Of course that would require knowing what should be happening.

We don't always know what should be happening. When I was a teen I had a car radio and a 12 volt transformer with a fin type rectifier. I connected them together and got horrible hum. Someone said I needed a filter cap. So I experimented with a 16V electrolytic cap. Put it after the diode. No noticeable improvement. Put it before the diode - even less improvement. Put it on the primary. BOOM! Live and learn.

This was the reason that the older, more common then, wound shunt field motors had field loss protection, if this occurred during operation, they could run uncontrollably up to extremely high RPM's to destruction and injury to personnel.

I've heard this before but have never understood why it was so. Was reading a manual on the Falcon 50 jet. Somewhere it mentioned a circulating fan. If the field voltage dropped to zero volts (going from a long time ago memory - probably stated wrong) the motor could spin up endlessly to destruction. Never got my head around that notion.

 

- probably stated wrong) the motor could spin up endlessly to destruction. Never got my head around that notion.

The same principal applies with a DC series field motor, and the AC/DC universal where both have series fields.
With a DC shunt motor, where there is a fixed voltage to the field, this creates a strong field, which also aid in the voltage self generated by the motor when this generated field equals the applied voltage, rpm increase ceases.
Now if the motor loses its dc fed field, all it relies on is the residual magnetism in frame, and because this field strength is much weaker the motor start to increase in RPM. because of the higher applied voltage and the weaker generated one, it is increasingly difficult for the two to become equal, and the result is the increasing rpm runaway of the motor, snowball effect.
With the series motor, the similar thing happens except as the motor rpm increases the current drops, because the field is also in series with the armature, its current also drops incrementally with RPM, so you have the same condition/effect as the shunt field motor.
The limiting rpm factor for these motors is usually friction/load/windage etc. Of course if you lose the (open) field on a series motor, the motor stops automatically.
This is why a series field motor is used in automotive starters, as they provided very high initial torque due to the very high current of the armature is also experienced by the the field, this is why it is never wise to run a starter off load on the bench.
Max.

 

The same principal applies with a DC series field motor, and the AC/DC universal where both have series fields.
With a DC shunt motor, where there is a fixed voltage to the field, this creates a strong field, which also aid in the voltage self generated by the motor when this generated field equals the applied voltage, rpm increase ceases.
Now if the motor loses its dc fed field, all it relies on is the residual magnetism in frame, and because this field strength is much weaker the motor start to increase in RPM. because of the higher applied voltage and the weaker generated one, it is increasingly difficult for the two to become equal, and the result is the increasing rpm runaway of the motor, snowball effect.
With the series motor, the similar thing happens except as the motor rpm increases the current drops, because the field is also in series with the armature, its current also drops incrementally with RPM, so you have the same condition/effect as the shunt field motor.
The limiting rpm factor for these motors is usually friction/load/windage etc. Of course if you lose the (open) field on a series motor, the motor stops automatically.
This is why a series field motor is used in automotive starters, as they provided very high initial torque due to the very high current of the armature is also experienced by the the field, this is why it is never wise to run a starter off load on the bench.
Max.

Max has it right.
To safely test run a starter motor on a bench, first it needs to be restrained because it will lurch when energised, and second it needs to be powered by a current limited source. When powering it off a 12 volt battery I use a #16 wire, which will burn up before anything runs away. A cheap fuse, really.

 

Hi guys, thank you very much for all your replies.

The supplier has refunded me for both the motors. I am waiting on the supplier to say whether they want me to return them or not. If they do not want them back, I will break one down and see what has happened. I don't really know what I am looking for, so I will post some images here...

Thanks again,
Harold

 

Hi guys, thank you very much for all your replies.

The supplier has refunded me for both the motors. I am waiting on the supplier to say whether they want me to return them or not. If they do not want them back, I will break one down and see what has happened. I don't really know what I am looking for, so I will post some images here...

Thanks again,
Harold

If they are replacing them you need to keep them untouched until you verify that they do not want them back. Very few accept customer modified returns.

 

I have found for the most part, no one bothers about returns now, especially low priced items, it is not worth the hassle, especially where return costs or borne by the receiver.
Max.