This is the first post in a series that will explore the basics behind electric motors. Note that this series is intended for a general audience and so makes some simplifying assumptions. For a more in-depth treatment of the topic please refer to the recommended reading page.
High-level overview
Electric motors are power converters, taking electrical power and converting it to mechanical power with some losses along the way. The faster the motor spins, or the more torque you require, the greater the electrical power that must be supplied. The contour plot below shows the mechanical power output of a typical electric motor.
Deeper explanation
What is an electric motor?
An electric motor is typically defined as a device that converts electrical power into mechanical power.
Power in
Electric Power P_{elec} entering the motor is defined as:
P_{elec}= VI
where
- Voltage (V) has the unit of Volt with the symbol of V
- Current (I) has the unit of Ampere with the symbol of A
Typically, the electrical power entering an electric motor is either:
- Direct Current (DC), where the source current is constant with time or
- Alternating current (AC), where the source current is changing with time
As the name may suggest, DC motors use a DC power source while AC motors use an AC power source. At Kite, we exclusively deal with AC motors and so will focus on these. If a single AC power source is used then the power delivery is not constant with time. This is visualised if we display the power entering (VI) a single-phase motor as it rotates through 360 electrical degrees.
This variation in input power can lead to the mechanical output power also being irregular, causing noise and vibration. For this reason, three separate AC power sources that are 120 degrees out of phase are typically supplied to an AC motor so that the power delivery is constant with time.
The source of an AC current may be directly from the mains power grid or, more typically, from a 3-phase power inverter. In either case, regardless of the number of phases, the timing of the AC current must be synced with that of the motor rotational speed and position.
Note that more than 3 phases can be used, with 6 or more phases being used in some applications.
Power out
Mechanical Power (P_{mech}) exiting the motor is defined as:
P_{mech} = {\displaystyle \tau }ω
where Torque (τ) has the unit of newton-metre with the symbol of N.m and is produced by a force (F) at some distance (r) from a point of rotation and is defined by the following assuming the force is always 90 degrees.
At Kite, you will always see newton-metre with the symbol to N.m, not nm or Nm. This is because nm implies nano-meter, or less commonly nautical mile, while Nm implies N*m. Instead, N.m is typically interpreted as a vector product of r and F.
At Kite, we refer to RPM most of the time as it’s more commonly used and will save rad/s for when we need to make a calculation.
Power losses
No electric motor is perfectly efficient and so will have some power losses in the electrical to the mechanical power conversion process. These losses mean the electrical power input is always greater than the mechanical power out. Just how much more power needs to be supplied to the motor depends on its efficiency (\eta) and the output mechanical power.
P_{elec}= VI = {\displaystyle \tau }ω\eta = P_{mech}
The majority of the energy lost in the motor is in the form of heat, with some also lost as vibration which then also turns into heat. We explore the sources of energy loss in a motor in more detail in a future post.
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Get in touch by emailing Richard at richard@kitemagnetics.com
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