hi,
i am in bit of confusion about dc motors.
i wanted to know what's the difference between 24 v and 48 v (1500 rpm or 3000 rpm) under 400 watts dc motor.
i want to know how they effect torque produced and speed.
is rated rpm is top speed of motor??
bldc or pmdc? does it effect in terms of speed and torque for same wattage.
You might get better info about electric motors elsewhere on the web.
In theory the conversion of electricity into mechanical power is 100% efficient (very different from converting heat into mechanical power). In practice, of course there are huge losses in small motors due to friction, heating etc.
Looked at from a theoretical point of view you could calculate the torque from the watts if you know the rpm. This is complicated by the fact that an electric motor draws less current (for a given voltage) as the RPM increases. Consequently at maximum rpm the motor can't produce any useful torque. (And it draws maximum current when it is stalled - the stall current).
All this is a long winded way of saying you need to get the specification sheet for a particular motor.
...R
strawchiu_2014:
hi,
i am in bit of confusion about dc motors.
i wanted to know what's the difference between 24 v and 48 v (1500 rpm or 3000 rpm) under 400 watts dc motor.
Is this the same motor powered at different voltages? - if so then its expected behaviour.
i want to know how they effect torque produced and speed.
is rated rpm is top speed of motor??
"Rated" normally means full continuous load - maximum power speed and torque for
continuous duty. It may well depend on nominal supply voltage.
bldc or pmdc? does it effect in terms of speed and torque for same wattage.
BLDC's are PMDCs. Brushed and brushless are two different ways of making
a permananent magnet DC motor and they share the same basic performance properties:
(no-load) speed is proportional to voltage, torque is proportional to current.
In fact most of a motor properties boil down to two figures, the torque-constant
(measured in newton-metres per amp) and the winding resistance in ohms.
Resistive losses in the motor are I-squared-R where R is the winding resistance -
you want to keep this well below the total power (otherwise you have a heater!)
The torque constant is also the voltage constant (measured in volts / (rad/s)).
This is because (for an ideally efficient motor) the electrical power is the same
as the mechanical power (volts x amps = torque x angular-velocity).
Often the voltage constant is presented the other way up as rpm/volt.
The third important property of a motor is the maximum thermal power it
can dissipate - related to the size.
The thermal power dissipated is usually dominated by the winding resistance
losses, called "copper losses", and there is also some magnetic heating from the
laminations called "iron losses", but they are usually secondary.
The mechanical power from a motor is ultimately limited by the maximum speed
it can safely run at and the thermal limit. All else being equal a faster spinning
motor can handle more power for the same losses (although bearing friction and
air-resistance eventually become dominant).
When a motor is doing significant work there is always inefficiency due to copper
losses - some of the supply voltage is "wasted" overcoming the IR drop. The rest
balances the "back-EMF" which is proportional to the speed.
From a cold start there is no speed, so no back EMF, so the motor winding
looks like a piece of wire of resistance R - the initial current will be as much
as V/R if the supply can provide that much, meaning a massive current and
torque spike - you shouldn't start a very large DC motor this way as the forces
involved can damage the motor windings or housing - either limit the current
or ramp up the effective supply voltage using PWM.