AC Series and Universal Motors

i Learning Objectives

By the end of this chapter you should be able to:

Explain why a DC series motor produces unidirectional torque on AC.
List the design changes that turn a DC series motor into an AC series or universal motor.
Draw the phasor diagram and compute speed and power factor on AC and DC.
Describe the speed–torque characteristic, speed control and applications.

Section 87-1

Why a Series Motor Works on AC

In a series motor the same current flows in the field and the armature. When the supply reverses, both the flux \(\phi\) and the armature current \(I_a\) reverse together, so the torque \(T = k\phi I_a\) keeps its direction.

\[T(t) = k\Phi_m I_m\sin^2\omega t = \frac{k\Phi_m I_m}{2}(1 - \cos 2\omega t)\]

The torque has a steady average and a component pulsating at twice supply frequency — the same \(2f\) pulsation as the single-phase induction motor.

Section 87-2

Design Modifications for AC Operation

Table 87.1 — Problems of a plain DC series motor on AC, and their remedies.
Problem on AC	Remedy
Eddy-current loss in the field poles and yoke	Laminate the entire magnetic circuit
High field reactance → low power factor and low current	Few field turns; strong armature (more conductors) to keep torque
Armature-reaction reactance	Compensating winding in the pole faces (larger machines)
Transformer EMF in coils under commutation → sparking	High-resistance leads and brushes; low frequency in traction
Saturation at the AC peak	Operate at lower flux density

Universal motor. A small (typically < 1 kW) series motor designed to run on either DC or single-phase AC at nearly the same speed is called a universal motor. Two forms exist: non-compensated (salient poles, cheap, for the smallest ratings) and compensated (distributed field and compensating winding, for larger ratings).

Section 87-3

Phasor Diagram and Speed on AC

The back EMF \(E_b\) is proportional to flux and speed and, since the flux is in phase with the current, \(E_b\) is in phase with \(I\). The supply voltage must also overcome the resistive drop \(IR\) and the reactive drop \(IX\) of the field and armature:

\[\vec V = (E_b + IR) + jIX \;\Rightarrow\; E_b = \sqrt{V^2 - (IX)^2} - IR, \qquad \cos\phi = \frac{E_b + IR}{V}\]

On DC, \(E_b = V - IR\). For the same current (and hence, roughly, the same flux), \(E_b\) on AC is smaller, so the motor runs slower on AC.
At high speed \(E_b\) is large compared with \(IX\), and the power factor approaches unity; at standstill it is poor.

1 Worked Example 87.1 — Speed on AC Versus DC

Problem. A 230 V universal motor has total resistance 20 Ω and total reactance 80 Ω at 50 Hz. It takes 1.0 A in both cases. Compare its speed on 230 V AC with that on 230 V DC, and find the AC power factor. Assume the same flux for the same current.

\[E_{b,\text{dc}} = 230 - 1(20) = 210~\text{V}\]

\[E_{b,\text{ac}} = \sqrt{230^2 - 80^2} - 20 = 215.6 - 20 = 195.6~\text{V}\]

\[\frac{N_\text{ac}}{N_\text{dc}} = \frac{195.6}{210} = 0.932, \qquad \cos\phi = \frac{215.6}{230} = 0.937~\text{lag}\]

The motor runs about 7 % slower on AC. (Strictly, the peak AC flux is \(\sqrt2\) times the RMS-equivalent value and may saturate, which narrows the difference.)

Section 87-4

Characteristics and Speed Control

Series characteristic: very high starting torque, and speed that falls steeply as load rises. At no load the speed is limited only by friction and windage — small universal motors run at 15 000–30 000 rev/min and are always connected to a load.
High power-to-weight ratio: because output is proportional to speed, a high-speed motor is small and light for its power.
Speed control: TRIAC phase-angle control of the applied voltage (most common), tapped field, series resistance, or a centrifugal governor that switches a resistor.

Applications

Mixers and grinders, vacuum cleaners, portable drills and saws, hair dryers, sewing machines, older washing-machine drives.

Limitations

Brush and commutator wear, sparking and radio interference (needs suppression capacitors), noise, and a relatively short life compared with induction motors.

Section 87-5

Summary and Common Mistakes

Key Points

Field and armature current reverse together, so series-motor torque is unidirectional on AC.
AC operation requires laminated poles, few field turns and, for larger ratings, compensation.
On AC: \(E_b = \sqrt{V^2 - (IX)^2} - IR\); the motor runs slower than on DC.

Common Mistakes

Subtracting \(IX\) algebraically from \(V\) instead of in quadrature.
Running a universal motor unloaded during testing — it may overspeed.
Thinking a shunt motor would work equally well on AC (its field current lags far behind its armature current).

Section 87-6

Chapter Review

Practice Problems

P87.1 A universal motor with \(R = 30~\Omega\) and \(X = 60~\Omega\) takes 0.8 A from 230 V AC. Find the back EMF and power factor.

Show answer
\(IX = 48\) V; \(\sqrt{230^2 - 48^2} = 224.9\) V; \(E_b = 224.9 - 24 = 200.9\) V; pf \(= 224.9/230 = 0.978\).
P87.2 The motor of P87.1 runs at 12 000 rev/min on AC. Estimate its speed on 230 V DC at the same current.

Show answer
\(E_{b,\text{dc}} = 230 - 24 = 206\) V; \(N = 12\,000 \times 206/200.9 = 12\,300\) rev/min.

Multiple-Choice Questions

MCQ 1. A universal motor is essentially a:
(a) shunt motor (b) series motor (c) induction motor (d) synchronous motor

Show answer
(b)
MCQ 2. The field poles of an AC series motor are laminated to reduce:
(a) copper loss (b) eddy-current loss (c) friction (d) armature reaction

Show answer
(b)

Conceptual Questions

Why does a universal motor have a better power factor at high speed?
What is the purpose of a compensating winding in an AC series motor?
Why are universal motors preferred in portable power tools?

Looking Ahead

The remaining chapters describe machines built for control rather than bulk power. Chapter 88 introduces the stepper motor, which turns digital pulses directly into precise angular steps.