Reliable 400 Hz HF-technology

400 Hz - High Frequency

   
High Frequency is a popular term used to describe a certain type of electric current, namely a current (Ampére) with a higher frequency than the standard 50 – 60 Hz found in most normal single or 3-phase supply.

Normal frequency = 50 or 60 HZ = 50 or 60 cycles per second.

High frequency, in our case, is maximum 500 Hz = 500 cycles per second.

High frequency, in the case of many of other suppliers of HF-concrete cutting equipment, is 1000 Hz = 1000 cycles per second.

 


More about 400 Hz vs. 1000 Hz »
 
 

Illustration of frequency
 

Why higher frequency than 50 Hz?


The reason we want to use a higher frequency than 50 Hz is that we want the electric motors to rotate at a higher speed than the standard motors.

If the torque is constant, then Higher Frequency (Hz) -> Higher speed (rpm) -> higher power.

An electric asynchronous 3-phase motor of a certain size, producing a certain torque, can produce a higher power output if the rotational speed of the motor is increased. To make the motor rotate quicker, the magnetic field must also rotate quicker and this is done by increasing the frequency of the current.

If the torque of the motor is kept constant, the power will increase proportionally with the increase in frequency. If the frequency is doubled, the speed of the motors is doubled, and if the torque is constant, also the power output is doubled.


400 Hz vs. 1000 Hz


As already explained, if the rotational speed of a motor is doubled, and the torque is kept constant, the output power is doubled. So, in theory, the higher the frequency the smaller and lighter the motor will be for the same output power.

But there is a practical side of everyting and we have looked more for reliability than to optimize the weight to power ratio. In contrast to most other manufacturers we have chosen to work with 400 Hz as our base frequency. This is an industry standard for motors used in the aircraft industry. A 4-pole motor driven by a 400 Hz current will rotate at a nominal speed of 12 000 rpm. A 4-pole motor driven by a 1000 Hz current will rotate at a nominal speed of 30 000 rpm.

  P = M x N
       9550

P = power in kW
M = torque in Nm
N = rotational speed in revolutions per minute
9550 = constant

Example for a 50 Hz system and a 4-pole motor, motor speed 1.450 rpm:

P = 11 Nm x 1450 rpm = 1.67 kW
                 9550

Example for a 440 Hz system and a 4-pole motor, motor speed 13.000 rpm:

P = 11 Nm x 13000 rpm = 15 kW
                 9550

Example for a 1000 Hz system and a 4-pole motor, motor speed 29.000 rpm:

P = 11 Nm x 29000 rpm = 33.4 kW
                 9550


Power is proportional to speed
As we at Tractive prefer to base our designs on sound principles, the 1000 Hz motors where never an option. To make bearings survive in this tough environment is not easy and it will cause the customer problems in the long run. We run our motors at a base frequency of 400 Hz and we go up to 440 Hz as a maximum. Thereby we can make the bearings live longer and there is no need for frequent maintenance.

To run a motor at 1000 Hz is a cheap way of getting a relatively powerful motor with a low weight. But there are too many disadvantages with such a system. As we run our motors at 12 000 rpm there is a weight penalty, but it can be compensated for by using a higher quality motor, namely a motor with a copper squirrel cage rotor. The hand built copper rotor offers about 30% higher output than a die cast aluminium rotor, but is more expensive to produce. Still, we know this is the right way, as reliablity always is a very important factor in our designs.
 
   

          
Stator and high quality copper squirrel cage rotor