Universal motors

[EpsilonRho]

Is it correct to assume that a typical universal motor like they ones used
in an AC drills for example, if supplied with rectified AC will increase
efficiency, due to a reduction in iron losses? My thinking is the following:

1. The area under the hysteresis loop is decreased
Ph = Kh × f × Bn watts m-3 (n~= 1.6) (Ph = power loss due
hysteresis)

2. Eddy currents are decreased

Pe= Ke × f2 × B2 (Pe = power loss due E. currents)

TIA for any pertinent comments.

Gene

 

[Bob Ferapples]

Is it correct to assume that a typical universal motor like they ones used
in an AC drills for example, if supplied with rectified AC will increase
efficiency, due to a reduction in iron losses? My thinking is the following:

1. The area under the hysteresis loop is decreased
Ph = Kh × f × Bn watts m-3 (n~= 1.6) (Ph = power loss due
hysteresis)

2. Eddy currents are decreased

Pe= Ke × f2 × B2 (Pe = power loss due E. currents)

TIA for any pertinent comments.

Gene

A universal motor is basically a DC motor anyway, so "rectifying" the
AC is already done in the motor. Besides, why bother trying to improve
the efficiency of something so seldom used anyway? Hardley worth the
effort.

 

[Don Kelly]

----------------------------

Is it correct to assume that a typical universal motor like they ones used
in an AC drills for example, if supplied with rectified AC will increase
efficiency, due to a reduction in iron losses? My thinking is the
following:

1. The area under the hysteresis loop is decreased
Ph = Kh × f × Bn watts m-3 (n~= 1.6) (Ph = power loss due
hysteresis)

2. Eddy currents are decreased

Pe= Ke × f2 × B2 (Pe = power loss due E. currents)

TIA for any pertinent comments.

Gene

The universal motor is simply a DC series motor which will run on both AC
and DC. It runs better on DC. The main culprit on AC is the inductance of
the field and armature coils as this does cause a phase shift and a decrease
in the back emf so the speed at a given torque will be less and performance
does suffer.
This effect will be much larger than a change in core losses.

Note- the area under the hysteresis loop depends on the peak flux and the
mechanical construction. The loss change that you have referred to is due to
frequency. Ditto for eddy currents. In the stator, you will get a reduction
of these losses but not in the rotor which sees a varying flux at a
frequency dependent on speed.
A motor intended for DC only doesn't need a laminated stator or pole pieces
but the rotor is laminated. --

Don Kelly @shawcross.ca
remove the X to answer

 

[Don Kelly]

----------------------------

I am not very familiar with universal motors. If they are actually to be
used with dc, is there not a good chance that excessive current will flow
under heavy load or stall? Do they have extra resistance built in because
there will be no inductive reactance to limit current?

Bill
-- Ferme le Bush

They are basically series DC motors, with the advantages and disadvantages
of the breed. Certainly the stall current and torque will be higher on DC
but, as with any motor, AC or DC, operating above rated current is not
recommended except for intermittent use.
I would be more worried about no load conditions and overspeeding.

What will happen using AC would be that for a given torque, the speed would
be lower than with DC and the maximum current and torque will be less.