors, armature current I, is given by;

 

When motor starts its speed remains zero so is the back emf E, since  and thus the starting current will be very high(Introduction : What Are DC Machines ?, n.d.).

For a DC shunt motor speed is directly proportional to the voltage applied across armature terminals and inversely proportional to the field flux, Ï•. Under constant supply voltage Vs, the flux remains approximately constant. The speed increases as supply voltage increases.

The torque of a DC shunt motor is directly proportional to the product of flux and armature current. Before magnetic saturation torque is proportional to the square armature current since field flux is directly proportional to armature current in the region. Power of the motor is not constant. The output power of the motor is the product of the speed and torque. The power will be zero at zero torque and zero speed. In between the power will increase, peak and then return to zero. Torque is inversely proportional to load current, because as load increases torque required to drive the load increases which in turn draws extra load current. Since power input is constant and torque requirement is increased, the speed will automatically decrease to maintain the power equation(Kissell, n.d.).

The locked rotor test otherwise known as short circuit test is used to determine current drawn by the motor when applied load is large enough such that the motor will stop rotating. It is important in determining the appropriate overcurrent protection device.

Methodology.

We followed the following procedure in conducting our experiment.

1. We first ensured all power supplies were turned off
2. We connected the induction machine to the 400V supply via the variac, with meters to measure line voltage, and line current.
3. We connected the DC machine in shunt mode, with meters to measure field current, armature current, and voltage.

Inrush Current Observation

1. We set the DC supply to 40-50V.
2. We turned on the DC power supply.
3. We then observed the response of the machine shaft and meters as the machine starts.
4. We then turned off the DC power supply and waited for the machine to stop.

DC Motor Voltage Speed Relationship

1. We set the DC supply voltage to zero volts.
2. We turned on the DC supply.
3. We then Increased the supply voltage and recorded shaft speed by using the tachometer (set to rpm).
4. We repeated the above until DC machine rated voltage was reached.

DC Motor Torque Response

1. We ensured the DC supply was (still) set to rated voltage.
2. We then recorded the current(s) drawn by the DC machine, speed, and the current drawn by the induction machine. We took note that the current drawn in the induction machine was a proxy for load applied to the DC machine.
3. We increased the load on the DC machine by increasing the voltage supplied to the induction machine, until the DC machine stalled.
4. We then turned off the DC supply

DC Motor Locked Rotor Test

1. We set the DC supply to a nominal amount
2. We turned on the DC supply
3. We then loaded the machine to point of stalling.
4. We recorded voltage and currents.
5. We then turned off the DC supply

Results And Discussions.

Data.

Supply:

Voltage = 42 V

Current = 5 A

Table 1: D.C motor voltage-speed relationship.

Motor voltage (V)	Motor speed (rpm)
39	1062
57	1218
77	1336
97	1470
128	1707

Table 2: D.C Motor torque response.

Load Current (A)	Speed (RPM)	IA (A)	%
1	1573	0.8	5
2	1713	1.2	10
3	1677	1.8	15
4	1632	2.4	20
5	1590	3.8	25
6	1510	6.0	30

Table 3: D.C motor locked rotor test.

Current drawn (A)	Voltage (V)	Short circuit current (mA)
9	78	150

Graphs.

Figure 1:Motor speed-voltage relationship.

Figure 2: Motor speed vs Load current.

Discussions.

From our results, we observed that at machine startup a very large current is drawn by the motor. This is the inrush current behavior we discussed in our theory.

From the speed-voltage curve we observed that on increasing supply voltage, the speed of the motor increases proportionally as discussed in our theory.

From the speed-load characteristics we observed that as load current increases the machine speed increases up to 1713 rpm at 1.8A and then starts decreasing as current increases until load current is equal to input current, at 6A. This behavior is explained in our theory.

From the locked rotor test,

Conclusions.

In conclusion we did four main parts in our experiment, first for the inrush motor current we studied the behavior of starting current when starting the motor. Second, we learned the relationship of motor speed as applied voltage varies. Third we learned about the behavior of torque and speed with increasing load and finally we studied the short circuit behavior of the D.C machine. So, we concluded many important relations among the motor parameters and learned how to take appropriate measurements while noting the effect it has on the others.

References.

Introduction : What are DC Machines ? (n.d.).

Kissell, T. E. (n.d.). DC Shunt Motors.

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