Electric Motors

What is a brushless PMAC motor?

PMAC is an acronym for Permanent Magnet Alternating Current.    Permanent Magnets are imbedded in the rotor.   There are no brushes because the commutation is electronic.   The controller replaces the brushes by picking up a signal from the hall effects sensor on the motor, which tells the controller where the rotor is, so the controller knows how to make the rotating magnetic field.

Why do Propulsion Marine Electric Drives use Brushless PMAC motors?

Brushless Permanent Magnet Electric Motors require no maintenance.    Motors rotate by creating a rotating magnetic force which pulls around a magnetic rotor.   In an induction motor, the rotor’s magnetic force is created by induction from the stator, hence two magnetic forces are being created, one pulling the other around.   In a permanent magnet motor, the rotor’s magnetic fields are created by rare earth Neodymium Permanent Magnets, and only the stator has to create a rotating electro magnetic force.    This results in higher efficiency.

Why are AC motors better than DC motors?

AC motors have three cables providing power to three phases, where DC motors have only two cables.   AC motors have electronic commutation outside of the motor and DC motors have mechanical commutation by the brushes.     AC motors are maintenance free.

What are the limiting factors that determine how powerful a motor is?

If the efficiency of a motor is 90%, then 10% of the power that gets to the motor is converted to heat.   If a motor consumes 5 KW, then 500 watts of that is converted to heat.  The motor must be able to dissipate that amount of heat.    On an intermittent basis, a motor can create enormous amounts of power and very little heat, but on a continuous basis the heat builds up in the motor and the motor must rely on a cooling system to dissipate the heat.

How does Propulsion Marine rate motors and how  is the different from the competition?

All Propulsion Marine motors are rated for continuous operation.   Many other electric propulsion companies rate motors for intermittent operation and do not describe the conditions of the intermittent operation.    Propulsion Marine rates Marine Electric Motors continuously because that is how they are used.    Consequently Propulsion Marine ratings are more conservative than the competition.

How does the number of windings of a motor affect the performance of the motor?

Motors with more windings turn slower with more torque than motors with higher number of windings.     For a given rpm, a motor with more windings will have a higher voltage to current ratio and will be more efficient until the ampacity of the windings is reached.    Our 5.5 KW motor has 28 turns on the stator for high efficiency at low power.

What is the relationship between power and rpm for a PMAC motor?

A PMAC motor develops power on an almost straight line relationship with rpm.    Double the rpm and you get double the power.

What is the relationship between voltage and rpm for a PMAC motor?

A PMAC motor has a voltage to rpm coefficient.     Double the AC motor voltage and you double the rpm of the motor.

What is the relationship between current and torque of a PMAC motor?

A PMAC motor has a current to torque coefficient.    Double the torque, and you double the current.

What is the relationship between current and heat generated by a PMAC motor?

The heat that is generated by a PMAC motor is a function of current.    Our 5.5 KW motor runs at approximately 1 degree C winding temperature for each amp of AC motor current.

How does a PMAC motor cool?

PMAC motors can be cooled by air or by water.   Our 5.5 KW motor is cooled by air so it is important to have adequate air flow in the compartment.    We provide a fan to introduce new air into the compartment.    The fan is turned on by a relay which turns on a 60 degrees C winding temperature.