Outline
Electrical Drive Systems 324
Synchronous Motors
1
Dr. P.J Randewijk
Stellenbosch University
Dep. of Electrical & Electronic Engineering
Chapman, Chapter 5
5.1 – Basic Principles of Motor Operation
5.2 – Steady-State Synchronous Motor Operation
5.3 – Starting Synchronous Motors
5.4 – Synchronous Generators and Motors
5.5 – Synchronous Motor Ratings
Stephan J. Chapman
Chapter 5 (5th Edition)
1 / 14
5.1 Basic Principles of Motor Operation
2 / 14
5.1 Basic Principles of Motor Operation (cont.)
Although the equivalent circuit is essentially the same,
the stator current now flows (per definition) into the
machine, i.e. the machine now absorbs electrical energy
Generator Operation – Fig. 5–3
The KVL equation for the machine is thus
Vφ = EA + jXS IA + RA IA
(5–1)
Motor Operation – Fig. 5–04
Magnetic Field Perspective
The biggest difference between motor operation and
generator operation is that:
for generator mode of operation, the rotor field, BR , pulls
the stator field, BS , and hence EA is leading. . .
but for motor operation, the stator field, BS , now pulls
the rotor field, BR , and hence Vφ is leading. . .
3 / 14
4 / 14
5.2 Steady-State Operation
5.2 Steady-State Operation (cont.)
The torque in terms of known phasor quantities
(remembering that P = ωτ
Torque-Speed Characteristic “Curve”
Because it is a synchronous machine, the speed is
constant regardless of the load – i.e. 0% speed
regulation
τind =
3Vφ EA sin δ
ωm XS
(4–22)
The maximum torque the motor can deliver is at δ = 90◦
and is called the pullout torque
If the mechanical load’s torque exceeds the torque the
motor can deliver,
the stator field will start to “lap” the rotor field with
disastrous consequences
this is called pole slipping
5 / 14
5.2 Steady-State Operation (cont.)
6 / 14
5.2 Steady-State Operation (cont.)
The effect of Load Changes on a Synchronous
Motor
From Fitzgerald Fig. 5–1, the torque, T (or τ in
Chapman) as a function of δ, can graphically be
depicted as
Similar to that of generator mode of operation, just with
EA lagging – see Fig. 5–6 (b)
For stability reasons, generators and motors are
operated at |δ| < 90◦ – see operating points g and m
respectively. . .
7 / 14
8 / 14
5.2 Steady-State Operation (cont.)
5.2 Steady-State Operation (cont.)
The Effect of Field Current Changes on a
Synchronous Motor
The Synchronous Motor and Power-Factor
Correctioning
Similar to that of generator mode of operation, just with
EA lagging – see Fig. 5–8 (b)
By over magnetising a synchronous motor, the power
factor for the rest of the plant can be corrected – see
Example 5–3
+ Ignore V-curves – Fig. 5–9
9 / 14
5.2 Steady-State Operation (cont.)
10 / 14
5.3 Starting Synchronous Motors
The Synchronous Capacitor or Synchronous
Condenser
The following methods can be used to start – i.e.
synchronise – a synchronous motor with the AC supply:
Power utilities (e.g. Eskom in South Africa) have large
synchronous motor connected to their power grid at
strategic places (e.g. Muldervlei, near Stellenbosch)
that are operated at no-load (mechanical) with the sole
purpose to do reactive power (i.e. VAR) compensation
on the power system – see Fig. 5–15
11 / 14
using a variable voltage, variable frequency AC supply –
i.e. basically a power electronic “drive”
using a non-synchronous motor, e.g. an induction motor,
as a pilot motor to start the synchronous motor
using a synchronous motor with damper – or
amortisseur windings – i.e. basically “squirrel cage”
windings (will be discussed when we do induction
machines)
12 / 14
5.4 Synchronous Generators and Motors
5.5 Synchronous Motor Ratings
The following is a nice summary of the four modes of
operation for synchronous machines – see Fig. 5–20
Similar to generator ratings. . .
With the only difference, that for a generator, the power
factor rating was given as lagging – for a motor it is
leading. . .
In both cases this has to do with how much the machine
can be over excited, i.e. the maximum value of the field
current. . .
13 / 14
14 / 14