jan_de_jong(at)casema.nl
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 Posted: Tue Jan 25, 2011 5:45 am Post subject: Starter motors |
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Having educated myself somewhat on 12V starter motors
and having looked at 13 starter motors rated from 0.85kW to 4.2kW in
more or less detail
I am sharing some generalized results:
1. voltage (12V) and power rating (in kW) are the only values on the
nameplate
2. the power rating indicates mechanical output power (torque times rpm)
3. a power rating is determined in a circuit with a certain lead-acid
battery and a fixed resistance,
not with a fixed supply voltage as one might expect;
this an industry protocol:
- 'largest battery allowed for this motor' Bosch says
(the protocol requires a certain minimum voltage drop I suspect)
- battery at 80% charge and at -20 degrees C
- with 1 mOhm in series
a 'performance curve' is produced, and in some cases made public
(fortunately)
4. at stall (rpm=0):
output power is 0,
input power is at maximum, about 2.7 to 3.5 times the rated power
at 5.5 to 7V the stall current (in A) is about half the power rating
(in W)
(the internal motor resistance is 11/power rating to 14/power rating
(in Ohm))
5. at maximum rpm (torque=0):
output power is 0,
input power is at minimum, 0.7 to 1.0 times rated power
at about 12V the idle current (in A) is 6 to 8% of the power rating
(in W)
6. maximum output power (rated power) occurs at half of maximum rpm and
about 45% of stall current
the input power is twice the output power or a little more
7. the normal start sequence of a 1.5kW (1hp) starter motor:
7.1. stall current buildup in a few tens of ms to 750A at 5.5 to 7V
7.2. speedup to maximum output power in several hundreds of ms,
the current drops to 350A at 9 or 10V
7.3. speedup to equilibrium rpm where output power generated meets power
required,
the current drops to a value not much above 100A at 11V, depending
on the load
8. during initial speedup the power required may soon rise faster than
the output power generated,
the motor will then turn in semi-stalled condition at very high
current - bad
causes are: excessive load, battery internal resistance, circuit
resistance
9. battery CA (cranking amps) rating to allow the motor to make its
rated power:
45% of half the power rating (units of A and W respectively)
-> not less than power rating / 5 (this is also a known rule of thumb)
Some starter motor sizes:
Micron: 0.5kW
Rotax 912: 0.6kW (option 0.9kW)
Jabiru: 1.0kW (option Bosch 1.5kW)
Ulpower: 1.1kW
Largest seen: Kelly 2.8kW
Cheers,
Jan de Jong
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