30 Questions
How can the balance of ampere-turns between the primary and secondary circuits be translated into apparent power balance in an ideal transformer?
Ip Ep = Is Es
What is the equation representing the balance of ampere-turns between the primary and secondary circuits in an ideal transformer on load?
IpNp = IsNs
What is the relationship between the primary and secondary power factors in a transformer on full-load?
They are always nearly the same
What is the term used to refer to the magnitude of complex power in a transformer?
Apparent power
In terms of complex power, what is the relationship between the power supplied to the primary winding and the power delivered to the load by the secondary winding in an ideal transformer?
IVp = IVs * p * s
What condition implies that the primary and secondary power factors are nearly the same on full-load in a transformer?
Vp Ip cosθp = Vs Is cosθs
Who built the first practical modern transformer in 1885?
William Stanley
What is the alternating flux produced at the primary voltage denoted by?
$\Phi$
What does the transformer E.M.F. equation describe?
Voltages induced in the two coils due to the alternating flux.
In the transformer E.M.F. equation, what does Φ represent?
Alternating flux produced at the primary voltage
What is the core made up of in the first practical modern transformer built by William Stanley?
Individual sheets of metal (laminations)
Why is only one high-voltage insulator and lightning arrester needed in the self-protected distribution transformer?
Because one side of the 7200-V line and one side of the primary are grounded.
What is the primary current like when the secondary winding of the transformer is on open-circuit?
The primary current is such that the primary ampere-turns are just sufficient to produce the necessary flux for inducing an e.m.f.
What is the usual percentage of the full-load primary current that the magnetising current represents?
The magnetising current is usually about 3 – 5 per cent of the full-load primary current.
When a load is connected across the secondary terminals, what effect does the secondary current produce?
The secondary current produces a demagnetising effect.
What happens to the e.m.f. induced in the primary winding when a load is connected to the transformer?
The e.m.f. induced in the primary winding is reduced slightly.
How does the primary current change when the e.m.f. induced in the primary winding is reduced?
The primary current increases appreciably.
What is the function of the current produced in the secondary winding when a load is connected?
The secondary current produces a magnetic flux that opposes the change in the flux in the primary winding.
Explain why the appearance of IsNs due to the secondary load current must be counter-balanced in a transformer.
To maintain the conservation of energy principle, the equal and opposite IpNp must be introduced to balance the energy extraction from the secondary.
What is the significance of the rate of EsIs in a transformer and how is it related to the rate of EpIp?
The rate of energy extraction from the secondary (EsIs) must be matched by the introduction of energy at an equal rate in the primary (EpIp) to maintain equilibrium.
Why is it important to have equal and opposite currents in the primary and secondary of a transformer?
Having equal and opposite currents (IpNp and IsNs) ensures that the energy balance required for efficient operation of the transformer is maintained.
How does the conservation of energy principle apply to the operation of an ideal transformer?
In an ideal transformer, the conservation of energy principle requires that the energy input in the primary side equals the energy output in the secondary side.
Explain the relationship between VsIs and VpIp in the context of energy transfer in a transformer.
VsIs represents the energy extraction rate from the secondary, which should be balanced by the energy introduction rate VpIp in the primary.
What role does the equal and opposite energy introduction rate EpIp play in the operation of a transformer?
EpIp, being equal and opposite to EsIs, ensures that the energy balance is maintained between the primary and secondary sides of the transformer.
What are the conditions in an ideal transformer where equation 4.6 holds exactly?
No voltage drops in resistance or leakage reactance, vanishingly small m.m.f. required to maintain main flux, and no core loss
Describe the phase relations in an ideal transformer according to Figure 4.3(c).
Primary voltage Vp is in phase with Ep, secondary terminal voltage Vs is equal to Es
What determines the phase and magnitude of the secondary current in an ideal transformer?
Nature and magnitude of the load
What is the relationship between secondary m.m.f. and primary m.m.f. in an ideal transformer?
Secondary m.m.f. IsNs is equal and opposite to primary m.m.f. IpNp
Explain the stability conditions in an ideal transformer represented in Figure 4.3(c).
Ep must always be equal to the applied voltage Vp, constant flux Φm inducing Ep, and no resultant m.m.f. in the common magnetic circuit
What are the implications of having no resistance and leakage reactance drops in an ideal transformer?
E.m.f. Ep must always be equal to the applied voltage Vp, constant flux Φm inducing Ep, and no resultant m.m.f. in the common magnetic circuit
Study Notes
Ideal Transformer
- An ideal transformer on load has a demagnetizing flux, Φs, which is neutralized by the increase in primary ampere-turns (IpNp).
- The primary ampere-turns (IpNp) and secondary ampere-turns (IsNs) are nearly equal, resulting in a magnetic field (m.m.f) balance: IpNp = IsNs.
Transformer Equations
- Equation 4.5: IpNp = IsNs
- Equation 4.6: Ep/Es = Vp/Vs = Np/Ns
- Equation 4.7: IpEp = IsEs (apparent power balance)
- Equation 4.7(b): IpVp = IsVs* (complex power balance)
Transformer Construction
- Core-type transformer: core surrounds the windings
- Shell-type transformer: windings surround the core
- Figure 2.2(a) and 2.2(b): shell-type transformer construction
- Figure 2.3(a): cutaway view of self-protected distribution transformer
- Figure 2.3(b): typical shell-type transformer
- Figure 2.3(c): the first practical modern transformer built by William Stanley in 1885
Transformer E.M.F. Equation
- Equation 3.1(a): Φ = Φmsin2πft (alternating flux produced at the primary voltage)
- Equation 4.3(b): Ep/Es = Np/Ns = α
- Equation 4.4: Ep/Es = Vp/Vs = Np/Ns
Ideal Transformer on Load
- When a load is connected, the secondary current produces a demagnetizing effect, reducing the flux and e.m.f. induced in the primary winding.
- The primary current increases to counterbalance the demagnetizing effect, resulting in a magnetic field (m.m.f) balance.
Equivalent Circuit and Phasor Diagrams
- In an ideal transformer, there are no voltage drops in resistance or leakage reactance, and no core loss.
- Equation 4.6 holds exactly, and the phase relations are simple: Vp = Ep, Vs = Es, and Ip = Is.
- The secondary m.m.f. is IsNs, and the primary m.m.f. is IpNp, which is equal and opposite to the secondary m.m.f.
Test your knowledge on transformer construction types such as core-type and shell-type transformers. Identify the differences between these constructions and learn about self-protected distribution transformers.
Make Your Own Quizzes and Flashcards
Convert your notes into interactive study material.
Get started for free
