Re: (ET) So how hot can a drive motor get?

["Konstanty, Walter (GE Energy Services)" <Walter Konstanty ge com>]

David,
   As usual, a good explanation.  I believe these motors were built with
primarily Class F insulation but with very low temperature rise for
traction (overload) duty.

E12  5BCE56KB5B  (model from my notes)   
 Rated 1.5HP  @36VDC   37 amps  3,500RPM   2.25 amps field
     40C rise at this rating

  So, the temperature rise is only 40C for the full load rating of 37
amps but as a traction motor it can be overloaded quite high producing
(theoretically) Class B to F temperature rises that David notes below.
  Motor torque is field flux (amps x turns) and armature amps..... with
fixed motor field volts, the field resistance increases from cold to
"hot" which lowers the field amps which is less torque and more motor
speed which can increase armature amps and temperature rise.
  The thermostat is probably rated for 110C and mounted on the outside
of the commutating field to indicate overload (shunt field heating
remains below the 40C rise for continuous duty).  If the thermostat
trips, the overload was too long for the motor to remain within Class B
rise for normal insulation life and the 10C rule noted below
applies..... 
  Motor frame temp is harder to predict but David's note about placing
your hand on it is pretty good.... about 65C with my paws is the limit.
Cooking one of these motors can get expensive so best to have the
thermostat in the circuit.  Don't wait for the smell or smoke test.

...Walt

-----Original Message-----
From: David Roden (Akron OH USA) [mailto:etpost drmm net] 
Sent: Monday, June 06, 2011 3:42 AM
To: elec-trak cosmos phy tufts edu
Subject: Re: (ET) So how hot can a drive motor get?

On 5 Jun 2011 at 15:13, Chris Zach wrote:

> So what's the max external and internal temps for an E12 shunt motor?

The motor's nameplate should have a temperature rating.  These are
letter codes from A to H (oddly, C, D, E, and G seem to be never used),
and they specify the manufacturer's limits on internal temperature rise
before the insulation breaks down and the motor lets its smoke out.  The
rule is that if you maintain these temperatures the motor is supposed to
last 20,000 
hours.   Each 10 deg C increase above the maximum in routine operation
cuts 
the expected life by half.  

Class A insulation shouldn't exceed 105 deg C

Class B (common in household appliances), 130 deg C

Class F (common in industrial motors), 155 deg C

Class H (often used in road EVs), 180 deg C

These are the temperatures measured at the windings.  It's a bit more
complex than this, with other limits on ambient temperature and motor
temperature rise, but that's the gist of it.  

Note that the armature windings may be hotter than field because the
field has that big hunk of metal attached to it, acting as a heatsink.
Thus to the cut to the chase, the general guideline in road EVs (usually
class H) is that if you can't keep your hand on your motor after a run,
you're getting 
it too hot.   



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