Motors -Single phase motors, Universal motors, NEMA Classification

Types of single phase motors.

Resistance Split Phase motors
It has Low Starting current.  The starting torque is moderate.  Applications include Fans, grinders, centrifugal pumps, washing machines

Capacitor Start Motors
The starting torque is high.  Hence, it is used for applications with heavy connected loads such as pumps, conveyors, compressors, etc.  It is the most widely used of all single phase motors.  It is used for motor sizes up to 6 kW.  The capacitor is in line during starting  The capacitor is disconnected by a centrifugal switch as the motor approaches the rated speed.

Permanent Capacitor Motor
The starting torque is high.  The power factor is good due to the presence of the capacitor at all times.  The efficiency is high and the torque produced is smoother.  Ceiling Fans use this type of motor

Two Value Capacitor Motors
This motor is also a capacitor based motor.  However, here there are two capacitors instead of one.  One capacitor is in line during start of the motor while the other capacitor comes in line when the motor is running.

Shaded pole motor
The Shaded pole motor works on the principle of a shading ring which is fitted to the stator poles. This ring varies the reactance and creates two fields.  This motor is simple in construction.  It is used for fractional kW applications such as in hair dryers, table fans, etc.  It has low starting torque and lower power factor.

Resistance split phase motor

The resistance split phase motor has two windings in parallel.  One of the winding is the main winding.  The other is the auxiliary winding.  There is a centrifugal switch in series with the auxiliary winding such that the winding is disconnected at around 75% of the synchronous speed.

The auxiliary winding is made of thin wire of a few turns such as that it has low reactance and high resistance. This creates a magnetic field with a displacement of 30 degrees.  This produces a torque which is moderate and can be used in motors up to 250 watts.

Permanent Split Phase motor

A permanent Split phase motor consists of a main winding and an auxiliary winding.  The auxiliary winding has a capacitor in series.  The auxiliary is always in circuit.  It produces a flux which is displaced from the main flux by 90 degrees.

The permanent split phase motor suffers from torque pulsations at main speed.  The permanent split phase motor is used up to 1/4 horse power.  The starting torque is relatively lower.  Hence, the permanent split phase motor is not used for applications which require high torque.

Universal Motor 

Universal Motors are motors which run on both AC as well as DC power.  These motors find wide application in household electric appliances such as mixers, blenders.

The small size, high power and speed of universal motors make them ideal in these applications.  The Universal Motor is the motor of choice for washing machines as they can be used to agitate the drum by easily reversing direction.

A universal motor is a dc series motor which usually has some modifications which enable it to run efficiently on AC as well.  Running the motor on AC leads to eddy currents being developed in the core of the motor.  Hence, the core and poles of universal motors are made of laminated sheets.  The windings of the motors are made thicker to reduce the reactance. 

Like all DC machines, universal motors suffer from the effects of commutation such as sparking.  Hence, universal motors are used mainly below 1000W.  The high speed of the universal motor enables it to deliver a higher power even while having a small size.  Universal motors are usually kept coupled to the loads such as gears or blowers as they tend to run at very high speeds on no load conditions.

Speed Control is usually done by electronic means.

NEMA Classification of motors

The National Electrical Manufacturers Association (NEMA)  has classified motors design in the following manner based on starting current, rotor torque, breakdown torque and slip.

They are

NEMA design A

maximum slip of 5 %
medium to high starting current
normal locked rotor torque
normal breakdown torque
suited for a broad variety of applications
Typical applications : fans and pumps

NEMA design B

maximum 5% slip
low starting current
high locked rotor torque
normal breakdown torque
Typical Applications
common in HVAC application with fans, blowers and pumps

NEMA design C

maximum 5% slip
low starting current
high locked rotor torque
normal breakdown torque

Typical application : High inertia starts - as positive displacement pumps

NEMA design D

maximum 5-13% slip
low starting current
very high locked rotor torque
Typical application : cranes, hoists etc.