Discovering the potential of regenerative testing or Hopkinson testing
Luciano Bertene
Regenerative or Hopkinson testing is a special method for evaluating the performance and characteristics of large electrical machines such as generators and DC motors. This testing is particularly valuable for understanding the behavior of devices under load conditions and provides information on efficiency, losses and other critical parameters. In the previous post we saw how to determine the efficiency of DC machines by Sweat Burn Test . The regenerative or Hopkinson test method is another method to determine the efficiency of a DC machine. This method saves electricity and provides much more accurate DC machine results. We need similar DC machines and supply voltage to perform this test.
Working principle
Two similar DC shunt machines are mechanically coupled and connected in parallel through the DC power supply. In both machines, one works as a motor and the other as a generator , varying the excitation of the shunt field. The electrical energy supplied to the internal combustion engine is converted into mechanical energy, the rest goes to the engine's other losses.
This converted mechanical energy is delivered to the generator. The electrical energy produced by the generator is returned to the engine, except for the energy lost as generator loss. Thus, the electrical energy extracted from the DC supply is the sum of the motor and generator losses, which can be read directly through the ammeter and voltmeter. Since the input power of the DC power supply corresponds to the power required to balance the losses of two machines, this test can only be performed with a small amount of current.
We can stress the machine by varying the intensity of the machine's field. Therefore, we can determine the total loss of the device under each load. The machine's switching performance and temperature rise can be observed as the device is tested under full load conditions.
Load distribution for DC shunt generators
The figure above shows the typical circuit diagram of a Hopkinson test. Two similar DC shunt machines are mechanically coupled and connected in parallel across the DC source. By varying the field strength of each machine 1 m, Machine M can work as a motor and machine G can work as a generator. The motor draws current I 1 from the generator and current I 2 from the DC power supply. Thus, the current input to the motor is (I 1 +I 2 ). The electrical energy of the DC source is V EU 2, which corresponds to the total losses (motor and generator losses). The shunt field current of the motor is I 4, and the generator is I 3 .
Calculation
Let V = supply voltage
Motor power input = V(I 1 + I 2 )
Generator power input = VI 1
We can determine the efficiency of DC machines in two cases.
Assuming equal machine efficiency
Assuming equal losses of iron, friction and wind
Assuming equal machine efficiency
Motor output power = η x motor input power
= ηV(I 1 +I 2 )
That is, engine input power = generator input
Now the generator output = η x generator input
= η x ηV(I 1 +I 2 )
= η2V(I 1 +I 2 )
VI 1 =η 2 V(i 1 +i 2 )
Generator output η = √ {I 1 / (I 1 + I 2 )
The above expression determines the efficiency satisfactorily and perfectly for an approximate test. If the case needs to be more precise, the efficiency of the two machines can be defined separately using the following expressions.
Assuming equal losses of iron, friction and wind
There is no need to assume that the efficiency of both machines is the same. The two DC machines do not have the same armature winding and field winding. However, since both machines are identical, the iron losses, friction losses, and air resistance losses of both devices are the same. Based on this, we can determine the efficiency of each machine.
R A = Armature winding resistance of individual machines.
EU 3 = G shunt field current generator
EU 4 = motor branch line current M
Copper losses in the generator armature = (I 1 +I 32 )R A
Motor armature copper losses = (I 1 + I 2 – EU 42 )R A
Copper loss in the shunt field at G = VI 3
Copper loss in the shunt field at M = VI 4
The energy consumed by the DC source is VI 2 and corresponds to the total losses of the motor and generator.
VI 2 = Total engine and generator losses
To obtain the iron loss, friction, and drag, subtract the armature copper loss and the shunt copper loss of both machines from VI 2 .
Total losses of 2 machines (M&G)
=VI 2 – ((EU 1 +eu 3 ) 2 R A + (eu 1 +eu 2 -EU 42 R A +VI 3 +VI 4 )) = W
To determine individual machine losses, divide by 2
i.e. total losses of each machine = C/2
Calculating the Efficiency of a Motor and Generator
Calculating the efficiency of a motor and generator is essential to evaluate their performance in converting electrical and mechanical energy. Efficiency serves as a valuable indicator of how effectively these machines work.
Let’s dive deeper into engine and generator efficiency calculations:
Engine efficiency
Input motor power : The motor input power is given by the expression V(I1 + I2), where V is the voltage and I1, I2 are the currents.
Total losses (Wm) : Total losses in the motor are calculated as (I1 + I2 – I4²)Ra + VI4 + (W/2), where Ra represents the armature resistance and W represents the mechanical losses.
Engine efficiency (ηm) : Engine efficiency is calculated using the formula: ηm = (V(I1 + I2) – Wm) / (V(I1 + I2)).
Input motor power = V(I 1 + I 2 )
Total losses = (I 1 +I 2 -I 42 )R A +VI 4 + (With 2)
=WM
Motor efficiency η M = (Input – Losses) / Input
= (V(I 1 +I 2 ) – C M ) / (V(I 1 +I 2 ))
Generator efficiency
Generator output power : The generator output power is given by VI1, where V is the voltage and I1 is the current.
Total losses (Wg) : Total losses in the generator are calculated as (W/2) + (I1 + I3²)Ra + VI3, where Ra is the armature resistance and W are the losses.
Generator efficiency (ηg) : Generator efficiency is calculated as ηg = VI1 / (VI1 + Wg).
Generator output power = VI 1
Total losses = (W/2) + (I 1 +I 32 )R A +VI 3
=W G
Generator efficiency η G =VI 1 / (VI 1 +W G )
Conclusion
In summary, regenerative or Hopkinson testing is an effective means of unlocking the potential of large electrical machines by providing accurate information about their efficiency, losses and performance characteristics under different load conditions. This specialized testing method allows engineers and researchers to make informed decisions regarding machine design, operation and application optimization.
